# Autoconf ¶

This manual (17 March 2026) is for GNU Autoconf (version 2.73), a package for creating scripts to configure source code packages using templates and an M4 macro package.

Copyright © 1992–1996, 1998–2017, 2020–2026 Free Software Foundation, Inc.

> Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.3 or any later version published by the Free Software Foundation; with no Invariant Sections, no Front-Cover texts, and no Back-Cover Texts. A copy of the license is included in the section entitled “GNU Free Documentation License.”

## Table of Contents

## 1 Introduction ¶

A physicist, an engineer, and a computer scientist were discussing the nature of God. “Surely a Physicist,” said the physicist, “because early in the Creation, God made Light; and you know, Maxwell’s equations, the dual nature of electromagnetic waves, the relativistic consequences...” “An Engineer!,” said the engineer, “because before making Light, God split the Chaos into Land and Water; it takes a hell of an engineer to handle that big amount of mud, and orderly separation of solids from liquids...” The computer scientist shouted: “And the Chaos, where do you think it was coming from, hmm?”

—Anonymous

Autoconf is a tool for producing shell scripts that automatically configure software source code packages to adapt to many kinds of POSIX-like systems. The configuration scripts produced by Autoconf are independent of Autoconf when they are run, so their users do not need to have Autoconf.

The configuration scripts produced by Autoconf require no manual user intervention when run; they do not normally even need an argument specifying the system type. Instead, they individually test for the presence of each feature that the software package they are for might need. (Before each check, they print a one-line message stating what they are checking for, so the user doesn’t get too bored while waiting for the script to finish.) As a result, they deal well with systems that are hybrids or customized from the more common POSIX variants. There is no need to maintain files that list the features supported by each release of each variant of POSIX.

For each software package that Autoconf is used with, it creates a configuration script from a template file that lists the system features that the package needs or can use. After the shell code to recognize and respond to a system feature has been written, Autoconf allows it to be shared by many software packages that can use (or need) that feature. If it later turns out that the shell code needs adjustment for some reason, it needs to be changed in only one place; all of the configuration scripts can be regenerated automatically to take advantage of the updated code.

Those who do not understand Autoconf are condemned to reinvent it, poorly. The primary goal of Autoconf is making the user’s life easier; making the maintainer’s life easier is only a secondary goal. Put another way, the primary goal is not to make the generation of `configure` automatic for package maintainers (although patches along that front are welcome, since package maintainers form the user base of Autoconf); rather, the goal is to make `configure` painless, portable, and predictable for the end user of each autoconfiscated package. And to this degree, Autoconf is highly successful at its goal—most complaints to the Autoconf list are about difficulties in writing Autoconf input, and not in the behavior of the resulting `configure`. Even packages that don’t use Autoconf will generally provide a `configure` script, and the most common complaint about these alternative home-grown scripts is that they fail to meet one or more of the GNU Coding Standards (see Configuration in The GNU Coding Standards) that users have come to expect from Autoconf-generated `configure` scripts.

The Metaconfig package is similar in purpose to Autoconf, but the scripts it produces require manual user intervention, which is quite inconvenient when configuring large source trees. Unlike Metaconfig scripts, Autoconf scripts can support cross-compiling, if some care is taken in writing them.

Autoconf does not solve all problems related to making portable software packages—for a more complete solution, it should be used in concert with other GNU build tools like Automake and Libtool. These other tools take on jobs like the creation of a portable, recursive makefile with all of the standard targets, linking of shared libraries, and so on. See The GNU Build System, for more information.

Autoconf imposes some restrictions on the names of macros used with `#if` in C programs (see Preprocessor Symbol Index).

Autoconf requires GNU M4 version 1.4.8 or later in order to generate the scripts. It uses features that some versions of M4, including GNU M4 1.3, do not have. Autoconf works better with GNU M4 version 1.4.16 or later, though this is not required.

See Upgrading From Version 1, for information about upgrading from version 1. See History of Autoconf, for the story of Autoconf’s development. See Frequent Autoconf Questions, with answers, for answers to some common questions about Autoconf.

See the Autoconf web page for up-to-date information, details on the mailing lists, pointers to a list of known bugs, etc.

Mail suggestions to the Autoconf mailing list. Past suggestions are archived.

Mail bug reports to the Autoconf Bugs mailing list. Past bug reports are archived.

If possible, first check that your bug is not already solved in current development versions, and that it has not been reported yet. Be sure to include all the needed information and a short `configure.ac` that demonstrates the problem.

Autoconf’s development tree is accessible via `git`; see the Autoconf Summary for details, or view the actual repository. Patches relative to the current `git` version can be sent for review to the Autoconf Patches mailing list, with discussion on prior patches archived; and all commits are posted in the read-only Autoconf Commit mailing list, which is also archived.

Because of its mission, the Autoconf package itself includes only a set of often-used macros that have already demonstrated their usefulness. Nevertheless, if you wish to share your macros, or find existing ones, see the Autoconf Macro Archive, which is kindly run by Peter Simons.

## 2 The GNU Build System ¶

Autoconf solves an important problem—reliable discovery of system-specific build and runtime information—but this is only one piece of the puzzle for the development of portable software. To this end, the GNU project has developed a suite of integrated utilities to finish the job Autoconf started: the GNU build system, whose most important components are Autoconf, Automake, and Libtool. In this chapter, we introduce you to those tools, point you to sources of more information, and try to convince you to use the entire GNU build system for your software.

### 2.1 Automake ¶

The ubiquity of `make` means that a makefile is almost the only viable way to distribute automatic build rules for software, but one quickly runs into its numerous limitations. Its lack of support for automatic dependency tracking, recursive builds in subdirectories, reliable timestamps (e.g., for network file systems), and so on, mean that developers must painfully (and often incorrectly) reinvent the wheel for each project. Portability is non-trivial, thanks to the quirks of `make` on many systems. On top of all this is the manual labor required to implement the many standard targets that users have come to expect (`make install`, `make distclean`, `make uninstall`, etc.). Since you are, of course, using Autoconf, you also have to insert repetitive code in your `Makefile.in` to recognize `@CC@`, `@CFLAGS@`, and other substitutions provided by `configure`. Into this mess steps Automake.

Automake allows you to specify your build needs in a `Makefile.am` file with a vastly simpler and more powerful syntax than that of a plain makefile, and then generates a portable `Makefile.in` for use with Autoconf. For example, the `Makefile.am` to build and install a simple “Hello world” program might look like:

    bin_PROGRAMS = hello
    hello_SOURCES = hello.c

The resulting `Makefile.in` (~400 lines) automatically supports all the standard targets, the substitutions provided by Autoconf, automatic dependency tracking, `VPATH` building, and so on. `make` builds the `hello` program, and `make install` installs it in `/usr/local/bin` (or whatever prefix was given to `configure`, if not `/usr/local`).

The benefits of Automake increase for larger packages (especially ones with subdirectories), but even for small programs the added convenience and portability can be substantial. And that’s not all...

### 2.2 Gnulib ¶

GNU software has a well-deserved reputation for running on many different types of systems. While our primary goal is to write software for the GNU system, many users and developers have been introduced to us through the systems that they were already using.

Gnulib is a central location for common GNU code, intended to be shared among free software packages. Its components are typically shared at the source level, rather than being a library that gets built, installed, and linked against. The idea is to copy files from Gnulib into your own source tree. There is no distribution tarball; developers should just grab source modules from the repository. The source files are available online, under various licenses, mostly GNU GPL or GNU LGPL.

Gnulib modules typically contain C source code along with Autoconf macros used to configure the source code. For example, the Gnulib `stdckdint` module implements a `stdckdint.h` header that nearly conforms to C23, even on older hosts that lack `stdckdint.h`. This module contains a source file for the replacement header, along with an Autoconf macro that arranges to use the replacement header on older systems.

For more information, consult the Gnulib website, https://www.gnu.org/software/gnulib/.

### 2.3 Libtool ¶

Often, one wants to build not only programs, but libraries, so that other programs can benefit from the fruits of your labor. Ideally, one would like to produce shared (dynamically linked) libraries, which can be used by multiple programs without duplication on disk or in memory and can be updated independently of the linked programs. Producing shared libraries portably, however, is the stuff of nightmares—each system has its own incompatible tools, compiler flags, and magic incantations. Fortunately, GNU provides a solution: Libtool.

Libtool handles all the requirements of building shared libraries for you, and at this time seems to be the only way to do so with any portability. It also handles many other headaches, such as: the interaction of Make rules with the variable suffixes of shared libraries, linking reliably with shared libraries before they are installed by the superuser, and supplying a consistent versioning system (so that different versions of a library can be installed or upgraded without breaking binary compatibility). Although Libtool, like Autoconf, can be used without Automake, it is most simply utilized in conjunction with Automake—there, Libtool is used automatically whenever shared libraries are needed, and you need not know its syntax.

### 2.4 Pointers ¶

Developers who are used to the simplicity of `make` for small projects on a single system might be daunted at the prospect of learning to use Automake and Autoconf. As your software is distributed to more and more users, however, you otherwise quickly find yourself putting lots of effort into reinventing the services that the GNU build tools provide, and making the same mistakes that they once made and overcame. (Besides, since you’re already learning Autoconf, Automake is a piece of cake.)

There are a number of places that you can go to for more information on the GNU build tools.

- Web

  The project home pages for Autoconf, Automake, Gnulib, and Libtool.

- Automake Manual

  See Automake in GNU Automake, for more information on Automake.

- Books

  The book GNU Autoconf, Automake and Libtool 1 describes the complete GNU build environment. You can also find the entire book on-line.

## 3 Making `configure` Scripts ¶

The configuration scripts that Autoconf produces are by convention called `configure`. When run, `configure` creates several files, replacing configuration parameters in them with appropriate values. The files that `configure` creates are:

- one or more `Makefile` files, usually one in each subdirectory of the package (see Substitutions in Makefiles);
- optionally, a C header file, the name of which is configurable, containing `#define` directives (see Configuration Header Files);
- a shell script called `config.status` that, when run, recreates the files listed above (see config.status Invocation);
- an optional shell script normally called `config.cache` (created when using ‘`configure --config-cache`’) that saves the results of running many of the tests (see Cache Files);
- a file called `config.log` containing any messages produced by compilers, to help debugging if `configure` makes a mistake.

To create a `configure` script with Autoconf, you need to write an Autoconf input file `configure.ac` and run `autoconf` on it. If you write your own feature tests to supplement those that come with Autoconf, you might also write files called `aclocal.m4` and `acsite.m4`. If you use a C header file to contain `#define` directives, you might also run `autoheader`, and you can distribute the generated file `config.h.in` with the package.

Here is a diagram showing how the files that can be used in configuration are produced. Programs that are executed are suffixed by ‘`*`’. Optional files are enclosed in square brackets (‘`[]`’). `autoconf` and `autoheader` also read the installed Autoconf macro files (by reading `autoconf.m4`).

Files used in preparing a software package for distribution, when using just Autoconf:

    your source files --> [autoscan*] --> [configure.scan] --> configure.ac

    configure.ac --.
                   |   .------> autoconf* -----> configure
    [aclocal.m4] --+---+
                   |   `-----> [autoheader*] --> [config.h.in]
    [acsite.m4] ---'

    Makefile.in

Additionally, if you use Automake, the following additional productions come into play:

    [acinclude.m4] --.
                     |
    [local macros] --+--> aclocal* --> aclocal.m4
                     |
    configure.ac ----'

    configure.ac --.
                   +--> automake* --> Makefile.in
    Makefile.am ---'

Files used in configuring a software package:

                           .-------------> [config.cache]
    configure* ------------+-------------> config.log
                           |
    [config.h.in] -.       v            .-> [config.h] -.
                   +--> config.status* -+               +--> make*
    Makefile.in ---'                    `-> Makefile ---'

### 3.1 Writing `configure.ac` ¶

To produce a `configure` script for a software package, create a file called `configure.ac` that contains invocations of the Autoconf macros that test the system features your package needs or can use. Autoconf macros already exist to check for many features; see Existing Tests, for their descriptions. For most other features, you can use Autoconf template macros to produce custom checks; see Writing Tests, for information about them. For especially tricky or specialized features, `configure.ac` might need to contain some hand-crafted shell commands; see Portable Shell Programming. The `autoscan` program can give you a good start in writing `configure.ac` (see Using `autoscan` to Create `configure.ac`, for more information).

Previous versions of Autoconf promoted the name `configure.in`, which is somewhat ambiguous (the tool needed to process this file is not described by its extension), and introduces a slight confusion with `config.h.in` and so on (for which ‘`.in`’ means “to be processed by `configure`”). Using `configure.ac` is now preferred, while the use of `configure.in` will cause warnings from `autoconf`.

#### 3.1.1 A Shell Script Compiler ¶

Just as for any other computer language, in order to properly program `configure.ac` in Autoconf you must understand what problem the language tries to address and how it does so.

The problem Autoconf addresses is that the world is a mess. After all, you are using Autoconf in order to have your package compile easily on all sorts of different systems, some of them being extremely hostile. Autoconf itself bears the price for these differences: `configure` must run on all those systems, and thus `configure` must limit itself to their lowest common denominator of features.

Naturally, you might then think of shell scripts; who needs `autoconf`? A set of properly written shell functions is enough to make it easy to write `configure` scripts by hand. Sigh! Unfortunately, even in 2008, where shells without any function support are far and few between, there are pitfalls to avoid when making use of them. Also, finding a Bourne shell that accepts shell functions is not trivial, even though there is almost always one on interesting porting targets.

So, what is really needed is some kind of compiler, `autoconf`, that takes an Autoconf program, `configure.ac`, and transforms it into a portable shell script, `configure`.

How does `autoconf` perform this task?

There are two obvious possibilities: creating a brand new language or extending an existing one. The former option is attractive: all sorts of optimizations could easily be implemented in the compiler and many rigorous checks could be performed on the Autoconf program (e.g., rejecting any non-portable construct). Alternatively, you can extend an existing language, such as the `sh` (Bourne shell) language.

Autoconf does the latter: it is a layer on top of `sh`. It was therefore most convenient to implement `autoconf` as a macro expander: a program that repeatedly performs macro expansions on text input, replacing macro calls with macro bodies and producing a pure `sh` script in the end. Instead of implementing a dedicated Autoconf macro expander, it is natural to use an existing general-purpose macro language, such as M4, and implement the extensions as a set of M4 macros.

#### 3.1.2 The Autoconf Language ¶

The Autoconf language differs from many other computer languages because it treats actual code the same as plain text. Whereas in C, for instance, data and instructions have different syntactic status, in Autoconf their status is rigorously the same. Therefore, we need a means to distinguish literal strings from text to be expanded: quotation.

When calling macros that take arguments, there must not be any white space between the macro name and the open parenthesis.

    AC_INIT ([oops], [1.0]) # incorrect
    AC_INIT([hello], [1.0]) # good

Arguments should be enclosed within the quote characters ‘`[`’ and ‘`]`’, and be separated by commas. Any leading blanks or newlines in arguments are ignored, unless they are quoted. You should always quote an argument that might contain a macro name, comma, parenthesis, or a leading blank or newline. This rule applies recursively for every macro call, including macros called from other macros. For more details on quoting rules, see Programming in M4.

For instance:

    AC_CHECK_HEADER([stdio.h],
                    [AC_DEFINE([HAVE_STDIO_H], [1],
                       [Define to 1 if you have <stdio.h>.])],
                    [AC_MSG_ERROR([sorry, can't do anything for you])])

is quoted properly. You may safely simplify its quotation to:

    AC_CHECK_HEADER([stdio.h],
                    [AC_DEFINE([HAVE_STDIO_H], 1,
                       [Define to 1 if you have <stdio.h>.])],
                    [AC_MSG_ERROR([sorry, can't do anything for you])])

because ‘`1`’ cannot contain a macro call. Here, the argument of `AC_MSG_ERROR` must be quoted; otherwise, its comma would be interpreted as an argument separator. Also, the second and third arguments of ‘`AC_CHECK_HEADER`’ must be quoted, since they contain macro calls. The three arguments ‘`HAVE_STDIO_H`’, ‘`stdio.h`’, and ‘`Define to 1 if you have <stdio.h>.`’ do not need quoting, but if you unwisely defined a macro with a name like ‘`Define`’ or ‘`stdio`’ then they would need quoting. Cautious Autoconf users would keep the quotes, but many Autoconf users find such precautions annoying, and would rewrite the example as follows:

    AC_CHECK_HEADER(stdio.h,
                    [AC_DEFINE(HAVE_STDIO_H, 1,
                       [Define to 1 if you have <stdio.h>.])],
                    [AC_MSG_ERROR([sorry, can't do anything for you])])

This is safe, so long as you adopt good naming conventions and do not define macros with names like ‘`HAVE_STDIO_H`’, ‘`stdio`’, or ‘`h`’. Though it is also safe here to omit the quotes around ‘`Define to 1 if you have <stdio.h>.`’ this is not recommended, as message strings are more likely to inadvertently contain commas.

The following example is wrong and dangerous, as it is underquoted:

    AC_CHECK_HEADER(stdio.h,
                    AC_DEFINE(HAVE_STDIO_H, 1,
                       Define to 1 if you have <stdio.h>.),
                    AC_MSG_ERROR([sorry, can't do anything for you]))

In other cases, you may want to use text that also resembles a macro call. You must quote that text (whether just the potential problem, or the entire line) even when it is not passed as a macro argument; and you may also have to use `m4_pattern_allow` (see Forbidden Patterns), to declare your intention that the resulting configure file will have a literal that resembles what would otherwise be reserved for a macro name. For example:

    dnl Simulate a possible future autoconf macro
    m4_define([AC_DC], [oops])
    dnl Underquoted:
    echo "Hard rock was here!  --AC_DC"
    dnl Correctly quoted:
    m4_pattern_allow([AC_DC])
    echo "Hard rock was here!  --[AC_DC]"
    [echo "Hard rock was here!  --AC_DC"]

which results in this text in `configure`:

    echo "Hard rock was here!  --oops"
    echo "Hard rock was here!  --AC_DC"
    echo "Hard rock was here!  --AC_DC"

When you use the same text in a macro argument, you must therefore have an extra quotation level (since one is stripped away by the macro substitution). In general, then, it is a good idea to use double quoting for all literal string arguments, either around just the problematic portions, or over the entire argument:

    m4_pattern_allow([AC_DC])
    AC_MSG_WARN([[AC_DC] stinks  --Iron Maiden])
    AC_MSG_WARN([[AC_DC stinks  --Iron Maiden]])

It is also possible to avoid the problematic patterns in the first place, by the use of additional escaping (either a quadrigraph, or creative shell constructs), in which case it is no longer necessary to use `m4_pattern_allow`:

    echo "Hard rock was here!  --AC""_DC"
    AC_MSG_WARN([[AC@&t@_DC stinks  --Iron Maiden]])

You are now able to understand one of the constructs of Autoconf that has been continually misunderstood... The rule of thumb is that whenever you expect macro expansion, expect quote expansion; i.e., expect one level of quotes to be lost. For instance:

    AC_COMPILE_IFELSE(AC_LANG_SOURCE([char b[10];]), [],
     [AC_MSG_ERROR([you lose])])

is incorrect: here, the first argument of `AC_LANG_SOURCE` is ‘`char b[10];`’ and is expanded once, which results in ‘`char b10;`’; and the `AC_LANG_SOURCE` is also expanded prior to being passed to `AC_COMPILE_IFELSE`. (There was an idiom common in Autoconf’s past to address this issue via the M4 `changequote` primitive, but do not use it!) Let’s take a closer look: the author meant the first argument to be understood as a literal, and therefore it must be quoted twice; likewise, the intermediate `AC_LANG_SOURCE` macro should be quoted once so that it is only expanded after the rest of the body of `AC_COMPILE_IFELSE` is in place:

    AC_COMPILE_IFELSE([AC_LANG_SOURCE([[char b[10];]])], [],
      [AC_MSG_ERROR([you lose])])

Voilà, you actually produce ‘`char b[10];`’ this time!

On the other hand, descriptions (e.g., the last parameter of `AC_DEFINE` or `AS_HELP_STRING`) are not literals—they are subject to line breaking, for example—and should not be double quoted. Even if these descriptions are short and are not actually broken, double quoting them yields weird results.

Some macros take optional arguments, which this documentation represents as [arg] (not to be confused with the quote characters). You may just leave them empty, or use ‘`[]`’ to make the emptiness of the argument explicit, or you may simply omit the trailing commas. The three lines below are equivalent:

    AC_CHECK_HEADERS([stdio.h], [], [], [])
    AC_CHECK_HEADERS([stdio.h],,,)
    AC_CHECK_HEADERS([stdio.h])

It is best to put each macro call on its own line in `configure.ac`. Most of the macros don’t add extra newlines; they rely on the newline after the macro call to terminate the commands. This approach makes the generated `configure` script a little easier to read by not inserting lots of blank lines. It is generally safe to set shell variables on the same line as a macro call, because the shell allows assignments without intervening newlines.

You can include comments in `configure.ac` files by starting them with the ‘`#`’. For example, it is helpful to begin `configure.ac` files with a line like this:

    # Process this file with autoconf to produce a configure script.

#### 3.1.3 Standard `configure.ac` Layout ¶

The order in which `configure.ac` calls the Autoconf macros is not important, with a few exceptions. Every `configure.ac` must contain a call to `AC_INIT` before the checks, and a call to `AC_OUTPUT` at the end (see Outputting Files). Additionally, some macros rely on other macros having been called first, because they check previously set values of some variables to decide what to do. These macros are noted in the individual descriptions (see Existing Tests), and they also warn you when `configure` is created if they are called out of order.

To encourage consistency, here is a suggested order for calling the Autoconf macros. Generally speaking, the things near the end of this list are those that could depend on things earlier in it. For example, library functions could be affected by types and libraries.

    Autoconf requirements
    AC_INIT(package, version, bug-report-address)
    information on the package
    checks for programs
    checks for libraries
    checks for header files
    checks for types
    checks for structures
    checks for compiler characteristics
    checks for library functions
    checks for system services
    AC_CONFIG_FILES([file...])
    AC_OUTPUT

### 3.2 Using `autoscan` to Create `configure.ac` ¶

The `autoscan` program can help you create and/or maintain a `configure.ac` file for a software package. `autoscan` examines source files in the directory tree rooted at a directory given as a command line argument, or the current directory if none is given. It searches the source files for common portability problems and creates a file `configure.scan` which is a preliminary `configure.ac` for that package, and checks a possibly existing `configure.ac` for completeness.

When using `autoscan` to create a `configure.ac`, you should manually examine `configure.scan` before renaming it to `configure.ac`; it probably needs some adjustments. Occasionally, `autoscan` outputs a macro in the wrong order relative to another macro, so that `autoconf` produces a warning; you need to move such macros manually. Also, if you want the package to use a configuration header file, you must add a call to `AC_CONFIG_HEADERS` (see Configuration Header Files). You might also have to change or add some `#if` directives to your program in order to make it work with Autoconf (see Using `ifnames` to List Conditionals, for information about a program that can help with that job).

When using `autoscan` to maintain a `configure.ac`, simply consider adding its suggestions. The file `autoscan.log` contains detailed information on why a macro is requested.

`autoscan` uses several data files (installed along with Autoconf) to determine which macros to output when it finds particular symbols in a package’s source files. These data files all have the same format: each line consists of a symbol, one or more blanks, and the Autoconf macro to output if that symbol is encountered. Lines starting with ‘`#`’ are comments.

`autoscan` accepts the following options:

`--help`
`-h`

  Print a summary of the command line options and exit.

`--version`
`-V`

  Print the version number of Autoconf and exit.

`--verbose`
`-v`

  Print the names of the files it examines and the potentially interesting symbols it finds in them. This output can be voluminous.

`--debug`
`-d`

  Don’t remove temporary files.

`--include=dir`
`-I dir`

  Append dir to the include path. Multiple invocations accumulate.

`--prepend-include=dir`
`-B dir`

  Prepend dir to the include path. Multiple invocations accumulate.

### 3.3 Using `ifnames` to List Conditionals ¶

`ifnames` can help you write `configure.ac` for a software package. It prints the identifiers that the package already uses in C preprocessor conditionals. If a package has already been set up to have some portability, `ifnames` can thus help you figure out what its `configure` needs to check for. It may help fill in some gaps in a `configure.ac` generated by `autoscan` (see Using `autoscan` to Create `configure.ac`).

`ifnames` scans all of the C source files named on the command line (or the standard input, if none are given) and writes to the standard output a sorted list of all the identifiers that appear in those files in `#if`, `#ifdef`, `#ifndef`, `#elif`, `#elifdef`, or `#elifndef` directives. It prints each identifier on a line, followed by a space-separated list of the files in which that identifier occurs.

`ifnames` accepts the following options:

`--help`
`-h`

  Print a summary of the command line options and exit.

`--version`
`-V`

  Print the version number of Autoconf and exit.

### 3.4 Using `autoconf` to Create `configure` ¶

To create `configure` from `configure.ac`, run the `autoconf` program with no arguments. `autoconf` processes `configure.ac` with the M4 macro processor, using the Autoconf macros. If you give `autoconf` an argument, it reads that file instead of `configure.ac` and writes the configuration script to the standard output instead of to `configure`. If you give `autoconf` the argument `-`, it reads from the standard input instead of `configure.ac` and writes the configuration script to the standard output.

The Autoconf macros are defined in several files. Some of the files are distributed with Autoconf; `autoconf` reads them first. Then it looks for the optional file `acsite.m4` in the directory that contains the distributed Autoconf macro files, and for the optional file `aclocal.m4` in the current directory. Those files can contain your site’s or the package’s own Autoconf macro definitions (see Writing Autoconf Macros, for more information). If a macro is defined in more than one of the files that `autoconf` reads, the last definition it reads overrides the earlier ones.

`autoconf` accepts the following options:

`--help`
`-h`

  Print a summary of the command line options and exit.

`--version`
`-V`

  Print the version number of Autoconf and exit.

`--verbose`
`-v`

  Report processing steps.

`--debug`
`-d`

  Don’t remove the temporary files.

`--force`
`-f`

  Remake `configure` even if newer than its input files.

`--include=dir`
`-I dir`

  Append dir to the include path. Multiple invocations accumulate.

`--prepend-include=dir`
`-B dir`

  Prepend dir to the include path. Multiple invocations accumulate.

`--output=file`
`-o file`

  Save output (script or trace) to file. The file `-` stands for the standard output.

`--warnings=category[,category...]` ¶
`-Wcategory[,category...]`

  Enable or disable warnings related to each category. See m4_warn, for a comprehensive list of categories. Special values include:

  ‘`all`’

    Enable all categories of warnings.

  ‘`none`’

    Disable all categories of warnings.

  ‘`error`’

    Treat all warnings as errors.

  ‘`no-category`’

    Disable warnings falling into category.

  The environment variable `WARNINGS` may also be set to a comma-separated list of warning categories to enable or disable. It is interpreted exactly the same way as the argument of `--warnings`, but unknown categories are silently ignored. The command line takes precedence; for instance, if `WARNINGS` is set to `obsolete`, but `-Wnone` is given on the command line, no warnings will be issued.

  Some categories of warnings are on by default. Again, for details see m4_warn.

`--trace=macro[:format]`
`-t macro[:format]`

  Do not create the `configure` script, but list the calls to macro according to the format. Multiple `--trace` arguments can be used to list several macros. Multiple `--trace` arguments for a single macro are not cumulative; instead, you should just make format as long as needed.

  The format is a regular string, with newlines if desired, and several special escape codes. It defaults to ‘`$f:$l:$n:$%`’; see Invoking `autom4te`, for details on the format.

`--initialization`
`-i`

  By default, `--trace` does not trace the initialization of the Autoconf macros (typically the `AC_DEFUN` definitions). This results in a noticeable speedup, but can be disabled by this option.

It is often necessary to check the content of a `configure.ac` file, but parsing it yourself is extremely fragile and error-prone. It is suggested that you rely upon `--trace` to scan `configure.ac`. For instance, to find the list of variables that are substituted, use:

    $ autoconf -t AC_SUBST
    configure.ac:28:AC_SUBST:SHELL
    configure.ac:28:AC_SUBST:PATH_SEPARATOR
    More traces deleted

The example below highlights the difference between ‘`$@`’, ‘`$*`’, and ‘`$%`’.

    $ cat configure.ac
    AC_DEFINE(This, is, [an
    [example]])
    $ autoconf -t 'AC_DEFINE:@: $@
    *: $*
    %: $%'
    @: [This],[is],[an
    [example]]
    *: This,is,an
    [example]
    %: This:is:an [example]

The format gives you a lot of freedom:

    $ autoconf -t 'AC_SUBST:$$ac_subst{"$1"} = "$f:$l";'
    $ac_subst{"SHELL"} = "configure.ac:28";
    $ac_subst{"PATH_SEPARATOR"} = "configure.ac:28";
    More traces deleted

A long separator can be used to improve the readability of complex structures, and to ease their parsing (for instance when no single character is suitable as a separator):

    $ autoconf -t 'AM_MISSING_PROG:${|:::::|}*'
    ACLOCAL|:::::|aclocal|:::::|$missing_dir
    AUTOCONF|:::::|autoconf|:::::|$missing_dir
    AUTOMAKE|:::::|automake|:::::|$missing_dir
    More traces deleted

### 3.5 Using `autoreconf` to Update `configure` Scripts ¶

Installing the various components of the GNU Build System can be tedious: running `autopoint` for Gettext, `automake` for `Makefile.in` etc. in each directory. It may be needed either because some tools such as `automake` have been updated on your system, or because some of the sources such as `configure.ac` have been updated, or finally, simply in order to install the GNU Build System in a fresh tree.

`autoreconf` runs `autoconf`, `autoheader`, `aclocal`, `automake`, `libtoolize`, `intltoolize`, `gtkdocize`, and `autopoint` (when appropriate) repeatedly to update the GNU Build System in the specified directories and their subdirectories (see Configuring Other Packages in Subdirectories). By default, it only remakes those files that are older than their sources.

If you install a new version of some tool, you can make `autoreconf` remake all of the files by giving it the `--force` option.

The environment variables `AUTOM4TE`, `AUTOCONF`, `AUTOHEADER`, `AUTOMAKE`, `ACLOCAL`, `AUTOPOINT`, `LIBTOOLIZE`, `INTLTOOLIZE`, `GTKDOCIZE`, `M4`, and `MAKE` may be used to override the invocation of the respective tools.

Except for `AUTOM4TE` and `M4`, setting any of these enviroment variables to ‘`true`’, ‘`false`’, ‘`:`’, or the empty string has the same effect as passing ‘`--exclude=cmd`’ on the command line, where cmd is the lowercased version of the command’s name; see the description of the `--exclude` option, below.

See Automatic Remaking, for Make rules to automatically rebuild `configure` scripts when their source files change. That method handles the timestamps of configuration header templates properly, but does not pass `--autoconf-dir=dir` or `--localdir=dir`.

Gettext supplies the `autopoint` command to add translation infrastructure to a source package. If you use `autopoint`, your `configure.ac` should invoke `AM_GNU_GETTEXT` and one of `AM_GNU_GETTEXT_VERSION(gettext-version)` or `AM_GNU_GETTEXT_REQUIRE_VERSION(min-gettext-version)`. See Invoking the `autopoint` Program in GNU `gettext` utilities, for further details.

`autoreconf` accepts the following options:

`--help`
`-h`

  Print a summary of the command line options and exit.

`--version`
`-V`

  Print the version number of Autoconf and exit.

`--verbose`
`-v`

  Print the name of each directory `autoreconf` examines and the commands it runs. If given two or more times, pass `--verbose` to subordinate tools that support it.

`--debug`
`-d`

  Don’t remove the temporary files.

`--force`
`-f`

  Consider all generated and standard auxiliary files to be obsolete. This remakes even `configure` scripts and configuration headers that are newer than their input files (`configure.ac` and, if present, `aclocal.m4`).

  If deemed appropriate, this option triggers calls to ‘`automake --force-missing`’. Passing both `--force` and `--install` to `autoreconf` will in turn undo any customizations to standard files. Note that the macro `AM_INIT_AUTOMAKE` has some options which change the set of files considered to be standard.

`--replace-handwritten`
`-R`

  Replace files that could be generated but appear to have been written by hand. Currently this only affects the treatment of the primary configuration header template (usually `config.h.in`; see Configuration Header Templates). In the future it may also affect `aclocal.m4` and others.

`--install`
`-i`

  Install any missing standard auxiliary files in the package. By default, files are copied; this can be changed with `--symlink`.

  If deemed appropriate, this option triggers calls to ‘`automake --add-missing`’, ‘`libtoolize`’, ‘`autopoint`’, etc.

`--no-recursive`

  Do not rebuild files in subdirectories to configure (see Configuring Other Packages in Subdirectories, macro `AC_CONFIG_SUBDIRS`).

`--symlink`
`-s`

  When used with `--install`, install symbolic links to the missing auxiliary files instead of copying them.

`--make`
`-m`

  When the directories were configured, update the configuration by running ‘`./config.status --recheck && ./config.status`’, and then run ‘`make`’.

`--exclude=cmd[,cmd...]`
`-x cmd[,cmd...]`

  Do not run any cmd in the list, even if it seems to be needed. cmd can be ‘`autoconf`’, ‘`autoheader`’, ‘`automake`’, ‘`aclocal`’, ‘`autopoint`’, ‘`libtoolize`’, ‘`intltoolize`’, ‘`gtkdocize`’, or ‘`make`’.

  ‘`--exclude make`’ has priority over ‘`--make`’.

`--include=dir`
`-I dir`

  Append dir to the include path. Multiple invocations accumulate. Passed on to `aclocal`, `autoconf` and `autoheader` internally.

`--prepend-include=dir`
`-B dir`

  Prepend dir to the include path. Multiple invocations accumulate. Passed on to `autoconf` and `autoheader` internally.

`--warnings=category[,category...]` ¶
`-Wcategory[,category...]`

  Enable or disable warnings related to each category. See m4_warn, for a comprehensive list of categories. Special values include:

  ‘`all`’

    Enable all categories of warnings.

  ‘`none`’

    Disable all categories of warnings.

  ‘`error`’

    Treat all warnings as errors.

  ‘`no-category`’

    Disable warnings falling into category.

  The environment variable `WARNINGS` may also be set to a comma-separated list of warning categories to enable or disable. It is interpreted exactly the same way as the argument of `--warnings`, but unknown categories are silently ignored. The command line takes precedence; for instance, if `WARNINGS` is set to `obsolete`, but `-Wnone` is given on the command line, no warnings will be issued.

  Some categories of warnings are on by default. Again, for details see m4_warn.

If you want `autoreconf` to pass flags that are not listed here on to `aclocal`, set `ACLOCAL_AMFLAGS` in your `Makefile.am`. Due to a limitation in the Autoconf implementation these flags currently must be set on a single line in `Makefile.am`, without any backslash-newlines or makefile comments. Also, be aware that future Automake releases might start flagging `ACLOCAL_AMFLAGS` as obsolescent, or even remove support for it.

## 4 Initialization and Output Files ¶

Autoconf-generated `configure` scripts need some information about how to initialize, such as how to find the package’s source files and about the output files to produce. The following sections describe the initialization and the creation of output files.

### 4.1 Initializing `configure` ¶

Every `configure` script must call `AC_INIT` before doing anything else that produces output. Calls to silent macros, such as `AC_DEFUN`, may also occur prior to `AC_INIT`, although these are generally used via `aclocal.m4`, since that is implicitly included before the start of `configure.ac`. The only other required macro is `AC_OUTPUT` (see Outputting Files).

Macro: AC_INIT(package, version, [bug-report], [tarname], [url]) ¶

  Process any command-line arguments and perform initialization and verification.

  Set the name of the package and its version. These are typically used in `--version` support, including that of `configure`. The optional argument bug-report should be the email to which users should send bug reports. The package tarname differs from package: the latter designates the full package name (e.g., ‘`GNU Autoconf`’), while the former is meant for distribution tar ball names (e.g., ‘`autoconf`’). It defaults to package with ‘`GNU` ’ stripped, lower-cased, and all characters other than alphanumerics and underscores are changed to ‘`-`’. If provided, url should be the home page for the package.

  Leading and trailing whitespace is stripped from all the arguments to `AC_INIT`, and interior whitespace is collapsed to a single space. This means that, for instance, if you want to put several email addresses in bug-report, you can put each one on its own line:

      # We keep having problems with the mail hosting for
      # gnomovision.example, so give people an alternative.
      AC_INIT([Gnomovision], [17.0.1], [
          bugs@gnomovision.example
          or gnomo-bugs@reliable-email.example
      ])

  The arguments to `AC_INIT` may be computed by M4, when `autoconf` is run. For instance, if you want to include the package’s version number in the tarname, but you don’t want to repeat it, you can use a helper macro:

      m4_define([gnomo_VERSION], [17.0.1])
      AC_INIT([Gnomovision],
              m4_defn([gnomo_VERSION]),
              [bugs@gnomovision.example],
              [gnomo-]m4_defn([gnomo_VERSION]))

  This uses `m4_defn` to produce the expansion of `gnomo_VERSION` as a quoted string, so that if there happen to be any more M4 macro names in `gnomo_VERSION`, they will not be expanded. See Renaming Macros in GNU m4 macro processor.

  Continuing this example, if you don’t want to embed the version number in `configure.ac` at all, you can use `m4_esyscmd` to look it up somewhere else when `autoconf` is run:

      m4_define([gnomo_VERSION],
        m4_esyscmd([build-aux/git-version-gen .tarball-version]))
      AC_INIT([Gnomovision],
              m4_defn([gnomo_VERSION]),
              [bugs@gnomovision.example],
              [gnomo-]m4_defn([gnomo_VERSION]))

  This uses the utility script `git-version-gen` to look up the package’s version in its version control metadata. This script is part of Gnulib (see Gnulib).

  The arguments to `AC_INIT` are written into `configure` in several different places. Therefore, we strongly recommend that you write any M4 logic in `AC_INIT` arguments to be evaluated before `AC_INIT` itself is evaluated. For instance, in the above example, the second argument to `m4_define` is not quoted, so the `m4_esyscmd` is evaluated only once, and `gnomo_VERSION` is defined to the output of the command. If the second argument to `m4_define` were quoted, `m4_esyscmd` would be evaluated each time the version or tarname arguments were written to `configure`, and the command would be run repeatedly.

  In some of the places where the arguments to `AC_INIT` are used, within `configure`, shell evaluation cannot happen. Therefore, the arguments to `AC_INIT` may not be computed when `configure` is run. If they contain any construct that isn’t always treated as literal by the shell (e.g. variable expansions), `autoconf` will issue an error.

  The tarname argument is used to construct filenames. It should not contain wildcard characters, white space, or anything else that could be troublesome as part of a file or directory name.

  Some of M4’s active characters (notably parentheses, square brackets, ‘`,`’ and ‘`#`’) commonly appear in URLs and lists of email addresses. If any of these characters appear in an argument to AC_INIT, that argument will probably need to be double-quoted to avoid errors and mistranscriptions. See M4 Quotation.

  The following M4 macros (e.g., `AC_PACKAGE_NAME`), output variables (e.g., `PACKAGE_NAME`), and preprocessor symbols (e.g., `PACKAGE_NAME`), are defined by `AC_INIT`:

  `AC_PACKAGE_NAME`, `PACKAGE_NAME` ¶

    Exactly package.

  `AC_PACKAGE_TARNAME`, `PACKAGE_TARNAME` ¶

    Exactly tarname, possibly generated from package.

  `AC_PACKAGE_VERSION`, `PACKAGE_VERSION` ¶

    Exactly version.

  `AC_PACKAGE_STRING`, `PACKAGE_STRING` ¶

    Exactly ‘`package version`’.

  `AC_PACKAGE_BUGREPORT`, `PACKAGE_BUGREPORT` ¶

    Exactly bug-report, if one was provided. Typically an email address, or URL to a bug management web page.

  `AC_PACKAGE_URL`, `PACKAGE_URL` ¶

    Exactly url, if one was provided. If url was empty, but package begins with ‘`GNU` ’, then this defaults to ‘`https://www.gnu.org/software/tarname/`’, otherwise, no URL is assumed.

If your `configure` script does its own option processing, it should inspect ‘`$@`’ or ‘`$*`’ immediately after calling `AC_INIT`, because other Autoconf macros liberally use the `set` command to process strings, and this has the side effect of updating ‘`$@`’ and ‘`$*`’. However, we suggest that you use standard macros like `AC_ARG_ENABLE` instead of attempting to implement your own option processing. See Site Configuration.

### 4.2 Dealing with Autoconf versions ¶

The following optional macros can be used to help choose the minimum version of Autoconf that can successfully compile a given `configure.ac`.

Macro: AC_PREREQ(version) ¶

  Ensure that a recent enough version of Autoconf is being used. If the version of Autoconf being used to create `configure` is earlier than version, print an error message to the standard error output and exit with failure (exit status is 63). For example:

      AC_PREREQ([2.73])

  This macro may be used before `AC_INIT`.

Macro: AC_AUTOCONF_VERSION ¶

  This macro was introduced in Autoconf 2.62. It identifies the version of Autoconf that is currently parsing the input file, in a format suitable for `m4_version_compare` (see m4_version_compare); in other words, for this release of Autoconf, its value is ‘`2.73`’. One potential use of this macro is for writing conditional fallbacks based on when a feature was added to Autoconf, rather than using `AC_PREREQ` to require the newer version of Autoconf. However, remember that the Autoconf philosophy favors feature checks over version checks.

  You should not expand this macro directly; use ‘`m4_defn([AC_AUTOCONF_VERSION])`’ instead. This is because some users might have a beta version of Autoconf installed, with arbitrary letters included in its version string. This means it is possible for the version string to contain the name of a defined macro, such that expanding `AC_AUTOCONF_VERSION` would trigger the expansion of that macro during rescanning, and change the version string to be different than what you intended to check.

### 4.3 Notices in `configure` ¶

The following macros manage version numbers for `configure` scripts. Using them is optional.

Macro: AC_COPYRIGHT(copyright-notice) ¶

  State that, in addition to the Free Software Foundation’s copyright on the Autoconf macros, parts of your `configure` are covered by the copyright-notice.

  The copyright-notice shows up in both the head of `configure` and in ‘`configure --version`’.

Macro: AC_REVISION(revision-info) ¶

  Copy revision stamp revision-info into the `configure` script, with any dollar signs or double-quotes removed. This macro lets you put a revision stamp from `configure.ac` into `configure` that the traditional version control systems RCS and CVS can update, without these systems changing it again when you check in the resulting `configure`. That way, you can determine easily which revision of `configure.ac` a particular `configure` corresponds to.

  For example, this line in `configure.ac`:

      AC_REVISION([$Revision: 1.30 $])

  produces this in `configure`:

      #!/bin/sh
      # From configure.ac Revision: 1.30

### 4.4 Configure Input: Source Code, Macros, and Auxiliary Files ¶

The following macros help you manage the contents of your source tree.

Macro: AC_CONFIG_SRCDIR(unique-file-in-source-dir) ¶

  Distinguish this package’s source directory from other source directories that might happen to exist in the file system. unique-file-in-source-dir should name a file that is unique to this package. `configure` will verify that this file exists in `srcdir`, before it runs any other checks.

  Use of this macro is strongly recommended. It protects against people accidentally specifying the wrong directory with `--srcdir`. See `configure` Invocation, for more information.

Packages that use `aclocal` to generate `aclocal.m4` should declare where local macros can be found using `AC_CONFIG_MACRO_DIRS`.

Macro: AC_CONFIG_MACRO_DIRS(dir1 [dir2 ... dirN]) ¶
Macro: AC_CONFIG_MACRO_DIR(dir) ¶

  Specify the given directories as the location of additional local Autoconf macros. These macros are intended for use by commands like `autoreconf` or `aclocal` that trace macro calls; they should be called directly from `configure.ac` so that tools that install macros for `aclocal` can find the macros’ declarations. Tools that want to learn which directories have been selected should trace `AC_CONFIG_MACRO_DIR_TRACE`, which will be called once per directory.

  AC_CONFIG_MACRO_DIRS is the preferred form, and can be called multiple times and with multiple arguments; in such cases, directories in earlier calls are expected to be searched before directories in later calls, and directories appearing in the same call are expected to be searched in the order in which they appear in the call. For historical reasons, the macro AC_CONFIG_MACRO_DIR can also be used once, if it appears first, for tools such as older `libtool` that weren’t prepared to handle multiple directories. For example, a usage like

      AC_CONFIG_MACRO_DIR([dir1])
      AC_CONFIG_MACRO_DIRS([dir2])
      AC_CONFIG_MACRO_DIRS([dir3 dir4])

  will cause the trace of AC_CONFIG_MACRO_DIR_TRACE to appear four times, and should cause the directories to be searched in this order: ‘`dir1 dir2 dir3 dir4`’.

  Note that if you use `aclocal` from an Automake release prior to 1.13 to generate `aclocal.m4`, you must also set `ACLOCAL_AMFLAGS = -I dir1 [-I dir2 ... -I dirN]` in your top-level `Makefile.am`. Due to a limitation in the Autoconf implementation of `autoreconf`, these include directives currently must be set on a single line in `Makefile.am`, without any backslash-newlines or makefile comments.

Some Autoconf macros require auxiliary scripts. `AC_PROG_INSTALL` (see Particular Program Checks) requires a fallback implementation of `install` called `install-sh`, and the `AC_CANONICAL` macros (see Manual Configuration) require the system-identification scripts `config.sub` and `config.guess`. Third-party tools, such as Automake and Libtool, may require additional auxiliary scripts.

By default, `configure` looks for these scripts next to itself, in `srcdir`. For convenience when working with subdirectories with their own configure scripts (see Configuring Other Packages in Subdirectories), if the scripts are not in `srcdir` it will also look in `srcdir/..` and `srcdir/../..`. All of the scripts must be found in the same directory.

If these default locations are not adequate, or simply to reduce clutter at the top level of the source tree, packages can use `AC_CONFIG_AUX_DIR` to declare where to look for auxiliary scripts.

Macro: AC_CONFIG_AUX_DIR(dir) ¶

  Look for auxiliary scripts in dir. Normally, dir should be a relative path, which is taken as relative to `srcdir`. If dir is an absolute path or contains shell variables, however, it is used as-is.

  When the goal of using `AC_CONFIG_AUX_DIR` is to reduce clutter at the top level of the source tree, the conventional name for dir is `build-aux`. If you need portability to DOS variants, do not name the auxiliary directory `aux`. See File System Conventions.

Macro: AC_REQUIRE_AUX_FILE(file) ¶

  Declare that file is an auxiliary script needed by this configure script, and set the shell variable `ac_aux_dir` to the directory where it can be found. The value of `ac_aux_dir` is guaranteed to end with a ‘`/`’.

  Macros that need auxiliary scripts must use this macro to register each script they need.

`configure` checks for all the auxiliary scripts it needs on startup, and exits with an error if any are missing.

`autoreconf` also detects missing auxiliary scripts. When used with the `--install` option, `autoreconf` will try to add missing scripts to the directory specified by `AC_CONFIG_AUX_DIR`, or to the top level of the source tree if `AC_CONFIG_AUX_DIR` was not used. It can always do this for the scripts needed by Autoconf core macros: `install-sh`, `config.sub`, and `config.guess`. Many other commonly-needed scripts are installed by the third-party tools that `autoreconf` knows how to run, such as `missing` for Automake and `ltmain.sh` for Libtool.

If you are using Automake, auxiliary scripts will automatically be included in the tarball created by `make dist`. If you are not using Automake you will need to arrange for auxiliary scripts to be included in tarballs yourself. Auxiliary scripts should normally not be checked into a version control system, for the same reasons that `configure` shouldn’t be.

The scripts needed by Autoconf core macros can be found in `$(datadir)/autoconf/build-aux` of the Autoconf installation (see Installation Directory Variables). `install-sh` can be downloaded from https://git.savannah.gnu.org/cgit/automake.git/plain/lib/install-sh. `config.sub` and `config.guess` can be downloaded from https://git.savannah.gnu.org/cgit/config.git/tree/.

### 4.5 Outputting Files ¶

Every Autoconf script, e.g., `configure.ac`, should finish by calling `AC_OUTPUT`. That is the macro that generates and runs `config.status`, which in turn creates the makefiles and any other files resulting from configuration. This is the only required macro besides `AC_INIT` (see Configure Input: Source Code, Macros, and Auxiliary Files).

Macro: AC_OUTPUT ¶

  Generate `config.status` and launch it. Call this macro once, at the end of `configure.ac`.

  `config.status` performs all the configuration actions: all the output files (see Creating Configuration Files, macro `AC_CONFIG_FILES`), header files (see Configuration Header Files, macro `AC_CONFIG_HEADERS`), commands (see Running Arbitrary Configuration Commands, macro `AC_CONFIG_COMMANDS`), links (see Creating Configuration Links, macro `AC_CONFIG_LINKS`), subdirectories to configure (see Configuring Other Packages in Subdirectories, macro `AC_CONFIG_SUBDIRS`) are honored.

  The location of your `AC_OUTPUT` invocation is the exact point where configuration actions are taken: any code afterwards is executed by `configure` once `config.status` was run. If you want to bind actions to `config.status` itself (independently of whether `configure` is being run), see Running Arbitrary Configuration Commands.

Historically, the usage of `AC_OUTPUT` was somewhat different. See Obsolete Macros, for a description of the arguments that `AC_OUTPUT` used to support.

If you run `make` in subdirectories, you should run it using the `make` variable `MAKE`. Most versions of `make` set `MAKE` to the name of the `make` program plus any options it was given. (But many do not include in it the values of any variables set on the command line, so those are not passed on automatically.) Some old versions of `make` do not set this variable. The following macro allows you to use it even with those versions.

Macro: AC_PROG_MAKE_SET ¶

  If the Make command, `$MAKE` if set or else ‘`make`’, predefines `$(MAKE)`, define output variable `SET_MAKE` to be empty. Otherwise, define `SET_MAKE` to a macro definition that sets `$(MAKE)`, such as ‘`MAKE=make`’. Calls `AC_SUBST` for `SET_MAKE`.

If you use this macro, place a line like this in each `Makefile.in` that runs `MAKE` on other directories:

    @SET_MAKE@

### 4.6 Performing Configuration Actions ¶

`configure` is designed so that it appears to do everything itself, but there is actually a hidden slave: `config.status`. `configure` is in charge of examining your system, but it is `config.status` that actually takes the proper actions based on the results of `configure`. The most typical task of `config.status` is to instantiate files.

This section describes the common behavior of the four standard instantiating macros: `AC_CONFIG_FILES`, `AC_CONFIG_HEADERS`, `AC_CONFIG_COMMANDS` and `AC_CONFIG_LINKS`. They all have this prototype:

    AC_CONFIG_ITEMS(tag..., [commands], [init-cmds])

where the arguments are:

tag…

  A blank-or-newline-separated list of tags, which are typically the names of the files to instantiate.

  You are encouraged to use literals as tags. In particular, you should avoid

      AS_IF([...], [my_foos="$my_foos fooo"])
      AS_IF([...], [my_foos="$my_foos foooo"])
      AC_CONFIG_ITEMS([$my_foos])

  and use this instead:

      AS_IF([...], [AC_CONFIG_ITEMS([fooo])])
      AS_IF([...], [AC_CONFIG_ITEMS([foooo])])

  The macros `AC_CONFIG_FILES` and `AC_CONFIG_HEADERS` use special tag values: they may have the form ‘`output`’ or ‘`output:inputs`’. The file output is instantiated from its templates, inputs (defaulting to ‘`output.in`’).

  ‘`AC_CONFIG_FILES([Makefile:boiler/top.mk:boiler/bot.mk])`’, for example, asks for the creation of the file `Makefile` that contains the expansion of the output variables in the concatenation of `boiler/top.mk` and `boiler/bot.mk`.

  The special value ‘`-`’ might be used to denote the standard output when used in output, or the standard input when used in the inputs. You most probably don’t need to use this in `configure.ac`, but it is convenient when using the command line interface of `./config.status`, see config.status Invocation, for more details.

  The inputs may be absolute or relative file names. In the latter case they are first looked for in the build tree, and then in the source tree. Input files should be text files, and a line length below 2000 bytes should be safe.

commands

  Shell commands output literally into `config.status`, and associated with a tag that the user can use to tell `config.status` which commands to run. The commands are run each time a tag request is given to `config.status`, typically each time the file `tag` is created.

  The variables set during the execution of `configure` are not available here: you first need to set them via the init-cmds. Nonetheless the following variables are pre-computed:

  `srcdir` ¶

    The name of the top source directory, assuming that the working directory is the top build directory. This is what `configure`’s `--srcdir` option sets.

  `ac_top_srcdir` ¶

    The name of the top source directory, assuming that the working directory is the current build directory.

  `ac_top_build_prefix` ¶

    The name of the top build directory, assuming that the working directory is the current build directory. It can be empty, or else ends with a slash, so that you may concatenate it.

  `ac_srcdir` ¶

    The name of the corresponding source directory, assuming that the working directory is the current build directory.

  `tmp` ¶

    The name of a temporary directory within the build tree, which you can use if you need to create additional temporary files. The directory is cleaned up when `config.status` is done or interrupted. Please use package-specific file name prefixes to avoid clashing with files that `config.status` may use internally.

  The current directory refers to the directory (or pseudo-directory) containing the input part of tags. For instance, running

      AC_CONFIG_COMMANDS([deep/dir/out:in/in.in], [...], [...])

  with `--srcdir=../package` produces the following values:

      # Argument of --srcdir
      srcdir='../package'
      # Reversing deep/dir
      ac_top_build_prefix='../../'
      # Concatenation of $ac_top_build_prefix and srcdir
      ac_top_srcdir='../../../package'
      # Concatenation of $ac_top_srcdir and deep/dir
      ac_srcdir='../../../package/deep/dir'

  independently of ‘`in/in.in`’.

init-cmds

  Shell commands output unquoted near the beginning of `config.status`, and executed each time `config.status` runs (regardless of the tag). Because they are unquoted, for example, ‘`$var`’ is output as the value of `var`. init-cmds is typically used by `configure` to give `config.status` some variables it needs to run the commands.

  You should be extremely cautious in your variable names: all the init-cmds share the same name space and may overwrite each other in unpredictable ways. Sorry...

All these macros can be called multiple times, with different tag values, of course!

### 4.7 Creating Configuration Files ¶

Be sure to read the previous section, Performing Configuration Actions.

Macro: AC_CONFIG_FILES(file…, [cmds], [init-cmds]) ¶

  Make `AC_OUTPUT` create each `file` by copying an input file (by default `file.in`), substituting the output variable values. This macro is one of the instantiating macros; see Performing Configuration Actions. See Substitutions in Makefiles, for more information on using output variables. See Setting Output Variables, for more information on creating them. This macro creates the directory that the file is in if it doesn’t exist. Usually, makefiles are created this way, but other files, such as `.gdbinit`, can be specified as well.

  Typical calls to `AC_CONFIG_FILES` look like this:

      AC_CONFIG_FILES([Makefile src/Makefile man/Makefile X/Imakefile])
      AC_CONFIG_FILES([autoconf], [chmod +x autoconf])

  You can override an input file name by appending to file a colon-separated list of input files. Examples:

      AC_CONFIG_FILES([Makefile:boiler/top.mk:boiler/bot.mk]
                      [lib/Makefile:boiler/lib.mk])

  Doing this allows you to keep your file names acceptable to DOS variants, or to prepend and/or append boilerplate to the file.

  The file names should not contain shell metacharacters. See Special Characters in Output Variables.

### 4.8 Substitutions in Makefiles ¶

Each subdirectory in a distribution that contains something to be compiled or installed should come with a file `Makefile.in`, from which `configure` creates a file `Makefile` in that directory. To create `Makefile`, `configure` performs a simple variable substitution, replacing occurrences of ‘`@variable@`’ in `Makefile.in` with the value that `configure` has determined for that variable. Variables that are substituted into output files in this way are called output variables. They are ordinary shell variables that are set in `configure`. To make `configure` substitute a particular variable into the output files, the macro `AC_SUBST` must be called with that variable name as an argument. Any occurrences of ‘`@variable@`’ for other variables are left unchanged. See Setting Output Variables, for more information on creating output variables with `AC_SUBST`.

A software package that uses a `configure` script should be distributed with a file `Makefile.in`, but no makefile; that way, the user has to properly configure the package for the local system before compiling it.

See Makefile Conventions in The GNU Coding Standards, for more information on what to put in makefiles.

#### 4.8.1 Preset Output Variables ¶

Some output variables are preset by the Autoconf macros. Some of the Autoconf macros set additional output variables, which are mentioned in the descriptions for those macros. See Output Variable Index, for a complete list of output variables. See Installation Directory Variables, for the list of the preset ones related to installation directories. Below are listed the other preset ones, many of which are precious variables (see Setting Output Variables, `AC_ARG_VAR`).

The preset variables which are available during `config.status` (see Performing Configuration Actions) may also be used during `configure` tests. For example, it is permissible to reference ‘`$srcdir`’ when constructing a list of directories to pass via the `-I` option during a compiler feature check. When used in this manner, coupled with the fact that `configure` is always run from the top build directory, it is sufficient to use just ‘`$srcdir`’ instead of ‘`$top_srcdir`’.

Variable: CFLAGS ¶

  Debugging and optimization options for the C compiler. If it is not set in the environment when `configure` runs, the default value is set when you call `AC_PROG_CC` (or empty if you don’t). `configure` uses this variable when compiling or linking programs to test for C features.

  If a compiler option affects only the behavior of the preprocessor (e.g., `-Dname`), it should be put into `CPPFLAGS` instead. If it affects only the linker (e.g., `-Ldirectory`), it should be put into `LDFLAGS` instead. If it affects only the compiler proper, `CFLAGS` is the natural home for it. If an option affects multiple phases of the compiler, though, matters get tricky:

  - If an option selects a 32-bit or 64-bit build on a bi-arch system, it must be put direcly into `CC`, e.g., `CC='gcc -m64'`. This is necessary for `config.guess` to work right.
  - Otherwise one approach is to put the option into `CC`. Another is to put it into both `CPPFLAGS` and `LDFLAGS`, but not into `CFLAGS`.

  However, remember that some `Makefile` variables are reserved by the GNU Coding Standards for the use of the “user”—the person building the package. For instance, `CFLAGS` is one such variable.

  Sometimes package developers are tempted to set user variables such as `CFLAGS` because it appears to make their job easier. However, the package itself should never set a user variable, particularly not to include switches that are required for proper compilation of the package. Since these variables are documented as being for the package builder, that person rightfully expects to be able to override any of these variables at build time. If the package developer needs to add switches without interfering with the user, the proper way to do that is to introduce an additional variable. Automake makes this easy by introducing `AM_CFLAGS` (see Flag Variables Ordering in GNU Automake), but the concept is the same even if Automake is not used.

Variable: configure_input ¶

  A comment saying that the file was generated automatically by `configure` and giving the name of the input file. `AC_OUTPUT` adds a comment line containing this variable to the top of every makefile it creates. For other files, you should reference this variable in a comment at the top of each input file. For example, an input shell script should begin like this:

      #!/bin/sh
      # @configure_input@

  The presence of that line also reminds people editing the file that it needs to be processed by `configure` in order to be used.

Variable: CPPFLAGS ¶

  Preprocessor options for the C, C++, Objective C, and Objective C++ preprocessors and compilers. If it is not set in the environment when `configure` runs, the default value is empty. `configure` uses this variable when preprocessing or compiling programs to test for C, C++, Objective C, and Objective C++ features.

  This variable’s contents should contain options like `-I`, `-D`, and `-U` that affect only the behavior of the preprocessor. Please see the explanation of `CFLAGS` for what you can do if an option affects other phases of the compiler as well.

  Currently, `configure` always links as part of a single invocation of the compiler that also preprocesses and compiles, so it uses this variable also when linking programs. However, it is unwise to depend on this behavior because the GNU Coding Standards do not require it and many packages do not use `CPPFLAGS` when linking programs.

  See Special Characters in Output Variables, for limitations that `CPPFLAGS` might run into.

Variable: CXXFLAGS ¶

  Debugging and optimization options for the C++ compiler. It acts like `CFLAGS`, but for C++ instead of C.

Variable: DEFS ¶

  `-D` options to pass to the C compiler. If `AC_CONFIG_HEADERS` is called, `configure` replaces ‘`@DEFS@`’ with `-DHAVE_CONFIG_H` instead (see Configuration Header Files). This variable is not defined while `configure` is performing its tests, only when creating the output files. See Setting Output Variables, for how to check the results of previous tests.

Variable: ECHO_C ¶
Variable: ECHO_N ¶
Variable: ECHO_T ¶

  These obsolescent variables let you suppress the trailing newline from `echo` for question-answer message pairs. Nowadays it is better to use `AS_ECHO_N`.

Variable: ERLCFLAGS ¶

  Debugging and optimization options for the Erlang compiler. If it is not set in the environment when `configure` runs, the default value is empty. `configure` uses this variable when compiling programs to test for Erlang features.

Variable: FCFLAGS ¶

  Debugging and optimization options for the Fortran compiler. If it is not set in the environment when `configure` runs, the default value is set when you call `AC_PROG_FC` (or empty if you don’t). `configure` uses this variable when compiling or linking programs to test for Fortran features.

Variable: FFLAGS ¶

  Debugging and optimization options for the Fortran 77 compiler. If it is not set in the environment when `configure` runs, the default value is set when you call `AC_PROG_F77` (or empty if you don’t). `configure` uses this variable when compiling or linking programs to test for Fortran 77 features.

Variable: LDFLAGS ¶

  Options for the linker. If it is not set in the environment when `configure` runs, the default value is empty. `configure` uses this variable when linking programs to test for features in C, C++, Objective C, Objective C++, Fortran, Go, and Algol 68.

  This variable’s contents should contain options like `-s` and `-L` that affect only the behavior of the linker. Please see the explanation of `CFLAGS` for what you can do if an option also affects other phases of the compiler.

  Don’t use this variable to pass library names (`-l`) to the linker; use `LIBS` instead.

Variable: LIBS ¶

  `-l` options to pass to the linker. The default value is empty, but some Autoconf macros may prepend extra libraries to this variable if those libraries are found and provide necessary functions, see Library Files. `configure` uses this variable when linking programs to test for C, C++, Objective C, Objective C++, Fortran, and Go features.

Variable: OBJCFLAGS ¶

  Debugging and optimization options for the Objective C compiler. It acts like `CFLAGS`, but for Objective C instead of C.

Variable: OBJCXXFLAGS ¶

  Debugging and optimization options for the Objective C++ compiler. It acts like `CXXFLAGS`, but for Objective C++ instead of C++.

Variable: GOFLAGS ¶

  Debugging and optimization options for the Go compiler. It acts like `CFLAGS`, but for Go instead of C.

Variable: A68FLAGS ¶

  Debugging and optimization options for the Algol 68 compiler. It acts like `CFLAGS`, but for Algol 68 instead of C.

Variable: builddir ¶

  Rigorously equal to ‘`.`’. Added for symmetry only.

Variable: abs_builddir ¶

  Absolute name of `builddir`.

Variable: top_builddir ¶

  The relative name of the top level of the current build tree. In the top-level directory, this is the same as `builddir`.

Variable: top_build_prefix ¶

  The relative name of the top level of the current build tree with final slash if nonempty. This is the same as `top_builddir`, except that it contains zero or more runs of `../`, so it should not be appended with a slash for concatenation. This helps for `make` implementations that otherwise do not treat `./file` and `file` as equal in the top-level build directory.

Variable: abs_top_builddir ¶

  Absolute name of `top_builddir`.

Variable: srcdir ¶

  The name of the directory that contains the source code for that makefile.

Variable: abs_srcdir ¶

  Absolute name of `srcdir`.

Variable: top_srcdir ¶

  The name of the top-level source code directory for the package. In the top-level directory, this is the same as `srcdir`.

Variable: abs_top_srcdir ¶

  Absolute name of `top_srcdir`.

#### 4.8.2 Installation Directory Variables ¶

The following variables specify the directories for package installation, see Variables for Installation Directories in The GNU Coding Standards, for more information. Each variable corresponds to an argument of `configure`; trailing slashes are stripped so that expressions such as ‘`${prefix}/lib`’ expand with only one slash between directory names. See the end of this section for details on when and how to use these variables.

Variable: bindir ¶

  The directory for installing executables that users run.

Variable: datadir ¶

  The directory for installing idiosyncratic read-only architecture-independent data.

Variable: datarootdir ¶

  The root of the directory tree for read-only architecture-independent data files.

Variable: docdir ¶

  The directory for installing documentation files (other than Info and man).

Variable: dvidir ¶

  The directory for installing documentation files in DVI format.

Variable: exec_prefix ¶

  The installation prefix for architecture-dependent files. By default it’s the same as `prefix`. You should avoid installing anything directly to `exec_prefix`. However, the default value for directories containing architecture-dependent files should be relative to `exec_prefix`.

Variable: htmldir ¶

  The directory for installing HTML documentation.

Variable: includedir ¶

  The directory for installing C header files.

Variable: infodir ¶

  The directory for installing documentation in Info format.

Variable: libdir ¶

  The directory for installing object code libraries.

Variable: libexecdir ¶

  The directory for installing executables that other programs run.

Variable: localedir ¶

  The directory for installing locale-dependent but architecture-independent data, such as message catalogs. This directory usually has a subdirectory per locale.

Variable: localstatedir ¶

  The directory for installing modifiable single-machine data. Content in this directory typically survives a reboot.

Variable: runstatedir ¶

  The directory for installing temporary modifiable single-machine data. Content in this directory survives as long as the process is running (such as pid files), as contrasted with `/tmp` that may be periodically cleaned. Conversely, this directory is typically cleaned on a reboot. By default, this is a subdirectory of `localstatedir`.

Variable: mandir ¶

  The top-level directory for installing documentation in man format.

Variable: oldincludedir ¶

  The directory for installing C header files for non-GCC compilers.

Variable: pdfdir ¶

  The directory for installing PDF documentation.

Variable: prefix ¶

  The common installation prefix for all files. If `exec_prefix` is defined to a different value, `prefix` is used only for architecture-independent files.

Variable: psdir ¶

  The directory for installing PostScript documentation.

Variable: sbindir ¶

  The directory for installing executables that system administrators run.

Variable: sharedstatedir ¶

  The directory for installing modifiable architecture-independent data.

Variable: sysconfdir ¶

  The directory for installing read-only single-machine data.

Most of these variables have values that rely on `prefix` or `exec_prefix`. It is deliberate that the directory output variables keep them unexpanded: typically ‘`@datarootdir@`’ is replaced by ‘`${prefix}/share`’, not ‘`/usr/local/share`’, and ‘`@datadir@`’ is replaced by ‘`${datarootdir}`’.

This behavior is mandated by the GNU Coding Standards, so that when the user runs:

‘`make`’

  she can still specify a different prefix from the one specified to `configure`, in which case, if needed, the package should hard code dependencies corresponding to the make-specified prefix.

‘`make install`’

  she can specify a different installation location, in which case the package must still depend on the location which was compiled in (i.e., never recompile when ‘`make install`’ is run). This is an extremely important feature, as many people may decide to install all the files of a package grouped together, and then install links from the final locations to there.

In order to support these features, it is essential that `datarootdir` remains defined as ‘`${prefix}/share`’, so that its value can be expanded based on the current value of `prefix`.

A corollary is that you should not use these variables except in makefiles. For instance, instead of trying to evaluate `datadir` in `configure` and hard-coding it in makefiles using e.g., ‘`AC_DEFINE_UNQUOTED([DATADIR], ["$datadir"], [Data directory.])`’, you should add `-DDATADIR='$(datadir)'` to your makefile’s definition of `CPPFLAGS` (`AM_CPPFLAGS` if you are also using Automake).

Similarly, you should not rely on `AC_CONFIG_FILES` to replace `bindir` and friends in your shell scripts and other files; instead, let `make` manage their replacement. For instance Autoconf ships templates of its shell scripts ending with ‘`.in`’, and uses a makefile snippet similar to the following to build scripts like `autoheader` and `autom4te`:

    edit = sed \
            -e 's|@bindir[@]|$(bindir)|g' \
            -e 's|@pkgdatadir[@]|$(pkgdatadir)|g' \
            -e 's|@prefix[@]|$(prefix)|g'

    autoheader autom4te: Makefile
            rm -f $@ $@.tmp
            srcdir=''; \
              test -f ./$@.in || srcdir=$(srcdir)/; \
              $(edit) $${srcdir}$@.in >$@.tmp
            chmod +x $@.tmp
            chmod a-w $@.tmp
            mv $@.tmp $@

    autoheader: $(srcdir)/autoheader.in
    autom4te: $(srcdir)/autom4te.in

Some details are noteworthy:

‘`@bindir[@]`’

  The brackets prevent `configure` from replacing ‘`@bindir@`’ in the Sed expression itself. Brackets are preferable to a backslash here, since POSIX says ‘`\@`’ is not portable.

‘`$(bindir)`’

  Don’t use ‘`@bindir@`’! Use the matching makefile variable instead.

‘`$(pkgdatadir)`’

  The example takes advantage of the variable ‘`$(pkgdatadir)`’ provided by Automake; it is equivalent to ‘`$(datadir)/$(PACKAGE)`’.

‘`/`’

  Don’t use ‘`/`’ in the Sed expressions that replace file names since most likely the variables you use, such as ‘`$(bindir)`’, contain ‘`/`’. Use a shell metacharacter instead, such as ‘`|`’.

special characters

  File names, file name components, and the value of `VPATH` should not contain shell metacharacters or white space. See Special Characters in Output Variables.

dependency on `Makefile`

  Since `edit` uses values that depend on the configuration specific values (`prefix`, etc.) and not only on `VERSION` and so forth, the output depends on `Makefile`, not `configure.ac`.

‘`$@`’

  The main rule is generic, and uses ‘`$@`’ extensively to avoid the need for multiple copies of the rule.

Separated dependencies and single suffix rules

  You can’t use them! The above snippet cannot be (portably) rewritten as:

      autoconf autoheader: Makefile

      .in:
              rm -f $@ $@.tmp
              $(edit) $< >$@.tmp
              chmod +x $@.tmp
              mv $@.tmp $@

  See Single Suffix Rules and Separated Dependencies, for details.

‘`$(srcdir)`’

  Be sure to specify the name of the source directory, otherwise the package won’t support separated builds.

For the more specific installation of Erlang libraries, the following variables are defined:

Variable: ERLANG_INSTALL_LIB_DIR ¶

  The common parent directory of Erlang library installation directories. This variable is set by calling the `AC_ERLANG_SUBST_INSTALL_LIB_DIR` macro in `configure.ac`.

Variable: ERLANG_INSTALL_LIB_DIR_library ¶

  The installation directory for Erlang library library. This variable is set by using the ‘`AC_ERLANG_SUBST_INSTALL_LIB_SUBDIR`’ macro in `configure.ac`.

See Erlang Libraries, for details.

#### 4.8.3 Changed Directory Variables ¶

In Autoconf 2.60, the set of directory variables has changed, and the defaults of some variables have been adjusted (see Installation Directory Variables) to changes in the GNU Coding Standards. Notably, `datadir`, `infodir`, and `mandir` are now expressed in terms of `datarootdir`. If you are upgrading from an earlier Autoconf version, you may need to adjust your files to ensure that the directory variables are substituted correctly (see How Do I `#define` Installation Directories?), and that a definition of `datarootdir` is in place. For example, in a `Makefile.in`, adding

    datarootdir = @datarootdir@

is usually sufficient. If you use Automake to create `Makefile.in`, it will add this for you.

To help with the transition, Autoconf warns about files that seem to use `datarootdir` without defining it. In some cases, it then expands the value of `$datarootdir` in substitutions of the directory variables. The following example shows such a warning:

    $ cat configure.ac
    AC_INIT
    AC_CONFIG_FILES([Makefile])
    AC_OUTPUT
    $ cat Makefile.in
    prefix = @prefix@
    datadir = @datadir@
    $ autoconf
    $ configure
    configure: creating ./config.status
    config.status: creating Makefile
    config.status: WARNING:
                   Makefile.in seems to ignore the --datarootdir setting
    $ cat Makefile
    prefix = /usr/local
    datadir = ${prefix}/share

Usually one can easily change the file to accommodate both older and newer Autoconf releases:

    $ cat Makefile.in
    prefix = @prefix@
    datarootdir = @datarootdir@
    datadir = @datadir@
    $ configure
    configure: creating ./config.status
    config.status: creating Makefile
    $ cat Makefile
    prefix = /usr/local
    datarootdir = ${prefix}/share
    datadir = ${datarootdir}

In some cases, however, the checks may not be able to detect that a suitable initialization of `datarootdir` is in place, or they may fail to detect that such an initialization is necessary in the output file. If, after auditing your package, there are still spurious `configure` warnings about `datarootdir`, you may add the line

    AC_DEFUN([AC_DATAROOTDIR_CHECKED])

to your `configure.ac` to disable the warnings. This is an exception to the usual rule that you should not define a macro whose name begins with `AC_` (see Macro Names).

#### 4.8.4 Build Directories ¶

You can support compiling a software package for several architectures simultaneously from the same copy of the source code. The object files for each architecture are kept in their own directory.

To support doing this, `make` uses the `VPATH` variable to find the files that are in the source directory. GNU Make can do this. Most other recent `make` programs can do this as well, though they may have difficulties and it is often simpler to recommend GNU `make` (see `VPATH` and Make). Older `make` programs do not support `VPATH`; when using them, the source code must be in the same directory as the object files.

If you are using GNU Automake, the remaining details in this section are already covered for you, based on the contents of your `Makefile.am`. But if you are using Autoconf in isolation, then supporting `VPATH` requires the following in your `Makefile.in`:

    srcdir = @srcdir@
    VPATH = @srcdir@

Do not set `VPATH` to the value of another variable (see Variables listed in `VPATH`.

`configure` substitutes the correct value for `srcdir` when it produces `Makefile`.

Do not use the `make` variable `$<`, which expands to the file name of the file in the source directory (found with `VPATH`), except in implicit rules. (An implicit rule is one such as ‘`.c.o`’, which tells how to create a `.o` file from a `.c` file.) Some versions of `make` do not set `$<` in explicit rules; they expand it to an empty value.

Instead, Make command lines should always refer to source files by prefixing them with ‘`$(srcdir)/`’. It’s safer to quote the source directory name, in case it contains characters that are special to the shell. Because ‘`$(srcdir)`’ is expanded by Make, single-quoting works and is safer than double-quoting. For example:

    time.info: time.texinfo
            $(MAKEINFO) '$(srcdir)/time.texinfo'

#### 4.8.5 Automatic Remaking ¶

You can put rules like the following in the top-level `Makefile.in` for a package to automatically update the configuration information when you change the configuration files. This example includes all of the optional files, such as `aclocal.m4` and those related to configuration header files. Omit from the `Makefile.in` rules for any of these files that your package does not use.

The ‘`$(srcdir)/`’ prefix is included because of limitations in the `VPATH` mechanism.

The `stamp-` files are necessary because the timestamps of `config.h.in` and `config.h` are not changed if remaking them does not change their contents. This feature avoids unnecessary recompilation. You should include the file `stamp-h.in` in your package’s distribution, so that `make` considers `config.h.in` up to date. Don’t use `touch` (see Limitations of Usual Tools); instead, use `echo` (using `date` would cause needless output differences).

    $(srcdir)/configure: configure.ac aclocal.m4
            cd '$(srcdir)' && autoconf

    # autoheader might not change config.h.in, so touch a stamp file.
    $(srcdir)/config.h.in: stamp-h.in ;
    $(srcdir)/stamp-h.in: configure.ac aclocal.m4
            cd '$(srcdir)' && autoheader
            echo timestamp > '$(srcdir)/stamp-h.in'

    config.h: stamp-h ;
    stamp-h: config.h.in config.status
            ./config.status

    Makefile: Makefile.in config.status
            ./config.status

    config.status: configure
            ./config.status --recheck

(Be careful if you copy these lines directly into your makefile, as you need to convert the indented lines to start with the tab character.)

In addition, you should use

    AC_CONFIG_FILES([stamp-h], [echo timestamp > stamp-h])

so `config.status` ensures that `config.h` is considered up to date. See Outputting Files, for more information about `AC_OUTPUT`.

See config.status Invocation, for more examples of handling configuration-related dependencies.

### 4.9 Configuration Header Files ¶

When a package contains more than a few tests that define C preprocessor symbols, the command lines to pass `-D` options to the compiler can get quite long. This causes two problems. One is that the `make` output is hard to visually scan for errors. More seriously, the command lines can exceed the length limits of some operating systems. As an alternative to passing `-D` options to the compiler, `configure` scripts can create a C header file containing ‘`#define`’ directives. The `AC_CONFIG_HEADERS` macro selects this kind of output. Though it can be called anywhere between `AC_INIT` and `AC_OUTPUT`, it is customary to call it right after `AC_INIT`.

The package should ‘`#include`’ the configuration header file before any other header files, to prevent inconsistencies in declarations (for example, if it redefines `const`, or if it defines a macro like `_FILE_OFFSET_BITS` that affects the behavior of system headers). Note that it is okay to only include `config.h` from `.c` files; the project’s `.h` files can rely on `config.h` already being included first by the corresponding `.c` file.

To provide for VPATH builds, remember to pass the C compiler a `-I.` option (or `-I..`; whichever directory contains `config.h`). Even if you use ‘`#include "config.h"`’, the preprocessor searches only the directory of the currently read file, i.e., the source directory, not the build directory.

With the appropriate `-I` option, you can use ‘`#include <config.h>`’. Actually, it’s a good habit to use it, because in the rare case when the source directory contains another `config.h`, the build directory should be searched first.

Macro: AC_CONFIG_HEADERS(header …, [cmds], [init-cmds]) ¶

  This macro is one of the instantiating macros; see Performing Configuration Actions. Make `AC_OUTPUT` create the file(s) in the blank-or-newline-separated list header containing C preprocessor `#define` statements, and replace ‘`@DEFS@`’ in generated files with `-DHAVE_CONFIG_H` instead of the value of `DEFS`. The usual name for header is `config.h`; header should not contain shell metacharacters. See Special Characters in Output Variables.

  If header already exists and its contents are identical to what `AC_OUTPUT` would put in it, it is left alone. Doing this allows making some changes in the configuration without needlessly causing object files that depend on the header file to be recompiled.

  Usually the input file is named `header.in`; however, you can override the input file name by appending to header a colon-separated list of input files. For example, you might need to make the input file name acceptable to DOS variants:

      AC_CONFIG_HEADERS([config.h:config.hin])

Macro: AH_HEADER ¶

  This macro is defined as the name of the first declared config header and undefined if no config headers have been declared up to this point. A third-party macro may, for example, require use of a config header without invoking AC_CONFIG_HEADERS twice, like this:

      AC_CONFIG_COMMANDS_PRE(
              [m4_ifndef([AH_HEADER], [AC_CONFIG_HEADERS([config.h])])])

See Performing Configuration Actions, for more details on header.

#### 4.9.1 Configuration Header Templates ¶

Your distribution should contain a template file that looks as you want the final header file to look, including comments, with `#undef` statements which are used as hooks. For example, suppose your `configure.ac` makes these calls:

    AC_CONFIG_HEADERS([conf.h])
    AC_CHECK_HEADERS([unistd.h])

Then you could have code like the following in `conf.h.in`. The `conf.h` created by `configure` defines ‘`HAVE_UNISTD_H`’ to 1, if and only if the system has `unistd.h`.

    /* Define as 1 if you have unistd.h.  */
    #undef HAVE_UNISTD_H

The format of the template file is stricter than what the C preprocessor is required to accept. A directive line should contain only whitespace, ‘`#undef`’, and ‘`HAVE_UNISTD_H`’. The use of ‘`#define`’ instead of ‘`#undef`’, or of comments on the same line as ‘`#undef`’, is strongly discouraged. Each hook should only be listed once. Other preprocessor lines, such as ‘`#ifdef`’ or ‘`#include`’, are copied verbatim from the template into the generated header.

Since it is a tedious task to keep a template header up to date, you may use `autoheader` to generate it, see Using `autoheader` to Create `config.h.in`.

During the instantiation of the header, each ‘`#undef`’ line in the template file for each symbol defined by ‘`AC_DEFINE`’ is changed to an appropriate ‘`#define`’. If the corresponding ‘`AC_DEFINE`’ has not been executed during the `configure` run, the ‘`#undef`’ line is commented out. (This is important, e.g., for ‘`_POSIX_SOURCE`’: on many systems, it can be implicitly defined by the compiler, and undefining it in the header would then break compilation of subsequent headers.)

Currently, all remaining ‘`#undef`’ lines in the header template are commented out, whether or not there was a corresponding ‘`AC_DEFINE`’ for the macro name; but this behavior is not guaranteed for future releases of Autoconf.

Generally speaking, since you should not use ‘`#define`’, and you cannot guarantee whether a ‘`#undef`’ directive in the header template will be converted to a ‘`#define`’ or commented out in the generated header file, the template file cannot be used for conditional definition effects. Consequently, if you need to use the construct

    #ifdef THIS
    # define THAT
    #endif

you must place it outside of the template. If you absolutely need to hook it to the config header itself, please put the directives to a separate file, and ‘`#include`’ that file from the config header template. If you are using `autoheader`, you would probably use ‘`AH_BOTTOM`’ to append the ‘`#include`’ directive.

#### 4.9.2 Using `autoheader` to Create `config.h.in` ¶

The `autoheader` program can create a template file of C ‘`#define`’ statements for `configure` to use. It searches for the first invocation of `AC_CONFIG_HEADERS` in `configure` sources to determine the name of the template. (If the first call of `AC_CONFIG_HEADERS` specifies more than one input file name, `autoheader` uses the first one.)

It is recommended that only one input file is used. If you want to append a boilerplate code, it is preferable to use ‘`AH_BOTTOM([#include <conf_post.h>])`’. File `conf_post.h` is not processed during the configuration then, which make things clearer. Analogically, `AH_TOP` can be used to prepend a boilerplate code.

In order to do its job, `autoheader` needs you to document all of the symbols that you might use. Typically this is done via an `AC_DEFINE` or `AC_DEFINE_UNQUOTED` call whose first argument is a literal symbol and whose third argument describes the symbol (see Defining C Preprocessor Symbols). Alternatively, you can use `AH_TEMPLATE` (see Autoheader Macros), or you can supply a suitable input file for a subsequent configuration header file. Symbols defined by Autoconf’s builtin tests are already documented properly; you need to document only those that you define yourself.

You might wonder why `autoheader` is needed: after all, why would `configure` need to “patch” a `config.h.in` to produce a `config.h` instead of just creating `config.h` from scratch? Well, when everything rocks, the answer is just that we are wasting our time maintaining `autoheader`: generating `config.h` directly is all that is needed. When things go wrong, however, you’ll be thankful for the existence of `autoheader`.

The fact that the symbols are documented is important in order to check that `config.h` makes sense. The fact that there is a well-defined list of symbols that should be defined (or not) is also important for people who are porting packages to environments where `configure` cannot be run: they just have to fill in the blanks.

But let’s come back to the point: the invocation of `autoheader`…

If you give `autoheader` an argument, it uses that file instead of `configure.ac` and writes the header file to the standard output instead of to `config.h.in`. If you give `autoheader` an argument of `-`, it reads the standard input instead of `configure.ac` and writes the header file to the standard output.

`autoheader` accepts the following options:

`--help`
`-h`

  Print a summary of the command line options and exit.

`--version`
`-V`

  Print the version number of Autoconf and exit.

`--verbose`
`-v`

  Report processing steps.

`--debug`
`-d`

  Don’t remove the temporary files.

`--force`
`-f`

  Remake the template file even if newer than its input files.

`--replace-handwritten`
`-R`

  Remake the template file even if it appears not to have been generated by `autoheader`.

`--include=dir`
`-I dir`

  Append dir to the include path. Multiple invocations accumulate.

`--prepend-include=dir`
`-B dir`

  Prepend dir to the include path. Multiple invocations accumulate.

`--warnings=category[,category...]` ¶
`-Wcategory[,category...]`

  Enable or disable warnings related to each category. See m4_warn, for a comprehensive list of categories. Special values include:

  ‘`all`’

    Enable all categories of warnings.

  ‘`none`’

    Disable all categories of warnings.

  ‘`error`’

    Treat all warnings as errors.

  ‘`no-category`’

    Disable warnings falling into category.

  The environment variable `WARNINGS` may also be set to a comma-separated list of warning categories to enable or disable. It is interpreted exactly the same way as the argument of `--warnings`, but unknown categories are silently ignored. The command line takes precedence; for instance, if `WARNINGS` is set to `obsolete`, but `-Wnone` is given on the command line, no warnings will be issued.

  Some categories of warnings are on by default. Again, for details see m4_warn.

#### 4.9.3 Autoheader Macros ¶

`autoheader` scans `configure.ac` and figures out which C preprocessor symbols it might define. It knows how to generate templates for symbols defined by `AC_CHECK_HEADERS`, `AC_CHECK_FUNCS` etc., but if you `AC_DEFINE` any additional symbol, you must define a template for it. If there are missing templates, `autoheader` fails with an error message.

The template for a symbol is created by `autoheader` from the description argument to an `AC_DEFINE`; see Defining C Preprocessor Symbols.

For special needs, you can use the following macros.

Macro: AH_TEMPLATE(key, description) ¶

  Tell `autoheader` to generate a template for key. This macro generates standard templates just like `AC_DEFINE` when a description is given.

  For example:

      AH_TEMPLATE([NULL_DEVICE],
        [Name of the file to open to get
         a null file, or a data sink.])

  generates the following template, with the description properly justified.

      /* Name of the file to open to get a null file, or a data sink. */
      #undef NULL_DEVICE

Macro: AH_VERBATIM(key, template) ¶

  Tell `autoheader` to include the template as-is in the header template file. This template is associated with the key, which is used to sort all the different templates and guarantee their uniqueness. It should be a symbol that can be defined via `AC_DEFINE`.

Macro: AH_TOP(text) ¶

  Include text at the top of the header template file.

Macro: AH_BOTTOM(text) ¶

  Include text at the bottom of the header template file.

Please note that text gets included “verbatim” to the template file, not to the resulting config header, so it can easily get mangled when the template is processed. There is rarely a need for something other than

    AH_BOTTOM([#include <custom.h>])

### 4.10 Running Arbitrary Configuration Commands ¶

You can execute arbitrary commands before, during, and after `config.status` is run. The three following macros accumulate the commands to run when they are called multiple times. `AC_CONFIG_COMMANDS` replaces the obsolete macro `AC_OUTPUT_COMMANDS`; see Obsolete Macros, for details.

Macro: AC_CONFIG_COMMANDS(tag…, [cmds], [init-cmds]) ¶

  Specify additional shell commands to run at the end of `config.status`, and shell commands to initialize any variables from `configure`. Associate the commands with tag. Since typically the cmds create a file, tag should naturally be the name of that file. If needed, the directory hosting tag is created. The tag should not contain shell metacharacters. See Special Characters in Output Variables. This macro is one of the instantiating macros; see Performing Configuration Actions.

  Here is an unrealistic example:

      fubar=42
      AC_CONFIG_COMMANDS([fubar],
        [AS_ECHO(["this is extra $fubar, and so on."])],
        [fubar=$fubar])

  Here is a better one:

      AC_CONFIG_COMMANDS([timestamp], [echo >timestamp])

The following two macros look similar, but in fact they are not of the same breed: they are executed directly by `configure`, so you cannot use `config.status` to rerun them.

Macro: AC_CONFIG_COMMANDS_PRE(cmds) ¶

  Execute the cmds right before creating `config.status`.

  This macro presents the last opportunity to call `AC_SUBST`, `AC_DEFINE`, or `AC_CONFIG_ITEMS` macros.

Macro: AC_CONFIG_COMMANDS_POST(cmds) ¶

  Execute the cmds right after creating `config.status`.

### 4.11 Creating Configuration Links ¶

You may find it convenient to create links whose destinations depend upon results of tests. One can use `AC_CONFIG_COMMANDS` but the creation of relative symbolic links can be delicate when the package is built in a directory different from the source directory.

Macro: AC_CONFIG_LINKS(dest:source…, [cmds], [init-cmds]) ¶

  Make `AC_OUTPUT` link each of the existing files source to the corresponding link name dest. Makes a symbolic link if possible, otherwise a hard link if possible, otherwise a copy. The dest and source names should be relative to the top level source or build directory, and should not contain shell metacharacters. See Special Characters in Output Variables.

  This macro is one of the instantiating macros; see Performing Configuration Actions.

  For example, this call:

      AC_CONFIG_LINKS([host.h:config/$machine.h
                      object.h:config/$obj_format.h])

  creates in the current directory `host.h` as a link to `srcdir/config/$machine.h`, and `object.h` as a link to `srcdir/config/$obj_format.h`.

  The tempting value ‘`.`’ for dest is invalid: it makes it impossible for ‘`config.status`’ to guess the links to establish.

  One can then run:

      ./config.status host.h object.h

  to create the links.

### 4.12 Configuring Other Packages in Subdirectories ¶

In most situations, calling `AC_OUTPUT` is sufficient to produce makefiles in subdirectories. However, `configure` scripts that control more than one independent package can use `AC_CONFIG_SUBDIRS` to run `configure` scripts for other packages in subdirectories.

Macro: AC_CONFIG_SUBDIRS(dir …) ¶

  Make `AC_OUTPUT` run `configure` in each subdirectory dir in the given blank-or-newline-separated list. Each dir should be a literal, i.e., please do not use:

      if test "x$package_foo_enabled" = xyes; then
        my_subdirs="$my_subdirs foo"
      fi
      AC_CONFIG_SUBDIRS([$my_subdirs])

  because this prevents ‘`./configure --help=recursive`’ from displaying the options of the package `foo`. Instead, you should write:

      AS_IF([test "x$package_foo_enabled" = xyes],
        [AC_CONFIG_SUBDIRS([foo])])

  If a given dir is not found at `configure` run time, a warning is reported; if the subdirectory is optional, write:

      AS_IF([test -d "$srcdir/foo"],
        [AC_CONFIG_SUBDIRS([foo])])

  These examples use `AS_IF` instead of ordinary shell `if` to avoid problems that Autoconf has with macro calls in shell conditionals outside macro definitions. See Common Shell Constructs.

  If a given dir contains `configure.gnu`, it is run instead of `configure`. This is for packages that might use a non-Autoconf script `Configure`, which can’t be called through a wrapper `configure` since it would be the same file on case-insensitive file systems.

  The subdirectory `configure` scripts are given the same command line options that were given to this `configure` script, with minor changes if needed, which include:

  - adjusting a relative name for the cache file;
  - adjusting a relative name for the source directory;
  - propagating the current value of `$prefix`, including if it was defaulted, and if the default values of the top level and of the subdirectory `configure` differ.

  This macro also sets the output variable `subdirs` to the list of directories ‘`dir …`’. Make rules can use this variable to determine which subdirectories to recurse into.

  This macro may be called multiple times.

### 4.13 Default Prefix ¶

By default, `configure` sets the prefix for files it installs to `/usr/local`. The user of `configure` can select a different prefix using the `--prefix` and `--exec-prefix` options. There are two ways to change the default: when creating `configure`, and when running it.

Some software packages might want to install in a directory other than `/usr/local` by default. To accomplish that, use the `AC_PREFIX_DEFAULT` macro.

Macro: AC_PREFIX_DEFAULT(prefix) ¶

  Set the default installation prefix to prefix instead of `/usr/local`.

It may be convenient for users to have `configure` guess the installation prefix from the location of a related program that they have already installed. If you wish to do that, you can call `AC_PREFIX_PROGRAM`.

Macro: AC_PREFIX_PROGRAM(program) ¶

  If the user did not specify an installation prefix (using the `--prefix` option), guess a value for it by looking for program in `PATH`, the way the shell does. If program is found, set the prefix to the parent of the directory containing program, else default the prefix as described above (`/usr/local` or `AC_PREFIX_DEFAULT`). For example, if program is `gcc` and the `PATH` contains `/usr/local/gnu/bin/gcc`, set the prefix to `/usr/local/gnu`.

## 5 Existing Tests ¶

These macros test for particular system features that packages might need or want to use. If you need to test for a kind of feature that none of these macros check for, you can probably do it by calling primitive test macros with appropriate arguments (see Writing Tests).

These tests print messages telling the user which feature they’re checking for, and what they find. They cache their results for future `configure` runs (see Caching Results).

Some of these macros set output variables. See Substitutions in Makefiles, for how to get their values. The phrase “define name” is used below as a shorthand to mean “define the C preprocessor symbol name to the value 1”. See Defining C Preprocessor Symbols, for how to get those symbol definitions into your program.

### 5.1 Common Behavior ¶

Much effort has been expended to make Autoconf easy to learn. The most obvious way to reach this goal is simply to enforce standard interfaces and behaviors, avoiding exceptions as much as possible. Because of history and inertia, unfortunately, there are still too many exceptions in Autoconf; nevertheless, this section describes some of the common rules.

#### 5.1.1 Standard Symbols ¶

All the generic macros that `AC_DEFINE` a symbol as a result of their test transform their argument values to a standard alphabet. First, argument is converted to upper case and any asterisks (‘`*`’) are each converted to ‘`P`’. Any remaining characters that are not alphanumeric are converted to underscores.

For instance,

    AC_CHECK_TYPES([struct $Expensive*])

defines the symbol ‘`HAVE_STRUCT__EXPENSIVEP`’ if the check succeeds.

#### 5.1.2 Default Includes ¶

Test programs frequently need to include headers that may or may not be available on the system whose features are being tested. Each test can use all the preprocessor macros that have been `AC_DEFINE`d by previous tests, so for example one may write

    #include <time.h>
    #ifdef HAVE_SYS_TIME_H
    # include <sys/time.h>
    #endif

if `sys/time.h` has already been tested for.

All hosted environments that are still of interest for portable code provide all of the headers specified in C89 (as amended in 1995): `assert.h`, `ctype.h`, `errno.h`, `float.h`, `iso646.h`, `limits.h`, `locale.h`, `math.h`, `setjmp.h`, `signal.h`, `stdarg.h`, `stddef.h`, `stdio.h`, `stdlib.h`, `string.h`, `time.h`, `wchar.h`, and `wctype.h`. Most programs can safely include these headers unconditionally. A program not intended to be portable to C89 can also safely include the C99-specified header `stdbool.h`. Other headers, including headers from C99 and later revisions of the C standard, might need to be tested for (see Header Files) or their bugs may need to be worked around (see Gnulib).

If your program needs to be portable to a freestanding environment, such as an embedded OS that doesn’t provide all of the facilities of the C89 standard library, you may need to test for some of the above headers as well. Note that many Autoconf macros internally assume that the complete set of C89 headers are available.

Most generic macros use the following macro to provide a default set of includes:

Macro: AC_INCLUDES_DEFAULT([include-directives]) ¶

  Expand to include-directives if present and nonempty, otherwise to:

      #include <stddef.h>
      #ifdef HAVE_STDIO_H
      # include <stdio.h>
      #endif
      #ifdef HAVE_STDLIB_H
      # include <stdlib.h>
      #endif
      #ifdef HAVE_STRING_H
      # include <string.h>
      #endif
      #ifdef HAVE_INTTYPES_H
      # include <inttypes.h>
      #endif
      #ifdef HAVE_STDINT_H
      # include <stdint.h>
      #endif
      #ifdef HAVE_STRINGS_H
      # include <strings.h>
      #endif
      #ifdef HAVE_SYS_TYPES_H
      # include <sys/types.h>
      #endif
      #ifdef HAVE_SYS_STAT_H
      # include <sys/stat.h>
      #endif
      #ifdef HAVE_UNISTD_H
      # include <unistd.h>
      #endif

  Using this macro without include-directives has the side effect of checking for `stdio.h`, `stdlib.h`, `string.h`, `inttypes.h`, `stdint.h`, `strings.h`, `sys/types.h`, `sys/stat.h`, and `unistd.h`, as if by `AC_CHECK_HEADERS_ONCE`. For backward compatibility, the macro `STDC_HEADERS` will be defined when both `stdlib.h` and `string.h` are available.

  Portability Note: It is safe for most programs to assume the presence of all of the headers required by the original 1990 C standard. `AC_INCLUDES_DEFAULT` checks for `stdio.h`, `stdlib.h`, and `string.h`, even though they are in that list, because they might not be available when compiling for a “freestanding environment” (in which most of the features of the C library are optional). You probably do not need to write ‘`#ifdef HAVE_STDIO_H`’ in your own code.

  `inttypes.h` and `stdint.h` were added to C in the 1999 revision of the standard, and `strings.h`, `sys/types.h`, `sys/stat.h`, and `unistd.h` are POSIX extensions. You should guard uses of these headers with appropriate conditionals.

Macro: AC_CHECK_INCLUDES_DEFAULT ¶

  Check for all the headers that `AC_INCLUDES_DEFAULT` would check for as a side-effect, if this has not already happened.

  This macro mainly exists so that `autoupdate` can replace certain obsolete constructs with it. You should not need to use it yourself; in fact, it is likely to be safe to delete it from any script in which it appears. (`autoupdate` does not know whether preprocessor macros such as `HAVE_STDINT_H` are used in the program, nor whether they would get defined as a side-effect of other checks.)

### 5.2 Alternative Programs ¶

These macros check for the presence or behavior of particular programs. They are used to choose between several alternative programs and to decide what to do once one has been chosen. If there is no macro specifically defined to check for a program you need, and you don’t need to check for any special properties of it, then you can use one of the general program-check macros.

#### 5.2.1 Particular Program Checks ¶

These macros check for particular programs—whether they exist, and in some cases whether they support certain features.

Macro: AC_PROG_AR ¶

  Set output variable `AR` to ‘`ar`’ if `ar` is found, and otherwise to ‘`:`’ (do nothing).

Macro: AC_PROG_AWK ¶

  Check for the commands `gawk`, `mawk`, `nawk`, `awk`, and `busybox awk`, in that order, and set output variable `AWK` to the first one that is found. Try `gawk` first because that is reported to be the best implementation. The result can be overridden by setting the variable `AWK` or the cache variable `ac_cv_prog_AWK`.

  Using this macro avoids some pitfalls of older systems that have both traditional `awk` (see Limitations of Usual Tools) and better alternatives like `gawk`. It also can help port to stripped-down systems that have only `busybox awk`, although these systems could well have problems with other build components that use plain `awk`.

Macro: AC_PROG_GREP ¶

  Look for the best available `grep` or `ggrep` that accepts the longest input lines possible, and that supports multiple `-e` options. Set the output variable `GREP` to whatever is chosen. See Limitations of Usual Tools, for more information about portability problems with the `grep` command family. The result can be overridden by setting the `GREP` variable and is cached in the `ac_cv_path_GREP` variable.

Macro: AC_PROG_EGREP ¶

  Check whether `$GREP -E` works, or else look for the best available `egrep` or `gegrep` that accepts the longest input lines possible. Set the output variable `EGREP` to whatever is chosen. The result can be overridden by setting the `EGREP` variable and is cached in the `ac_cv_path_EGREP` variable.

Macro: AC_PROG_FGREP ¶

  Check whether `$GREP -F` works, or else look for the best available `fgrep` or `gfgrep` that accepts the longest input lines possible. Set the output variable `FGREP` to whatever is chosen. The result can be overridden by setting the `FGREP` variable and is cached in the `ac_cv_path_FGREP` variable.

Macro: AC_PROG_INSTALL ¶

  Set output variable `INSTALL` to the name of a BSD-compatible `install` program, if one is found in the current `PATH`. Otherwise, set `INSTALL` to ‘`dir/install-sh -c`’, checking the directories specified to `AC_CONFIG_AUX_DIR` (or its default directories) to determine dir (see Outputting Files). Also set the variables `INSTALL_PROGRAM` and `INSTALL_SCRIPT` to ‘`${INSTALL}`’ and `INSTALL_DATA` to ‘`${INSTALL} -m 644`’.

  ‘`@INSTALL@`’ is special, as its value may vary for different configuration files.

  This macro screens out various instances of `install` known not to work. It prefers to find a C program rather than a shell script, for speed. Instead of `install-sh`, it can also use `install.sh`, but that name is obsolete because some `make` programs have a rule that creates `install` from it if there is no makefile. Further, this macro requires `install` to be able to install multiple files into a target directory in a single invocation.

  Autoconf comes with a copy of `install-sh` that you can use. If you use `AC_PROG_INSTALL`, you must include `install-sh` in your distribution; otherwise `autoreconf` and `configure` will produce an error message saying they can’t find it—even if the system you’re on has a good `install` program. This check is a safety measure to prevent you from accidentally leaving that file out, which would prevent your package from installing on systems that don’t have a BSD-compatible `install` program.

  If you need to use your own installation program because it has features not found in standard `install` programs, there is no reason to use `AC_PROG_INSTALL`; just put the file name of your program into your `Makefile.in` files.

  The result of the test can be overridden by setting the variable `INSTALL` or the cache variable `ac_cv_path_install`.

Macro: AC_PROG_MKDIR_P ¶

  Set output variable `MKDIR_P` to a program that ensures that for each argument, a directory named by this argument exists, creating it and its parent directories if needed, and without race conditions when two instances of the program attempt to make the same directory at nearly the same time.

  This macro uses the equivalent of the ‘`mkdir -p`’ command. Ancient versions of `mkdir` are vulnerable to race conditions, so if you want to support parallel installs from different packages into the same directory you should use a non-ancient `mkdir`.

  This macro is related to the `AS_MKDIR_P` macro (see Programming in M4sh), but it sets an output variable intended for use in other files, whereas `AS_MKDIR_P` is intended for use in scripts like `configure`. Also, `AS_MKDIR_P` does not accept options, but `MKDIR_P` supports the `-m` option, e.g., a makefile might invoke `$(MKDIR_P) -m 0 dir` to create an inaccessible directory, and conversely a makefile should use `$(MKDIR_P) -- $(FOO)` if FOO might yield a value that begins with ‘`-`’.

  The result of the test can be overridden by setting the variable `MKDIR_P` or the cache variable `ac_cv_path_mkdir`.

Macro: AC_PROG_LEX(options) ¶

  Search for a lexical analyzer generator, preferring `flex` to plain `lex`. Output variable `LEX` is set to whichever program is available. If neither program is available, `LEX` is set to ‘`:`’; for packages that ship the generated `file.yy.c` alongside the source `file.l`, this default allows users without a lexer generator to still build the package even if the timestamp for `file.l` is inadvertently changed.

  The name of the program to use can be overridden by setting the output variable `LEX` or the cache variable `ac_cv_prog_LEX` when running `configure`.

  If a lexical analyzer generator is found, this macro performs additional checks for common portability pitfalls. If these additional checks fail, `LEX` is reset to ‘`:`’; otherwise the following additional macros and variables are provided.

  Preprocessor macro `YYTEXT_POINTER` is defined if the lexer skeleton, by default, declares `yytext` as a ‘`char *`’ rather than a ‘`char []`’.

  Output variable `LEX_OUTPUT_ROOT` is set to the base of the file name that the lexer generates; this is usually either `lex.yy` or `lexyy`.

  If generated lexers need a library to work, output variable `LEXLIB` is set to a link option for that library (e.g., `-ll`), otherwise it is set to empty.

  The options argument modifies the behavior of `AC_PROG_LEX`. It should be a whitespace-separated list of options. Currently there are only two options, and they are mutually exclusive:

  `yywrap`

    Indicate that the library in `LEXLIB` needs to define the function `yywrap`. If a library that defines this function cannot be found, `LEX` will be reset to ‘`:`’.

  `noyywrap`

    Indicate that the library in `LEXLIB` does not need to define the function `yywrap`. `configure` will not search for it at all.

  Prior to Autoconf 2.70, `AC_PROG_LEX` did not take any arguments, and its behavior was different from either of the above possibilities: it would search for a library that defines `yywrap`, and would set `LEXLIB` to that library if it finds one. However, if a library that defines this function could not be found, `LEXLIB` would be left empty and `LEX` would not be reset. This behavior was due to a bug, but several packages came to depend on it, so `AC_PROG_LEX` still does this if neither the `yywrap` nor the `noyywrap` option is given.

  Usage of `AC_PROG_LEX` without choosing one of the `yywrap` or `noyywrap` options is deprecated. It is usually better to use `noyywrap` and define the `yywrap` function yourself, as this almost always renders the `LEXLIB` unnecessary.

  Caution: As a side-effect of the test, this macro may delete any file in the configure script’s current working directory named `lex.yy.c` or `lexyy.c`.

  Caution: Packages that ship a generated `lex.yy.c` cannot assume that the definition of `YYTEXT_POINTER` matches the code in that file. They also cannot assume that `LEXLIB` provides the library routines required by the code in that file.

  If you use Flex to generate `lex.yy.c`, you can work around these limitations by defining `yywrap` and `main` yourself (rendering `-lfl` unnecessary), and by using either the `--array` or `--pointer` options to control how `yytext` is declared. The code generated by Flex is also more portable than the code generated by historical versions of Lex.

  If you have used Flex to generate `lex.yy.c`, and especially if your scanner depends on Flex features, we recommend you use this Autoconf snippet to prevent the scanner being regenerated with historical Lex:

      AC_PROG_LEX
      AS_IF([test "x$LEX" != xflex],
        [LEX="$SHELL $missing_dir/missing flex"
         AC_SUBST([LEX_OUTPUT_ROOT], [lex.yy])
         AC_SUBST([LEXLIB], [''])])

  The shell script `missing` can be found in the Automake distribution.

  Remember that the user may have supplied an alternate location in `LEX`, so if Flex is required, it is better to check that the user provided something sufficient by parsing the output of ‘`$LEX --version`’ than by simply relying on `test "x$LEX" = xflex`.

Macro: AC_PROG_LN_S ¶

  If ‘`ln -s`’ works on the current file system (the operating system and file system support symbolic links), set the output variable `LN_S` to ‘`ln -s`’; otherwise, if ‘`ln`’ works, set `LN_S` to ‘`ln`’, and otherwise set it to ‘`cp -pR`’.

  If you make a link in a directory other than the current directory, its meaning depends on whether ‘`ln`’ or ‘`ln -s`’ is used. To safely create links using ‘`$(LN_S)`’, either find out which form is used and adjust the arguments, or always invoke `ln` in the directory where the link is to be created.

  In other words, it does not work to do:

      $(LN_S) foo /x/bar

  Instead, do:

      (cd /x && $(LN_S) foo bar)

Macro: AC_PROG_RANLIB ¶

  Set output variable `RANLIB` to ‘`ranlib`’ if `ranlib` is found, and otherwise to ‘`:`’ (do nothing).

Macro: AC_PROG_SED ¶

  Set output variable `SED` to a Sed implementation that conforms to POSIX and does not have arbitrary length limits. Report an error if no acceptable Sed is found. See Limitations of Usual Tools.

  The result of this test can be overridden by setting the `SED` variable and is cached in the `ac_cv_path_SED` variable.

Macro: AC_PROG_YACC ¶

  If `bison` is found, set output variable `YACC` to ‘`bison -y`’. Otherwise, if `byacc` is found, set `YACC` to ‘`byacc`’. Otherwise set `YACC` to ‘`yacc`’. The result of this test can be influenced by setting the variable `YACC` or the cache variable `ac_cv_prog_YACC`.

#### 5.2.2 Generic Program and File Checks ¶

These macros are used to find programs not covered by the “particular” test macros. If you need to check the behavior of a program as well as find out whether it is present, you have to write your own test for it (see Writing Tests). By default, these macros use the environment variable `PATH`. If you need to check for a program that might not be in the user’s `PATH`, you can pass a modified path to use instead, like this:

    AC_PATH_PROG([INETD], [inetd], [/usr/libexec/inetd],
                 [$PATH$PATH_SEPARATOR/usr/libexec$PATH_SEPARATOR]dnl
    [/usr/sbin$PATH_SEPARATOR/usr/etc$PATH_SEPARATOR/etc])

You are strongly encouraged to declare the variable passed to `AC_CHECK_PROG` etc. as precious. See Setting Output Variables, `AC_ARG_VAR`, for more details.

Macro: AC_CHECK_PROG(variable, prog-to-check-for, value-if-found, [value-if-not-found], [path = ‘`$PATH`’], [reject]) ¶

  Check whether program prog-to-check-for exists in path. If it is found, set variable to value-if-found, otherwise to value-if-not-found, if given. Always pass over reject (an absolute file name) even if it is the first found in the search path; in that case, set variable using the absolute file name of the prog-to-check-for found that is not reject. If variable was already set, do nothing. Calls `AC_SUBST` for variable. The result of this test can be overridden by setting the variable variable or the cache variable `ac_cv_prog_variable`.

Macro: AC_CHECK_PROGS(variable, progs-to-check-for, [value-if-not-found], [path = ‘`$PATH`’]) ¶

  Check for each program in the blank-separated list progs-to-check-for existing in the path. If one is found, set variable to the name of that program. Otherwise, continue checking the next program in the list. If none of the programs in the list are found, set variable to value-if-not-found; if value-if-not-found is not specified, the value of variable is not changed. Calls `AC_SUBST` for variable. The result of this test can be overridden by setting the variable variable or the cache variable `ac_cv_prog_variable`.

Macro: AC_CHECK_TARGET_TOOL(variable, prog-to-check-for, [value-if-not-found], [path = ‘`$PATH`’]) ¶

  Like `AC_CHECK_PROG`, but first looks for prog-to-check-for with a prefix of the target type as determined by `AC_CANONICAL_TARGET`, followed by a dash (see Getting the Canonical System Type). If the tool cannot be found with a prefix, and if the build and target types are equal, then it is also searched for without a prefix.

  As noted in Specifying target triplets, the target is rarely specified, because most of the time it is the same as the host: it is the type of system for which any compiler tool in the package produces code. What this macro looks for is, for example, a tool (assembler, linker, etc.) that the compiler driver (`gcc` for the GNU C Compiler) uses to produce objects, archives or executables.

Macro: AC_CHECK_TOOL(variable, prog-to-check-for, [value-if-not-found], [path = ‘`$PATH`’]) ¶

  Like `AC_CHECK_PROG`, but first looks for prog-to-check-for with a prefix of the host type as specified by `--host`, followed by a dash. For example, if the user runs ‘`configure --build=x86_64-gnu --host=aarch64-linux-gnu`’, then this call:

      AC_CHECK_TOOL([RANLIB], [ranlib], [:])

  sets `RANLIB` to `aarch64-linux-gnu-ranlib` if that program exists in path, or otherwise to ‘`ranlib`’ if that program exists in path, or to ‘`:`’ if neither program exists.

  When cross-compiling, this macro will issue a warning if no program prefixed with the host type could be found. For more information, see Specifying target triplets.

Macro: AC_CHECK_TARGET_TOOLS(variable, progs-to-check-for, [value-if-not-found], [path = ‘`$PATH`’]) ¶

  Like `AC_CHECK_TARGET_TOOL`, each of the tools in the list progs-to-check-for are checked with a prefix of the target type as determined by `AC_CANONICAL_TARGET`, followed by a dash (see Getting the Canonical System Type). If none of the tools can be found with a prefix, and if the build and target types are equal, then the first one without a prefix is used. If a tool is found, set variable to the name of that program. If none of the tools in the list are found, set variable to value-if-not-found; if value-if-not-found is not specified, the value of variable is not changed. Calls `AC_SUBST` for variable.

Macro: AC_CHECK_TOOLS(variable, progs-to-check-for, [value-if-not-found], [path = ‘`$PATH`’]) ¶

  Like `AC_CHECK_TOOL`, each of the tools in the list progs-to-check-for are checked with a prefix of the host type as determined by `AC_CANONICAL_HOST`, followed by a dash (see Getting the Canonical System Type). If none of the tools can be found with a prefix, then the first one without a prefix is used. If a tool is found, set variable to the name of that program. If none of the tools in the list are found, set variable to value-if-not-found; if value-if-not-found is not specified, the value of variable is not changed. Calls `AC_SUBST` for variable.

  When cross-compiling, this macro will issue a warning if no program prefixed with the host type could be found. For more information, see Specifying target triplets.

Macro: AC_PATH_PROG(variable, prog-to-check-for, [value-if-not-found], [path = ‘`$PATH`’]) ¶

  Like `AC_CHECK_PROG`, but set variable to the absolute name of prog-to-check-for if found. The result of this test can be overridden by setting the variable variable. A positive result of this test is cached in the `ac_cv_path_variable` variable.

Macro: AC_PATH_PROGS(variable, progs-to-check-for, [value-if-not-found], [path = ‘`$PATH`’]) ¶

  Like `AC_CHECK_PROGS`, but if any of progs-to-check-for are found, set variable to the absolute name of the program found. The result of this test can be overridden by setting the variable variable. A positive result of this test is cached in the `ac_cv_path_variable` variable.

Macro: AC_PATH_PROGS_FEATURE_CHECK(variable, progs-to-check-for, feature-test, [action-if-not-found], [path = ‘`$PATH`’]) ¶

  This macro was introduced in Autoconf 2.62. If variable is not empty, then set the cache variable `ac_cv_path_variable` to its value. Otherwise, check for each program in the blank-separated list progs-to-check-for existing in path. For each program found, execute feature-test with `ac_path_variable` set to the absolute name of the candidate program. If no invocation of feature-test sets the shell variable `ac_cv_path_variable`, then action-if-not-found is executed. feature-test will be run even when `ac_cv_path_variable` is set, to provide the ability to choose a better candidate found later in path; to accept the current setting and bypass all further checks, feature-test can execute `ac_path_variable_found=:`.

  Note that this macro has some subtle differences from `AC_CHECK_PROGS`. It is designed to be run inside `AC_CACHE_VAL`, therefore, it should have no side effects. In particular, variable is not set to the final value of `ac_cv_path_variable`, nor is `AC_SUBST` automatically run. Also, on failure, any action can be performed, whereas `AC_CHECK_PROGS` only performs `variable=value-if-not-found`.

  Here is an example that searches for an implementation of `m4` that supports the `indir` builtin, even if it goes by the name `gm4` or is not the first implementation on `PATH`.

      AC_CACHE_CHECK([for m4 that supports indir], [ac_cv_path_M4],
        [AC_PATH_PROGS_FEATURE_CHECK([M4], [m4 gm4],
          [[m4out=`echo 'changequote([,])indir([divnum])' | $ac_path_M4`
            test "x$m4out" = x0 \
            && ac_cv_path_M4=$ac_path_M4 ac_path_M4_found=:]],
          [AC_MSG_ERROR([could not find m4 that supports indir])])])
      AC_SUBST([M4], [$ac_cv_path_M4])

Macro: AC_PATH_TARGET_TOOL(variable, prog-to-check-for, [value-if-not-found], [path = ‘`$PATH`’]) ¶

  Like `AC_CHECK_TARGET_TOOL`, but set variable to the absolute name of the program if it is found.

Macro: AC_PATH_TOOL(variable, prog-to-check-for, [value-if-not-found], [path = ‘`$PATH`’]) ¶

  Like `AC_CHECK_TOOL`, but set variable to the absolute name of the program if it is found.

  When cross-compiling, this macro will issue a warning if no program prefixed with the host type could be found. For more information, see Specifying target triplets.

### 5.3 Files ¶

You might also need to check for the existence of files. Before using these macros, ask yourself whether a runtime test might not be a better solution. Be aware that, like most Autoconf macros, they test a feature of the host machine, and therefore, they die when cross-compiling.

Macro: AC_CHECK_FILE(file, [action-if-found], [action-if-not-found]) ¶

  Check whether file file exists on the native system. If it is found, execute action-if-found, otherwise do action-if-not-found, if given. Cache the result of this test in the `ac_cv_file_file` variable, with characters not suitable for a variable name mapped to underscores.

Macro: AC_CHECK_FILES(files, [action-if-found], [action-if-not-found]) ¶

  For each file listed in files, execute `AC_CHECK_FILE` and perform either action-if-found or action-if-not-found. Like `AC_CHECK_FILE`, this defines ‘`HAVE_file`’ (see Standard Symbols) for each file found and caches the results of each test in the `ac_cv_file_file` variable, with characters not suitable for a variable name mapped to underscores.

### 5.4 Library Files ¶

The following macros check for the presence of certain C, C++, Fortran, or Go library archive files.

Macro: AC_CHECK_LIB(library, function, [action-if-found], [action-if-not-found], [other-libraries]) ¶

  Test whether the library library is available by trying to link a test program that calls function function with the library. function should be a function provided by the library. Use the base name of the library; e.g., to check for `-lmp`, use ‘`mp`’ as the library argument.

  action-if-found is a list of shell commands to run if the link with the library succeeds; action-if-not-found is a list of shell commands to run if the link fails. If action-if-found is not specified, the default action prepends `-llibrary` to `LIBS` and defines ‘`HAVE_LIBlibrary`’ (in all capitals). This macro is intended to support building `LIBS` in a right-to-left (least-dependent to most-dependent) fashion such that library dependencies are satisfied as a natural side effect of consecutive tests. Linkers are sensitive to library ordering so the order in which `LIBS` is generated is important to reliable detection of libraries.

  If linking with library results in unresolved symbols that would be resolved by linking with additional libraries, give those libraries as the other-libraries argument, separated by spaces: e.g., `-lXt -lX11`. Otherwise, this macro may fail to detect that library is present, because linking the test program can fail with unresolved symbols. The other-libraries argument should be limited to cases where it is desirable to test for one library in the presence of another that is not already in `LIBS`.

  `AC_CHECK_LIB` requires some care in usage, and should be avoided in some common cases. Many standard functions like `gethostbyname` appear in the standard C library on some hosts, and in special libraries like `nsl` on other hosts. On some hosts the special libraries contain variant implementations that you may not want to use. These days it is normally better to use `AC_SEARCH_LIBS([gethostbyname], [nsl])` instead of `AC_CHECK_LIB([nsl], [gethostbyname])`.

  The result of this test is cached in the `ac_cv_lib_library_function` variable.

Macro: AC_SEARCH_LIBS(function, search-libs, [action-if-found], [action-if-not-found], [other-libraries]) ¶

  Search for a library defining function if it’s not already available. This equates to calling ‘`AC_LINK_IFELSE([AC_LANG_CALL([], [function])])`’ first with no libraries, then for each library listed in search-libs.

  Prepend `-llibrary` to `LIBS` for the first library found to contain function, and run action-if-found. If the function is not found, run action-if-not-found.

  If linking with library results in unresolved symbols that would be resolved by linking with additional libraries, give those libraries as the other-libraries argument, separated by spaces: e.g., `-lXt -lX11`. Otherwise, this macro fails to detect that function is present, because linking the test program always fails with unresolved symbols.

  The result of this test is cached in the `ac_cv_search_function` variable as ‘`none required`’ if function is already available, as ‘`no`’ if no library containing function was found, otherwise as the `-llibrary` option that needs to be prepended to `LIBS`.

### 5.5 Library Functions ¶

The following macros check for particular C library functions. If there is no macro specifically defined to check for a function you need, and you don’t need to check for any special properties of it, then you can use one of the general function-check macros.

#### 5.5.1 Portability of C Functions ¶

Most usual functions can either be missing, or be buggy, or be limited on some architectures. This section tries to make an inventory of these portability issues. By definition, this list always requires additions. A much more complete list is maintained by the Gnulib project (see Gnulib), covering Current POSIX Functions in Gnulib, Legacy Functions in Gnulib, and Glibc Functions in Gnulib. Please help us keep the Gnulib list as complete as possible.

`exit`

  On ancient hosts, `exit` returned `int`. This is because `exit` predates `void`, and there was a long tradition of it returning `int`.

  On current hosts, the problem more likely is that `exit` is not declared, due to C++ problems of some sort or another. For this reason we suggest that test programs not invoke `exit`, but return from `main` instead.

`malloc`

  The C standard says a successful call `malloc (0)` is implementation dependent. It can return either `NULL` or a new non-null pointer. The latter is more common (e.g., the GNU C Library) but is by no means universal. `AC_FUNC_MALLOC` can be used to insist on non-`NULL` (see Particular Function Checks).

`putenv`

  POSIX prefers `setenv` to `putenv`; among other things, `putenv` is not required of all POSIX implementations, but `setenv` is.

  POSIX specifies that `putenv` puts the given string directly in `environ`, but some systems make a copy of it instead (e.g., glibc 2.0, or BSD). And when a copy is made, `unsetenv` might not free it, causing a memory leak (e.g., FreeBSD 4).

  On some systems `putenv ("FOO")` removes ‘`FOO`’ from the environment, but this is not standard usage and it dumps core on some systems (e.g., AIX).

  On MinGW, a call `putenv ("FOO=")` removes ‘`FOO`’ from the environment, rather than inserting it with an empty value.

`realloc`

  It is problematic to call `realloc` with a zero size. The C standard says `realloc (NULL, 0)` is equivalent to `malloc (0)`, which means one cannot portably tell whether the call has succeeded if it returns a null pointer. If `ptr` is non-null, the C standard says `realloc (ptr, 0)` has undefined behavior.

  The `AC_FUNC_REALLOC` macro avoids some of these portability issues. See Particular Function Checks.

`signal` handler

  In most cases, it is more robust to use `sigaction` when it is available, rather than `signal`.

`snprintf`

  In C99 and later, if the output array isn’t big enough and if no other errors occur, `snprintf` and `vsnprintf` truncate the output and return the number of bytes that ought to have been produced. Some older systems, notably Microsoft Windows before Visual Studio 2015 and Windows 10, do not null-terminate the output and return −1 instead.

  Portable code can check the return value of `snprintf (buf, sizeof buf, ...)`: if the value is negative or is not less than `sizeof buf`, an error occurred and the contents of `buf` can be ignored. Alternatively, one of the Gnulib modules related to `snprintf` can be used. See Gnulib.

`strerror_r`

  POSIX specifies that `strerror_r` returns an `int`, but many systems (e.g., the GNU C Library) provide a different version returning a `char *`. `AC_FUNC_STRERROR_R` can detect which is in use (see Particular Function Checks).

`strnlen`

  Android 5.0’s strnlen was broken, because it assumed the addressed array always had at least the specified number of bytes. For example, `strnlen ("", SIZE_MAX)` should return 0 but on Android 5.0 it crashed.

  AIX 4.3 provided a broken version which produces the following results:

      strnlen ("foobar", 0) = 0
      strnlen ("foobar", 1) = 3
      strnlen ("foobar", 2) = 2
      strnlen ("foobar", 3) = 1
      strnlen ("foobar", 4) = 0
      strnlen ("foobar", 5) = 6
      strnlen ("foobar", 6) = 6
      strnlen ("foobar", 7) = 6
      strnlen ("foobar", 8) = 6
      strnlen ("foobar", 9) = 6

`sysconf`

  `_SC_PAGESIZE` is standard, but some older systems (e.g., HP-UX 9) have `_SC_PAGE_SIZE` instead. This can be tested with `#ifdef`.

`unlink`

  The POSIX spec says that `unlink` causes the given file to be removed only after there are no more open file handles for it. Some non-POSIX hosts have trouble with this requirement, though, and some DOS variants even corrupt the file system.

`unsetenv`

  On MinGW, `unsetenv` is not available, but a variable ‘`FOO`’ can be removed with a call `putenv ("FOO=")`, as described under `putenv` above.

`va_copy`

  C99 and later provide `va_copy` for copying `va_list` variables. It may be available in older environments too, though possibly as `__va_copy` (e.g., `gcc` in strict pre-C99 mode). These can be tested with `#ifdef`. A fallback to `memcpy (&dst, &src, sizeof (va_list))` gives maximum portability.

`va_list`

  `va_list` is not necessarily just a pointer. It can be a `struct`, which means `NULL` is not portable. Or it can be an array, which means as a function parameter it can be effectively call-by-reference and library routines might modify the value back in the caller.

Signed `>>`

  Normally the C `>>` right shift of a signed type replicates the high bit, giving a so-called “arithmetic” shift. But care should be taken since Standard C doesn’t require that behavior. On a few platforms (e.g., Cray C by default) zero bits are shifted in, the same as a shift of an unsigned type.

Integer `/`

  C divides signed integers by truncating their quotient toward zero, yielding the same result as Fortran. However, before C99 the standard allowed C implementations to take the floor or ceiling of the quotient in some cases. Hardly any implementations took advantage of this freedom, though, and it’s probably not worth worrying about this issue nowadays.

#### 5.5.2 Particular Function Checks ¶

These macros check for particular C functions—whether they exist, and in some cases how they respond when given certain arguments.

Macro: AC_FUNC_ALLOCA ¶

  Check for the `alloca` function. Define `HAVE_ALLOCA_H` if `alloca.h` defines a working `alloca`. If not, look for a builtin alternative. If either method succeeds, define `HAVE_ALLOCA`. Otherwise, set the output variable `ALLOCA` to ‘`${LIBOBJDIR}alloca.o`’ and define `C_ALLOCA` (so programs can periodically call ‘`alloca (0)`’ to garbage collect). This variable is separate from `LIBOBJS` so multiple programs can share the value of `ALLOCA` without needing to create an actual library, in case only some of them use the code in `LIBOBJS`. The ‘`${LIBOBJDIR}`’ prefix serves the same purpose as in `LIBOBJS` (see `AC_LIBOBJ` vs. `LIBOBJS`).

  Source files that use `alloca` should start with a piece of code like the following, to declare it properly.

      #include <stdlib.h>
      #include <stddef.h>
      #ifdef HAVE_ALLOCA_H
      # include <alloca.h>
      #elif !defined alloca
      # ifdef __GNUC__
      #  define alloca __builtin_alloca
      # elif defined _MSC_VER
      #  include <malloc.h>
      #  define alloca _alloca
      # elif !defined HAVE_ALLOCA
      #  ifdef  __cplusplus
      extern "C"
      #  endif
      void *alloca (size_t);
      # endif
      #endif

  If you don’t want to maintain this piece of code in your package manually, you can instead use the Gnulib module `alloca-opt` or `alloca`. See Gnulib.

Macro: AC_FUNC_CHOWN ¶

  If the `chown` function is available and works (in particular, it should accept `-1` for `uid` and `gid`), define `HAVE_CHOWN`. The result of this macro is cached in the `ac_cv_func_chown_works` variable.

  If you want a workaround, that is, a `chown` function that is available and works, you can use the Gnulib module `chown`. See Gnulib.

Macro: AC_FUNC_CLOSEDIR_VOID ¶

  If the `closedir` function does not return a meaningful value, define `CLOSEDIR_VOID`. Otherwise, callers ought to check its return value for an error indicator.

  Currently this test is implemented by running a test program. When cross compiling the pessimistic assumption that `closedir` does not return a meaningful value is made.

  The result of this macro is cached in the `ac_cv_func_closedir_void` variable.

  This macro is obsolescent, as `closedir` returns a meaningful value on current systems. New programs need not use this macro.

Macro: AC_FUNC_ERROR_AT_LINE ¶

  If the `error_at_line` function is not found, require an `AC_LIBOBJ` replacement of ‘`error`’.

  The result of this macro is cached in the `ac_cv_lib_error_at_line` variable.

  The `AC_FUNC_ERROR_AT_LINE` macro is obsolescent. New programs should use Gnulib’s `error` module. See Gnulib.

Macro: AC_FUNC_FNMATCH ¶

  If the `fnmatch` function conforms to POSIX, define `HAVE_FNMATCH`.

  Unlike the other specific `AC_FUNC` macros, `AC_FUNC_FNMATCH` does not replace a broken/missing `fnmatch`. This is for historical reasons. See `AC_REPLACE_FNMATCH` below.

  The result of this macro is cached in the `ac_cv_func_fnmatch_works` variable.

  This macro is obsolescent. New programs should use Gnulib’s `fnmatch-posix` module. See Gnulib.

Macro: AC_FUNC_FNMATCH_GNU ¶

  Behave like `AC_REPLACE_FNMATCH` (replace) but also test whether `fnmatch` supports GNU extensions. Detect common implementation bugs, for example, the bugs in the GNU C Library 2.1.

  The result of this macro is cached in the `ac_cv_func_fnmatch_gnu` variable.

  This macro is obsolescent. New programs should use Gnulib’s `fnmatch-gnu` module. See Gnulib.

Macro: AC_FUNC_FORK ¶

  This macro checks for the `fork` and `vfork` functions. If a working `fork` is found, define `HAVE_WORKING_FORK`. This macro checks whether `fork` is just a stub by trying to run it.

  If `vfork.h` is found, define `HAVE_VFORK_H`. If a working `vfork` is found, define `HAVE_WORKING_VFORK`. Otherwise, define `vfork` to be `fork` for backward compatibility with previous versions of `autoconf`. This macro checks for several known errors in implementations of `vfork` and considers the system to not have a working `vfork` if it detects any of them.

  Since this macro defines `vfork` only for backward compatibility with previous versions of `autoconf` you’re encouraged to define it yourself in new code:

      #ifndef HAVE_WORKING_VFORK
      # define vfork fork
      #endif

  The results of this macro are cached in the `ac_cv_func_fork_works` and `ac_cv_func_vfork_works` variables. In order to override the test, you also need to set the `ac_cv_func_fork` and `ac_cv_func_vfork` variables.

Macro: AC_FUNC_FSEEKO ¶

  If the `fseeko` and `ftello` functions are available, define `HAVE_FSEEKO`. Define `_LARGEFILE_SOURCE` if necessary to make the prototype visible.

  Configure scripts that use `AC_FUNC_FSEEKO` should normally also use `AC_SYS_LARGEFILE` to ensure that `off_t` can represent all supported file sizes. See AC_SYS_LARGEFILE.

  The Gnulib module `fseeko` invokes `AC_FUNC_FSEEKO` and also contains workarounds for other portability problems of `fseeko`. See Gnulib.

Macro: AC_FUNC_GETGROUPS ¶

  Perform all the checks performed by `AC_TYPE_GETGROUPS` (see AC_TYPE_GETGROUPS). Then, if the `getgroups` function is available and known to work correctly, define `HAVE_GETGROUPS`. Set the output variable `GETGROUPS_LIB` to any libraries needed to get that function.

  This macro relies on a list of systems with known, serious bugs in `getgroups`. If this list mis-identifies your system’s `getgroups` as buggy, or as not buggy, you can override it by setting the cache variable `ac_cv_func_getgroups_works` in a `config.site` file (see Setting Site Defaults). Please also report the error to the Autoconf Bugs mailing list.

  The Gnulib module `getgroups` provides workarounds for additional, less severe portability problems with this function.

Macro: AC_FUNC_GETLOADAVG ¶

  Check how to get the system load averages. To perform its tests properly, this macro needs the file `getloadavg.c`; therefore, be sure to set the `AC_LIBOBJ` replacement directory properly (see Generic Function Checks, `AC_CONFIG_LIBOBJ_DIR`).

  If the system has the `getloadavg` function, define `HAVE_GETLOADAVG`, and set `GETLOADAVG_LIBS` to any libraries necessary to get that function. Also add `GETLOADAVG_LIBS` to `LIBS`. Otherwise, require an `AC_LIBOBJ` replacement for ‘`getloadavg`’ and possibly define several other C preprocessor macros and output variables:

  1. Define `C_GETLOADAVG`.
  2. Define `SVR4`, `DGUX`, `UMAX`, or `UMAX4_3` if on those systems.
  3. If `nlist.h` is found, define `HAVE_NLIST_H`.
  4. If ‘`struct nlist`’ has an ‘`n_un.n_name`’ member, define `HAVE_STRUCT_NLIST_N_UN_N_NAME`. The obsolete symbol `NLIST_NAME_UNION` is still defined, but do not depend upon it.
  5. Programs may need to be installed set-group-ID (or set-user-ID) for `getloadavg` to work. In this case, define `GETLOADAVG_PRIVILEGED`, set the output variable `NEED_SETGID` to ‘`true`’ (and otherwise to ‘`false`’), and set `KMEM_GROUP` to the name of the group that should own the installed program.

  The `AC_FUNC_GETLOADAVG` macro is obsolescent. New programs should use Gnulib’s `getloadavg` module. See Gnulib.

Macro: AC_FUNC_GETMNTENT ¶

  Check for `getmntent` in the standard C library, and then in the `sun`, `seq`, and `gen` libraries. Then, if `getmntent` is available, define `HAVE_GETMNTENT` and set `ac_cv_func_getmntent` to `yes`. Otherwise set `ac_cv_func_getmntent` to `no`.

  The result of this macro can be overridden by setting the cache variable `ac_cv_search_getmntent`.

  The `AC_FUNC_GETMNTENT` macro is obsolescent. New programs should use Gnulib’s `mountlist` module. See Gnulib.

Macro: AC_FUNC_GETPGRP ¶

  Define `GETPGRP_VOID` if it is an error to pass 0 to `getpgrp`; this is the POSIX behavior. On older BSD systems, you must pass 0 to `getpgrp`, as it takes an argument and behaves like POSIX’s `getpgid`.

      #ifdef GETPGRP_VOID
        pid = getpgrp ();
      #else
        pid = getpgrp (0);
      #endif

  This macro does not check whether `getpgrp` exists at all; if you need to work in that situation, first call `AC_CHECK_FUNC` for `getpgrp`.

  The result of this macro is cached in the `ac_cv_func_getpgrp_void` variable.

  This macro is obsolescent, as current systems have a `getpgrp` whose signature conforms to POSIX. New programs need not use this macro.

Macro: AC_FUNC_LSTAT_FOLLOWS_SLASHED_SYMLINK ¶

  If `link` is a symbolic link, then `lstat` should treat `link/` the same as `link/.`. However, many older `lstat` implementations incorrectly ignore trailing slashes.

  It is safe to assume that if `lstat` incorrectly ignores trailing slashes, then other symbolic-link-aware functions like `unlink` also incorrectly ignore trailing slashes.

  If `lstat` behaves properly, define `LSTAT_FOLLOWS_SLASHED_SYMLINK`, otherwise require an `AC_LIBOBJ` replacement of `lstat`.

  The result of this macro is cached in the `ac_cv_func_lstat_dereferences_slashed_symlink` variable.

  The `AC_FUNC_LSTAT_FOLLOWS_SLASHED_SYMLINK` macro is obsolescent. New programs should use Gnulib’s `lstat` module. See Gnulib.

Macro: AC_FUNC_MALLOC ¶

  If the `malloc` function is compatible with the GNU C library `malloc` (i.e., ‘`malloc (0)`’ returns a valid pointer), define `HAVE_MALLOC` to 1. Otherwise define `HAVE_MALLOC` to 0, ask for an `AC_LIBOBJ` replacement for ‘`malloc`’, and define `malloc` to `rpl_malloc` so that the native `malloc` is not used in the main project.

  Typically, the replacement file `malloc.c` should look like (note the ‘`#undef malloc`’):

      #include <config.h>
      #undef malloc

      #include <stdlib.h>

      /* Allocate an N-byte block of memory from the heap.
         If N is zero, allocate a 1-byte block.  */

      void *
      rpl_malloc (size_t n)
      {
        if (n == 0)
          n = 1;
        return malloc (n);
      }

  The result of this macro is cached in the `ac_cv_func_malloc_0_nonnull` variable. The result might contain spaces, e.g., `guessing yes`.

  If you don’t want to maintain a `malloc.c` file in your package manually, you can instead use the Gnulib module `malloc-gnu`.

Macro: AC_FUNC_MBRTOWC ¶

  Define `HAVE_MBRTOWC` to 1 if the function `mbrtowc` and the type `mbstate_t` are properly declared.

  The result of this macro is cached in the `ac_cv_func_mbrtowc` variable.

  The Gnulib module `mbrtowc` not only ensures that the function is declared, but also works around other portability problems of this function.

Macro: AC_FUNC_MEMCMP ¶

  If the `memcmp` function is not available or does not work, require an `AC_LIBOBJ` replacement for ‘`memcmp`’.

  The result of this macro is cached in the `ac_cv_func_memcmp_working` variable.

  This macro is obsolescent, as current systems have a working `memcmp`. New programs need not use this macro.

Macro: AC_FUNC_MKTIME ¶

  If the `mktime` function is not available, or does not work correctly, require an `AC_LIBOBJ` replacement for ‘`mktime`’. For the purposes of this test, `mktime` should conform to the POSIX standard and should be the inverse of `localtime`.

  The result of this macro is cached in the `ac_cv_func_working_mktime` variable.

  The `AC_FUNC_MKTIME` macro is obsolescent. New programs should use Gnulib’s `mktime` module. See Gnulib.

Macro: AC_FUNC_MMAP ¶

  If the `mmap` function exists and works correctly, define `HAVE_MMAP`. This checks only private fixed mapping of already-mapped memory.

  The result of this macro is cached in the `ac_cv_func_mmap_fixed_mapped` variable.

  Note: This macro asks for more than what an average program needs from `mmap`. In particular, the use of `MAP_FIXED` fails on HP-UX 11, whereas `mmap` otherwise works fine on this platform.

Macro: AC_FUNC_OBSTACK ¶

  If the obstacks are found, define `HAVE_OBSTACK`, else require an `AC_LIBOBJ` replacement for ‘`obstack`’.

  The result of this macro is cached in the `ac_cv_func_obstack` variable.

  The `AC_FUNC_OBSTACK` macro is obsolescent. New programs should use Gnulib’s `obstack` module. See Gnulib.

Macro: AC_FUNC_REALLOC ¶

  If a successful call to ‘`realloc (NULL, 0)`’ returns a non-null pointer, define `HAVE_REALLOC` to 1. Otherwise define `HAVE_REALLOC` to 0, ask for an `AC_LIBOBJ` replacement for ‘`realloc`’, and define `realloc` to `rpl_realloc` so that the native `realloc` is not used in the main project. See `AC_FUNC_MALLOC` for details.

  The result of this macro is cached in the `ac_cv_func_realloc_0_nonnull` variable. The result might contain spaces, e.g., `guessing yes`.

  This macro does not check compatibility with glibc `realloc (p, 0)` when p is non-null, as glibc 1–2.1 behaves differently from glibc 2.1.1–2.40 (at least), and the C standard says behavior is undefined.

Macro: AC_FUNC_SELECT_ARGTYPES ¶

  Determines the correct type to be passed for each of the `select` function’s arguments, and defines those types in `SELECT_TYPE_ARG1`, `SELECT_TYPE_ARG234`, and `SELECT_TYPE_ARG5` respectively. `SELECT_TYPE_ARG1` defaults to ‘`int`’, `SELECT_TYPE_ARG234` defaults to ‘`int *`’, and `SELECT_TYPE_ARG5` defaults to ‘`struct timeval *`’.

  This macro is obsolescent, as current systems have a `select` whose signature conforms to POSIX. New programs need not use this macro.

Macro: AC_FUNC_SETPGRP ¶

  If `setpgrp` takes no argument (the POSIX version), define `SETPGRP_VOID`. Otherwise, it is the BSD version, which takes two process IDs as arguments. This macro does not check whether `setpgrp` exists at all; if you need to work in that situation, first call `AC_CHECK_FUNC` for `setpgrp`. This macro also does not check for the Solaris variant of `setpgrp`, which returns a `pid_t` instead of an `int`; portable code should only use the return value by comparing it against `-1` to check for errors.

  The result of this macro is cached in the `ac_cv_func_setpgrp_void` variable.

  This macro is obsolescent, as all forms of `setpgrp` are also obsolescent. New programs should use the POSIX function `setpgid`, which takes two process IDs as arguments (like the BSD `setpgrp`).

Macro: AC_FUNC_STAT ¶
Macro: AC_FUNC_LSTAT ¶

  Determine whether `stat` or `lstat` have the bug that it succeeds when given the zero-length file name as argument.

  If it does, then define `HAVE_STAT_EMPTY_STRING_BUG` (or `HAVE_LSTAT_EMPTY_STRING_BUG`) and ask for an `AC_LIBOBJ` replacement of it.

  The results of these macros are cached in the `ac_cv_func_stat_empty_string_bug` and the `ac_cv_func_lstat_empty_string_bug` variables, respectively.

  These macros are obsolescent, as no current systems have the bug. New programs need not use these macros.

Macro: AC_FUNC_STRCOLL ¶

  If the `strcoll` function exists and works correctly, define `HAVE_STRCOLL`. This does a bit more than ‘`AC_CHECK_FUNCS(strcoll)`’, because some systems have incorrect definitions of `strcoll` that should not be used. But it does not check against a known bug of this function on Solaris 10.

  The result of this macro is cached in the `ac_cv_func_strcoll_works` variable.

Macro: AC_FUNC_STRERROR_R ¶

  If `strerror_r` is available, define `HAVE_STRERROR_R`, and if it is declared, define `HAVE_DECL_STRERROR_R`. If it returns a `char *` message, define `STRERROR_R_CHAR_P`; otherwise it returns an `int` error number. The Thread-Safe Functions option of POSIX requires `strerror_r` to return `int`, but many systems (including, for example, the GNU C Library) return a `char *` value that is not necessarily equal to the buffer argument.

  The result of this macro is cached in the `ac_cv_func_strerror_r_char_p` variable.

  The Gnulib module `strerror_r` not only ensures that the function has the return type specified by POSIX, but also works around other portability problems of this function.

Macro: AC_FUNC_STRFTIME ¶

  Check for `strftime` in the `intl` library. Then, if `strftime` is available, define `HAVE_STRFTIME`.

  This macro is obsolescent, as no current systems require the `intl` library for `strftime`. New programs need not use this macro.

Macro: AC_FUNC_STRTOD ¶

  If the `strtod` function does not exist or doesn’t work correctly, ask for an `AC_LIBOBJ` replacement of ‘`strtod`’. In this case, because `strtod.c` is likely to need ‘`pow`’, set the output variable `POW_LIB` to the extra library needed.

  This macro caches its result in the `ac_cv_func_strtod` variable and depends upon the result in the `ac_cv_func_pow` variable.

  The `AC_FUNC_STRTOD` macro is obsolescent. New programs should use Gnulib’s `strtod` module. See Gnulib.

Macro: AC_FUNC_STRTOLD ¶

  If the `strtold` function exists and conforms to C99 or later, define `HAVE_STRTOLD`.

  This macro caches its result in the `ac_cv_func_strtold` variable.

  The Gnulib module `strtold` not only ensures that the function exists, but also works around other portability problems of this function.

Macro: AC_FUNC_STRNLEN ¶

  If the `strnlen` function is not available, or is buggy (like the one from Android 5.0 or AIX 4.3), require an `AC_LIBOBJ` replacement for it.

  This macro caches its result in the `ac_cv_func_strnlen_working` variable.

  The `AC_FUNC_STRNLEN` macro is obsolescent. New programs should use Gnulib’s `strnlen` module. See Gnulib.

Macro: AC_FUNC_UTIME_NULL ¶

  If ‘`utime (file, NULL)`’ sets file’s timestamp to the present, define `HAVE_UTIME_NULL`.

  This macro caches its result in the `ac_cv_func_utime_null` variable.

  This macro is obsolescent, as all current systems have a `utime` that behaves this way. New programs need not use this macro.

Macro: AC_FUNC_VPRINTF ¶

  If `vprintf` is found, define `HAVE_VPRINTF`. Otherwise, if `_doprnt` is found, define `HAVE_DOPRNT`. (If `vprintf` is available, you may assume that `vfprintf` and `vsprintf` are also available.)

  This macro is obsolescent, as all current systems have `vprintf`. New programs need not use this macro.

Macro: AC_REPLACE_FNMATCH ¶

  If the `fnmatch` function does not conform to POSIX (see `AC_FUNC_FNMATCH`), ask for its `AC_LIBOBJ` replacement.

  The files `fnmatch.c`, `fnmatch_loop.c`, and `fnmatch_.h` in the `AC_LIBOBJ` replacement directory are assumed to contain a copy of the source code of GNU `fnmatch`. If necessary, this source code is compiled as an `AC_LIBOBJ` replacement, and the `fnmatch_.h` file is linked to `fnmatch.h` so that it can be included in place of the system `<fnmatch.h>`.

  This macro caches its result in the `ac_cv_func_fnmatch_works` variable.

  This macro is obsolescent, as it assumes the use of particular source files. New programs should use Gnulib’s `fnmatch-posix` module, which provides this macro along with the source files. See Gnulib.

#### 5.5.3 Generic Function Checks ¶

These macros are used to find functions not covered by the “particular” test macros. If the functions might be in libraries other than the default C library, first call `AC_CHECK_LIB` for those libraries. If you need to check the behavior of a function as well as find out whether it is present, you have to write your own test for it (see Writing Tests).

Macro: AC_CHECK_FUNC(function, [action-if-found], [action-if-not-found]) ¶

  If C function function is available, run shell commands action-if-found, otherwise action-if-not-found. If you just want to define a symbol if the function is available, consider using `AC_CHECK_FUNCS` instead. This macro checks for functions with C linkage even when `AC_LANG(C++)` has been called, since C is more standardized than C++. (see Language Choice, for more information about selecting the language for checks.)

  This macro caches its result in the `ac_cv_func_function` variable.

Macro: AC_CHECK_FUNCS(function…, [action-if-found], [action-if-not-found]) ¶

  For each function enumerated in the blank-or-newline-separated argument list, define `HAVE_function` (in all capitals) if it is available. If action-if-found is given, it is additional shell code to execute when one of the functions is found. You can give it a value of ‘`break`’ to break out of the loop on the first match. If action-if-not-found is given, it is executed when one of the functions is not found.

  Results are cached for each function as in `AC_CHECK_FUNC`.

Macro: AC_CHECK_FUNCS_ONCE(function…) ¶

  For each function enumerated in the blank-or-newline-separated argument list, define `HAVE_function` (in all capitals) if it is available. This is a once-only variant of `AC_CHECK_FUNCS`. It generates the checking code at most once, so that `configure` is smaller and faster; but the checks cannot be conditionalized and are always done once, early during the `configure` run.

Autoconf follows a philosophy that was formed over the years by those who have struggled for portability: isolate the portability issues in specific files, and then program as if you were in a POSIX environment. Some functions may be missing or unfixable, and your package must be ready to replace them.

Suitable replacements for many such problem functions are available from Gnulib (see Gnulib).

Macro: AC_LIBOBJ(function) ¶

  Specify that ‘`function.c`’ must be included in the executables to replace a missing or broken implementation of function.

  Technically, it adds ‘`function.$ac_objext`’ to the output variable `LIBOBJS` if it is not already in, and calls `AC_LIBSOURCE` for ‘`function.c`’. You should not directly change `LIBOBJS`, since this is not traceable.

Macro: AC_LIBSOURCE(file) ¶

  Specify that file might be needed to compile the project. If you need to know what files might be needed by a `configure.ac`, you should trace `AC_LIBSOURCE`. file must be a literal.

  This macro is called automatically from `AC_LIBOBJ`, but you must call it explicitly if you pass a shell variable to `AC_LIBOBJ`. In that case, since shell variables cannot be traced statically, you must pass to `AC_LIBSOURCE` any possible files that the shell variable might cause `AC_LIBOBJ` to need. For example, if you want to pass a variable `$foo_or_bar` to `AC_LIBOBJ` that holds either `"foo"` or `"bar"`, you should do:

      AC_LIBSOURCE([foo.c])
      AC_LIBSOURCE([bar.c])
      AC_LIBOBJ([$foo_or_bar])

  There is usually a way to avoid this, however, and you are encouraged to simply call `AC_LIBOBJ` with literal arguments.

  Note that this macro replaces the obsolete `AC_LIBOBJ_DECL`, with slightly different semantics: the old macro took the function name, e.g., `foo`, as its argument rather than the file name.

Macro: AC_LIBSOURCES(files) ¶

  Like `AC_LIBSOURCE`, but accepts one or more files in a comma-separated M4 list. Thus, the above example might be rewritten:

      AC_LIBSOURCES([foo.c, bar.c])
      AC_LIBOBJ([$foo_or_bar])

Macro: AC_CONFIG_LIBOBJ_DIR(directory) ¶

  Specify that `AC_LIBOBJ` replacement files are to be found in directory, a name relative to the top level of the source tree. The replacement directory defaults to `.`, the top level directory, and the most typical value is `lib`, corresponding to ‘`AC_CONFIG_LIBOBJ_DIR([lib])`’.

  `configure` might need to know the replacement directory for the following reasons: (i) some checks use the replacement files, (ii) some macros bypass broken system headers by installing links to the replacement headers (iii) when used in conjunction with Automake, within each makefile, directory is used as a relative path from `$(top_srcdir)` to each object named in `LIBOBJS` and `LTLIBOBJS`, etc.

It is common to merely check for the existence of a function, and ask for its `AC_LIBOBJ` replacement if missing. The following macro is a convenient shorthand.

Macro: AC_REPLACE_FUNCS(function…) ¶

  Like `AC_CHECK_FUNCS`, but uses ‘`AC_LIBOBJ(function)`’ as action-if-not-found. You can declare your replacement function by enclosing the prototype in ‘`#ifndef HAVE_function`’. If the system has the function, it probably declares it in a header file you should be including, so you shouldn’t redeclare it lest your declaration conflict.

### 5.6 Header Files ¶

The following macros check for the presence of certain C header files. If there is no macro specifically defined to check for a header file you need, and you don’t need to check for any special properties of it, then you can use one of the general header-file check macros.

#### 5.6.1 Portability of Headers ¶

This section documents some collected knowledge about common headers, and the problems they cause. By definition, this list always requires additions. A much more complete list is maintained by the Gnulib project (see Gnulib), covering POSIX Headers in Gnulib and Glibc Headers in Gnulib. Please help us keep the Gnulib list as complete as possible.

When we say that a header “may require” some set of other headers, we mean that it may be necessary for you to manually include those other headers first, or the contents of the header under test will fail to compile. When checking for these headers, you must provide the potentially-required headers in the includes argument to `AC_CHECK_HEADER` or `AC_CHECK_HEADERS`, or the check will fail spuriously. `AC_INCLUDES_DEFAULT` (see Default Includes) arranges to include a number of common requirements and should normally come first in your includes. For example, `net/if.h` may require `sys/types.h`, `sys/socket.h`, or both, and `AC_INCLUDES_DEFAULT` handles `sys/types.h` but not `sys/socket.h`, so you should check for it like this:

    AC_CHECK_HEADERS([sys/socket.h])
    AC_CHECK_HEADERS([net/if.h], [], [],
    [AC_INCLUDES_DEFAULT[
    #ifdef HAVE_SYS_SOCKET_H
    # include <sys/socket.h>
    #endif
    ]])

Note that the example mixes single quoting (for`AC_INCLUDES_DEFAULT`, so that it gets expanded) and double quoting (to ensure that each preprocessor `#` gets treated as a literal string rather than a comment).

`limits.h`

  In C99 and later, `limits.h` defines `LLONG_MIN`, `LLONG_MAX`, and `ULLONG_MAX`, but many almost-C99 environments (e.g., default GCC 4.0.2 + glibc 2.4) do not define them.

`memory.h` ¶

  This header file is obsolete; use `string.h` instead.

`strings.h` ¶

  On some systems, this is the only header that declares `strcasecmp`, `strncasecmp`, and `ffs`.

  This header may or may not include `string.h` for you. However, on all recent systems it is safe to include both `string.h` and `strings.h`, in either order, in the same source file.

`inttypes.h` vs. `stdint.h` ¶

  C99 specifies that `inttypes.h` includes `stdint.h`, so there’s no need to include `stdint.h` separately in a standard environment. However, some implementations have `stdint.h` but not `inttypes.h` (e.g. MSVC 2012). Therefore, it is necessary to check for each and include each only if available.

`linux/irda.h` ¶

  This header may require `linux/types.h` and/or `sys/socket.h`.

`linux/random.h` ¶

  This header may require `linux/types.h`.

`net/if.h` ¶

  This header may require `sys/types.h` and/or `sys/socket.h`.

`netinet/if_ether.h` ¶

  This header may require some combination of `sys/types.h`, `sys/socket.h`, `netinet/in.h`, and `net/if.h`.

`sys/mount.h` ¶

  This header may require `sys/params.h`.

`sys/ptem.h` ¶

  This header may require `sys/stream.h`.

`sys/socket.h` ¶

  This header may require `sys/types.h`.

`sys/ucred.h` ¶

  This header may require `sys/types.h`.

`X11/extensions/scrnsaver.h` ¶

  Using XFree86, this header requires `X11/Xlib.h`, which is probably so required that you might not even consider looking for it.

#### 5.6.2 Particular Header Checks ¶

These macros check for particular system header files—whether they exist, and in some cases whether they declare certain symbols.

Macro: AC_CHECK_HEADER_STDBOOL ¶

  Check whether `stdbool.h` exists and conforms to C99 or later, and cache the result in the `ac_cv_header_stdbool_h` variable. If the type `_Bool` is defined, define `HAVE__BOOL` to 1.

  This macro is obsolescent, as all current C compilers have `stdbool.h`, a header that is itself obsolescent as of C23.

  This macro is intended for use by Gnulib (see Gnulib) and other packages that supply a substitute `stdbool.h` on platforms lacking a conforming one. The `AC_HEADER_STDBOOL` macro is better for code that explicitly checks for `stdbool.h`.

Macro: AC_HEADER_ASSERT ¶

  Check whether to enable assertions in the style of `assert.h`. Assertions are enabled by default, but the user can override this by invoking `configure` with the `--disable-assert` option.

Macro: AC_HEADER_DIRENT ¶

  Check for the following header files. For the first one that is found and defines ‘`DIR`’, define the listed C preprocessor macro:

  `dirent.h` | `HAVE_DIRENT_H`
  `sys/ndir.h` | `HAVE_SYS_NDIR_H`
  `sys/dir.h` | `HAVE_SYS_DIR_H`
  `ndir.h` | `HAVE_NDIR_H`

  The directory-library declarations in your source code should look something like the following:

      #include <sys/types.h>
      #ifdef HAVE_DIRENT_H
      # include <dirent.h>
      # define NAMLEN(dirent) strlen ((dirent)->d_name)
      #else
      # define dirent direct
      # define NAMLEN(dirent) ((dirent)->d_namlen)
      # ifdef HAVE_SYS_NDIR_H
      #  include <sys/ndir.h>
      # endif
      # ifdef HAVE_SYS_DIR_H
      #  include <sys/dir.h>
      # endif
      # ifdef HAVE_NDIR_H
      #  include <ndir.h>
      # endif
      #endif

  Using the above declarations, the program would declare variables to be of type `struct dirent`, not `struct direct`, and would access the length of a directory entry name by passing a pointer to a `struct dirent` to the `NAMLEN` macro.

  This macro also checks for the obsolete `dir` and `x` libraries.

  This macro is obsolescent, as all current systems with directory libraries have `<dirent.h>`. New programs need not use this macro.

  Also see `AC_STRUCT_DIRENT_D_INO` and `AC_STRUCT_DIRENT_D_TYPE` (see Particular Structure Checks).

Macro: AC_HEADER_MAJOR ¶

  Detect the headers required to use `makedev`, `major`, and `minor`. These functions may be defined by `sys/mkdev.h`, `sys/sysmacros.h`, or `sys/types.h`.

  `AC_HEADER_MAJOR` defines `MAJOR_IN_MKDEV` if they are in `sys/mkdev.h`, or `MAJOR_IN_SYSMACROS` if they are in `sys/sysmacros.h`. If neither macro is defined, they are either in `sys/types.h` or unavailable.

  To properly use these functions, your code should contain something like:

      #include <sys/types.h>
      #ifdef MAJOR_IN_MKDEV
      # include <sys/mkdev.h>
      #elif defined MAJOR_IN_SYSMACROS
      # include <sys/sysmacros.h>
      #endif

  Note: Configure scripts built with Autoconf 2.69 or earlier will not detect a problem if `sys/types.h` contains definitions of `major`, `minor`, and/or `makedev` that trigger compiler warnings upon use. This is known to occur with GNU libc 2.25, where those definitions are being deprecated to reduce namespace pollution. If it is not practical to use Autoconf 2.70 to regenerate the configure script of affected software, you can work around the problem by setting ‘`ac_cv_header_sys_types_h_makedev=no`’, as an argument to `configure` or as part of a `config.site` site default file (see Setting Site Defaults).

Macro: AC_HEADER_RESOLV ¶

  Checks for header `resolv.h`, checking for prerequisites first. To properly use `resolv.h`, your code should contain something like the following:

      #ifdef HAVE_SYS_TYPES_H
      #  include <sys/types.h>
      #endif
      #ifdef HAVE_NETINET_IN_H
      #  include <netinet/in.h>   /* inet_ functions / structs */
      #endif
      #ifdef HAVE_ARPA_NAMESER_H
      #  include <arpa/nameser.h> /* DNS HEADER struct */
      #endif
      #ifdef HAVE_NETDB_H
      #  include <netdb.h>
      #endif
      #include <resolv.h>

Macro: AC_HEADER_STAT ¶

  If the macros `S_ISDIR`, `S_ISREG`, etc. defined in `sys/stat.h` do not work properly (returning false positives), define `STAT_MACROS_BROKEN`. This is the case on Tektronix UTekV, Amdahl UTS and Motorola System V/88.

  This macro is obsolescent, as no current systems have the bug. New programs need not use this macro.

Macro: AC_HEADER_STDBOOL ¶

  If `stdbool.h` exists and conforms to C99 or later, define `HAVE_STDBOOL_H` to 1; if the type `_Bool` is defined, define `HAVE__BOOL` to 1.

  This macro is obsolescent, as all current C compilers have `stdbool.h`, a header that is itself obsolescent as of C23. Nowadays programs that need `bool`, `true` and `false` can include `stdbool.h` unconditionally, without using `AC_HEADER_STDBOOL`, and if such a program needs to be portable only to C23 or later it need not even include `stdbool.h`.

  This macro caches its result in the `ac_cv_header_stdbool_h` variable.

  This macro differs from `AC_CHECK_HEADER_STDBOOL` only in that it defines `HAVE_STDBOOL_H` whereas `AC_CHECK_HEADER_STDBOOL` does not.

Macro: AC_HEADER_STDC ¶

  This macro is obsolescent. Its sole effect is to make sure that all the headers that are included by `AC_INCLUDES_DEFAULT` (see Default Includes), but not part of C89, have been checked for.

  All hosted environments that are still of interest for portable code provide all of the headers specified in C89 (as amended in 1995).

Macro: AC_HEADER_SYS_WAIT ¶

  If `sys/wait.h` exists and is compatible with POSIX, define `HAVE_SYS_WAIT_H`. Incompatibility can occur if `sys/wait.h` does not exist, or if it uses the old BSD `union wait` instead of `int` to store a status value. If `sys/wait.h` is not POSIX compatible, then instead of including it, define the POSIX macros with their usual interpretations. Here is an example:

      #include <sys/types.h>
      #ifdef HAVE_SYS_WAIT_H
      # include <sys/wait.h>
      #endif
      #ifndef WEXITSTATUS
      # define WEXITSTATUS(stat_val) ((unsigned int) (stat_val) >> 8)
      #endif
      #ifndef WIFEXITED
      # define WIFEXITED(stat_val) (((stat_val) & 255) == 0)
      #endif

  This macro caches its result in the `ac_cv_header_sys_wait_h` variable.

  This macro is obsolescent, as current systems are compatible with POSIX. New programs need not use this macro.

`_POSIX_VERSION` is defined when `unistd.h` is included on POSIX systems. If there is no `unistd.h`, it is definitely not a POSIX system. However, some non-POSIX systems do have `unistd.h`.

The way to check whether the system says it supports POSIX is:

    #ifdef HAVE_UNISTD_H
    # include <sys/types.h>
    # include <unistd.h>
    #endif

    #ifdef _POSIX_VERSION
    /* Code for POSIX systems.  */
    #endif

Macro: AC_HEADER_TIOCGWINSZ ¶

  If the use of `TIOCGWINSZ` requires `<sys/ioctl.h>`, then define `GWINSZ_IN_SYS_IOCTL`. Otherwise `TIOCGWINSZ` can be found in `<termios.h>`.

  Use:

      #ifdef HAVE_TERMIOS_H
      # include <termios.h>
      #endif

      #ifdef GWINSZ_IN_SYS_IOCTL
      # include <sys/ioctl.h>
      #endif

#### 5.6.3 Generic Header Checks ¶

These macros are used to find system header files not covered by the “particular” test macros. If you need to check the contents of a header as well as find out whether it is present, you have to write your own test for it (see Writing Tests).

Macro: AC_CHECK_HEADER(header-file, [action-if-found], [action-if-not-found], [includes]) ¶

  If the system header file header-file is compilable, execute shell commands action-if-found, otherwise execute action-if-not-found. If you just want to define a symbol if the header file is available, consider using `AC_CHECK_HEADERS` instead.

  includes should be the appropriate prerequisite code, i.e. whatever might be required to appear above ‘`#include <header-file>`’ for it to compile without error. This can be anything, but will normally be additional ‘`#include`’ directives. If includes is omitted or empty, `configure` will use the contents of the macro `AC_INCLUDES_DEFAULT`. See Default Includes.

  This macro used to check only for the presence of a header, not whether its contents were acceptable to the compiler. Some older `configure` scripts rely on this behavior, so it is still available by specifying ‘`-`’ as includes. This mechanism is deprecated as of Autoconf 2.70; situations where a preprocessor-only check is required should use `AC_PREPROC_IFELSE`. See Running the Preprocessor.

  This macro caches its result in the `ac_cv_header_header-file` variable, with characters not suitable for a variable name mapped to underscores.

Macro: AC_CHECK_HEADERS(header-file…, [action-if-found], [action-if-not-found], [includes]) ¶

  For each given system header file header-file in the blank-separated argument list that exists, define `HAVE_header-file` (in all capitals). If action-if-found is given, it is additional shell code to execute when one of the header files is found. You can give it a value of ‘`break`’ to break out of the loop on the first match. If action-if-not-found is given, it is executed when one of the header files is not found.

  includes is interpreted as in `AC_CHECK_HEADER`, in order to choose the set of preprocessor directives supplied before the header under test.

  This macro caches its result in the `ac_cv_header_header-file` variable, with characters not suitable for a variable name mapped to underscores.

Macro: AC_CHECK_HEADERS_ONCE(header-file…) ¶

  For each given system header file header-file in the blank-separated argument list that exists, define `HAVE_header-file` (in all capitals).

  If you do not need the full power of `AC_CHECK_HEADERS`, this variant generates smaller, faster `configure` files. All headers passed to `AC_CHECK_HEADERS_ONCE` are checked for in one pass, early during the `configure` run. The checks cannot be conditionalized, you cannot specify an action-if-found or action-if-not-found, and `AC_INCLUDES_DEFAULT` is always used for the prerequisites.

In previous versions of Autoconf, these macros merely checked whether the header was accepted by the preprocessor. This was changed because the old test was inappropriate for typical uses. Headers are typically used to compile, not merely to preprocess, and the old behavior sometimes accepted headers that clashed at compile-time (see Header Present But Cannot Be Compiled). If for some reason it is inappropriate to check whether a header is compilable, you should use `AC_PREPROC_IFELSE` (see Running the Preprocessor) instead of these macros.

Requiring each header to compile improves the robustness of the test, but it also requires you to make sure that the includes are correct. Most system headers nowadays make sure to `#include` whatever they require, or else have their dependencies satisfied by `AC_INCLUDES_DEFAULT` (see Default Includes), but see Portability of Headers, for known exceptions. In general, if you are looking for `bar.h`, which requires that `foo.h` be included first if it exists, you should do something like this:

    AC_CHECK_HEADERS([foo.h])
    AC_CHECK_HEADERS([bar.h], [], [],
    [#ifdef HAVE_FOO_H
    # include <foo.h>
    #endif
    ])

### 5.7 Declarations ¶

The following macros check for the declaration of variables and functions. If there is no macro specifically defined to check for a symbol you need, then you can use the general macros (see Generic Declaration Checks) or, for more complex tests, you may use `AC_COMPILE_IFELSE` (see Running the Compiler).

#### 5.7.1 Particular Declaration Checks ¶

There are no specific macros for declarations.

#### 5.7.2 Generic Declaration Checks ¶

These macros are used to find declarations not covered by the “particular” test macros.

Macro: AC_CHECK_DECL(symbol, [action-if-found], [action-if-not-found], [includes = ‘`AC_INCLUDES_DEFAULT`’]) ¶

  If symbol (a function, variable, or constant) is not declared in includes and a declaration is needed, run the shell commands action-if-not-found, otherwise action-if-found. includes is a series of include directives, defaulting to `AC_INCLUDES_DEFAULT` (see Default Includes), which are used prior to the declaration under test.

  This macro actually tests whether symbol is defined as a macro or can be used as an r-value, not whether it is really declared, because it is much safer to avoid introducing extra declarations when they are not needed. In order to facilitate use of C++ and overloaded function declarations, it is possible to specify function argument types in parentheses for types which can be zero-initialized:

      AC_CHECK_DECL([basename(char *)])

  This macro caches its result in the `ac_cv_have_decl_symbol` variable, with characters not suitable for a variable name mapped to underscores.

Macro: AC_CHECK_DECLS(symbols, [action-if-found], [action-if-not-found], [includes = ‘`AC_INCLUDES_DEFAULT`’]) ¶

  For each of the symbols (comma-separated list with optional function argument types for C++ overloads), define `HAVE_DECL_symbol` (in all capitals) to ‘`1`’ if symbol is declared, otherwise to ‘`0`’. If action-if-not-found is given, it is additional shell code to execute when one of the function declarations is needed, otherwise action-if-found is executed.

  includes is a series of include directives, defaulting to `AC_INCLUDES_DEFAULT` (see Default Includes), which are used prior to the declarations under test.

  This macro uses an M4 list as first argument:

      AC_CHECK_DECLS([strdup])
      AC_CHECK_DECLS([strlen])
      AC_CHECK_DECLS([malloc, realloc, calloc, free])
      AC_CHECK_DECLS([j0], [], [], [[#include <math.h>]])
      AC_CHECK_DECLS([[basename(char *)], [dirname(char *)]])

  Unlike the other ‘`AC_CHECK_*S`’ macros, when a symbol is not declared, `HAVE_DECL_symbol` is defined to ‘`0`’ instead of leaving `HAVE_DECL_symbol` undeclared. When you are sure that the check was performed, use `HAVE_DECL_symbol` in `#if`:

      #if !HAVE_DECL_SYMBOL
      extern char *symbol;
      #endif

  If the test may have not been performed, however, because it is safer not to declare a symbol than to use a declaration that conflicts with the system’s one, you should use:

      #if defined HAVE_DECL_MALLOC && !HAVE_DECL_MALLOC
      void *malloc (size_t *s);
      #endif

  You fall into the second category only in extreme situations: either your files may be used without being configured, or they are used during the configuration. In most cases the traditional approach is enough.

  This macro caches its results in `ac_cv_have_decl_symbol` variables, with characters not suitable for a variable name mapped to underscores.

Macro: AC_CHECK_DECLS_ONCE(symbols) ¶

  For each of the symbols (comma-separated list), define `HAVE_DECL_symbol` (in all capitals) to ‘`1`’ if symbol is declared in the default include files, otherwise to ‘`0`’. This is a once-only variant of `AC_CHECK_DECLS`. It generates the checking code at most once, so that `configure` is smaller and faster; but the checks cannot be conditionalized and are always done once, early during the `configure` run.

### 5.8 Structures ¶

The following macros check for the presence of certain members in C structures. If there is no macro specifically defined to check for a member you need, then you can use the general structure-member macros (see Generic Structure Checks) or, for more complex tests, you may use `AC_COMPILE_IFELSE` (see Running the Compiler).

#### 5.8.1 Particular Structure Checks ¶

The following macros check for certain structures or structure members.

Macro: AC_STRUCT_DIRENT_D_INO ¶

  Perform all the actions of `AC_HEADER_DIRENT` (see Particular Header Checks). Then, if `struct dirent` contains a `d_ino` member, define `HAVE_STRUCT_DIRENT_D_INO`.

  `HAVE_STRUCT_DIRENT_D_INO` indicates only the presence of `d_ino`, not whether its contents are always reliable. Traditionally, a zero `d_ino` indicated a deleted directory entry, though current systems hide this detail from the user and never return zero `d_ino` values. Many current systems report an incorrect `d_ino` for a directory entry that is a mount point.

Macro: AC_STRUCT_DIRENT_D_TYPE ¶

  Perform all the actions of `AC_HEADER_DIRENT` (see Particular Header Checks). Then, if `struct dirent` contains a `d_type` member, define `HAVE_STRUCT_DIRENT_D_TYPE`.

Macro: AC_STRUCT_ST_BLOCKS ¶

  If `struct stat` contains an `st_blocks` member, define `HAVE_STRUCT_STAT_ST_BLOCKS`. Otherwise, require an `AC_LIBOBJ` replacement of ‘`fileblocks`’. The former name, `HAVE_ST_BLOCKS` is to be avoided, as its support will cease in the future.

  This macro caches its result in the `ac_cv_member_struct_stat_st_blocks` variable.

Macro: AC_STRUCT_TM ¶

  If `time.h` does not define `struct tm`, define `TM_IN_SYS_TIME`, which means that including `sys/time.h` had better define `struct tm`.

  This macro is obsolescent, as `time.h` defines `struct tm` in current systems. New programs need not use this macro.

Macro: AC_STRUCT_TIMEZONE ¶

  Figure out how to get the current timezone. If `struct tm` has a `tm_zone` member, define `HAVE_STRUCT_TM_TM_ZONE` (and the obsoleted `HAVE_TM_ZONE`). Otherwise, if the external array `tzname` is found, define `HAVE_TZNAME`; if it is declared, define `HAVE_DECL_TZNAME`.

#### 5.8.2 Generic Structure Checks ¶

These macros are used to find structure members not covered by the “particular” test macros.

Macro: AC_CHECK_MEMBER(aggregate.member, [action-if-found], [action-if-not-found], [includes = ‘`AC_INCLUDES_DEFAULT`’]) ¶

  Check whether member is a member of the aggregate aggregate. If no includes are specified, the default includes are used (see Default Includes).

      AC_CHECK_MEMBER([struct passwd.pw_gecos], [],
                      [AC_MSG_ERROR([we need 'passwd.pw_gecos'])],
                      [[#include <pwd.h>]])

  You can use this macro for submembers:

      AC_CHECK_MEMBER(struct top.middle.bot)

  This macro caches its result in the `ac_cv_member_aggregate_member` variable, with characters not suitable for a variable name mapped to underscores.

Macro: AC_CHECK_MEMBERS(members, [action-if-found], [action-if-not-found], [includes = ‘`AC_INCLUDES_DEFAULT`’]) ¶

  Check for the existence of each ‘`aggregate.member`’ of members using the previous macro. When member belongs to aggregate, define `HAVE_aggregate_member` (in all capitals, with spaces and dots replaced by underscores). If action-if-found is given, it is executed for each of the found members. If action-if-not-found is given, it is executed for each of the members that could not be found.

  includes is a series of include directives, defaulting to `AC_INCLUDES_DEFAULT` (see Default Includes), which are used prior to the members under test.

  This macro uses M4 lists:

      AC_CHECK_MEMBERS([struct stat.st_rdev, struct stat.st_blksize])

### 5.9 Types ¶

The following macros check for C types, either builtin or typedefs. If there is no macro specifically defined to check for a type you need, and you don’t need to check for any special properties of it, then you can use a general type-check macro.

#### 5.9.1 Particular Type Checks ¶

These macros check for particular C types in `sys/types.h`, `stdlib.h`, `stdint.h`, `inttypes.h` and others, if they exist.

The Gnulib `stdint` module is an alternate way to define many of these symbols; it is useful if you prefer your code to assume a C99-or-better environment. See Gnulib.

Macro: AC_TYPE_GETGROUPS ¶

  Define `GETGROUPS_T` to be whichever of `gid_t` or `int` is the base type of the array argument to `getgroups`.

  This macro caches the base type in the `ac_cv_type_getgroups` variable.

Macro: AC_TYPE_INT8_T ¶

  If `stdint.h` or `inttypes.h` does not define the type `int8_t`, define `int8_t` to a signed integer type that is exactly 8 bits wide and that uses two’s complement representation, if such a type exists. If you are worried about porting to hosts that lack such a type, you can use the results of this macro as follows:

      #if HAVE_STDINT_H
      # include <stdint.h>
      #endif
      #if defined INT8_MAX || defined int8_t
       code using int8_t
      #else
       complicated alternative using >8-bit 'signed char'
      #endif

  This macro caches the type in the `ac_cv_c_int8_t` variable.

Macro: AC_TYPE_INT16_T ¶

  This is like `AC_TYPE_INT8_T`, except for 16-bit integers.

Macro: AC_TYPE_INT32_T ¶

  This is like `AC_TYPE_INT8_T`, except for 32-bit integers.

Macro: AC_TYPE_INT64_T ¶

  This is like `AC_TYPE_INT8_T`, except for 64-bit integers.

Macro: AC_TYPE_INTMAX_T ¶

  If `stdint.h` or `inttypes.h` defines the type `intmax_t`, define `HAVE_INTMAX_T`. Otherwise, define `intmax_t` to the widest signed integer type.

Macro: AC_TYPE_INTPTR_T ¶

  If `stdint.h` or `inttypes.h` defines the type `intptr_t`, define `HAVE_INTPTR_T`. Otherwise, define `intptr_t` to a signed integer type wide enough to hold a pointer, if such a type exists.

Macro: AC_TYPE_LONG_DOUBLE ¶

  If the C compiler supports a working `long double` type, define `HAVE_LONG_DOUBLE`. The `long double` type might have the same range and precision as `double`.

  This macro caches its result in the `ac_cv_type_long_double` variable.

  This macro is obsolescent, as current C compilers support `long double`. New programs need not use this macro.

Macro: AC_TYPE_LONG_DOUBLE_WIDER ¶

  If the C compiler supports a working `long double` type with more range or precision than the `double` type, define `HAVE_LONG_DOUBLE_WIDER`.

  This macro caches its result in the `ac_cv_type_long_double_wider` variable.

Macro: AC_TYPE_LONG_LONG_INT ¶

  If the C compiler supports a working `long long int` type, define `HAVE_LONG_LONG_INT`. However, this test does not test `long long int` values in preprocessor `#if` expressions, because too many compilers mishandle such expressions. See Preprocessor Arithmetic.

  This macro caches its result in the `ac_cv_type_long_long_int` variable.

Macro: AC_TYPE_MBSTATE_T ¶

  Define `HAVE_MBSTATE_T` if `<wchar.h>` declares the `mbstate_t` type. Also, define `mbstate_t` to be a type if `<wchar.h>` does not declare it.

  This macro caches its result in the `ac_cv_type_mbstate_t` variable.

Macro: AC_TYPE_MODE_T ¶

  Define `mode_t` to a suitable type, if standard headers do not define it.

  This macro caches its result in the `ac_cv_type_mode_t` variable.

Macro: AC_TYPE_OFF_T ¶

  Define `off_t` to a suitable type, if standard headers do not define it.

  This macro caches its result in the `ac_cv_type_off_t` variable.

Macro: AC_TYPE_PID_T ¶

  Define `pid_t` to a suitable type, if standard headers do not define it.

  This macro caches its result in the `ac_cv_type_pid_t` variable.

Macro: AC_TYPE_SIZE_T ¶

  Define `size_t` to a suitable type, if standard headers do not define it.

  This macro caches its result in the `ac_cv_type_size_t` variable.

Macro: AC_TYPE_SSIZE_T ¶

  Define `ssize_t` to a suitable type, if standard headers do not define it.

  This macro caches its result in the `ac_cv_type_ssize_t` variable.

Macro: AC_TYPE_UID_T ¶

  Define `uid_t` and `gid_t` to suitable types, if standard headers do not define them.

  This macro caches its result in the `ac_cv_type_uid_t` variable.

Macro: AC_TYPE_UINT8_T ¶

  If `stdint.h` or `inttypes.h` does not define the type `uint8_t`, define `uint8_t` to an unsigned integer type that is exactly 8 bits wide, if such a type exists. This is like `AC_TYPE_INT8_T`, except for unsigned integers.

Macro: AC_TYPE_UINT16_T ¶

  This is like `AC_TYPE_UINT8_T`, except for 16-bit integers.

Macro: AC_TYPE_UINT32_T ¶

  This is like `AC_TYPE_UINT8_T`, except for 32-bit integers.

Macro: AC_TYPE_UINT64_T ¶

  This is like `AC_TYPE_UINT8_T`, except for 64-bit integers.

Macro: AC_TYPE_UINTMAX_T ¶

  If `stdint.h` or `inttypes.h` defines the type `uintmax_t`, define `HAVE_UINTMAX_T`. Otherwise, define `uintmax_t` to the widest unsigned integer type.

Macro: AC_TYPE_UINTPTR_T ¶

  If `stdint.h` or `inttypes.h` defines the type `uintptr_t`, define `HAVE_UINTPTR_T`. Otherwise, define `uintptr_t` to an unsigned integer type wide enough to hold a pointer, if such a type exists.

Macro: AC_TYPE_UNSIGNED_LONG_LONG_INT ¶

  If the C compiler supports a working `unsigned long long int` type, define `HAVE_UNSIGNED_LONG_LONG_INT`. However, this test does not test `unsigned long long int` values in preprocessor `#if` expressions, because too many compilers mishandle such expressions. See Preprocessor Arithmetic.

  This macro caches its result in the `ac_cv_type_unsigned_long_long_int` variable.

#### 5.9.2 Generic Type Checks ¶

These macros are used to check for types not covered by the “particular” test macros.

Macro: AC_CHECK_TYPE(type, [action-if-found], [action-if-not-found], [includes = ‘`AC_INCLUDES_DEFAULT`’]) ¶

  Check whether type is defined. It may be a compiler builtin type or defined by the includes. includes is a series of include directives, defaulting to `AC_INCLUDES_DEFAULT` (see Default Includes), which are used prior to the type under test.

  In C, type must be a type-name, so that the expression ‘`sizeof (type)`’ is valid (but ‘`sizeof ((type))`’ is not). The same test is applied when compiling for C++, which means that in C++ type should be a type-id and should not be an anonymous ‘`struct`’ or ‘`union`’.

  This macro caches its result in the `ac_cv_type_type` variable, with ‘`*`’ mapped to ‘`p`’ and other characters not suitable for a variable name mapped to underscores.

Macro: AC_CHECK_TYPES(types, [action-if-found], [action-if-not-found], [includes = ‘`AC_INCLUDES_DEFAULT`’]) ¶

  For each type of the types that is defined, define `HAVE_type` (in all capitals). Each type must follow the rules of `AC_CHECK_TYPE`. If no includes are specified, the default includes are used (see Default Includes). If action-if-found is given, it is additional shell code to execute when one of the types is found. If action-if-not-found is given, it is executed when one of the types is not found.

  This macro uses M4 lists:

      AC_CHECK_TYPES([ptrdiff_t])
      AC_CHECK_TYPES([unsigned long long int, uintmax_t])
      AC_CHECK_TYPES([float_t], [], [], [[#include <math.h>]])

Autoconf, up to 2.13, used to provide to another version of `AC_CHECK_TYPE`, broken by design. In order to keep backward compatibility, a simple heuristic, quite safe but not totally, is implemented. In case of doubt, read the documentation of the former `AC_CHECK_TYPE`, see Obsolete Macros.

### 5.10 Compilers and Preprocessors ¶

All the tests for compilers (`AC_PROG_CC`, `AC_PROG_CXX`, `AC_PROG_F77`) define the output variable `EXEEXT` based on the output of the compiler, typically to the empty string if POSIX and ‘`.exe`’ if a DOS variant.

They also define the output variable `OBJEXT` based on the output of the compiler, after `.c` files have been excluded, typically to ‘`o`’ if POSIX, ‘`obj`’ if a DOS variant.

If the compiler being used does not produce executables, the tests fail. If the executables can’t be run, and cross-compilation is not enabled, they fail too. See Manual Configuration, for more on support for cross compiling.

#### 5.10.1 Specific Compiler Characteristics ¶

Some compilers exhibit different behaviors.

Static/Dynamic Expressions

  Autoconf relies on a trick to extract one bit of information from the C compiler: using negative array sizes. For instance the following excerpt of a C source demonstrates how to test whether ‘`int`’ objects are 4 bytes wide:

      static int test_array[sizeof (int) == 4 ? 1 : -1];

  To our knowledge, there is a single compiler that does not support this trick: the HP C compilers (the real ones, not only the “bundled”) on HP-UX 11.00. They incorrectly reject the above program with the diagnostic “Variable-length arrays cannot have static storage.” This bug comes from HP compilers’ mishandling of `sizeof (int)`, not from the `? 1 : -1`, and Autoconf works around this problem by casting `sizeof (int)` to `long int` before comparing it.

#### 5.10.2 Generic Compiler Characteristics ¶

Macro: AC_CHECK_SIZEOF(type-or-expr, [unused], [includes = ‘`AC_INCLUDES_DEFAULT`’]) ¶

  Define `SIZEOF_type-or-expr` (see Standard Symbols) to be the size in bytes of type-or-expr, which may be either a type or an expression returning a value that has a size. If the expression ‘`sizeof (type-or-expr)`’ is invalid, the result is 0. includes is a series of include directives, defaulting to `AC_INCLUDES_DEFAULT` (see Default Includes), which are used prior to the expression under test.

  This macro now works even when cross-compiling. The unused argument was used when cross-compiling.

  For example, the call

      AC_CHECK_SIZEOF([int *])

  defines `SIZEOF_INT_P` to be 8 on x86-64 systems.

  This macro caches its result in the `ac_cv_sizeof_type-or-expr` variable, with ‘`*`’ mapped to ‘`p`’ and other characters not suitable for a variable name mapped to underscores.

Macro: AC_CHECK_ALIGNOF(type, [includes = ‘`AC_INCLUDES_DEFAULT`’]) ¶

  Define `ALIGNOF_type` (see Standard Symbols) to be the alignment in bytes of type. ‘`type y;`’ must be valid as a structure member declaration. If ‘`type`’ is unknown, the result is 0. If no includes are specified, the default includes are used (see Default Includes).

  This macro caches its result in the `ac_cv_alignof_type-or-expr` variable, with ‘`*`’ mapped to ‘`p`’ and other characters not suitable for a variable name mapped to underscores.

Macro: AC_COMPUTE_INT(var, expression, [includes = ‘`AC_INCLUDES_DEFAULT`’], [action-if-fails]) ¶

  Store into the shell variable var the value of the integer expression. The value should fit in an initializer in a C variable of type `signed long`. To support cross compilation, it should be possible to evaluate the expression at compile-time. If no includes are specified, the default includes are used (see Default Includes).

  Execute action-if-fails if the value cannot be determined correctly.

Macro: AC_LANG_WERROR ¶

  Normally Autoconf ignores warnings generated by the compiler, linker, and preprocessor. If this macro is used, warnings count as fatal errors for the current language. This macro is useful when the results of configuration are used where warnings are unacceptable; for instance, if parts of a program are built with the GCC `-Werror` option. If the whole program is built using `-Werror` it is often simpler to put `-Werror` in the compiler flags (`CFLAGS`, etc.).

Macro: AC_OPENMP ¶

  OpenMP specifies extensions of C, C++, and Fortran that simplify optimization of shared memory parallelism, which is a common problem on multi-core CPUs.

  If the current language is C, the macro `AC_OPENMP` sets the variable `OPENMP_CFLAGS` to the C compiler flags needed for supporting OpenMP. `OPENMP_CFLAGS` is set to empty if the compiler already supports OpenMP, if it has no way to activate OpenMP support, or if the user rejects OpenMP support by invoking ‘`configure`’ with the ‘`--disable-openmp`’ option.

  `OPENMP_CFLAGS` needs to be used when compiling programs, when preprocessing program source, and when linking programs. Therefore you need to add `$(OPENMP_CFLAGS)` to the `CFLAGS` of C programs that use OpenMP. If you preprocess OpenMP-specific C code, you also need to add `$(OPENMP_CFLAGS)` to `CPPFLAGS`. The presence of OpenMP support is revealed at compile time by the preprocessor macro `_OPENMP`.

  Linking a program with `OPENMP_CFLAGS` typically adds one more shared library to the program’s dependencies, so its use is recommended only on programs that actually require OpenMP.

  If the current language is C++, `AC_OPENMP` sets the variable `OPENMP_CXXFLAGS`, suitably for the C++ compiler. The same remarks hold as for C.

  If the current language is Fortran 77 or Fortran, `AC_OPENMP` sets the variable `OPENMP_FFLAGS` or `OPENMP_FCFLAGS`, respectively. Similar remarks as for C hold, except that `CPPFLAGS` is not used for Fortran, and no preprocessor macro signals OpenMP support.

  For portability, it is best to avoid spaces between ‘`#`’ and ‘`pragma omp`’. That is, write ‘`#pragma omp`’, not ‘`# pragma omp`’. The Sun WorkShop 6.2 C compiler chokes on the latter.

  This macro caches its result in the `ac_cv_prog_c_openmp`, `ac_cv_prog_cxx_openmp`, `ac_cv_prog_f77_openmp`, or `ac_cv_prog_fc_openmp` variable, depending on the current language.

  Caution: Some of the compiler options that `AC_OPENMP` tests, mean “enable OpenMP” to one compiler, but “write output to a file named `mp` or `penmp`” to other compilers. We cannot guarantee that the implementation of `AC_OPENMP` will not overwrite an existing file with either of these names.

  Therefore, as a defensive measure, a `configure` script that uses `AC_OPENMP` will issue an error and stop (before doing any of the operations that might overwrite these files) upon encountering either of these files in its working directory. `autoconf` will also issue an error if it finds either of these files in the same directory as a `configure.ac` that uses `AC_OPENMP`.

  If you have files with either of these names at the top level of your source tree, and you need to use `AC_OPENMP`, we recommend you either change their names or move them into a subdirectory.

#### 5.10.3 C Compiler Characteristics ¶

The following macros provide ways to find and exercise a C Compiler. There are a few constructs that ought to be avoided, but do not deserve being checked for, since they can easily be worked around.

Don’t use lines containing solitary backslashes

  They tickle a bug in the HP-UX C compiler (checked on HP-UX 10.20, 11.00, and 11i). When given the following source:

      #ifdef __STDC__
      /\
      * A comment with backslash-newlines in it.  %{ %} *\
      \
      /
      char str[] = "\\
      " A string with backslash-newlines in it %{ %} \\
      "";
      char apostrophe = '\\
      \
      '\
      ';
      #endif

  the compiler incorrectly fails with the diagnostics “Non-terminating comment at end of file” and “Missing ‘`#endif`’ at end of file.” Removing the lines with solitary backslashes solves the problem.

Don’t compile several files at once if output matters to you

  Some compilers, such as HP’s, report names of files being compiled when given more than one file operand. For instance:

      $ cc a.c b.c
      a.c:
      b.c:

  This can cause problems if you observe the output of the compiler to detect failures. Invoking ‘`cc -c a.c && cc -c b.c && cc -o c a.o b.o`’ solves the issue.

Don’t rely on correct `#line` support

  On Solaris, `c89` (at least through Oracle Developer Studio 12.6) diagnoses `#line` directives whose line numbers are greater than 32767. Nothing in POSIX makes this invalid. That is why Autoconf stopped issuing `#line` directives.

Macro: AC_PROG_CC([compiler-search-list]) ¶

  Determine a C compiler to use.

  If the environment variable `CC` is set, its value will be taken as the name of the C compiler to use. Otherwise, search for a C compiler under a series of likely names, trying `gcc` and `cc` first. Regardless, the output variable `CC` is set to the chosen compiler.

  If the optional first argument to the macro is used, it must be a whitespace-separated list of potential names for a C compiler, which overrides the built-in list.

  If no C compiler can be found, `configure` will error out.

  If the selected C compiler is found to be GNU C (regardless of its name), the shell variable `GCC` will be set to ‘`yes`’. If the shell variable `CFLAGS` was not already set, it is set to `-g -O2` for the GNU C compiler (`-O2` on systems where GCC does not accept `-g`), or `-g` for other compilers. `CFLAGS` is then made an output variable. You can override the default for `CFLAGS` by inserting a shell default assignment between `AC_INIT` and `AC_PROG_CC`:

      : ${CFLAGS="options"}

  where options are the appropriate set of options to use by default. (It is important to use this construct rather than a normal assignment, so that `CFLAGS` can still be overridden by the person building the package. See Preset Output Variables.)

  If necessary, options are added to `CC` to enable support for ISO Standard C features with extensions, preferring the newest edition of the C standard for which detection is supported. Currently the newest edition Autoconf knows how to detect support for is C23. After calling this macro you can check whether the C compiler has been set to accept standard C by inspecting the shell variable `ac_prog_cc_stdc`. Its value is ‘`c23`’, ‘`c11`’, ‘`c99`’, or ‘`c89`’, respectively, if the C compiler has been set to use the 2023, 2011, 1999, or 1990 edition of the C standard, and ‘`no`’ if the compiler does not support compiling standard C at all. (There is no special value for the 2017 edition of the C standard, as it is a minor revision that does not introduce new language features.)

  The tests for standard conformance are not comprehensive. They test the value of `__STDC_VERSION__`, and a representative sample of the language features added in each version of the C standard. They do not examine `__STDC__` because some compilers by default leave it undefined. They do not test for variable-length arrays, a C99 feature that was made optional in C11; if you need to use this feature when available, use `AC_C_VARARRAYS`. They do not test the C standard library, because the C compiler might be generating code for a “freestanding environment” (in which most of the standard library is optional). If you need to know whether a particular C standard header exists, use `AC_CHECK_HEADER`.

  None of the options that may be added to `CC` by this macro enable strict conformance to the C standard. In particular, system-specific extensions are not disabled. (For example, for GNU C, the `-std=gnunn` options may be used, but not the `-std=cnn` options.)

  Many Autoconf macros use a compiler, and thus call ‘`AC_REQUIRE([AC_PROG_CC])`’ to ensure that the compiler has been determined before the body of the outermost `AC_DEFUN` macro. Although `AC_PROG_CC` is safe to directly expand multiple times, it performs certain checks (such as the proper value of `EXEEXT`) only on the first invocation. Therefore, care must be used when invoking this macro from within another macro rather than at the top level (see Expanded Before Required).

Macro: AC_PROG_CC_C_O ¶

  If the C compiler does not accept the `-c` and `-o` options simultaneously, define `NO_MINUS_C_MINUS_O`. This macro actually tests both the compiler found by `AC_PROG_CC`, and, if different, the first `cc` in the path. The test fails if one fails. This macro was created for GNU Make to choose the default C compilation rule.

  For the compiler compiler, this macro caches its result in the `ac_cv_prog_cc_compiler_c_o` variable.

Macro: AC_PROG_CPP ¶

  Set output variable `CPP` to a command that runs the C preprocessor. If ‘`$CC -E`’ doesn’t work, tries `cpp` and `/lib/cpp`, in that order.

  It is only portable to run `CPP` on files with a `.c` extension.

  Some preprocessors don’t indicate missing include files by the error status. For such preprocessors an internal variable is set that causes other macros to check the standard error from the preprocessor and consider the test failed if any warnings have been reported. For most preprocessors, though, warnings do not cause include-file tests to fail unless `AC_PROG_CPP_WERROR` is also specified.

Macro: AC_PROG_CPP_WERROR ¶

  This acts like `AC_PROG_CPP`, except it treats warnings from the preprocessor as errors even if the preprocessor exit status indicates success. This is useful for avoiding headers that generate mandatory warnings, such as deprecation notices.

The following macros check for C compiler or machine architecture features. To check for characteristics not listed here, use `AC_COMPILE_IFELSE` (see Running the Compiler) or `AC_RUN_IFELSE` (see Checking Runtime Behavior).

Macro: AC_C_BACKSLASH_A ¶

  Define ‘`HAVE_C_BACKSLASH_A`’ to 1 if the C compiler understands ‘`\a`’.

  This macro is obsolescent, as current C compilers understand ‘`\a`’. New programs need not use this macro.

Macro: AC_C_BIGENDIAN([action-if-true], [action-if-false], [action-if-unknown], [action-if-universal]) ¶

  If words are stored with the most significant byte first (like Motorola and SPARC CPUs), execute action-if-true. If words are stored with the least significant byte first (like Intel and VAX CPUs), execute action-if-false.

  This macro runs a test-case if endianness cannot be determined from the system header files. When cross-compiling, the test-case is not run but grep’ed for some magic values. action-if-unknown is executed if the latter case fails to determine the byte sex of the host system.

  In some cases a single run of a compiler can generate code for multiple architectures. This can happen, for example, when generating Mac OS X universal binary files, which work on both PowerPC and Intel architectures. In this case, the different variants might be for architectures with differing endianness. If `configure` detects this, it executes action-if-universal instead of action-if-unknown.

  The default for action-if-true is to define ‘`WORDS_BIGENDIAN`’. The default for action-if-false is to do nothing. The default for action-if-unknown is to abort configure and tell the installer how to bypass this test. And finally, the default for action-if-universal is to ensure that ‘`WORDS_BIGENDIAN`’ is defined if and only if a universal build is detected and the current code is big-endian; this default works only if `autoheader` is used (see Using `autoheader` to Create `config.h.in`).

  If you use this macro without specifying action-if-universal, you should also use `AC_CONFIG_HEADERS`; otherwise ‘`WORDS_BIGENDIAN`’ may be set incorrectly for Mac OS X universal binary files.

Macro: AC_C_CONST ¶

  If the C compiler does not fully support the `const` keyword, define `const` to be empty. Some C compilers that do not define `__STDC__` do support `const`; some compilers that define `__STDC__` do not completely support `const`. Programs can simply use `const` as if every C compiler supported it; for those that don’t, the makefile or configuration header file defines it as empty.

  Occasionally installers use a C++ compiler to compile C code, typically because they lack a C compiler. This causes problems with `const`, because C and C++ treat `const` differently. For example:

      const int foo;

  is valid in C but not in C++. These differences unfortunately cannot be papered over by defining `const` to be empty.

  If `autoconf` detects this situation, it leaves `const` alone, as this generally yields better results in practice. However, using a C++ compiler to compile C code is not recommended or supported, and installers who run into trouble in this area should get a C compiler like GCC to compile their C code.

  This macro caches its result in the `ac_cv_c_const` variable.

  This macro is obsolescent, as current C compilers support `const`. New programs need not use this macro.

Macro: AC_C__GENERIC ¶

  If the C compiler supports C11-style generic selection using the `_Generic` keyword, define `HAVE_C__GENERIC`.

Macro: AC_C_RESTRICT ¶

  If the C compiler recognizes a variant spelling for the `restrict` keyword (`__restrict`, `__restrict__`, or `_Restrict`), then define `restrict` to that; this is more likely to do the right thing with compilers that support language variants where plain `restrict` is not a keyword. Otherwise, if the C compiler recognizes the `restrict` keyword, don’t do anything. Otherwise, define `restrict` to be empty. Thus, programs may simply use `restrict` as if every C compiler supported it; for those that do not, the makefile or configuration header defines it away.

  Although support in C++ for the `restrict` keyword is not required, several C++ compilers do accept the keyword. This macro works for them, too.

  This macro caches ‘`no`’ in the `ac_cv_c_restrict` variable if `restrict` is not supported, and a supported spelling otherwise.

Macro: AC_C_VOLATILE ¶

  If the C compiler does not understand the keyword `volatile`, define `volatile` to be empty. Programs can simply use `volatile` as if every C compiler supported it; for those that do not, the makefile or configuration header defines it as empty.

  If the correctness of your program depends on the semantics of `volatile`, simply defining it to be empty does, in a sense, break your code. However, given that the compiler does not support `volatile`, you are at its mercy anyway. At least your program compiles, when it wouldn’t before. See Volatile Objects, for more about `volatile`.

  This macro is obsolescent, as current C compilers support `volatile`. New programs need not use this macro.

Macro: AC_C_INLINE ¶

  If the C compiler supports the keyword `inline`, do nothing. Otherwise define `inline` to `__inline__` or `__inline` if it accepts one of those, otherwise define `inline` to be empty.

Macro: AC_C_CHAR_UNSIGNED ¶

  If the C type `char` is unsigned, define `__CHAR_UNSIGNED__`, unless the C compiler predefines it.

  These days, using this macro is not necessary. The same information can be determined by this portable alternative, thus avoiding the use of preprocessor macros in the namespace reserved for the implementation.

      #include <limits.h>
      #if CHAR_MIN == 0
      # define CHAR_UNSIGNED 1
      #endif

Macro: AC_C_STRINGIZE ¶

  If the C preprocessor supports the stringizing operator, define `HAVE_STRINGIZE`. The stringizing operator is ‘`#`’ and is found in macros such as this:

      #define x(y) #y

  This macro is obsolescent, as current C compilers support the stringizing operator. New programs need not use this macro.

Macro: AC_C_FLEXIBLE_ARRAY_MEMBER ¶

  If the C compiler supports flexible array members, define `FLEXIBLE_ARRAY_MEMBER` to nothing; otherwise define it to 1. That way, a declaration like this:

      struct s
        {
          size_t n_vals;
          double val[FLEXIBLE_ARRAY_MEMBER];
        };

  will let applications use the “struct hack” even with compilers that do not support flexible array members. To allocate and use such an object, you can use code like this:

      size_t i;
      size_t n = compute_value_count ();
      struct s *p =
         malloc (offsetof (struct s, val)
                 + n * sizeof (double));
      p->n_vals = n;
      for (i = 0; i < n; i++)
        p->val[i] = compute_value (i);

Macro: AC_C_VARARRAYS ¶

  If the C compiler does not support variable-length arrays, define the macro `__STDC_NO_VLA__` to be 1 if it is not already defined. A variable-length array is an array of automatic storage duration whose length is determined at run time, when the array is declared. For backward compatibility this macro also defines `HAVE_C_VARARRAYS` if the C compiler supports variable-length arrays, but this usage is obsolescent and new programs should use `__STDC_NO_VLA__`.

Macro: AC_C_TYPEOF ¶

  If the C compiler supports GNU C’s `typeof` syntax either directly or through a different spelling of the keyword (e.g., `__typeof__`), define `HAVE_TYPEOF`. If the support is available only through a different spelling, define `typeof` to that spelling.

Macro: AC_C_PROTOTYPES ¶

  If function prototypes are understood by the compiler (as determined by `AC_PROG_CC`), define `PROTOTYPES` and `__PROTOTYPES`. Defining `__PROTOTYPES` is for the benefit of header files that cannot use macros that infringe on user name space.

  This macro is obsolescent, as current C compilers support prototypes. New programs need not use this macro.

#### 5.10.4 C++ Compiler Characteristics ¶

Macro: AC_PROG_CXX([compiler-search-list]) ¶

  Determine a C++ compiler to use.

  If either the environment variable `CXX` or the environment variable `CCC` is set, its value will be taken as the name of a C++ compiler. If both are set, `CXX` is preferred. If neither are set, search for a C++ compiler under a series of likely names, trying `g++` and `c++` first. Regardless, the output variable `CXX` is set to the chosen compiler.

  If the optional first argument to the macro is used, it must be a whitespace-separated list of potential names for a C++ compiler, which overrides the built-in list.

  If no C++ compiler can be found, as a last resort `CXX` is set to `g++` (and subsequent tests will probably fail).

  If the selected C++ compiler is found to be GNU C++ (regardless of its name), the shell variable `GXX` will be set to ‘`yes`’. If the shell variable `CXXFLAGS` was not already set, it is set to `-g -O2` for the GNU C++ compiler (`-O2` on systems where G++ does not accept `-g`), or `-g` for other compilers. `CXXFLAGS` is then made an output variable. You can override the default for `CXXFLAGS` by inserting a shell default assignment between `AC_INIT` and `AC_PROG_CXX`:

      : ${CXXFLAGS="options"}

  where options are the appropriate set of options to use by default. (It is important to use this construct rather than a normal assignment, so that `CXXFLAGS` can still be overridden by the person building the package. See Preset Output Variables.)

Macro: AC_PROG_CXXCPP ¶

  Set output variable `CXXCPP` to a command that runs the C++ preprocessor. If ‘`$CXX -E`’ doesn’t work, tries `cpp` and `/lib/cpp`, in that order. Because of this fallback, `CXXCPP` may or may not set C++-specific predefined macros (such as `__cplusplus`).

  It is portable to run `CXXCPP` only on files with a `.c`, `.C`, `.cc`, or `.cpp` extension.

  Some preprocessors don’t indicate missing include files by the error status. For such preprocessors an internal variable is set that causes other macros to check the standard error from the preprocessor and consider the test failed if any warnings have been reported. However, it is not known whether such broken preprocessors exist for C++.

Macro: AC_PROG_CXX_C_O ¶

  Test whether the C++ compiler accepts the options `-c` and `-o` simultaneously, and define `CXX_NO_MINUS_C_MINUS_O`, if it does not.

#### 5.10.5 Objective C Compiler Characteristics ¶

Macro: AC_PROG_OBJC([compiler-search-list]) ¶

  Determine an Objective C compiler to use. If `OBJC` is not already set in the environment, check for Objective C compilers. Set output variable `OBJC` to the name of the compiler found.

  This macro may, however, be invoked with an optional first argument which, if specified, must be a blank-separated list of Objective C compilers to search for. This just gives the user an opportunity to specify an alternative search list for the Objective C compiler. For example, if you didn’t like the default order, then you could invoke `AC_PROG_OBJC` like this:

      AC_PROG_OBJC([gcc objcc objc])

  If using a compiler that supports GNU Objective C, set shell variable `GOBJC` to ‘`yes`’. If output variable `OBJCFLAGS` was not already set, set it to `-g -O2` for a GNU Objective C compiler (`-O2` on systems where the compiler does not accept `-g`), or `-g` for other compilers.

Macro: AC_PROG_OBJCPP ¶

  Set output variable `OBJCPP` to a command that runs the Objective C preprocessor. If ‘`$OBJC -E`’ doesn’t work, tries `cpp` and `/lib/cpp`, in that order. Because of this fallback, `CXXCPP` may or may not set Objective-C-specific predefined macros (such as `__OBJC__`).

#### 5.10.6 Objective C++ Compiler Characteristics ¶

Macro: AC_PROG_OBJCXX([compiler-search-list]) ¶

  Determine an Objective C++ compiler to use. If `OBJCXX` is not already set in the environment, check for Objective C++ compilers. Set output variable `OBJCXX` to the name of the compiler found.

  This macro may, however, be invoked with an optional first argument which, if specified, must be a blank-separated list of Objective C++ compilers to search for. This just gives the user an opportunity to specify an alternative search list for the Objective C++ compiler. For example, if you didn’t like the default order, then you could invoke `AC_PROG_OBJCXX` like this:

      AC_PROG_OBJCXX([gcc g++ objcc++ objcxx])

  If using a compiler that supports GNU Objective C++, set shell variable `GOBJCXX` to ‘`yes`’. If output variable `OBJCXXFLAGS` was not already set, set it to `-g -O2` for a GNU Objective C++ compiler (`-O2` on systems where the compiler does not accept `-g`), or `-g` for other compilers.

Macro: AC_PROG_OBJCXXCPP ¶

  Set output variable `OBJCXXCPP` to a command that runs the Objective C++ preprocessor. If ‘`$OBJCXX -E`’ doesn’t work, tries `cpp` and `/lib/cpp`, in that order. Because of this fallback, `CXXCPP` may or may not set Objective-C++-specific predefined macros (such as `__cplusplus` and `__OBJC__`).

#### 5.10.7 Erlang Compiler and Interpreter Characteristics ¶

Autoconf defines the following macros for determining paths to the essential Erlang/OTP programs:

Macro: AC_ERLANG_PATH_ERLC([value-if-not-found], [path = ‘`$PATH`’]) ¶

  Determine an Erlang compiler to use. If `ERLC` is not already set in the environment, check for `erlc`. Set output variable `ERLC` to the complete path of the compiler command found. In addition, if `ERLCFLAGS` is not set in the environment, set it to an empty value.

  The two optional arguments have the same meaning as the two last arguments of macro `AC_PATH_PROG` for looking for the `erlc` program. For example, to look for `erlc` only in the `/usr/lib/erlang/bin` directory:

      AC_ERLANG_PATH_ERLC([not found], [/usr/lib/erlang/bin])

Macro: AC_ERLANG_NEED_ERLC([path = ‘`$PATH`’]) ¶

  A simplified variant of the `AC_ERLANG_PATH_ERLC` macro, that prints an error message and exits the `configure` script if the `erlc` program is not found.

Macro: AC_ERLANG_PATH_ERL([value-if-not-found], [path = ‘`$PATH`’]) ¶

  Determine an Erlang interpreter to use. If `ERL` is not already set in the environment, check for `erl`. Set output variable `ERL` to the complete path of the interpreter command found.

  The two optional arguments have the same meaning as the two last arguments of macro `AC_PATH_PROG` for looking for the `erl` program. For example, to look for `erl` only in the `/usr/lib/erlang/bin` directory:

      AC_ERLANG_PATH_ERL([not found], [/usr/lib/erlang/bin])

Macro: AC_ERLANG_NEED_ERL([path = ‘`$PATH`’]) ¶

  A simplified variant of the `AC_ERLANG_PATH_ERL` macro, that prints an error message and exits the `configure` script if the `erl` program is not found.

#### 5.10.8 Fortran Compiler Characteristics ¶

The Autoconf Fortran support is divided into two categories: legacy Fortran 77 macros (`F77`), and modern Fortran macros (`FC`). The former are intended for traditional Fortran 77 code, and have output variables like `F77`, `FFLAGS`, and `FLIBS`. The latter are for newer programs that can (or must) compile under the newer Fortran standards, and have output variables like `FC`, `FCFLAGS`, and `FCLIBS`.

Except for the macros `AC_FC_SRCEXT`, `AC_FC_FREEFORM`, `AC_FC_FIXEDFORM`, and `AC_FC_LINE_LENGTH` (see below), the `FC` and `F77` macros behave almost identically, and so they are documented together in this section.

Macro: AC_PROG_F77([compiler-search-list]) ¶

  Determine a Fortran 77 compiler to use. If `F77` is not already set in the environment, then check for `g77` and `f77`, and then some other names. Set the output variable `F77` to the name of the compiler found.

  This macro may, however, be invoked with an optional first argument which, if specified, must be a blank-separated list of Fortran 77 compilers to search for. This just gives the user an opportunity to specify an alternative search list for the Fortran 77 compiler. For example, if you didn’t like the default order, then you could invoke `AC_PROG_F77` like this:

      AC_PROG_F77([fl32 f77 fort77 xlf g77 f90 xlf90])

  If using a compiler that supports GNU Fortran 77, set the shell variable `G77` to ‘`yes`’. If the output variable `FFLAGS` was not already set in the environment, set it to `-g -02` for `g77` (or `-O2` where the GNU Fortran 77 compiler does not accept `-g`), or `-g` for other compilers.

  The result of the GNU test is cached in the `ac_cv_f77_compiler_gnu` variable, acceptance of `-g` in the `ac_cv_prog_f77_g` variable.

Macro: AC_PROG_FC([compiler-search-list], [dialect]) ¶

  Determine a Fortran compiler to use. If `FC` is not already set in the environment, then `dialect` is a hint to indicate what Fortran dialect to search for; the default is to search for the newest available dialect. Set the output variable `FC` to the name of the compiler found.

  By default, newer dialects are preferred over older dialects, but if `dialect` is specified then older dialects are preferred starting with the specified dialect. `dialect` can currently be one of Fortran 77, Fortran 90, or Fortran 95. However, this is only a hint of which compiler name to prefer (e.g., `f90` or `f95`), and no attempt is made to guarantee that a particular language standard is actually supported. Thus, it is preferable that you avoid the `dialect` option, and use AC_PROG_FC only for code compatible with the latest Fortran standard.

  This macro may, alternatively, be invoked with an optional first argument which, if specified, must be a blank-separated list of Fortran compilers to search for, just as in `AC_PROG_F77`.

  If using a compiler that supports GNU Fortran, set the shell variable `GFC` to ‘`yes`’. If the output variable `FCFLAGS` was not already set in the environment, then set it to `-g -02` for a GNU Fortran compiler (or `-O2` where the compiler does not accept `-g`), or `-g` for other compilers.

  The result of the GNU test is cached in the `ac_cv_fc_compiler_gnu` variable, acceptance of `-g` in the `ac_cv_prog_fc_g` variable.

Macro: AC_PROG_F77_C_O ¶
Macro: AC_PROG_FC_C_O ¶

  Test whether the Fortran compiler accepts the options `-c` and `-o` simultaneously, and define `F77_NO_MINUS_C_MINUS_O` or `FC_NO_MINUS_C_MINUS_O`, respectively, if it does not.

  The result of the test is cached in the `ac_cv_prog_f77_c_o` or `ac_cv_prog_fc_c_o` variable, respectively.

The following macros check for Fortran compiler characteristics. To check for characteristics not listed here, use `AC_COMPILE_IFELSE` (see Running the Compiler) or `AC_RUN_IFELSE` (see Checking Runtime Behavior), making sure to first set the current language to Fortran 77 or Fortran via `AC_LANG([Fortran 77])` or `AC_LANG(Fortran)` (see Language Choice).

Macro: AC_F77_LIBRARY_LDFLAGS ¶
Macro: AC_FC_LIBRARY_LDFLAGS ¶

  Determine the linker flags (e.g., `-L` and `-l`) for the Fortran intrinsic and runtime libraries that are required to successfully link a Fortran program or shared library. The output variable `FLIBS` or `FCLIBS` is set to these flags (which should be included after `LIBS` when linking).

  This macro is intended to be used in those situations when it is necessary to mix, e.g., C++ and Fortran source code in a single program or shared library (see Mixing Fortran 77 With C and C++ in GNU Automake).

  For example, if object files from a C++ and Fortran compiler must be linked together, then the C++ compiler/linker must be used for linking (since special C++-ish things need to happen at link time like calling global constructors, instantiating templates, enabling exception support, etc.).

  However, the Fortran intrinsic and runtime libraries must be linked in as well, but the C++ compiler/linker doesn’t know by default how to add these Fortran 77 libraries. Hence, this macro was created to determine these Fortran libraries.

  The macros `AC_F77_DUMMY_MAIN` and `AC_FC_DUMMY_MAIN` or `AC_F77_MAIN` and `AC_FC_MAIN` are probably also necessary to link C/C++ with Fortran; see below. Further, it is highly recommended that you use `AC_CONFIG_HEADERS` (see Configuration Header Files) because the complex defines that the function wrapper macros create may not work with C/C++ compiler drivers.

  These macros internally compute the flag needed to verbose linking output and cache it in `ac_cv_prog_f77_v` or `ac_cv_prog_fc_v` variables, respectively. The computed linker flags are cached in `ac_cv_f77_libs` or `ac_cv_fc_libs`, respectively.

Macro: AC_F77_DUMMY_MAIN([action-if-found], [action-if-not-found = ‘`AC_MSG_FAILURE`’]) ¶
Macro: AC_FC_DUMMY_MAIN([action-if-found], [action-if-not-found = ‘`AC_MSG_FAILURE`’]) ¶

  With many compilers, the Fortran libraries detected by `AC_F77_LIBRARY_LDFLAGS` or `AC_FC_LIBRARY_LDFLAGS` provide their own `main` entry function that initializes things like Fortran I/O, and which then calls a user-provided entry function named (say) `MAIN__` to run the user’s program. The `AC_F77_DUMMY_MAIN` and `AC_FC_DUMMY_MAIN` or `AC_F77_MAIN` and `AC_FC_MAIN` macros figure out how to deal with this interaction.

  When using Fortran for purely numerical functions (no I/O, etc.) often one prefers to provide one’s own `main` and skip the Fortran library initializations. In this case, however, one may still need to provide a dummy `MAIN__` routine in order to prevent linking errors on some systems. `AC_F77_DUMMY_MAIN` or `AC_FC_DUMMY_MAIN` detects whether any such routine is required for linking, and what its name is; the shell variable `F77_DUMMY_MAIN` or `FC_DUMMY_MAIN` holds this name, `unknown` when no solution was found, and `none` when no such dummy main is needed.

  By default, action-if-found defines `F77_DUMMY_MAIN` or `FC_DUMMY_MAIN` to the name of this routine (e.g., `MAIN__`) if it is required. action-if-not-found defaults to exiting with an error.

  In order to link with Fortran routines, the user’s C/C++ program should then include the following code to define the dummy main if it is needed:

      #ifdef F77_DUMMY_MAIN
      #  ifdef __cplusplus
           extern "C"
      #  endif
         int F77_DUMMY_MAIN (void) { return 1; }
      #endif

  (Replace `F77` with `FC` for Fortran instead of Fortran 77.)

  Note that this macro is called automatically from `AC_F77_WRAPPERS` or `AC_FC_WRAPPERS`; there is generally no need to call it explicitly unless one wants to change the default actions.

  The result of this macro is cached in the `ac_cv_f77_dummy_main` or `ac_cv_fc_dummy_main` variable, respectively.

Macro: AC_F77_MAIN ¶
Macro: AC_FC_MAIN ¶

  As discussed above, many Fortran libraries allow you to provide an entry point called (say) `MAIN__` instead of the usual `main`, which is then called by a `main` function in the Fortran libraries that initializes things like Fortran I/O. The `AC_F77_MAIN` and `AC_FC_MAIN` macros detect whether it is possible to utilize such an alternate main function, and defines `F77_MAIN` and `FC_MAIN` to the name of the function. (If no alternate main function name is found, `F77_MAIN` and `FC_MAIN` are simply defined to `main`.)

  Thus, when calling Fortran routines from C that perform things like I/O, one should use this macro and declare the "main" function like so:

      #ifdef __cplusplus
        extern "C"
      #endif
      int F77_MAIN (int argc, char *argv[]);

  (Again, replace `F77` with `FC` for Fortran instead of Fortran 77.)

  The result of this macro is cached in the `ac_cv_f77_main` or `ac_cv_fc_main` variable, respectively.

Macro: AC_F77_WRAPPERS ¶
Macro: AC_FC_WRAPPERS ¶

  Defines C macros `F77_FUNC (name, NAME)`, `FC_FUNC (name, NAME)`, `F77_FUNC_(name, NAME)`, and `FC_FUNC_(name, NAME)` to properly mangle the names of C/C++ identifiers, and identifiers with underscores, respectively, so that they match the name-mangling scheme used by the Fortran compiler.

  Fortran is case-insensitive, and in order to achieve this the Fortran compiler converts all identifiers into a canonical case and format. To call a Fortran subroutine from C or to write a C function that is callable from Fortran, the C program must explicitly use identifiers in the format expected by the Fortran compiler. In order to do this, one simply wraps all C identifiers in one of the macros provided by `AC_F77_WRAPPERS` or `AC_FC_WRAPPERS`. For example, suppose you have the following Fortran 77 subroutine:

            subroutine foobar (x, y)
            double precision x, y
            y = 3.14159 * x
            return
            end

  You would then declare its prototype in C or C++ as:

      #define FOOBAR_F77 F77_FUNC (foobar, FOOBAR)
      #ifdef __cplusplus
      extern "C"  /* prevent C++ name mangling */
      #endif
      void FOOBAR_F77 (double *x, double *y);

  Note that we pass both the lowercase and uppercase versions of the function name to `F77_FUNC` so that it can select the right one. Note also that all parameters to Fortran 77 routines are passed as pointers (see Mixing Fortran 77 With C and C++ in GNU Automake).

  (Replace `F77` with `FC` for Fortran instead of Fortran 77.)

  Although Autoconf tries to be intelligent about detecting the name-mangling scheme of the Fortran compiler, there may be Fortran compilers that it doesn’t support yet. In this case, the above code generates a compile-time error, but some other behavior (e.g., disabling Fortran-related features) can be induced by checking whether `F77_FUNC` or `FC_FUNC` is defined.

  Now, to call that routine from a C program, we would do something like:

      {
          double x = 2.7183, y;
          FOOBAR_F77 (&x, &y);
      }

  If the Fortran identifier contains an underscore (e.g., `foo_bar`), you should use `F77_FUNC_` or `FC_FUNC_` instead of `F77_FUNC` or `FC_FUNC` (with the same arguments). This is because some Fortran compilers mangle names differently if they contain an underscore.

  The name mangling scheme is encoded in the `ac_cv_f77_mangling` or `ac_cv_fc_mangling` cache variable, respectively, and also used for the `AC_F77_FUNC` and `AC_FC_FUNC` macros described below.

Macro: AC_F77_FUNC(name, [shellvar]) ¶
Macro: AC_FC_FUNC(name, [shellvar]) ¶

  Given an identifier name, set the shell variable shellvar to hold the mangled version name according to the rules of the Fortran linker (see also `AC_F77_WRAPPERS` or `AC_FC_WRAPPERS`). shellvar is optional; if it is not supplied, the shell variable is simply name. The purpose of this macro is to give the caller a way to access the name-mangling information other than through the C preprocessor as above, for example, to call Fortran routines from some language other than C/C++.

Macro: AC_FC_SRCEXT(ext, [action-if-success], [action-if-failure = ‘`AC_MSG_FAILURE`’]) ¶
Macro: AC_FC_PP_SRCEXT(ext, [action-if-success], [action-if-failure = ‘`AC_MSG_FAILURE`’]) ¶

  By default, the `FC` macros perform their tests using a `.f` extension for source-code files. Some compilers, however, only enable newer language features for appropriately named files, e.g., Fortran 90 features only for `.f90` files, or preprocessing only with `.F` files or maybe other upper-case extensions. On the other hand, some other compilers expect all source files to end in `.f` and require special flags to support other file name extensions. The `AC_FC_SRCEXT` and `AC_FC_PP_SRCEXT` macros deal with these issues.

  The `AC_FC_SRCEXT` macro tries to get the `FC` compiler to accept files ending with the extension `.ext` (i.e., ext does not contain the dot). If any special compiler flags are needed for this, it stores them in the output variable `FCFLAGS_ext`. This extension and these flags are then used for all subsequent `FC` tests (until `AC_FC_SRCEXT` or `AC_FC_PP_SRCEXT` is called another time).

  For example, you would use `AC_FC_SRCEXT(f90)` to employ the `.f90` extension in future tests, and it would set the `FCFLAGS_f90` output variable with any extra flags that are needed to compile such files.

  Similarly, the `AC_FC_PP_SRCEXT` macro tries to get the `FC` compiler to preprocess and compile files with the extension `.ext`. When both `fpp` and `cpp` style preprocessing are provided, the former is preferred, as the latter may treat continuation lines, `//` tokens, and white space differently from what some Fortran dialects expect. Conversely, if you do not want files to be preprocessed, use only lower-case characters in the file name extension. Like with `AC_FC_SRCEXT(f90)`, any needed flags are stored in the `FCFLAGS_ext` variable.

  The `FCFLAGS_ext` flags can not be simply absorbed into `FCFLAGS`, for two reasons based on the limitations of some compilers. First, only one `FCFLAGS_ext` can be used at a time, so files with different extensions must be compiled separately. Second, `FCFLAGS_ext` must appear immediately before the source-code file name when compiling. So, continuing the example above, you might compile a `foo.f90` file in your makefile with the command:

      foo.o: foo.f90
             $(FC) -c $(FCFLAGS) $(FCFLAGS_f90) '$(srcdir)/foo.f90'

  If `AC_FC_SRCEXT` or `AC_FC_PP_SRCEXT` succeeds in compiling files with the ext extension, it calls action-if-success (defaults to nothing). If it fails, and cannot find a way to make the `FC` compiler accept such files, it calls action-if-failure (defaults to exiting with an error message).

  The `AC_FC_SRCEXT` and `AC_FC_PP_SRCEXT` macros cache their results in `ac_cv_fc_srcext_ext` and `ac_cv_fc_pp_srcext_ext` variables, respectively.

Macro: AC_FC_PP_DEFINE([action-if-success], [action-if-failure = ‘`AC_MSG_FAILURE`’]) ¶

  Find a flag to specify defines for preprocessed Fortran. Not all Fortran compilers use `-D`. Substitute `FC_DEFINE` with the result and call action-if-success (defaults to nothing) if successful, and action-if-failure (defaults to failing with an error message) if not.

  This macro calls `AC_FC_PP_SRCEXT([F])` in order to learn how to preprocess a `conftest.F` file, but restores a previously used Fortran source file extension afterwards again.

  The result of this test is cached in the `ac_cv_fc_pp_define` variable.

Macro: AC_FC_FREEFORM([action-if-success], [action-if-failure = ‘`AC_MSG_FAILURE`’]) ¶

  Try to ensure that the Fortran compiler (`$FC`) allows free-format source code (as opposed to the older fixed-format style from Fortran 77). If necessary, it may add some additional flags to `FCFLAGS`.

  This macro is most important if you are using the default `.f` extension, since many compilers interpret this extension as indicating fixed-format source unless an additional flag is supplied. If you specify a different extension with `AC_FC_SRCEXT`, such as `.f90`, then `AC_FC_FREEFORM` ordinarily succeeds without modifying `FCFLAGS`. For extensions which the compiler does not know about, the flag set by the `AC_FC_SRCEXT` macro might let the compiler assume Fortran 77 by default, however.

  If `AC_FC_FREEFORM` succeeds in compiling free-form source, it calls action-if-success (defaults to nothing). If it fails, it calls action-if-failure (defaults to exiting with an error message).

  The result of this test, or ‘`none`’ or ‘`unknown`’, is cached in the `ac_cv_fc_freeform` variable.

Macro: AC_FC_FIXEDFORM([action-if-success], [action-if-failure = ‘`AC_MSG_FAILURE`’]) ¶

  Try to ensure that the Fortran compiler (`$FC`) allows the old fixed-format source code (as opposed to free-format style). If necessary, it may add some additional flags to `FCFLAGS`.

  This macro is needed for some compilers alias names like `xlf95` which assume free-form source code by default, and in case you want to use fixed-form source with an extension like `.f90` which many compilers interpret as free-form by default. If you specify a different extension with `AC_FC_SRCEXT`, such as `.f`, then `AC_FC_FIXEDFORM` ordinarily succeeds without modifying `FCFLAGS`.

  If `AC_FC_FIXEDFORM` succeeds in compiling fixed-form source, it calls action-if-success (defaults to nothing). If it fails, it calls action-if-failure (defaults to exiting with an error message).

  The result of this test, or ‘`none`’ or ‘`unknown`’, is cached in the `ac_cv_fc_fixedform` variable.

Macro: AC_FC_LINE_LENGTH([length], [action-if-success], [action-if-failure = ‘`AC_MSG_FAILURE`’]) ¶

  Try to ensure that the Fortran compiler (`$FC`) accepts long source code lines. The length argument may be given as 80, 132, or unlimited, and defaults to 132. Note that line lengths above 250 columns are not portable, and some compilers do not accept more than 132 columns at least for fixed format source. If necessary, it may add some additional flags to `FCFLAGS`.

  If `AC_FC_LINE_LENGTH` succeeds in compiling fixed-form source, it calls action-if-success (defaults to nothing). If it fails, it calls action-if-failure (defaults to exiting with an error message).

  The result of this test, or ‘`none`’ or ‘`unknown`’, is cached in the `ac_cv_fc_line_length` variable.

Macro: AC_FC_CHECK_BOUNDS([action-if-success], [action-if-failure = ‘`AC_MSG_FAILURE`’]) ¶

  The `AC_FC_CHECK_BOUNDS` macro tries to enable array bounds checking in the Fortran compiler. If successful, the action-if-success is called and any needed flags are added to `FCFLAGS`. Otherwise, action-if-failure is called, which defaults to failing with an error message. The macro currently requires Fortran 90 or a newer dialect.

  The result of the macro is cached in the `ac_cv_fc_check_bounds` variable.

Macro: AC_F77_IMPLICIT_NONE([action-if-success], [action-if-failure = ‘`AC_MSG_FAILURE`’]) ¶
Macro: AC_FC_IMPLICIT_NONE([action-if-success], [action-if-failure = ‘`AC_MSG_FAILURE`’]) ¶

  Try to disallow implicit declarations in the Fortran compiler. If successful, action-if-success is called and any needed flags are added to `FFLAGS` or `FCFLAGS`, respectively. Otherwise, action-if-failure is called, which defaults to failing with an error message.

  The result of these macros are cached in the `ac_cv_f77_implicit_none` and `ac_cv_fc_implicit_none` variables, respectively.

Macro: AC_FC_MODULE_EXTENSION ¶

  Find the Fortran 90 module file name extension. Most Fortran 90 compilers store module information in files separate from the object files. The module files are usually named after the name of the module rather than the source file name, with characters possibly turned to upper case, plus an extension, often `.mod`.

  Not all compilers use module files at all, or by default. The Cray Fortran compiler requires `-e m` in order to store and search module information in `.mod` files rather than in object files. Likewise, the Fujitsu Fortran compilers uses the `-Am` option to indicate how module information is stored.

  The `AC_FC_MODULE_EXTENSION` macro computes the module extension without the leading dot, and stores that in the `FC_MODEXT` variable. If the compiler does not produce module files, or the extension cannot be determined, `FC_MODEXT` is empty. Typically, the result of this macro may be used in cleanup `make` rules as follows:

      clean-modules:
              -test -z "$(FC_MODEXT)" || rm -f *.$(FC_MODEXT)

  The extension, or ‘`unknown`’, is cached in the `ac_cv_fc_module_ext` variable.

Macro: AC_FC_MODULE_FLAG([action-if-success], [action-if-failure = ‘`AC_MSG_FAILURE`’]) ¶

  Find the compiler flag to include Fortran 90 module information from another directory, and store that in the `FC_MODINC` variable. Call action-if-success (defaults to nothing) if successful, and set `FC_MODINC` to empty and call action-if-failure (defaults to exiting with an error message) if not.

  Most Fortran 90 compilers provide a way to specify module directories. Some have separate flags for the directory to write module files to, and directories to search them in, whereas others only allow writing to the current directory or to the first directory specified in the include path. Further, with some compilers, the module search path and the preprocessor search path can only be modified with the same flag. Thus, for portability, write module files to the current directory only and list that as first directory in the search path.

  There may be no whitespace between `FC_MODINC` and the following directory name, but `FC_MODINC` may contain trailing white space. For example, if you use Automake and would like to search `../lib` for module files, you can use the following:

      AM_FCFLAGS = $(FC_MODINC). $(FC_MODINC)../lib

  Inside `configure` tests, you can use:

      if test -n "$FC_MODINC"; then
        FCFLAGS="$FCFLAGS $FC_MODINC. $FC_MODINC../lib"
      fi

  The flag is cached in the `ac_cv_fc_module_flag` variable. The substituted value of `FC_MODINC` may refer to the `ac_empty` dummy placeholder empty variable, to avoid losing the significant trailing whitespace in a `Makefile`.

Macro: AC_FC_MODULE_OUTPUT_FLAG([action-if-success], [action-if-failure = ‘`AC_MSG_FAILURE`’]) ¶

  Find the compiler flag to write Fortran 90 module information to another directory, and store that in the `FC_MODOUT` variable. Call action-if-success (defaults to nothing) if successful, and set `FC_MODOUT` to empty and call action-if-failure (defaults to exiting with an error message) if not.

  Not all Fortran 90 compilers write module files, and of those that do, not all allow writing to a directory other than the current one, nor do all have separate flags for writing and reading; see the description of `AC_FC_MODULE_FLAG` above. If you need to be able to write to another directory, for maximum portability use `FC_MODOUT` before any `FC_MODINC` and include both the current directory and the one you write to in the search path:

      AM_FCFLAGS = $(FC_MODOUT)../mod $(FC_MODINC)../mod $(FC_MODINC). ...

  The flag is cached in the `ac_cv_fc_module_output_flag` variable. The substituted value of `FC_MODOUT` may refer to the `ac_empty` dummy placeholder empty variable, to avoid losing the significant trailing whitespace in a `Makefile`.

Macro: AC_F77_CRAY_POINTERS([action-if-success], [action-if-failure = ‘`AC_MSG_FAILURE`’]) ¶
Macro: AC_FC_CRAY_POINTERS([action-if-success], [action-if-failure = ‘`AC_MSG_FAILURE`’]) ¶

  Try to ensure that the Fortran compiler (`$F77` or `$FC`) accepts Cray pointers. If successful, the action-if-success is called and any needed flags are added to `FFLAGS` or `FCFLAGS`. Otherwise, action-if-failure is called, which defaults to failing with an error message.

  Cray pointers are a non-standard extension supported by many Fortran compilers which allow an integer to be declared as C-like pointer to a target variable.

  The result of this test, or ‘`none`’ or ‘`unknown`’, is cached in the `ac_cv_f77_cray_ptr` or `ac_cv_fc_cray_ptr` variable.

#### 5.10.9 Go Compiler Characteristics ¶

Autoconf provides basic support for the Go programming language when using the `gccgo` compiler (there is currently no support for the `6g` and `8g` compilers).

Macro: AC_PROG_GO([compiler-search-list]) ¶

  Find the Go compiler to use. Check whether the environment variable `GOC` is set; if so, then set output variable `GOC` to its value.

  Otherwise, if the macro is invoked without an argument, then search for a Go compiler named `gccgo`. If it is not found, then as a last resort set `GOC` to `gccgo`.

  This macro may be invoked with an optional first argument which, if specified, must be a blank-separated list of Go compilers to search for.

  If output variable `GOFLAGS` was not already set, set it to `-g -O2`. If your package does not like this default, `GOFLAGS` may be set before `AC_PROG_GO`.

#### 5.10.10 Algol 68 Compiler ¶

Autoconf provides basic support for the Algol 68 programming language when using the `ga68` compiler.

Note that the GCC Algol 68 front-end is not yet integrated in the main compiler, so it is developed and distributed off-tree. See https://gcc.gnu.org/wiki/Algol68FrontEnd. Additional information about the Algol 68 programming language, and how it is being evolved by the GNU Algol 68 Working Group can be found at https://algol68-lang.org.

Macro: AC_PROG_A68([compiler-search-list]) ¶

  Find the Algol 68 compiler to use. Check whether the environment variable `A68` is set; if so, then set output variable `A68` to its value.

  Otherwise, if the macro is invoked without an argument, then search for an Algol 68 compiler named `ga68`. If it is not found, then as a last resort set `A68` to `ga68`.

  This macro may be invoked with an optional first argument which, if specified, must be a blank-separated list of Algol 68 compilers to search for.

  If output variable `A68FLAGS` was not already set, set it to `-g -O2`. If your package does not like this default, `A68FLAGS` may be set before `AC_PROG_A68`.

### 5.11 System Services ¶

The following macros check for operating system services or capabilities.

Macro: AC_PATH_X ¶

  Try to locate the X Window System include files and libraries. If the user gave the command line options `--x-includes=dir` and `--x-libraries=dir`, use those directories.

  If either or both were not given, get the missing values by running `xmkmf` (or an executable pointed to by the `XMKMF` environment variable) on a trivial `Imakefile` and examining the makefile that it produces. Setting `XMKMF` to ‘`false`’ disables this method.

  If this method fails to find the X Window System, `configure` looks for the files in several directories where they often reside. If either method is successful, set the shell variables `x_includes` and `x_libraries` to their locations, unless they are in directories the compiler searches by default.

  If both methods fail, or the user gave the command line option `--without-x`, set the shell variable `no_x` to ‘`yes`’; otherwise set it to the empty string.

Macro: AC_PATH_XTRA ¶

  An enhanced version of `AC_PATH_X`. It adds the C compiler flags that X needs to output variable `X_CFLAGS`, and the X linker flags to `X_LIBS`. Define `X_DISPLAY_MISSING` if X is not available.

  This macro also checks for special libraries that some systems need in order to compile X programs. It adds any that the system needs to output variable `X_EXTRA_LIBS`. And it checks for special X11R6 libraries that need to be linked with before `-lX11`, and adds any found to the output variable `X_PRE_LIBS`.

Macro: AC_SYS_INTERPRETER ¶

  Check whether the system supports starting scripts with a line of the form ‘`#!/bin/sh`’ to select the interpreter to use for the script. After running this macro, shell code in `configure.ac` can check the shell variable `interpval`; it is set to ‘`yes`’ if the system supports ‘`#!`’, ‘`no`’ if not.

Macro: AC_SYS_LARGEFILE ¶

  If the default `off_t` type is a 32-bit integer, and therefore cannot be used with files 2 GiB or larger, make a wider `off_t` available if the system supports it. Similarly, widen other types related to sizes of files and file systems if possible. These types may include `blkcnt_t`, `dev_t`, `ino_t`, `fsblkcnt_t`, `fsfilcnt_t`, and `rlim_t`.

  Also, arrange for a `configure` option `--enable-year2038` to request widening the type `time_t` as needed to represent file wand other timestamps after mid-January 2038. This widening is possible only on 32-bit GNU/Linux x86 and ARM systems with glibc 2.34 or later. If year-2038 support is requested but `configure` fails to find a way to widen `time_t` and inspection of the system suggests that this feature is available somehow, `configure` will error out. If you want the default to be `--enable-year2038`, you can use `AC_SYS_YEAR2038` or `AC_SYS_YEAR2038_RECOMMENDED` instead of `AC_SYS_LARGEFILE`. In other words, older packages that have long used `AC_SYS_LARGEFILE` can have year-2038 support on 32-bit GNU/Linux x86 and ARM systems either by regenerating `configure` with current Autoconf and configuring with `--enable-year2038`, or by using `AC_SYS_YEAR2038` or `AC_SYS_YEAR2038_RECOMMENDED` and configuring without `--disable-year2038`. A future version of Autoconf might change the `AC_SYS_LARGEFILE` default to `--enable-year2038`; if and when that happens, `AC_SYS_LARGEFILE` and `AC_SYS_YEAR2038` will become equivalent. See AC_SYS_YEAR2038.

  Set the shell variable `ac_have_largefile` to ‘`yes`’ or `no` depending on whether a wide `off_t` is available, regardless of whether arrangements were necessary. Similarly, set the shell variable `ac_have_year2038` to `yes` or `no` depending on whether a wide-enough `time_t` is available.

  Define preprocessor macros if necessary to make types wider; for example, on GNU/Linux systems the macros `_FILE_OFFSET_BITS` and `_TIME_BITS` can be defined. Some of these macros work only if defined before the first system header is included; therefore, when using this macro in concert with `AC_CONFIG_HEADERS`, make sure that `config.h` is included before any system headers.

  Large-file support can be disabled by configuring with the `--disable-largefile` option, and year-2038 support can be enabled and disabled via the `--enable-year2038` and `--disable-year2038` options. These options have no effect on systems where types are wide enough by default. Large-file support is required for year-2038 support: if you configure with `--disable-largefile` on a platform with 32-bit `time_t`, then year-2038 support is not available.

  Disabling large-file or year-2038 support can have surprising effects, such as causing functions like `readdir` and `stat` to fail even on a small file because its inode number or timestamp is out of range.

  Regardless of whether you use this macro, portable programs should not assume that any of the types listed above fit into a `long int`. For example, it is not portable to print an arbitrary `off_t` or `time_t` value `X` with `printf ("%ld", (long int) X)`.

  The standard C library functions `fseek` and `ftell` do not use `off_t`. If you need to use either of these functions, you should use `AC_FUNC_FSEEKO` as well as `AC_SYS_LARGEFILE`, and then use their POSIX replacements `fseeko` and `ftello`. See AC_FUNC_FSEEKO.

  When using `AC_SYS_LARGEFILE` in different packages that are linked together and that have interfaces that depend on the width of `off_t`, `time_t` or related types, the simplest thing is to configure all components the same way. For example, if an application uses `AC_SYS_LARGEFILE` and is configured with `--enable-year2038`, libraries it links to with an `off_t`- or `time_t`-dependent interface should be configured equivalently. Alternatively, you can modify libraries to support both 32- and 64-bit interfaces though this is more work and few libraries other than the C library itself are modified in this way.

  Applications and libraries should be configured compatibly. If `off_t`, `time_t` or related types appear in a library’s public interface, enabling or disabling the library’s large-file or year-2038 support may break binary compatibility with applications or with other libraries. Similarly, if an application links to a such a library, enabling or disabling the application’s large-file support may break binary compatibility with that library.

Macro: AC_SYS_LONG_FILE_NAMES ¶

  If the system supports file names longer than 14 characters, define `HAVE_LONG_FILE_NAMES`.

Macro: AC_SYS_POSIX_TERMIOS ¶

  Check to see if the POSIX termios headers and functions are available on the system. If so, set the shell variable `ac_cv_sys_posix_termios` to ‘`yes`’. If not, set the variable to ‘`no`’.

Macro: AC_SYS_YEAR2038 ¶

  This is like `AC_SYS_LARGEFILE` except it defaults to enabling instead of disabling year-2038 support. Year-2038 support for applications and libraries should be configured compatibly. See AC_SYS_LARGEFILE.

Macro: AC_SYS_YEAR2038_RECOMMENDED ¶

  This macro has the same effect as `AC_SYS_YEAR2038`, but also declares that the program being configured should support timestamps after mid-January 2038. If a large `time_t` is unavailable, `configure` will error out unless the `--disable-year2038` option is specified.

  Year-2038 support for applications and libraries should be configured compatibly. See AC_SYS_YEAR2038.

### 5.12 C and POSIX Variants ¶

The following macro makes it possible to use C language and library extensions defined by the C standards committee, features of POSIX that are extensions to C, and platform extensions not defined by POSIX.

Macro: AC_USE_SYSTEM_EXTENSIONS ¶

  If possible, enable extensions to C or POSIX on hosts that normally disable the extensions, typically due to standards-conformance namespace issues. This should be called before any macros that run the C compiler. Also, when using this macro in concert with `AC_CONFIG_HEADERS`, be sure that `config.h` is included before any system header.

  Define the following preprocessor macros unconditionally:

  `_ALL_SOURCE` ¶

    Enable extensions on AIX and z/OS.

  `_COSMO_SOURCE` ¶

    Enable extensions on Cosmopolitan Libc.

  `_DARWIN_C_SOURCE` ¶

    Enable extensions on macOS.

  `_GNU_SOURCE` ¶

    Enable extensions on GNU systems.

  `_NETBSD_SOURCE` ¶

    Enable general extensions on NetBSD. Enable NetBSD compatibility extensions on Minix.

  `_OPENBSD_SOURCE` ¶

    Enable OpenBSD compatibility extensions on NetBSD. Oddly enough, this does nothing on OpenBSD.

  `_POSIX_PTHREAD_SEMANTICS` ¶

    Enable POSIX-compatible threading on Solaris.

  `__STDC_WANT_IEC_60559_ATTRIBS_EXT__` ¶

    Enable extensions specified by ISO/IEC TS 18661-5:2014.

  `__STDC_WANT_IEC_60559_BFP_EXT__` ¶

    Enable extensions specified by ISO/IEC TS 18661-1:2014.

  `__STDC_WANT_IEC_60559_DFP_EXT__` ¶

    Enable extensions specified by ISO/IEC TS 18661-2:2015.

  `__STDC_WANT_IEC_60559_EXT__` ¶

    Enable extensions specified by C23 Annex F.

  `__STDC_WANT_IEC_60559_FUNCS_EXT__` ¶

    Enable extensions specified by ISO/IEC TS 18661-4:2015.

  `__STDC_WANT_IEC_60559_TYPES_EXT__` ¶

    Enable extensions specified by C23 Annex H and by ISO/IEC TS 18661-3:2015.

  `__STDC_WANT_LIB_EXT2__` ¶

    Enable extensions specified by ISO/IEC TR 24731-2:2010.

  `__STDC_WANT_MATH_SPEC_FUNCS__` ¶

    Enable extensions specified by ISO/IEC 24747:2009.

  `_TANDEM_SOURCE` ¶

    Enable extensions on HP NonStop systems.

  The following preprocessor macros are defined only when necessary; they enable access to extensions on some operating systems but disable extensions on other operating systems.

  `__EXTENSIONS__` ¶

    Enable general extensions on Solaris. This macro is defined only if the headers included by `AC_INCLUDES_DEFAULT` (see Default Includes) work correctly with it defined.

  `_MINIX` ¶
  `_POSIX_SOURCE`
  `_POSIX_1_SOURCE`

    Defined only on MINIX. `_POSIX_SOURCE` and `_POSIX_1_SOURCE` are needed to enable a number of POSIX features on this OS. `_MINIX` does not affect the system headers’ behavior; future versions of Autoconf may stop defining it. Programs that need to recognize Minix should use `AC_CANONICAL_HOST`.

  `_XOPEN_SOURCE` ¶

    Defined (with value 500) only if needed to make `wchar.h` declare `mbstate_t`. This is known to be necessary on some versions of HP-UX.

  The C preprocessor macro `__STDC_WANT_DEC_FP__` is not defined. ISO/IEC TR 24732:2009 was superseded by ISO/IEC TS 18661-2:2015.

  The C preprocessor macro `__STDC_WANT_LIB_EXT1__` is not defined, as the C standard’s Annex K is problematic. See: O’Donell C, Sebor M. Field Experience With Annex K—Bounds Checking Interfaces.

  The Autoconf macro `AC_USE_SYSTEM_EXTENSIONS` was introduced in Autoconf 2.60.

### 5.13 Erlang Libraries ¶

The following macros check for an installation of Erlang/OTP, and for the presence of certain Erlang libraries. All those macros require the configuration of an Erlang interpreter and an Erlang compiler (see Erlang Compiler and Interpreter Characteristics).

Macro: AC_ERLANG_SUBST_ERTS_VER ¶

  Set the output variable `ERLANG_ERTS_VER` to the version of the Erlang runtime system (as returned by Erlang’s `erlang:system_info(version)` function). The result of this test is cached if caching is enabled when running `configure`. The `ERLANG_ERTS_VER` variable is not intended to be used for testing for features of specific ERTS versions, but to be used for substituting the ERTS version in Erlang/OTP release resource files (`.rel` files), as shown below.

Macro: AC_ERLANG_SUBST_ROOT_DIR ¶

  Set the output variable `ERLANG_ROOT_DIR` to the path to the base directory in which Erlang/OTP is installed (as returned by Erlang’s `code:root_dir/0` function). The result of this test is cached if caching is enabled when running `configure`.

Macro: AC_ERLANG_SUBST_LIB_DIR ¶

  Set the output variable `ERLANG_LIB_DIR` to the path of the library directory of Erlang/OTP (as returned by Erlang’s `code:lib_dir/0` function), which subdirectories each contain an installed Erlang/OTP library. The result of this test is cached if caching is enabled when running `configure`.

Macro: AC_ERLANG_CHECK_LIB(library, [action-if-found], [action-if-not-found]) ¶

  Test whether the Erlang/OTP library library is installed by calling Erlang’s `code:lib_dir/1` function. The result of this test is cached if caching is enabled when running `configure`. action-if-found is a list of shell commands to run if the library is installed; action-if-not-found is a list of shell commands to run if it is not. Additionally, if the library is installed, the output variable ‘`ERLANG_LIB_DIR_library`’ is set to the path to the library installation directory, and the output variable ‘`ERLANG_LIB_VER_library`’ is set to the version number that is part of the subdirectory name, if it is in the standard form (`library-version`). If the directory name does not have a version part, ‘`ERLANG_LIB_VER_library`’ is set to the empty string. If the library is not installed, ‘`ERLANG_LIB_DIR_library`’ and ‘`ERLANG_LIB_VER_library`’ are set to `"not found"`. For example, to check if library `stdlib` is installed:

      AC_ERLANG_CHECK_LIB([stdlib],
        [AS_ECHO(["stdlib version \"$ERLANG_LIB_VER_stdlib\""])
         AS_ECHO(["is installed in \"$ERLANG_LIB_DIR_stdlib\""])],
        [AC_MSG_ERROR([stdlib was not found!])])

  The ‘`ERLANG_LIB_VER_library`’ variables (set by `AC_ERLANG_CHECK_LIB`) and the `ERLANG_ERTS_VER` variable (set by `AC_ERLANG_SUBST_ERTS_VER`) are not intended to be used for testing for features of specific versions of libraries or of the Erlang runtime system. Those variables are intended to be substituted in Erlang release resource files (`.rel` files). For instance, to generate a `example.rel` file for an application depending on the `stdlib` library, `configure.ac` could contain:

      AC_ERLANG_SUBST_ERTS_VER
      AC_ERLANG_CHECK_LIB([stdlib],
        [],
        [AC_MSG_ERROR([stdlib was not found!])])
      AC_CONFIG_FILES([example.rel])

  The `example.rel.in` file used to generate `example.rel` should contain:

      {release,
          {"@PACKAGE@", "@VERSION@"},
          {erts, "@ERLANG_ERTS_VER@"},
          [{stdlib, "@ERLANG_LIB_VER_stdlib@"},
           {@PACKAGE@, "@VERSION@"}]}.

In addition to the above macros, which test installed Erlang libraries, the following macros determine the paths to the directories into which newly built Erlang libraries are to be installed:

Macro: AC_ERLANG_SUBST_INSTALL_LIB_DIR ¶

  Set the `ERLANG_INSTALL_LIB_DIR` output variable to the directory into which every built Erlang library should be installed in a separate subdirectory. If this variable is not set in the environment when `configure` runs, its default value is `${libdir}/erlang/lib`.

Macro: AC_ERLANG_SUBST_INSTALL_LIB_SUBDIR(library, version) ¶

  Set the ‘`ERLANG_INSTALL_LIB_DIR_library`’ output variable to the directory into which the built Erlang library library version version should be installed. If this variable is not set in the environment when `configure` runs, its default value is ‘`$ERLANG_INSTALL_LIB_DIR/library-version`’, the value of the `ERLANG_INSTALL_LIB_DIR` variable being set by the `AC_ERLANG_SUBST_INSTALL_LIB_DIR` macro.

## 6 Writing Tests ¶

If the existing feature tests don’t do something you need, you have to write new ones. These macros are the building blocks. They provide ways for other macros to check whether various kinds of features are available and report the results.

This chapter contains some suggestions and some of the reasons why the existing tests are written the way they are. You can also learn a lot about how to write Autoconf tests by looking at the existing ones. If something goes wrong in one or more of the Autoconf tests, this information can help you understand the assumptions behind them, which might help you figure out how to best solve the problem.

These macros check the output of the compiler system of the current language (see Language Choice). They do not cache the results of their tests for future use (see Caching Results), because they don’t know enough about the information they are checking for to generate a cache variable name. They also do not print any messages, for the same reason. The checks for particular kinds of features call these macros and do cache their results and print messages about what they’re checking for.

When you write a feature test that could be applicable to more than one software package, the best thing to do is encapsulate it in a new macro. See Writing Autoconf Macros, for how to do that.

### 6.1 Language Choice ¶

Autoconf-generated `configure` scripts check for the C compiler and its features by default. Packages that use other programming languages (maybe more than one, e.g., C and C++) need to test features of the compilers for the respective languages. The following macros determine which programming language is used in the subsequent tests in `configure.ac`.

Macro: AC_LANG(language) ¶

  Do compilation tests using the compiler, preprocessor, and file extensions for the specified language.

  Supported languages are:

  ‘`C`’

    Do compilation tests using `CC` and `CPP` and use extension `.c` for test programs. Use compilation flags: `CPPFLAGS` with `CPP`, and both `CPPFLAGS` and `CFLAGS` with `CC`.

  ‘`C++`’

    Do compilation tests using `CXX` and `CXXCPP` and use extension `.C` for test programs. Use compilation flags: `CPPFLAGS` with `CXXCPP`, and both `CPPFLAGS` and `CXXFLAGS` with `CXX`.

  ‘`Fortran 77`’

    Do compilation tests using `F77` and use extension `.f` for test programs. Use compilation flags: `FFLAGS`.

  ‘`Fortran`’

    Do compilation tests using `FC` and use extension `.f` (or whatever has been set by `AC_FC_SRCEXT`) for test programs. Use compilation flags: `FCFLAGS`.

  ‘`Erlang`’ ¶

    Compile and execute tests using `ERLC` and `ERL` and use extension `.erl` for test Erlang modules. Use compilation flags: `ERLCFLAGS`.

  ‘`Objective C`’

    Do compilation tests using `OBJC` and `OBJCPP` and use extension `.m` for test programs. Use compilation flags: `CPPFLAGS` with `OBJCPP`, and both `CPPFLAGS` and `OBJCFLAGS` with `OBJC`.

  ‘`Objective C++`’

    Do compilation tests using `OBJCXX` and `OBJCXXCPP` and use extension `.mm` for test programs. Use compilation flags: `CPPFLAGS` with `OBJCXXCPP`, and both `CPPFLAGS` and `OBJCXXFLAGS` with `OBJCXX`.

  ‘`Go`’

    Do compilation tests using `GOC` and use extension `.go` for test programs. Use compilation flags `GOFLAGS`.

  ‘`Algol 68`’

    Do compilation tests using `A68` and use extension `.a68` for test programs. Use compilation flags `A68FLAGS`.

Macro: AC_LANG_PUSH(language) ¶

  Remember the current language (as set by `AC_LANG`) on a stack, and then select the language. Use this macro and `AC_LANG_POP` in macros that need to temporarily switch to a particular language.

Macro: AC_LANG_POP([language]) ¶

  Select the language that is saved on the top of the stack, as set by `AC_LANG_PUSH`, and remove it from the stack.

  If given, language specifies the language we just quit. It is a good idea to specify it when it’s known (which should be the case…), since Autoconf detects inconsistencies.

      AC_LANG_PUSH([Fortran 77])
      # Perform some tests on Fortran 77.
      # ...
      AC_LANG_POP([Fortran 77])

Macro: AC_LANG_ASSERT(language) ¶

  Check statically that the current language is language. You should use this in your language specific macros to avoid that they be called with an inappropriate language.

  This macro runs only at `autoconf` time, and incurs no cost at `configure` time. Sadly enough and because Autoconf is a two layer language 2, the macros `AC_LANG_PUSH` and `AC_LANG_POP` cannot be “optimizing”, therefore as much as possible you ought to avoid using them to wrap your code, rather, require from the user to run the macro with a correct current language, and check it with `AC_LANG_ASSERT`. And anyway, that may help the user understand she is running a Fortran macro while expecting a result about her Fortran 77 compiler...

Macro: AC_REQUIRE_CPP ¶

  Ensure that whichever preprocessor would currently be used for tests has been found. Calls `AC_REQUIRE` (see Prerequisite Macros) with an argument of either `AC_PROG_CPP` or `AC_PROG_CXXCPP`, depending on which language is current.

### 6.2 Writing Test Programs ¶

Autoconf tests follow a common scheme: feed some program with some input, and most of the time, feed a compiler with some source file. This section is dedicated to these source samples.

#### 6.2.1 Guidelines for Test Programs ¶

The most important rule to follow when writing testing samples is:

Look for realism.

This motto means that testing samples must be written with the same strictness as real programs are written. In particular, you should avoid “shortcuts” and simplifications.

Don’t just play with the preprocessor if you want to prepare a compilation. For instance, using `cpp` to check whether a header is functional might let your `configure` accept a header which causes some compiler error. Do not hesitate to check a header with other headers included before, especially required headers.

Make sure the symbols you use are properly defined, i.e., refrain from simply declaring a function yourself instead of including the proper header.

Test programs should not write to standard output. They should exit with status 0 if the test succeeds, and with status 1 otherwise, so that success can be distinguished easily from a core dump or other failure; segmentation violations and other failures produce a nonzero exit status. Unless you arrange for `exit` to be declared, test programs should `return`, not `exit`, from `main`, because on many systems `exit` is not declared by default.

Test programs can use `#if` or `#ifdef` to check the values of preprocessor macros defined by tests that have already run. For example, if you call `AC_HEADER_STDBOOL`, then later on in `configure.ac` you can have a test program that includes `stdbool.h` conditionally:

    #ifdef HAVE_STDBOOL_H
    # include <stdbool.h>
    #endif

Both `#if HAVE_STDBOOL_H` and `#ifdef HAVE_STDBOOL_H` will work with any standard C compiler. Some developers prefer `#if` because it is easier to read, while others prefer `#ifdef` because it avoids diagnostics with picky compilers like GCC with the `-Wundef` option.

If a test program needs to use or create a data file, give it a name that starts with `conftest`, such as `conftest.data`. The `configure` script cleans up by running ‘`rm -f -r conftest*`’ after running test programs and if the script is interrupted.

#### 6.2.2 Test Functions ¶

Functions in test code should use function prototypes, introduced in C89 and required in C23.

Functions that test programs declare should also be conditionalized for C++, which requires ‘`extern "C"`’ prototypes. Make sure to not include any header files containing clashing prototypes.

    #ifdef __cplusplus
    extern "C"
    #endif
    void *valloc (size_t);

If a test program calls a function with invalid parameters (just to see whether it exists), organize the program to ensure that it never invokes that function. You can do this by calling it in another function that is never invoked. You can’t do it by putting it after a call to `exit`, because GCC version 2 knows that `exit` never returns and optimizes out any code that follows it in the same block.

If you include any header files, be sure to call the functions relevant to them with the correct number of arguments, even if they are just 0, to avoid compilation errors due to prototypes. GCC version 2 has internal prototypes for several functions that it automatically inlines; for example, `memcpy`. To avoid errors when checking for them, either pass them the correct number of arguments or redeclare them with a different return type (such as `char`).

#### 6.2.3 Generating Sources ¶

Autoconf provides a set of macros that can be used to generate test source files. They are written to be language generic, i.e., they actually depend on the current language (see Language Choice) to “format” the output properly.

Macro: AC_LANG_CONFTEST(source) ¶

  Save the source text in the current test source file: `conftest.extension` where the extension depends on the current language. As of Autoconf 2.63b, the source file also contains the results of all of the `AC_DEFINE` performed so far.

  Note that the source is evaluated exactly once, like regular Autoconf macro arguments, and therefore (i) you may pass a macro invocation, (ii) if not, be sure to double quote if needed.

  The source text is expanded as an unquoted here-document, so ‘`$`’, ‘```’ and some ‘`\`’s should be backslash-escaped. See Here-Documents.

  This macro issues a warning during `autoconf` processing if source does not include an expansion of the macro `AC_LANG_DEFINES_PROVIDED` (note that both `AC_LANG_SOURCE` and `AC_LANG_PROGRAM` call this macro, and thus avoid the warning).

  This macro is seldom called directly, but is used under the hood by more common macros such as `AC_COMPILE_IFELSE` and `AC_RUN_IFELSE`.

Macro: AC_LANG_DEFINES_PROVIDED ¶

  This macro is called as a witness that the file `conftest.extension` appropriate for the current language is complete, including all previously determined results from `AC_DEFINE`. This macro is seldom called directly, but exists if you have a compelling reason to write a conftest file without using `AC_LANG_SOURCE`, yet still want to avoid a syntax warning from `AC_LANG_CONFTEST`.

Macro: AC_LANG_SOURCE(source) ¶

  Expands into the source, with the definition of all the `AC_DEFINE` performed so far. This macro includes an expansion of `AC_LANG_DEFINES_PROVIDED`.

  In many cases, you may find it more convenient to use the wrapper `AC_LANG_PROGRAM`.

For instance, executing (observe the double quotation!):

    AC_INIT([Hello], [1.0], [bug-hello@example.org], [],
            [https://www.example.org/])
    AC_DEFINE([HELLO_WORLD], ["Hello, World\n"],
      [Greetings string.])
    AC_LANG([C])
    AC_LANG_CONFTEST(
       [AC_LANG_SOURCE([[const char hw[] = "Hello, World\n";]])])
    gcc -E -dD conftest.c

on a system with `gcc` installed, results in:

    ...
    # 1 "conftest.c"

    #define PACKAGE_NAME "Hello"
    #define PACKAGE_TARNAME "hello"
    #define PACKAGE_VERSION "1.0"
    #define PACKAGE_STRING "Hello 1.0"
    #define PACKAGE_BUGREPORT "bug-hello@example.org"
    #define PACKAGE_URL "https://www.example.org/"
    #define HELLO_WORLD "Hello, World\n"

    const char hw[] = "Hello, World\n";

When the test language is Fortran, Erlang, Go, or Algol 68, the `AC_DEFINE` definitions are not automatically translated into constants in the source code by this macro.

Macro: AC_LANG_PROGRAM([prologue], [body]) ¶

  Expands into a source file which consists of the prologue, and then body as body of the main function (e.g., `main` in C). Since it uses `AC_LANG_SOURCE`, the features of the latter are available.

For instance:

    AC_INIT([Hello], [1.0], [bug-hello@example.org], [],
            [https://www.example.org/])
    AC_DEFINE([HELLO_WORLD], ["Hello, World\n"],
      [Greetings string.])
    AC_LANG_CONFTEST(
    [AC_LANG_PROGRAM([[const char hw[] = "Hello, World\n";]],
                     [[fputs (hw, stdout);]])])
    gcc -E -dD conftest.c

on a system with `gcc` installed, results in:

    ...
    # 1 "conftest.c"

    #define PACKAGE_NAME "Hello"
    #define PACKAGE_TARNAME "hello"
    #define PACKAGE_VERSION "1.0"
    #define PACKAGE_STRING "Hello 1.0"
    #define PACKAGE_BUGREPORT "bug-hello@example.org"
    #define PACKAGE_URL "https://www.example.org/"
    #define HELLO_WORLD "Hello, World\n"

    const char hw[] = "Hello, World\n";
    int
    main (void)
    {
    fputs (hw, stdout);
      ;
      return 0;
    }

In Erlang tests, the created source file is that of an Erlang module called `conftest` (`conftest.erl`). This module defines and exports at least one `start/0` function, which is called to perform the test. The prologue is optional code that is inserted between the module header and the `start/0` function definition. body is the body of the `start/0` function without the final period (see Checking Runtime Behavior, about constraints on this function’s behavior).

For instance:

    AC_INIT([Hello], [1.0], [bug-hello@example.org])
    AC_LANG(Erlang)
    AC_LANG_CONFTEST(
    [AC_LANG_PROGRAM([[-define(HELLO_WORLD, "Hello, world!").]],
                     [[io:format("~s~n", [?HELLO_WORLD])]])])
    cat conftest.erl

results in:

    -module(conftest).
    -export([start/0]).
    -define(HELLO_WORLD, "Hello, world!").
    start() ->
    io:format("~s~n", [?HELLO_WORLD])
    .

Macro: AC_LANG_CALL(prologue, function) ¶

  Expands into a source file which consists of the prologue, and then a call to the function as body of the main function (e.g., `main` in C). Since it uses `AC_LANG_PROGRAM`, the feature of the latter are available.

  This function will probably be replaced in the future by a version which would enable specifying the arguments. The use of this macro is not encouraged, as it violates strongly the typing system.

  This macro cannot be used for Erlang tests.

Macro: AC_LANG_FUNC_LINK_TRY(function) ¶

  Expands into a source file which uses the function in the body of the main function (e.g., `main` in C). Since it uses `AC_LANG_PROGRAM`, the features of the latter are available.

  As `AC_LANG_CALL`, this macro is documented only for completeness. It is considered to be severely broken, and in the future will be removed in favor of actual function calls (with properly typed arguments).

  This macro cannot be used for Erlang tests.

### 6.3 Running the Preprocessor ¶

Sometimes one might need to run the preprocessor on some source file. Usually it is a bad idea, as you typically need to compile your project, not merely run the preprocessor on it; therefore you certainly want to run the compiler, not the preprocessor. Resist the temptation of following the easiest path.

Nevertheless, if you need to run the preprocessor, then use `AC_PREPROC_IFELSE`.

The macros described in this section cannot be used for tests in Erlang, Fortran, Go, or Algol 68 since those languages require no preprocessor.

Macro: AC_PREPROC_IFELSE(input, [action-if-true], [action-if-false]) ¶

  Run the preprocessor of the current language (see Language Choice) on the input, run the shell commands action-if-true on success, action-if-false otherwise.

  If input is nonempty use the equivalent of `AC_LANG_CONFTEST(input)` to generate the current test source file; otherwise reuse the already-existing test source file. The input can be made by `AC_LANG_PROGRAM` and friends. The input text is expanded as an unquoted here-document, so ‘`$`’, ‘```’ and some ‘`\`’s should be backslash-escaped. See Here-Documents.

  This macro uses `CPPFLAGS`, but not `CFLAGS`, because `-g`, `-O`, etc. are not valid options to many C preprocessors.

  It is customary to report unexpected failures with `AC_MSG_FAILURE`. If needed, action-if-true can further access the preprocessed output in the file `conftest.i`.

For instance:

    AC_INIT([Hello], [1.0], [bug-hello@example.org])
    AC_DEFINE([HELLO_WORLD], ["Hello, World\n"],
      [Greetings string.])
    AC_PREPROC_IFELSE(
       [AC_LANG_PROGRAM([[const char hw[] = "Hello, World\n";]],
                        [[fputs (hw, stdout);]])],
       [AC_MSG_RESULT([OK])],
       [AC_MSG_FAILURE([unexpected preprocessor failure])])

might result in:

    checking for gcc... gcc
    checking whether the C compiler works... yes
    checking for C compiler default output file name... a.out
    checking for suffix of executables...
    checking whether we are cross compiling... no
    checking for suffix of object files... o
    checking whether the compiler supports GNU C... yes
    checking whether gcc accepts -g... yes
    checking for gcc option to enable C23 features... -std=gnu23
    checking how to run the C preprocessor... gcc -std=gnu23 -E
    OK

The macro `AC_TRY_CPP` (see Obsolete Macros) used to play the role of `AC_PREPROC_IFELSE`, but double quotes its argument, making it impossible to use it to elaborate sources. You are encouraged to get rid of your old use of the macro `AC_TRY_CPP` in favor of `AC_PREPROC_IFELSE`, but, in the first place, are you sure you need to run the preprocessor and not the compiler?

Macro: AC_EGREP_HEADER(pattern, header-file, action-if-found, [action-if-not-found]) ¶

  pattern, after being expanded as if in a double-quoted shell string, is an extended regular expression. If the output of running the preprocessor on the system header file header-file contains a line matching pattern, execute shell commands action-if-found, otherwise execute action-if-not-found.

  See below for some problems involving this macro.

Macro: AC_EGREP_CPP(pattern, program, [action-if-found], [action-if-not-found]) ¶

  pattern, after being expanded as if in a double-quoted shell string, is an extended regular expression. program is the text of a C or C++ program, which is expanded as an unquoted here-document (see Here-Documents). If the output of running the preprocessor on program contains a line matching pattern, execute shell commands action-if-found, otherwise execute action-if-not-found.

  See below for some problems involving this macro.

`AC_EGREP_CPP` and `AC_EGREP_HEADER` should be used with care, as preprocessors can insert line breaks between output tokens. For example, the preprocessor might transform this:

    #define MAJOR 2
    #define MINOR 23
    Version MAJOR . MINOR

into this:

    Version
           2
                     .
                       23

Because preprocessors are allowed to insert white space, change escapes in string constants, insert backlash-newline pairs, or do any of a number of things that do not change the meaning of the preprocessed program, it is better to rely on `AC_PREPROC_IFELSE` than to resort to `AC_EGREP_CPP` or `AC_EGREP_HEADER`.

For more information about what can appear in portable extended regular expressions, see Problematic Expressions in GNU Grep.

### 6.4 Running the Compiler ¶

To check for a syntax feature of the current language’s (see Language Choice) compiler, such as whether it recognizes a certain keyword, or simply to try some library feature, use `AC_COMPILE_IFELSE` to try to compile a small program that uses that feature.

Macro: AC_COMPILE_IFELSE(input, [action-if-true], [action-if-false]) ¶

  Run the compiler and compilation flags of the current language (see Language Choice) on the input, run the shell commands action-if-true on success, action-if-false otherwise.

  If input is nonempty use the equivalent of `AC_LANG_CONFTEST(input)` to generate the current test source file; otherwise reuse the already-existing test source file. The input can be made by `AC_LANG_PROGRAM` and friends. The input text is expanded as an unquoted here-document, so ‘`$`’, ‘```’ and some ‘`\`’s should be backslash-escaped. See Here-Documents.

  It is customary to report unexpected failures with `AC_MSG_FAILURE`. This macro does not try to link; use `AC_LINK_IFELSE` if you need to do that (see Running the Linker). If needed, action-if-true can further access the just-compiled object file `conftest.$OBJEXT`.

  This macro uses `AC_REQUIRE` for the compiler associated with the current language, which means that if the compiler has not yet been determined, the compiler determination will be made prior to the body of the outermost `AC_DEFUN` macro that triggered this macro to expand (see Expanded Before Required).

For tests in Erlang, the input must be the source code of a module named `conftest`. `AC_COMPILE_IFELSE` generates a `conftest.beam` file that can be interpreted by the Erlang virtual machine (`ERL`). It is recommended to use `AC_LANG_PROGRAM` to specify the test program, to ensure that the Erlang module has the right name.

### 6.5 Running the Linker ¶

To check for a library, a function, or a global variable, Autoconf `configure` scripts try to compile and link a small program that uses it. This is unlike Metaconfig, which by default uses `nm` or `ar` on the C library to try to figure out which functions are available. Trying to link with the function is usually a more reliable approach because it avoids dealing with the variations in the options and output formats of `nm` and `ar` and in the location of the standard libraries. It also allows configuring for cross-compilation or checking a function’s runtime behavior if needed. On the other hand, it can be slower than scanning the libraries once, but accuracy is more important than speed.

`AC_LINK_IFELSE` is used to compile test programs to test for functions and global variables. It is also used by `AC_CHECK_LIB` to check for libraries (see Library Files), by adding the library being checked for to `LIBS` temporarily and trying to link a small program.

Macro: AC_LINK_IFELSE(input, [action-if-true], [action-if-false]) ¶

  Run the compiler (and compilation flags) and the linker of the current language (see Language Choice) on the input, run the shell commands action-if-true on success, action-if-false otherwise. If needed, action-if-true can further access the just-linked program file `conftest$EXEEXT`.

  If input is nonempty use the equivalent of `AC_LANG_CONFTEST(input)` to generate the current test source file; otherwise reuse the already-existing test source file. The input can be made by `AC_LANG_PROGRAM` and friends. The input text is expanded as an unquoted here-document, so ‘`$`’, ‘```’ and some ‘`\`’s should be backslash-escaped. See Here-Documents.

  `LDFLAGS` and `LIBS` are used for linking, in addition to the current compilation flags.

  It is customary to report unexpected failures with `AC_MSG_FAILURE`. This macro does not try to execute the program; use `AC_RUN_IFELSE` if you need to do that (see Checking Runtime Behavior).

The `AC_LINK_IFELSE` macro cannot be used for Erlang tests, since Erlang programs are interpreted and do not require linking.

### 6.6 Checking Runtime Behavior ¶

Sometimes you need to find out how a system performs at runtime, such as whether a given function has a certain capability or bug. If you can, make such checks when your program runs instead of when it is configured. You can check for things like the machine’s endianness when your program initializes itself.

If you really need to test for a runtime behavior while configuring, you can write a test program to determine the result, and compile and run it using `AC_RUN_IFELSE`. Avoid running test programs if possible, because this prevents people from configuring your package for cross-compiling.

Macro: AC_RUN_IFELSE(input, [action-if-true], [action-if-false], [action-if-cross-compiling = ‘`AC_MSG_FAILURE`’]) ¶

  Run the compiler (and compilation flags) and the linker of the current language (see Language Choice) on the input, then execute the resulting program. If the program returns an exit status of 0 when executed, run shell commands action-if-true. Otherwise, run shell commands action-if-false.

  If input is nonempty use the equivalent of `AC_LANG_CONFTEST(input)` to generate the current test source file; otherwise reuse the already-existing test source file. The input can be made by `AC_LANG_PROGRAM` and friends. The input text is expanded as an unquoted here-document, so ‘`$`’, ‘```’ and some ‘`\`’s should be backslash-escaped. See Here-Documents.

  `LDFLAGS` and `LIBS` are used for linking, in addition to the compilation flags of the current language (see Language Choice). Additionally, action-if-true can run `./conftest$EXEEXT` for further testing.

  In the action-if-false section, the failing exit status is available in the shell variable ‘`$?`’. This exit status might be that of a failed compilation, or it might be that of a failed program execution.

  If cross-compilation mode is enabled (this is the case if either the compiler being used does not produce executables that run on the system where `configure` is being run, or if the options `--build` and `--host` were both specified and their values are different), then the test program is not run. If the optional shell commands action-if-cross-compiling are given, those commands are run instead; typically these commands provide pessimistic defaults that allow cross-compilation to work even if the guess was wrong. If the fourth argument is empty or omitted, but cross-compilation is detected, then `configure` prints an error message and exits. If you want your package to be useful in a cross-compilation scenario, you should provide a non-empty action-if-cross-compiling clause, as well as wrap the `AC_RUN_IFELSE` compilation inside an `AC_CACHE_CHECK` (see Caching Results) which allows the user to override the pessimistic default if needed.

  It is customary to report unexpected failures with `AC_MSG_FAILURE`.

`autoconf` prints a warning message when creating `configure` each time it encounters a call to `AC_RUN_IFELSE` with no action-if-cross-compiling argument given. If you are not concerned about users configuring your package for cross-compilation, you may ignore the warning. A few of the macros distributed with Autoconf produce this warning message; but if this is a problem for you, please report it as a bug, along with an appropriate pessimistic guess to use instead.

To configure for cross-compiling you can also choose a value for those parameters based on the canonical system name (see Manual Configuration). Alternatively, set up a test results cache file with the correct values for the host system (see Caching Results).

To provide a default for calls of `AC_RUN_IFELSE` that are embedded in other macros, including a few of the ones that come with Autoconf, you can test whether the shell variable `cross_compiling` is set to ‘`yes`’, and then use an alternate method to get the results instead of calling the macros.

It is also permissible to temporarily assign to `cross_compiling` in order to force tests to behave as though they are in a cross-compilation environment, particularly since this provides a way to test your action-if-cross-compiling even when you are not using a cross-compiler.

    # We temporarily set cross-compile mode to force AC_COMPUTE_INT
    # to use the slow link-only method
    save_cross_compiling=$cross_compiling
    cross_compiling=yes
    AC_COMPUTE_INT([...])
    cross_compiling=$save_cross_compiling

A C or C++ runtime test should be portable. See Portable C and C++ Programming.

Erlang tests must exit themselves the Erlang VM by calling the `halt/1` function: the given status code is used to determine the success of the test (status is `0`) or its failure (status is different than `0`), as explained above. It must be noted that data output through the standard output (e.g., using `io:format/2`) may be truncated when halting the VM. Therefore, if a test must output configuration information, it is recommended to create and to output data into the temporary file named `conftest.out`, using the functions of module `file`. The `conftest.out` file is automatically deleted by the `AC_RUN_IFELSE` macro. For instance, a simplified implementation of Autoconf’s `AC_ERLANG_SUBST_LIB_DIR` macro is:

    AC_INIT([LibdirTest], [1.0], [bug-libdirtest@example.org])
    AC_ERLANG_NEED_ERL
    AC_LANG(Erlang)
    AC_RUN_IFELSE(
      [AC_LANG_PROGRAM([], [dnl
        file:write_file("conftest.out", code:lib_dir()),
        halt(0)])],
      [AS_ECHO(["code:lib_dir() returned: `cat conftest.out`"])],
      [AC_MSG_FAILURE([test Erlang program execution failed])])

### 6.7 Multiple Cases ¶

Some operations are accomplished in several possible ways, depending on the OS variant. Checking for them essentially requires a “case statement”. Autoconf does not directly provide one; however, it is easy to simulate by using a shell variable to keep track of whether a way to perform the operation has been found yet.

Here is an example that uses the shell variable `fstype` to keep track of whether the remaining cases need to be checked. Note that since the value of `fstype` is under our control, we don’t have to use the longer ‘`test "x$fstype" = xno`’.

    AC_MSG_CHECKING([how to get file system type])
    fstype=no
    # The order of these tests is important.
    AC_COMPILE_IFELSE([AC_LANG_PROGRAM([[#include <sys/statvfs.h>
                                         #include <sys/fstyp.h>
                                       ]])],
      [AC_DEFINE([FSTYPE_STATVFS], [1],
         [Define if statvfs exists.])
       fstype=SVR4])
    AS_IF([test $fstype = no],
      [AC_COMPILE_IFELSE([AC_LANG_PROGRAM([[#include <sys/statfs.h>
                                            #include <sys/fstyp.h>
                                          ]])],
         [AC_DEFINE([FSTYPE_USG_STATFS], [1],
            [Define if USG statfs.])
          fstype=SVR3])])
    AS_IF([test $fstype = no],
      [AC_COMPILE_IFELSE([AC_LANG_PROGRAM([[#include <sys/statfs.h>
                                            #include <sys/vmount.h>
                                          ]])],
         [AC_DEFINE([FSTYPE_AIX_STATFS], [1],
            [Define if AIX statfs.])
          fstype=AIX])])
    # (more cases omitted here)
    AC_MSG_RESULT([$fstype])

## 7 Results of Tests ¶

Once `configure` has determined whether a feature exists, what can it do to record that information? There are four sorts of things it can do: define a C preprocessor symbol, set a variable in the output files, save the result in a cache file for future `configure` runs, and print a message letting the user know the result of the test.

### 7.1 Defining C Preprocessor Symbols ¶

A common action to take in response to a feature test is to define a C preprocessor symbol indicating the results of the test. That is done by calling `AC_DEFINE` or `AC_DEFINE_UNQUOTED`.

By default, `AC_OUTPUT` places the symbols defined by these macros into the output variable `DEFS`, which contains an option `-Dsymbol=value` for each symbol defined. Unlike in Autoconf version 1, there is no variable `DEFS` defined while `configure` is running. To check whether Autoconf macros have already defined a certain C preprocessor symbol, test the value of the appropriate cache variable, as in this example:

    AC_CHECK_FUNC([vprintf],
      [AC_DEFINE([HAVE_VPRINTF], [1],
         [Define if vprintf exists.])])
    AS_IF([test "x$ac_cv_func_vprintf" != xyes],
      [AC_CHECK_FUNC([_doprnt],
         [AC_DEFINE([HAVE_DOPRNT], [1],
            [Define if _doprnt exists.])])])

If `AC_CONFIG_HEADERS` has been called, then instead of creating `DEFS`, `AC_OUTPUT` creates a header file by substituting the correct values into `#define` statements in a template file. See Configuration Header Files, for more information about this kind of output.

Macro: AC_DEFINE(variable, value, [description]) ¶
Macro: AC_DEFINE(variable) ¶

  Define variable to value (verbatim), by defining a C preprocessor macro for variable. variable should be a C identifier, optionally suffixed by a parenthesized argument list to define a C preprocessor macro with arguments. The macro argument list, if present, should be a comma-separated list of C identifiers, possibly terminated by an ellipsis ‘`...`’ if C99-or-later syntax is employed. variable should not contain comments, white space, trigraphs, backslash-newlines, universal character names, or non-ASCII characters.

  value may contain backslash-escaped newlines, which will be preserved if you use `AC_CONFIG_HEADERS` but flattened if passed via `@DEFS@` (with no effect on the compilation, since the preprocessor sees only one line in the first place). value should not contain raw newlines. If you are not using `AC_CONFIG_HEADERS`, value should not contain any ‘`#`’ characters, as `make` tends to eat them. To use a shell variable, use `AC_DEFINE_UNQUOTED` instead.

  description is only useful if you are using `AC_CONFIG_HEADERS`. In this case, description is put into the generated `config.h.in` as the comment before the macro define. The following example defines the C preprocessor variable `EQUATION` to be the string constant ‘`"$a > $b"`’:

      AC_DEFINE([EQUATION], ["$a > $b"],
        [Equation string.])

  If neither value nor description are given, then value defaults to 1 instead of to the empty string. This is for backwards compatibility with older versions of Autoconf, but this usage is obsolescent and may be withdrawn in future versions of Autoconf.

  If the variable is a literal string, it is passed to `m4_pattern_allow` (see Forbidden Patterns).

  If multiple `AC_DEFINE` statements are executed for the same variable name (not counting any parenthesized argument list), the last one wins.

Macro: AC_DEFINE_UNQUOTED(variable, value, [description]) ¶
Macro: AC_DEFINE_UNQUOTED(variable) ¶

  Like `AC_DEFINE`, but three shell expansions are performed—once—on variable and value: variable expansion (‘`$`’), command substitution (‘```’), and backslash escaping (‘`\`’), as if in an unquoted here-document. Single and double quote characters in the value have no special meaning. Use this macro instead of `AC_DEFINE` when variable or value is a shell variable. Examples:

      AC_DEFINE_UNQUOTED([config_machfile], ["$machfile"],
        [Configuration machine file.])
      AC_DEFINE_UNQUOTED([GETGROUPS_T], [$ac_cv_type_getgroups],
        [getgroups return type.])
      AC_DEFINE_UNQUOTED([$ac_tr_hdr], [1],
        [Translated header name.])

Due to a syntactical oddity of the Bourne shell, do not use semicolons to separate `AC_DEFINE` or `AC_DEFINE_UNQUOTED` calls from other macro calls or shell code; that can cause syntax errors in the resulting `configure` script. Use either blanks or newlines. That is, do this:

    AC_CHECK_HEADER([elf.h],
      [AC_DEFINE([SVR4], [1], [System V Release 4]) LIBS="-lelf $LIBS"])

or this:

    AC_CHECK_HEADER([elf.h],
      [AC_DEFINE([SVR4], [1], [System V Release 4])
       LIBS="-lelf $LIBS"])

instead of this:

    AC_CHECK_HEADER([elf.h],
      [AC_DEFINE([SVR4], [1], [System V Release 4]); LIBS="-lelf $LIBS"])

### 7.2 Setting Output Variables ¶

Another way to record the results of tests is to set output variables, which are shell variables whose values are substituted into files that `configure` outputs. The two macros below create new output variables. See Preset Output Variables, for a list of output variables that are always available.

Macro: AC_SUBST(variable, [value]) ¶

  Create an output variable from a shell variable. Make `AC_OUTPUT` substitute the variable variable into output files (typically one or more makefiles). This means that `AC_OUTPUT` replaces instances of ‘`@variable@`’ in input files with the value that the shell variable variable has when `AC_OUTPUT` is called. The value can contain any non-`NUL` character, including newline. If you are using Automake 1.11 or newer, for newlines in values you might want to consider using `AM_SUBST_NOTMAKE` to prevent `automake` from adding a line `variable = @variable@` to the `Makefile.in` files (see Automake in Other things Automake recognizes).

  Variable occurrences should not overlap: e.g., an input file should not contain ‘`@var1@var2@`’ if var1 and var2 are variable names. The substituted value is not rescanned for more output variables; occurrences of ‘`@variable@`’ in the value are inserted literally into the output file. (The algorithm uses the special marker `|#_!!_#|` internally, so neither the substituted value nor the output file may contain `|#_!!_#|`.)

  If value is given, in addition assign it to variable.

  The string variable is passed to `m4_pattern_allow` (see Forbidden Patterns). variable is not further expanded, even if there is another macro by the same name.

Macro: AC_SUBST_FILE(variable) ¶

  Another way to create an output variable from a shell variable. Make `AC_OUTPUT` insert (without substitutions) the contents of the file named by shell variable variable into output files. This means that `AC_OUTPUT` replaces instances of ‘`@variable@`’ in output files (such as `Makefile.in`) with the contents of the file that the shell variable variable names when `AC_OUTPUT` is called. Set the variable to `/dev/null` for cases that do not have a file to insert. This substitution occurs only when the ‘`@variable@`’ is on a line by itself, optionally surrounded by spaces and tabs. The substitution replaces the whole line, including the spaces, tabs, and the terminating newline.

  This macro is useful for inserting makefile fragments containing special dependencies or other `make` directives for particular host or target types into makefiles. For example, `configure.ac` could contain:

      AC_SUBST_FILE([host_frag])
      host_frag=$srcdir/conf/sun4.mh

  and then a `Makefile.in` could contain:

      @host_frag@

  The string variable is passed to `m4_pattern_allow` (see Forbidden Patterns).

Running `configure` in varying environments can be extremely dangerous. If for instance the user runs ‘`CC=bizarre-cc ./configure`’, then the cache, `config.h`, and many other output files depend upon `bizarre-cc` being the C compiler. If for some reason the user runs `./configure` again, or if it is run via ‘`./config.status --recheck`’, (See Automatic Remaking, and see config.status Invocation), then the configuration can be inconsistent, composed of results depending upon two different compilers.

Environment variables that affect this situation, such as ‘`CC`’ above, are called precious variables, and can be declared as such by `AC_ARG_VAR`.

Macro: AC_ARG_VAR(variable, description) ¶

  Declare variable is a precious variable, and include its description in the variable section of ‘`./configure --help`’.

  Being precious means that

  - variable is substituted via `AC_SUBST`.
  - The value of variable when `configure` was launched is saved in the cache, including if it was not specified on the command line but via the environment. Indeed, while `configure` can notice the definition of `CC` in ‘`./configure CC=bizarre-cc`’, it is impossible to notice it in ‘`CC=bizarre-cc ./configure`’, which, unfortunately, is what most users do.

    We emphasize that it is the initial value of variable which is saved, not that found during the execution of `configure`. Indeed, specifying ‘`./configure FOO=foo`’ and letting ‘`./configure`’ guess that `FOO` is `foo` can be two different things.

  - variable is checked for consistency between two `configure` runs. For instance:

        $ ./configure --silent --config-cache
        $ CC=cc ./configure --silent --config-cache
        configure: error: 'CC' was not set in the previous run
        configure: error: changes in the environment can compromise \
        the build
        configure: error: run 'make distclean' and/or \
        'rm config.cache' and start over

    and similarly if the variable is unset, or if its content is changed. If the content has white space changes only, then the error is degraded to a warning only, but the old value is reused.

  - variable is kept during automatic reconfiguration (see config.status Invocation) as if it had been passed as a command line argument, including when no cache is used:

        $ CC=/usr/bin/cc ./configure var=raboof --silent
        $ ./config.status --recheck
        running CONFIG_SHELL=/bin/sh /bin/sh ./configure var=raboof \
          CC=/usr/bin/cc  --no-create --no-recursion

### 7.3 Special Characters in Output Variables ¶

Many output variables are intended to be evaluated both by `make` and by the shell. Some characters are expanded differently in these two contexts, so to avoid confusion these variables’ values should not contain any of the following characters:

    " # $ & ' ( ) * ; < > ? [ \ ^ ` |

Also, these variables’ values should neither contain newlines, nor start with ‘`~`’, nor contain white space or ‘`:`’ immediately followed by ‘`~`’. The values can contain nonempty sequences of white space characters like tabs and spaces, but each such sequence might arbitrarily be replaced by a single space during substitution.

These restrictions apply both to the values that `configure` computes, and to the values set directly by the user. For example, the following invocations of `configure` are problematic, since they attempt to use special characters within `CPPFLAGS` and white space within `$(srcdir)`:

    CPPFLAGS='-DOUCH="&\"#$*?"' '../My Source/ouch-1.0/configure'

    '../My Source/ouch-1.0/configure' CPPFLAGS='-DOUCH="&\"#$*?"'

### 7.4 Caching Results ¶

To avoid checking for the same features repeatedly in various `configure` scripts (or in repeated runs of one script), `configure` can optionally save the results of many checks in a cache file (see Cache Files). If a `configure` script runs with caching enabled and finds a cache file, it reads the results of previous runs from the cache and avoids rerunning those checks. As a result, `configure` can then run much faster than if it had to perform all of the checks every time.

Macro: AC_CACHE_VAL(cache-id, commands-to-set-it) ¶

  Ensure that the results of the check identified by cache-id are available. If the results of the check were in the cache file that was read, and `configure` was not given the `--quiet` or `--silent` option, print a message saying that the result was cached; otherwise, run the shell commands commands-to-set-it. If the shell commands are run to determine the value, the value is saved in the cache file just before `configure` creates its output files. See Cache Variable Names, for how to choose the name of the cache-id variable.

  The commands-to-set-it must have no side effects except for setting the variable cache-id, see below.

Macro: AC_CACHE_CHECK(message, cache-id, commands-to-set-it) ¶

  A wrapper for `AC_CACHE_VAL` that takes care of printing the messages. This macro provides a convenient shorthand for the most common way to use these macros. It calls `AC_MSG_CHECKING` for message, then `AC_CACHE_VAL` with the cache-id and commands arguments, and `AC_MSG_RESULT` with cache-id.

  The commands-to-set-it must have no side effects except for setting the variable cache-id, see below.

It is common to find buggy macros using `AC_CACHE_VAL` or `AC_CACHE_CHECK`, because people are tempted to call `AC_DEFINE` in the commands-to-set-it. Instead, the code that follows the call to `AC_CACHE_VAL` should call `AC_DEFINE`, by examining the value of the cache variable. For instance, the following macro is broken:

    AC_DEFUN([AC_SHELL_TRUE],
    [AC_CACHE_CHECK([whether true(1) works], [my_cv_shell_true_works],
                    [my_cv_shell_true_works=no
                     (true) 2>/dev/null && my_cv_shell_true_works=yes
                     if test "x$my_cv_shell_true_works" = xyes; then
                       AC_DEFINE([TRUE_WORKS], [1],
                                 [Define if 'true(1)' works properly.])
                     fi])
    ])

This fails if the cache is enabled: the second time this macro is run, `TRUE_WORKS` will not be defined. The proper implementation is:

    AC_DEFUN([AC_SHELL_TRUE],
    [AC_CACHE_CHECK([whether true(1) works], [my_cv_shell_true_works],
                    [my_cv_shell_true_works=no
                     (true) 2>/dev/null && my_cv_shell_true_works=yes])
     if test "x$my_cv_shell_true_works" = xyes; then
       AC_DEFINE([TRUE_WORKS], [1],
                 [Define if 'true(1)' works properly.])
     fi
    ])

Also, commands-to-set-it should not print any messages, for example with `AC_MSG_CHECKING`; do that before calling `AC_CACHE_VAL`, so the messages are printed regardless of whether the results of the check are retrieved from the cache or determined by running the shell commands.

#### 7.4.1 Cache Variable Names ¶

The names of cache variables should have the following format:

    package-prefix_cv_value-type_specific-value_[additional-options]

for example, ‘`ac_cv_header_stat_broken`’ or ‘`ac_cv_prog_gcc_traditional`’. The parts of the variable name are:

package-prefix

  An abbreviation for your package or organization; the same prefix you begin local Autoconf macros with, except lowercase by convention. For cache values used by the distributed Autoconf macros, this value is ‘`ac`’.

`_cv_`

  Indicates that this shell variable is a cache value. This string must be present in the variable name, including the leading underscore.

value-type

  A convention for classifying cache values, to produce a rational naming system. The values used in Autoconf are listed in Macro Names.

specific-value

  Which member of the class of cache values this test applies to. For example, which function (‘`alloca`’), program (‘`gcc`’), or output variable (‘`INSTALL`’).

additional-options

  Any particular behavior of the specific member that this test applies to. For example, ‘`broken`’ or ‘`set`’. This part of the name may be omitted if it does not apply.

The values assigned to cache variables may not contain newlines. Usually, their values are Boolean (‘`yes`’ or ‘`no`’) or the names of files or functions; so this is not an important restriction. Cache Variable Index for an index of cache variables with documented semantics.

#### 7.4.2 Cache Files ¶

A cache file is a shell script that caches the results of configure tests run on one system so they can be shared between configure scripts and configure runs. It is not useful on other systems. If its contents are invalid for some reason, the user may delete or edit it, or override documented cache variables on the `configure` command line.

By default, `configure` uses no cache file, to avoid problems caused by accidental use of stale cache files.

To enable caching, `configure` accepts `--config-cache` (or `-C`) to cache results in the file `config.cache`. Alternatively, `--cache-file=file` specifies that file be the cache file. The cache file is created if it does not exist already. When `configure` calls `configure` scripts in subdirectories, it uses the `--cache-file` argument so that they share the same cache. See Configuring Other Packages in Subdirectories, for information on configuring subdirectories with the `AC_CONFIG_SUBDIRS` macro.

`config.status` only pays attention to the cache file if it is given the `--recheck` option, which makes it rerun `configure`.

It is wrong to try to distribute cache files for particular system types. There is too much room for error in doing that, and too much administrative overhead in maintaining them. For any features that can’t be guessed automatically, use the standard method of the canonical system type and linking files (see Manual Configuration).

The site initialization script can specify a site-wide cache file to use, instead of the usual per-program cache. In this case, the cache file gradually accumulates information whenever someone runs a new `configure` script. (Running `configure` merges the new cache results with the existing cache file.) This may cause problems, however, if the system configuration (e.g., the installed libraries or compilers) changes and the stale cache file is not deleted.

If `configure` is interrupted at the right time when it updates a cache file outside of the build directory where the `configure` script is run, it may leave behind a temporary file named after the cache file with digits following it. You may safely delete such a file.

#### 7.4.3 Cache Checkpointing ¶

If your configure script, or a macro called from `configure.ac`, happens to abort the configure process, it may be useful to checkpoint the cache a few times at key points using `AC_CACHE_SAVE`. Doing so reduces the amount of time it takes to rerun the configure script with (hopefully) the error that caused the previous abort corrected.

Macro: AC_CACHE_LOAD ¶

  Loads values from existing cache file, or creates a new cache file if a cache file is not found. Called automatically from `AC_INIT`.

Macro: AC_CACHE_SAVE ¶

  Flushes all cached values to the cache file. Called automatically from `AC_OUTPUT`, but it can be quite useful to call `AC_CACHE_SAVE` at key points in `configure.ac`.

For instance:

     ... AC_INIT, etc. ...

    # Checks for programs.
    AC_PROG_CC
    AC_PROG_AWK
     ... more program checks ...
    AC_CACHE_SAVE

    # Checks for libraries.
    AC_CHECK_LIB([nsl], [gethostbyname])
    AC_CHECK_LIB([socket], [connect])
     ... more lib checks ...
    AC_CACHE_SAVE

    # Might abort...
    AM_PATH_GTK([1.0.2], [], [AC_MSG_ERROR([GTK not in path])])
    AM_PATH_GTKMM([0.9.5], [], [AC_MSG_ERROR([GTK not in path])])

     ... AC_OUTPUT, etc. ...

### 7.5 Printing Messages ¶

`configure` scripts need to give users running them several kinds of information. The following macros print messages in ways appropriate for each kind. The arguments to all of them get enclosed in shell double quotes, so the shell performs variable and back-quote substitution on them.

These macros are all wrappers around the `printf` shell command. They direct output to the appropriate file descriptor (see File Descriptor Macros). `configure` scripts should rarely need to run `printf` directly to print messages for the user. Using these macros makes it easy to change how and when each kind of message is printed; such changes need only be made to the macro definitions and all the callers change automatically.

To diagnose static issues, i.e., when `autoconf` is run, see Diagnostic messages from M4sugar.

Macro: AC_MSG_CHECKING(feature-description) ¶

  Notify the user that `configure` is checking for a particular feature. This macro prints a message that starts with ‘`checking` ’ and ends with ‘`...`’ and no newline. It must be followed by a call to `AC_MSG_RESULT` to print the result of the check and the newline. The feature-description should be something like ‘`whether the Fortran compiler accepts C++ comments`’ or ‘`for _Alignof`’.

  This macro prints nothing if `configure` is run with the `--quiet` or `--silent` option.

Macro: AC_MSG_RESULT(result-description) ¶

  Notify the user of the results of a check. result-description is almost always the value of the cache variable for the check, typically ‘`yes`’, ‘`no`’, or a file name. This macro should follow a call to `AC_MSG_CHECKING`, and the result-description should be the completion of the message printed by the call to `AC_MSG_CHECKING`.

  This macro prints nothing if `configure` is run with the `--quiet` or `--silent` option.

Macro: AC_MSG_NOTICE(message) ¶

  Deliver the message to the user. It is useful mainly to print a general description of the overall purpose of a group of feature checks, e.g.,

      AC_MSG_NOTICE([checking if stack overflow is detectable])

  This macro prints nothing if `configure` is run with the `--quiet` or `--silent` option.

Macro: AC_MSG_ERROR(error-description, [exit-status = ‘`$?/1`’]) ¶

  Notify the user of an error that prevents `configure` from completing. This macro prints an error message to the standard error output and exits `configure` with exit-status (‘`$?`’ by default, except that ‘`0`’ is converted to ‘`1`’). error-description should be something like ‘`invalid value $HOME for \$HOME`’.

  The error-description should start with a lower-case letter, and “cannot” is preferred to “can’t”.

Macro: AC_MSG_FAILURE(error-description, [exit-status]) ¶

  This `AC_MSG_ERROR` wrapper notifies the user of an error that prevents `configure` from completing and that additional details are provided in `config.log`. This is typically used when abnormal results are found during a compilation.

Macro: AC_MSG_WARN(problem-description) ¶

  Notify the `configure` user of a possible problem. This macro prints the message to the standard error output; `configure` continues running afterward, so macros that call `AC_MSG_WARN` should provide a default (back-up) behavior for the situations they warn about. problem-description should be something like ‘`ln -s seems to make hard links`’.

## 8 Programming in M4 ¶

Autoconf is written on top of two layers: M4sugar, which provides convenient macros for pure M4 programming, and M4sh, which provides macros dedicated to shell script generation.

As of this version of Autoconf, these two layers still contain experimental macros, whose interface might change in the future. As a matter of fact, anything that is not documented must not be used.

### 8.1 M4 Quotation ¶

The most common problem with existing macros is an improper quotation. This section, which users of Autoconf can skip, but which macro writers must read, first justifies the quotation scheme that was chosen for Autoconf and then ends with a rule of thumb. Understanding the former helps one to follow the latter.

#### 8.1.1 Active Characters ¶

To fully understand where proper quotation is important, you first need to know what the special characters are in Autoconf: ‘`#`’ introduces a comment inside which no macro expansion is performed, ‘`,`’ separates arguments, ‘`[`’ and ‘`]`’ are the quotes themselves3, ‘`(`’ and ‘`)`’ (which M4 tries to match by pairs), and finally ‘`$`’ inside a macro definition.

In order to understand the delicate case of macro calls, we first have to present some obvious failures. Below they are “obvious-ified”, but when you find them in real life, they are usually in disguise.

Comments, introduced by a hash and running up to the newline, are opaque tokens to the top level: active characters are turned off, and there is no macro expansion:

    # define([def], ine)
    ⇒# define([def], ine)

Each time there can be a macro expansion, there is a quotation expansion, i.e., one level of quotes is stripped:

    int tab[10];
    ⇒int tab10;
    [int tab[10];]
    ⇒int tab[10];

Without this in mind, the reader might try hopelessly to use her macro `array`:

    define([array], [int tab[10];])
    array
    ⇒int tab10;
    [array]
    ⇒array

How can you correctly output the intended results4?

#### 8.1.2 One Macro Call ¶

Let’s proceed on the interaction between active characters and macros with this small macro, which just returns its first argument:

    define([car], [$1])

The two pairs of quotes above are not part of the arguments of `define`; rather, they are understood by the top level when it tries to find the arguments of `define`. Therefore, assuming `car` is not already defined, it is equivalent to write:

    define(car, $1)

But, while it is acceptable for a `configure.ac` to avoid unnecessary quotes, it is bad practice for Autoconf macros which must both be more robust and also advocate perfect style.

At the top level, there are only two possibilities: either you quote or you don’t:

    car(foo, bar, baz)
    ⇒foo
    [car(foo, bar, baz)]
    ⇒car(foo, bar, baz)

Let’s pay attention to the special characters:

    car(#)
    error→EOF in argument list

The closing parenthesis is hidden in the comment; with a hypothetical quoting, the top level understood it this way:

    car([#)]

Proper quotation, of course, fixes the problem:

    car([#])
    ⇒#

Here are more examples:

    car(foo, bar)
    ⇒foo
    car([foo, bar])
    ⇒foo, bar
    car((foo, bar))
    ⇒(foo, bar)
    car([(foo], [bar)])
    ⇒(foo
    define([a], [b])
    ⇒
    car(a)
    ⇒b
    car([a])
    ⇒b
    car([[a]])
    ⇒a
    car([[[a]]])
    ⇒[a]

#### 8.1.3 Quoting and Parameters ¶

When M4 encounters ‘`$`’ within a macro definition, followed immediately by a character it recognizes (‘`0`’…‘`9`’, ‘`#`’, ‘`@`’, or ‘`*`’), it will perform M4 parameter expansion. This happens regardless of how many layers of quotes the parameter expansion is nested within, or even if it occurs in text that will be rescanned as a comment.

    define([none], [$1])
    ⇒
    define([one], [[$1]])
    ⇒
    define([two], [[[$1]]])
    ⇒
    define([comment], [# $1])
    ⇒
    define([active], [ACTIVE])
    ⇒
    none([active])
    ⇒ACTIVE
    one([active])
    ⇒active
    two([active])
    ⇒[active]
    comment([active])
    ⇒# active

On the other hand, since autoconf generates shell code, you often want to output shell variable expansion, rather than performing M4 parameter expansion. To do this, you must use M4 quoting to separate the ‘`$`’ from the next character in the definition of your macro. If the macro definition occurs in single-quoted text, then insert another level of quoting; if the usage is already inside a double-quoted string, then split it into concatenated strings.

    define([foo], [a single-quoted $[]1 definition])
    ⇒
    define([bar], [[a double-quoted $][1 definition]])
    ⇒
    foo
    ⇒a single-quoted $1 definition
    bar
    ⇒a double-quoted $1 definition

POSIX states that M4 implementations are free to provide implementation extensions when ‘`${`’ is encountered in a macro definition. Autoconf reserves the longer sequence ‘`${{`’ for use with planned extensions that will be available in the future GNU M4 2.0, but guarantees that all other instances of ‘`${`’ will be output literally. Therefore, this idiom can also be used to output shell code parameter references:

    define([first], [${1}])first
    ⇒${1}

POSIX also states that ‘`$11`’ should expand to the first parameter concatenated with a literal ‘`1`’, although some versions of GNU M4 expand the eleventh parameter instead. For portability, you should only use single-digit M4 parameter expansion.

With this in mind, we can explore the cases where macros invoke macros...

#### 8.1.4 Quotation and Nested Macros ¶

The examples below use the following macros:

    define([car], [$1])
    define([active], [ACT, IVE])
    define([array], [int tab[10]])

Each additional embedded macro call introduces other possible interesting quotations:

    car(active)
    ⇒ACT
    car([active])
    ⇒ACT, IVE
    car([[active]])
    ⇒active

In the first case, the top level looks for the arguments of `car`, and finds ‘`active`’. Because M4 evaluates its arguments before applying the macro, ‘`active`’ is expanded, which results in:

    car(ACT, IVE)
    ⇒ACT

In the second case, the top level gives ‘`active`’ as first and only argument of `car`, which results in:

    active
    ⇒ACT, IVE

i.e., the argument is evaluated after the macro that invokes it. In the third case, `car` receives ‘`[active]`’, which results in:

    [active]
    ⇒active

exactly as we already saw above.

The example above, applied to a more realistic example, gives:

    car(int tab[10];)
    ⇒int tab10;
    car([int tab[10];])
    ⇒int tab10;
    car([[int tab[10];]])
    ⇒int tab[10];

Huh? The first case is easily understood, but why is the second wrong, and the third right? To understand that, you must know that after M4 expands a macro, the resulting text is immediately subjected to macro expansion and quote removal. This means that the quote removal occurs twice—first before the argument is passed to the `car` macro, and second after the `car` macro expands to the first argument.

As the author of the Autoconf macro `car`, you then consider it to be incorrect that your users have to double-quote the arguments of `car`, so you “fix” your macro. Let’s call it `qar` for quoted car:

    define([qar], [[$1]])

and check that `qar` is properly fixed:

    qar([int tab[10];])
    ⇒int tab[10];

Ahhh! That’s much better.

But note what you’ve done: now that the result of `qar` is always a literal string, the only time a user can use nested macros is if she relies on an unquoted macro call:

    qar(active)
    ⇒ACT
    qar([active])
    ⇒active

leaving no way for her to reproduce what she used to do with `car`:

    car([active])
    ⇒ACT, IVE

Worse yet: she wants to use a macro that produces a set of `cpp` macros:

    define([my_includes], [#include <stdio.h>])
    car([my_includes])
    ⇒#include <stdio.h>
    qar(my_includes)
    error→EOF in argument list

This macro, `qar`, because it double quotes its arguments, forces its users to leave their macro calls unquoted, which is dangerous. Commas and other active symbols are interpreted by M4 before they are given to the macro, often not in the way the users expect. Also, because `qar` behaves differently from the other macros, it’s an exception that should be avoided in Autoconf.

#### 8.1.5 `changequote` is Evil ¶

The temptation is often high to bypass proper quotation, in particular when it’s late at night. Then, many experienced Autoconf hackers finally surrender to the dark side of the force and use the ultimate weapon: `changequote`.

The M4 builtin `changequote` belongs to a set of primitives that allow one to adjust the syntax of the language to adjust it to one’s needs. For instance, by default M4 uses ‘```’ and ‘`'`’ as quotes, but in the context of shell programming (and actually of most programming languages), that’s about the worst choice one can make: because of strings and back-quoted expressions in shell code (such as ‘`'this'`’ and ‘``that``’), and because of literal characters in usual programming languages (as in ‘`'0'`’), there are many unbalanced ‘```’ and ‘`'`’. Proper M4 quotation then becomes a nightmare, if not impossible. In order to make M4 useful in such a context, its designers have equipped it with `changequote`, which makes it possible to choose another pair of quotes. M4sugar, M4sh, Autoconf, and Autotest all have chosen to use ‘`[`’ and ‘`]`’. Not especially because they are unlikely characters, but because they are characters unlikely to be unbalanced.

There are other magic primitives, such as `changecom` to specify what syntactic forms are comments (it is common to see ‘`changecom(<!--, -->)`’ when M4 is used to produce HTML pages), `changeword` and `changesyntax` to change other syntactic details (such as the character to denote the nth argument, ‘`$`’ by default, the parentheses around arguments, etc.).

These primitives are really meant to make M4 more useful for specific domains: they should be considered like command line options: `--quotes`, `--comments`, `--words`, and `--syntax`. Nevertheless, they are implemented as M4 builtins, as it makes M4 libraries self contained (no need for additional options).

There lies the problem...

The problem is that it is then tempting to use them in the middle of an M4 script, as opposed to its initialization. This, if not carefully thought out, can lead to disastrous effects: you are changing the language in the middle of the execution. Changing and restoring the syntax is often not enough: if you happened to invoke macros in between, these macros are lost, as the current syntax is probably not the one they were implemented with.

#### 8.1.6 Quadrigraphs ¶

When writing an Autoconf macro you may occasionally need to generate special characters that are difficult to express with the standard Autoconf quoting rules. For example, you may need to output the regular expression ‘`[^[]`’, which matches any character other than ‘`[`’. This expression contains unbalanced brackets so it cannot be put easily into an M4 macro.

Additionally, there are a few m4sugar macros (such as `m4_split` and `m4_expand`) which internally use special markers in addition to the regular quoting characters. If the arguments to these macros contain the literal strings ‘`-=<{(`’ or ‘`)}>=-`’, the macros might behave incorrectly.

You can work around these problems by using one of the following quadrigraphs:

‘`@<:@`’

  ‘`[`’

‘`@:>@`’

  ‘`]`’

‘`@S|@`’

  ‘`$`’

‘`@%:@`’

  ‘`#`’

‘`@{:@`’

  ‘`(`’

‘`@:}@`’

  ‘`)`’

‘`@&t@`’

  Expands to nothing.

Quadrigraphs are replaced at a late stage of the translation process, after `m4` is run, so they do not get in the way of M4 quoting. For example, the string ‘`^@<:@`’, independently of its quotation, appears as ‘`^[`’ in the output.

The empty quadrigraph can be used:

- to mark trailing spaces explicitly

  Trailing spaces are smashed by `autom4te`. This is a feature.

- to produce quadrigraphs and other strings reserved by m4sugar

  For instance ‘`@<@&t@:@`’ produces ‘`@<:@`’. For a more contrived example:

      m4_define([a], [A])m4_define([b], [B])m4_define([c], [C])dnl
      m4_split([a )}>=- b -=<{( c])
      ⇒[a], [], [B], [], [c]
      m4_split([a )}@&t@>=- b -=<@&t@{( c])
      ⇒[a], [)}>=-], [b], [-=<{(], [c]

- to escape occurrences of forbidden patterns

  For instance you might want to mention `AC_FOO` in a comment, while still being sure that `autom4te` still catches unexpanded ‘`AC_*`’. Then write ‘`AC@&t@_FOO`’.

The name ‘`@&t@`’ was suggested by Paul Eggert:

> I should give some credit to the ‘`@&t@`’ pun. The ‘`&`’ is my own invention, but the ‘`t`’ came from the source code of the ALGOL68C compiler, written by Steve Bourne (of Bourne shell fame), and which used ‘`mt`’ to denote the empty string. In C, it would have looked like something like:

>     char const mt[] = "";

> but of course the source code was written in Algol 68.

> I don’t know where he got ‘`mt`’ from: it could have been his own invention, and I suppose it could have been a common pun around the Cambridge University computer lab at the time.

#### 8.1.7 Dealing with unbalanced parentheses ¶

One of the pitfalls of portable shell programming is that if you intend your script to run with obsolescent shells, `case` statements require unbalanced parentheses. See Limitations of Shell Builtins. With syntax highlighting editors, the presence of unbalanced ‘`)`’ can interfere with editors that perform syntax highlighting of macro contents based on finding the matching ‘`(`’. Another concern is how much editing must be done when transferring code snippets between shell scripts and macro definitions. But most importantly, the presence of unbalanced parentheses can introduce expansion bugs.

For an example, here is an underquoted attempt to use the macro `my_case`, which happens to expand to a portable `case` statement:

    AC_DEFUN([my_case],
    [case $file_name in
      *.c) file_type='C source code';;
    esac])
    AS_IF(:, my_case)

In the above example, the `AS_IF` call under-quotes its arguments. As a result, the unbalanced ‘`)`’ generated by the premature expansion of `my_case` results in expanding `AS_IF` with a truncated parameter, and the expansion is syntactically invalid:

    if :
    then :
      case $file_name in
      *.c
    fi file_type='C source code';;
    esac)

If nothing else, this should emphasize the importance of the quoting arguments to macro calls. On the other hand, there are several variations for defining `my_case` to be more robust, even when used without proper quoting, each with some benefits and some drawbacks.

- Use left parenthesis before pattern

      AC_DEFUN([my_case],
      [case $file_name in
        (*.c) file_type='C source code';;
      esac])

  This is simple and provides balanced parentheses. Although this is not portable to obsolescent shells (notably Solaris 10 `/bin/sh`), platforms with these shells invariably have a more-modern shell available somewhere so this approach typically suffices nowadays.

- Creative literal shell comment

      AC_DEFUN([my_case],
      [case $file_name in #(
        *.c) file_type='C source code';;
      esac])

  This version provides balanced parentheses to several editors, and can be copied and pasted into a terminal as is. Unfortunately, it is still unbalanced as an Autoconf argument, since ‘`#(`’ is an M4 comment that masks the normal properties of ‘`(`’.

- Quadrigraph shell comment

      AC_DEFUN([my_case],
      [case $file_name in @%:@(
        *.c) file_type='C source code';;
      esac])

  This version provides balanced parentheses to even more editors, and can be used as a balanced Autoconf argument. Unfortunately, it requires some editing before it can be copied and pasted into a terminal, and the use of the quadrigraph ‘`@%:@`’ for ‘`#`’ reduces readability.

- Quoting just the parenthesis

      AC_DEFUN([my_case],
      [case $file_name in
        *.c[)] file_type='C source code';;
      esac])

  This version quotes the ‘`)`’, so that it can be used as a balanced Autoconf argument. As written, this is not balanced to an editor, but it can be coupled with ‘`[#(]`’ to meet that need, too. However, it still requires some edits before it can be copied and pasted into a terminal.

- Double-quoting the entire statement

      AC_DEFUN([my_case],
      [[case $file_name in #(
        *.c) file_type='C source code';;
      esac]])

  Since the entire macro is double-quoted, there is no problem with using this as an Autoconf argument; and since the double-quoting is over the entire statement, this code can be easily copied and pasted into a terminal. However, the double quoting prevents the expansion of any macros inside the case statement, which may cause its own set of problems.

- Using `AS_CASE`

      AC_DEFUN([my_case],
      [AS_CASE([$file_name],
        [*.c], [file_type='C source code'])])

  This version avoids the balancing issue altogether, by relying on `AS_CASE` (see Common Shell Constructs); it also allows for the expansion of `AC_REQUIRE` to occur prior to the entire case statement, rather than within a branch of the case statement that might not be taken. However, the abstraction comes with a penalty that it is no longer a quick copy, paste, and edit to get back to shell code.

#### 8.1.8 Quotation Rule Of Thumb ¶

To conclude, the quotation rule of thumb is:

One pair of quotes per pair of parentheses.

Never over-quote, never under-quote, in particular in the definition of macros. In the few places where the macros need to use brackets (usually in C program text or regular expressions), properly quote the arguments!

It is common to read Autoconf programs with snippets like:

    AC_TRY_LINK(
    changequote(<<, >>)dnl
    <<#include <time.h>
    #ifndef tzname /* For SGI.  */
    extern char *tzname[]; /* RS6000 and others reject char **tzname.  */
    #endif>>,
    changequote([, ])dnl
    [atoi (*tzname);], ac_cv_var_tzname=yes, ac_cv_var_tzname=no)

which is incredibly useless since `AC_TRY_LINK` is already double quoting, so you just need:

    AC_TRY_LINK(
    [#include <time.h>
    #ifndef tzname /* For SGI.  */
    extern char *tzname[]; /* RS6000 and others reject char **tzname.  */
    #endif],
                [atoi (*tzname);],
                [ac_cv_var_tzname=yes],
                [ac_cv_var_tzname=no])

The M4-fluent reader might note that these two examples are rigorously equivalent, since M4 swallows both the ‘`changequote(<<, >>)`’ and ‘`<<`’ ‘`>>`’ when it collects the arguments: these quotes are not part of the arguments!

Simplified, the example above is just doing this:

    changequote(<<, >>)dnl
    <<[]>>
    changequote([, ])dnl

instead of simply:

    [[]]

With macros that do not double quote their arguments (which is the rule), double-quote the (risky) literals:

    AC_LINK_IFELSE([AC_LANG_PROGRAM(
    [[#include <time.h>
    #ifndef tzname /* For SGI.  */
    extern char *tzname[]; /* RS6000 and others reject char **tzname.  */
    #endif]],
                                    [atoi (*tzname);])],
                   [ac_cv_var_tzname=yes],
                   [ac_cv_var_tzname=no])

Please note that the macro `AC_TRY_LINK` is obsolete, so you really should be using `AC_LINK_IFELSE` instead.

See Quadrigraphs, for what to do if you run into a hopeless case where quoting does not suffice.

When you create a `configure` script using newly written macros, examine it carefully to check whether you need to add more quotes in your macros. If one or more words have disappeared in the M4 output, you need more quotes. When in doubt, quote.

However, it’s also possible to put on too many layers of quotes. If this happens, the resulting `configure` script may contain unexpanded macros. The `autoconf` program checks for this problem by looking for the string ‘`AC_`’ in `configure`. However, this heuristic does not work in general: for example, it does not catch overquoting in `AC_DEFINE` descriptions.

### 8.2 Using `autom4te` ¶

The Autoconf suite, including M4sugar, M4sh, and Autotest, in addition to Autoconf per se, heavily rely on M4. All these different uses revealed common needs factored into a layer over M4: `autom4te` 5.

`autom4te` is a preprocessor that is like `m4`. It supports M4 extensions designed for use in tools like Autoconf.

#### 8.2.1 Invoking `autom4te` ¶

The command line arguments are modeled after M4’s:

    autom4te options files

where the files are directly passed to `m4`. By default, GNU M4 is found during configuration, but the environment variable `M4` can be set to tell `autom4te` where to look. In addition to the regular expansion, it handles the replacement of the quadrigraphs (see Quadrigraphs), and of ‘`__oline__`’, the current line in the output. It supports an extended syntax for the files:

`file.m4f`

  This file is an M4 frozen file. Note that all the previous files are ignored. See the `--melt` option for the rationale.

`file?`

  If found in the library path, the file is included for expansion, otherwise it is ignored instead of triggering a failure.

Of course, it supports the Autoconf common subset of options:

`--help`
`-h`

  Print a summary of the command line options and exit.

`--version`
`-V`

  Print the version number of Autoconf and exit.

`--verbose`
`-v`

  Report processing steps.

`--debug`
`-d`

  Don’t remove the temporary files and be even more verbose.

`--include=dir`
`-I dir`

  Also look for input files in dir. Multiple invocations accumulate.

`--output=file`
`-o file`

  Save output (script or trace) to file. The file `-` stands for the standard output.

As an extension of `m4`, it includes the following options:

`--warnings=category[,category...]` ¶
`-Wcategory[,category...]`

  Enable or disable warnings related to each category. See m4_warn, for a comprehensive list of categories. Special values include:

  ‘`all`’

    Enable all categories of warnings.

  ‘`none`’

    Disable all categories of warnings.

  ‘`error`’

    Treat all warnings as errors.

  ‘`no-category`’

    Disable warnings falling into category.

  The environment variable `WARNINGS` may also be set to a comma-separated list of warning categories to enable or disable. It is interpreted exactly the same way as the argument of `--warnings`, but unknown categories are silently ignored. The command line takes precedence; for instance, if `WARNINGS` is set to `obsolete`, but `-Wnone` is given on the command line, no warnings will be issued.

  Some categories of warnings are on by default. Again, for details see m4_warn.

`--melt`
`-M`

  Do not use frozen files. Any argument `file.m4f` is replaced by `file.m4`. This helps tracing the macros which are executed only when the files are frozen, typically `m4_define`. For instance, running:

      autom4te --melt 1.m4 2.m4f 3.m4 4.m4f input.m4

  is roughly equivalent to running:

      m4 1.m4 2.m4 3.m4 4.m4 input.m4

  while

      autom4te 1.m4 2.m4f 3.m4 4.m4f input.m4

  is equivalent to:

      m4 --reload-state=4.m4f input.m4

`--freeze`
`-F`

  Produce a frozen state file. `autom4te` freezing is stricter than M4’s: it must produce no warnings, and no output other than empty lines (a line with white space is not empty) and comments (starting with ‘`#`’). Unlike `m4`’s similarly-named option, this option takes no argument:

      autom4te 1.m4 2.m4 3.m4 --freeze --output=3.m4f

  corresponds to

      m4 1.m4 2.m4 3.m4 --freeze-state=3.m4f

`--mode=octal-mode`
`-m octal-mode`

  Set the mode of the non-traces output to octal-mode; by default ‘`0666`’.

As another additional feature over `m4`, `autom4te` caches its results. GNU M4 is able to produce a regular output and traces at the same time. Traces are heavily used in the GNU Build System: `autoheader` uses them to build `config.h.in`, `autoreconf` to determine what GNU Build System components are used, `automake` to “parse” `configure.ac` etc. To avoid recomputation, traces are cached while performing regular expansion, and conversely. This cache is (actually, the caches are) stored in the directory `autom4te.cache`. It can safely be removed at any moment (especially if for some reason `autom4te` considers it trashed).

`--cache=directory`
`-C directory`

  Specify the name of the directory where the result should be cached. Passing an empty value disables caching. Be sure to pass a relative file name, as for the time being, global caches are not supported.

`--no-cache`

  Don’t cache the results.

`--force`
`-f`

  If a cache is used, consider it obsolete (but update it anyway).

Because traces are so important to the GNU Build System, `autom4te` provides high level tracing features as compared to M4, and helps exploiting the cache:

`--trace=macro[:format]`
`-t macro[:format]`

  Trace the invocations of macro according to the format. Multiple `--trace` arguments can be used to list several macros. Multiple `--trace` arguments for a single macro are not cumulative; instead, you should just make format as long as needed.

  The format is a regular string, with newlines if desired, and several special escape codes. It defaults to ‘`$f:$l:$n:$%`’. It can use the following special escapes:

  ‘`$$`’

    The character ‘`$`’.

  ‘`$f`’

    The file name from which macro is called.

  ‘`$l`’

    The line number from which macro is called.

  ‘`$d`’

    The depth of the macro call. This is an M4 technical detail that you probably don’t want to know about.

  ‘`$n`’

    The name of the macro.

  ‘`$num`’

    The numth argument of the call to macro.

  ‘`$@`’
  ‘`$sep@`’
  ‘`${separator}@`’

    All the arguments passed to macro, separated by the character sep or the string separator (‘`,`’ by default). Each argument is quoted, i.e., enclosed in a pair of square brackets.

  ‘`$*`’
  ‘`$sep*`’
  ‘`${separator}*`’

    As above, but the arguments are not quoted.

  ‘`$%`’
  ‘`$sep%`’
  ‘`${separator}%`’

    As above, but the arguments are not quoted, all new line characters in the arguments are smashed, and the default separator is ‘`:`’.

    The escape ‘`$%`’ produces single-line trace outputs (unless you put newlines in the ‘`separator`’), while ‘`$@`’ and ‘`$*`’ do not.

  See Using `autoconf` to Create `configure`, for examples of trace uses.

`--preselect=macro`
`-p macro`

  Cache the traces of macro, but do not enable traces. This is especially important to save CPU cycles in the future. For instance, when invoked, `autoconf` pre-selects all the macros that `autoheader`, `automake`, `autoreconf`, etc., trace, so that running `m4` is not needed to trace them: the cache suffices. This results in a huge speed-up.

Finally, `autom4te` introduces the concept of Autom4te libraries. They consists in a powerful yet extremely simple feature: sets of combined command line arguments:

`--language=language`
`-l language`

  Use the language Autom4te library. Current languages include:

  `M4sugar`

    create M4sugar output.

  `M4sh`

    create M4sh executable shell scripts.

  `Autotest`

    create Autotest executable test suites.

  `Autoconf-without-aclocal-m4`

    create Autoconf executable configure scripts without reading `aclocal.m4`.

  `Autoconf`

    create Autoconf executable configure scripts. This language inherits all the characteristics of `Autoconf-without-aclocal-m4` and additionally reads `aclocal.m4`.

`--prepend-include=dir`
`-B dir`

  Prepend directory dir to the search path. This is used to include the language-specific files before any third-party macros.

As an example, if Autoconf is installed in its default location, `/usr/local`, the command ‘`autom4te -l m4sugar foo.m4`’ is strictly equivalent to the command:

    autom4te --prepend-include /usr/local/share/autoconf \
      m4sugar/m4sugar.m4f foo.m4

Recursive expansion applies here: the command ‘`autom4te -l m4sh foo.m4`’ is the same as ‘`autom4te --language M4sugar m4sugar/m4sh.m4f foo.m4`’, i.e.:

    autom4te --prepend-include /usr/local/share/autoconf \
      m4sugar/m4sugar.m4f m4sugar/m4sh.m4f --mode 777 foo.m4

The definition of the languages is stored in `autom4te.cfg`.

#### 8.2.2 Customizing `autom4te` ¶

One can customize `autom4te` via `~/.autom4te.cfg` (i.e., as found in the user home directory), and `./.autom4te.cfg` (i.e., as found in the directory from which `autom4te` is run). The order is first reading `autom4te.cfg`, then `~/.autom4te.cfg`, then `./.autom4te.cfg`, and finally the command line arguments.

In these text files, comments are introduced with `#`, and empty lines are ignored. Customization is performed on a per-language basis, wrapped in between a ‘`begin-language: "language"`’, ‘`end-language: "language"`’ pair.

Customizing a language stands for appending options (see Invoking `autom4te`) to the current definition of the language. Options, and more generally arguments, are introduced by ‘`args: arguments`’. You may use the traditional shell syntax to quote the arguments.

As an example, to disable Autoconf caches (`autom4te.cache`) globally, include the following lines in `~/.autom4te.cfg`:

    ## ------------------ ##
    ## User Preferences.  ##
    ## ------------------ ##

    begin-language: "Autoconf-without-aclocal-m4"
    args: --no-cache
    end-language: "Autoconf-without-aclocal-m4"

### 8.3 Programming in M4sugar ¶

M4 by itself provides only a small, but sufficient, set of all-purpose macros. M4sugar introduces additional generic macros. Its name was coined by Lars J. Aas: “Readability And Greater Understanding Stands 4 M4sugar”.

M4sugar reserves the macro namespace ‘`^_m4_`’ for internal use, and the macro namespace ‘`^m4_`’ for M4sugar macros. You should not define your own macros into these namespaces.

#### 8.3.1 Redefined M4 Macros ¶

With a few exceptions, all the M4 native macros are moved in the ‘`m4_`’ pseudo-namespace, e.g., M4sugar renames `define` as `m4_define` etc.

The list of macros unchanged from M4, except for their name, is:

- m4_builtin
- m4_changecom
- m4_changequote
- m4_debugfile
- m4_debugmode
- m4_decr
- m4_define
- m4_divnum
- m4_errprint
- m4_esyscmd
- m4_eval
- m4_format
- m4_ifdef
- m4_incr
- m4_index
- m4_indir
- m4_len
- m4_pushdef
- m4_shift
- m4_substr
- m4_syscmd
- m4_sysval
- m4_traceoff
- m4_traceon
- m4_translit

Some M4 macros are redefined, and are slightly incompatible with their native equivalent.

Macro: __file__ ¶
Macro: __line__ ¶

  All M4 macros starting with ‘`__`’ retain their original name: for example, no `m4__file__` is defined.

Macro: __oline__ ¶

  This is not technically a macro, but a feature of Autom4te. The sequence `__oline__` can be used similarly to the other m4sugar location macros, but rather than expanding to the location of the input file, it is translated to the line number where it appears in the output file after all other M4 expansions.

Macro: dnl ¶

  This macro kept its original name: no `m4_dnl` is defined.

Macro: m4_bpatsubst(string, regexp, [replacement]) ¶

  This is the M4sugar name for the M4 native macro `patsubst`.

  It uses the same regular expression syntax as `m4_bregexp`; see m4_bregexp. Also see Substituting text by regular expression in GNU m4 macro processor, for details of how the replacement is processed.

  The name `m4_patsubst` is reserved for use by a future version of M4sugar, when there is a version of GNU M4 that supports POSIX extended regular expression syntax.

Macro: m4_bregexp(string, regexp, [replacement]) ¶

  This is the M4sugar name for the M4 native macro `regexp`.

  The only regular expression syntax understood by GNU M4 is similar to, but not the same as, POSIX basic regular expression syntax. See Searching for regular expressions in GNU m4 macro processor, for the precise syntax.

  The name `m4_regexp` is reserved for use by a future version of M4sugar, when there is a version of GNU M4 that supports POSIX extended regular expression syntax.

Macro: m4_copy(source, dest) ¶
Macro: m4_copy_force(source, dest) ¶
Macro: m4_rename(source, dest) ¶
Macro: m4_rename_force(source, dest) ¶

  These macros aren’t directly builtins, but are closely related to `m4_pushdef` and `m4_defn`. `m4_copy` and `m4_rename` ensure that dest is undefined, while `m4_copy_force` and `m4_rename_force` overwrite any existing definition. All four macros then proceed to copy the entire pushdef stack of definitions of source over to dest. `m4_copy` and `m4_copy_force` preserve the source (including in the special case where source is undefined), while `m4_rename` and `m4_rename_force` undefine the original macro name (making it an error to rename an undefined source).

  Note that attempting to invoke a renamed macro might not work, since the macro may have a dependence on helper macros accessed via composition of ‘`$0`’ but that were not also renamed; likewise, other macros may have a hard-coded dependence on source and could break if source has been deleted. On the other hand, it is always safe to rename a macro to temporarily move it out of the way, then rename it back later to restore original semantics.

Macro: m4_defn(macro…) ¶

  This macro fails if macro is not defined, even when using older versions of M4 that did not warn. See `m4_undefine`. Unfortunately, in order to support these older versions of M4, there are some situations involving unbalanced quotes where concatenating multiple macros together will work in newer M4 but not in m4sugar; use quadrigraphs to work around this.

Macro: m4_divert(diversion) ¶

  M4sugar relies heavily on diversions, so rather than behaving as a primitive, `m4_divert` behaves like:

      m4_divert_pop()m4_divert_push([diversion])

  See Diversion support, for more details about the use of the diversion stack. In particular, this implies that diversion should be a named diversion rather than a raw number. But be aware that it is seldom necessary to explicitly change the diversion stack, and that when done incorrectly, it can lead to syntactically invalid scripts.

Macro: m4_dumpdef(name…) ¶
Macro: m4_dumpdefs(name…) ¶

  `m4_dumpdef` is like the M4 builtin, except that this version requires at least one argument, output always goes to standard error rather than the current debug file, no sorting is done on multiple arguments, and an error is issued if any name is undefined. `m4_dumpdefs` is a convenience macro that calls `m4_dumpdef` for all of the `m4_pushdef` stack of definitions, starting with the current, and silently does nothing if name is undefined.

  Unfortunately, due to a limitation in M4 1.4.x, any macro defined as a builtin is output as the empty string. This behavior is rectified by using M4 1.6 or newer. However, this behavior difference means that `m4_dumpdef` should only be used while developing m4sugar macros, and never in the final published form of a macro.

Macro: m4_esyscmd_s(command) ¶

  Like `m4_esyscmd`, this macro expands to the result of running command in a shell. The difference is that any trailing newlines are removed, so that the output behaves more like shell command substitution.

Macro: m4_exit(exit-status) ¶

  This macro corresponds to `m4exit`.

Macro: m4_if(comment) ¶
Macro: m4_if(string-1, string-2, equal, [not-equal]) ¶
Macro: m4_if(string-1, string-2, equal-1, string-3, string-4, equal-2, …, [not-equal]) ¶

  This macro corresponds to `ifelse`. string-1 and string-2 are compared literally, so usually one of the two arguments is passed unquoted. See Conditional constructs, for more conditional idioms.

Macro: m4_include(file) ¶
Macro: m4_sinclude(file) ¶

  Like the M4 builtins, but warn against multiple inclusions of file.

Macro: m4_mkstemp(template) ¶
Macro: m4_maketemp(template) ¶

  Older versions of POSIX specified a macro `maketemp`, which replaced the trailing ‘`X`’ characters in template with the process id, without regards to the existence of a file by that name. This was a security hole, so current POSIX instead specifies a macro `mkstemp` that always creates a uniquely named file. In M4sugar, `m4_maketemp` and `m4_mkstemp` are synonyms for each other, and both have the secure semantics regardless of which macro the underlying M4 provides.

Macro: m4_popdef(macro…) ¶

  This macro fails if macro is not defined, even when using older versions of M4 that did not warn. See `m4_undefine`.

Macro: m4_undefine(macro…) ¶

  This macro fails if macro is not defined, even when using older versions of M4 that did not warn. Use

      m4_ifdef([macro], [m4_undefine([macro])])

  if you are not sure whether macro is defined.

Macro: m4_undivert(diversion…) ¶

  Unlike the M4 builtin, at least one diversion must be specified. Also, since the M4sugar diversion stack prefers named diversions, the use of `m4_undivert` to include files is risky. See Diversion support, for more details about the use of the diversion stack. But be aware that it is seldom necessary to explicitly change the diversion stack, and that when done incorrectly, it can lead to syntactically invalid scripts.

Macro: m4_wrap(text) ¶
Macro: m4_wrap_lifo(text) ¶

  These macros correspond to `m4wrap`. POSIX requires arguments of multiple wrap calls to be reprocessed at EOF in the same order as the original calls (first-in, first-out). GNU M4 versions through 1.4.10, however, reprocess them in reverse order (last-in, first-out). Both orders are useful, therefore, you can rely on `m4_wrap` to provide FIFO semantics and `m4_wrap_lifo` for LIFO semantics, regardless of the underlying GNU M4 version.

  Unlike the GNU M4 builtin, these macros only recognize one argument, and avoid token pasting between consecutive invocations. On the other hand, nested calls to `m4_wrap` from within wrapped text work just as in the builtin.

#### 8.3.2 Diagnostic messages from M4sugar ¶

When macros statically diagnose abnormal situations, benign or fatal, they should report them using these macros. For issuing dynamic issues, i.e., when `configure` is run, see Printing Messages.

Macro: m4_assert(expression, [exit-status = ‘`1`’]) ¶

  Assert that the arithmetic expression evaluates to non-zero. Otherwise, issue a fatal error, and exit `autom4te` with exit-status.

Macro: m4_errprintn(message) ¶

  Similar to the builtin `m4_errprint`, except that a newline is guaranteed after message.

Macro: m4_fatal(message) ¶

  Report a severe error message prefixed with the current location, and have `autom4te` die.

Macro: m4_location ¶

  Useful as a prefix in a message line. Short for:

      __file__:__line__

Macro: m4_warn(category, message) ¶

  Report message as a warning (or as an error if requested by the user) if warnings of the category are turned on. If the message is emitted, it is prefixed with the current location, and followed by a call trace of all macros defined via `AC_DEFUN` used to get to the current expansion.

  The category must be one of:

  ‘`cross`’

    Warnings about constructs that may interfere with cross-compilation, such as using `AC_RUN_IFELSE` without a default.

  ‘`gnu`’

    Warnings related to the GNU Coding Standards (see The GNU Coding Standards). On by default.

  ‘`obsolete`’

    Warnings about obsolete features. On by default.

  ‘`override`’

    Warnings about redefinitions of Autoconf internals.

  ‘`portability`’

    Warnings about non-portable constructs.

  ‘`portability-recursive`’

    Warnings about recursive Make variable expansions (`$(foo$(x))`).

  ‘`extra-portability`’

    Extra warnings about non-portable constructs, covering rarely-used tools.

  ‘`syntax`’

    Warnings about questionable syntactic constructs, incorrectly ordered macro calls, typos, etc. On by default.

  ‘`unsupported`’

    Warnings about unsupported features. On by default.

  Hacking Note: The set of categories is defined by code in `autom4te`, not by M4sugar itself. Additions should be coordinated with Automake, so that both sets of tools accept the same options.

#### 8.3.3 Diversion support ¶

M4sugar makes heavy use of diversions under the hood, because it is often the case that text that must appear early in the output is not discovered until late in the input. Additionally, some of the topological sorting algorithms used in resolving macro dependencies use diversions. However, most macros should not need to change diversions directly, but rather rely on higher-level M4sugar macros to manage diversions transparently. If you change diversions improperly, you risk generating a syntactically invalid script, because an incorrect diversion will violate assumptions made by many macros about whether prerequisite text has been previously output. In short, if you manually change the diversion, you should not expect any macros provided by the Autoconf package to work until you have restored the diversion stack back to its original state.

In the rare case that it is necessary to write a macro that explicitly outputs text to a different diversion, it is important to be aware of an M4 limitation regarding diversions: text only goes to a diversion if it is not part of argument collection. Therefore, any macro that changes the current diversion cannot be used as an unquoted argument to another macro, but must be expanded at the top level. The macro `m4_expand` will diagnose any attempt to change diversions, since it is generally useful only as an argument to another macro. The following example shows what happens when diversion manipulation is attempted within macro arguments:

    m4_do([normal text]
    m4_divert_push([KILL])unwanted[]m4_divert_pop([KILL])
    [m4_divert_push([KILL])discarded[]m4_divert_pop([KILL])])dnl
    ⇒normal text
    ⇒unwanted

Notice that the unquoted text `unwanted` is output, even though it was processed while the current diversion was `KILL`, because it was collected as part of the argument to `m4_do`. However, the text `discarded` disappeared as desired, because the diversion changes were single-quoted, and were not expanded until the top-level rescan of the output of `m4_do`.

To make diversion management easier, M4sugar uses the concept of named diversions. Rather than using diversion numbers directly, it is nicer to associate a name with each diversion. The diversion number associated with a particular diversion name is an implementation detail, and a syntax warning is issued if a diversion number is used instead of a name. In general, you should not output text to a named diversion until after calling the appropriate initialization routine for your language (`m4_init`, `AS_INIT`, `AT_INIT`, …), although there are some exceptions documented below.

M4sugar defines two named diversions.

`KILL`

  Text written to this diversion is discarded. This is the default diversion once M4sugar is initialized.

`GROW`

  This diversion is used behind the scenes by topological sorting macros, such as `AC_REQUIRE`.

M4sh adds several more named diversions.

`BINSH`

  This diversion is reserved for the ‘`#!`’ interpreter line.

`HEADER-REVISION`

  This diversion holds text from `AC_REVISION`.

`HEADER-COMMENT`

  This diversion holds comments about the purpose of a file.

`HEADER-COPYRIGHT`

  This diversion is managed by `AC_COPYRIGHT`.

`M4SH-SANITIZE`

  This diversion contains M4sh sanitization code, used to ensure M4sh is executing in a reasonable shell environment.

`M4SH-INIT`

  This diversion contains M4sh initialization code, initializing variables that are required by other M4sh macros.

`BODY`

  This diversion contains the body of the shell code, and is the default diversion once M4sh is initialized.

Autotest inherits diversions from M4sh, and changes the default diversion from `BODY` back to `KILL`. It also adds several more named diversions, with the following subset designed for developer use.

`PREPARE_TESTS`

  This diversion contains initialization sequences which are executed after `atconfig` and `atlocal`, and after all command line arguments have been parsed, but prior to running any tests. It can be used to set up state that is required across all tests. This diversion will work even before `AT_INIT`.

Autoconf inherits diversions from M4sh, and adds the following named diversions which developers can utilize.

`DEFAULTS`

  This diversion contains shell variable assignments to set defaults that must be in place before arguments are parsed. This diversion is placed early enough in `configure` that it is unsafe to expand any autoconf macros into this diversion.

`HELP_ENABLE`

  If `AC_PRESERVE_HELP_ORDER` was used, then text placed in this diversion will be included as part of a quoted here-doc providing all of the `--help` output of `configure` related to options created by `AC_ARG_WITH` and `AC_ARG_ENABLE`.

`INIT_PREPARE`

  This diversion occurs after all command line options have been parsed, but prior to the main body of the `configure` script. This diversion is the last chance to insert shell code such as variable assignments or shell function declarations that will used by the expansion of other macros.

For now, the remaining named diversions of Autoconf, Autoheader, and Autotest are not documented. In other words, intentionally outputting text into an undocumented diversion is subject to breakage in a future release of Autoconf.

Macro: m4_cleardivert(diversion…) ¶

  Permanently discard any text that has been diverted into diversion.

Macro: m4_divert_once(diversion, [content]) ¶

  Similar to `m4_divert_text`, except that content is only output to diversion if this is the first time that `m4_divert_once` has been called with its particular arguments.

Macro: m4_divert_pop([diversion]) ¶

  If provided, check that the current diversion is indeed diversion. Then change to the diversion located earlier on the stack, giving an error if an attempt is made to pop beyond the initial m4sugar diversion of `KILL`.

Macro: m4_divert_push(diversion) ¶

  Remember the former diversion on the diversion stack, and output subsequent text into diversion. M4sugar maintains a diversion stack, and issues an error if there is not a matching pop for every push.

Macro: m4_divert_text(diversion, [content]) ¶

  Output content and a newline into diversion, without affecting the current diversion. Shorthand for:

      m4_divert_push([diversion])content
      m4_divert_pop([diversion])dnl

  One use of `m4_divert_text` is to develop two related macros, where macro ‘`MY_A`’ does the work, but adjusts what work is performed based on whether the optional macro ‘`MY_B`’ has also been expanded. Of course, it is possible to use `AC_BEFORE` within `MY_A` to require that ‘`MY_B`’ occurs first, if it occurs at all. But this imposes an ordering restriction on the user; it would be nicer if macros ‘`MY_A`’ and ‘`MY_B`’ can be invoked in either order. The trick is to let ‘`MY_B`’ leave a breadcrumb in an early diversion, which ‘`MY_A`’ can then use to determine whether ‘`MY_B`’ has been expanded.

      AC_DEFUN([MY_A],
      [# various actions
      if test -n "$b_was_used"; then
        # extra action
      fi])
      AC_DEFUN([MY_B],
      [AC_REQUIRE([MY_A])dnl
      m4_divert_text([INIT_PREPARE], [b_was_used=true])])

Macro: m4_init ¶

  Initialize the M4sugar environment, setting up the default named diversion to be `KILL`.

#### 8.3.4 Conditional constructs ¶

The following macros provide additional conditional constructs as convenience wrappers around `m4_if`.

Macro: m4_bmatch(string, regex-1, value-1, [regex-2], [value-2], …, [default]) ¶

  The string string is repeatedly compared against a series of regex arguments; if a match is found, the expansion is the corresponding value, otherwise, the macro moves on to the next regex. If no regex match, then the result is the optional default, or nothing.

  The regular expression syntax understood by `m4_bmatch` is the same as that understood by `m4_bregexp`; see m4_bregexp. The name `m4_match` is reserved for use by a future version of M4sugar, when there is a version of GNU M4 that supports POSIX extended regular expression syntax.

Macro: m4_bpatsubsts(string, regex-1, subst-1, [regex-2], [subst-2], …) ¶

  The string string is altered by regex-1 and subst-1, as if by:

      m4_bpatsubst([[string]], [regex], [subst])

  The result of the substitution is then passed through the next set of regex and subst, and so forth. An empty subst implies deletion of any matched portions in the current string. Note that this macro over-quotes string; this behavior is intentional, so that the result of each step of the recursion remains as a quoted string. However, it means that anchors (‘`^`’ and ‘`$`’ in the regex will line up with the extra quotations, and not the characters of the original string. The overquoting is removed after the final substitution.

  The regular expression and replacement syntax understood by `m4_bpatsubsts` is the same as that understood by `m4_bpatsubst`; see m4_bpatsubst. The name `m4_patsubsts` is reserved for use by future version of M4sugar, when there is a version of GNU M4 that supports POSIX extended regular expression syntax.

Macro: m4_case(string, value-1, if-value-1, [value-2], [if-value-2], …, [default]) ¶

  Test string against multiple value possibilities, resulting in the first if-value for a match, or in the optional default. This is shorthand for:

      m4_if([string], [value-1], [if-value-1],
            [string], [value-2], [if-value-2], ...,
            [default])

Macro: m4_cond(test-1, value-1, if-value-1, [test-2], [value-2], [if-value-2], …, [default]) ¶

  This macro was introduced in Autoconf 2.62. Similar to `m4_if`, except that each test is expanded only when it is encountered. This is useful for short-circuiting expensive tests; while `m4_if` requires all its strings to be expanded up front before doing comparisons, `m4_cond` only expands a test when all earlier tests have failed.

  For an example, these two sequences give the same result, but in the case where ‘`$1`’ does not contain a backslash, the `m4_cond` version only expands `m4_index` once, instead of five times, for faster computation if this is a common case for ‘`$1`’. Notice that every third argument is unquoted for `m4_if`, and quoted for `m4_cond`:

      m4_if(m4_index([$1], [\]), [-1], [$2],
            m4_eval(m4_index([$1], [\\]) >= 0), [1], [$2],
            m4_eval(m4_index([$1], [\$]) >= 0), [1], [$2],
            m4_eval(m4_index([$1], [\`]) >= 0), [1], [$3],
            m4_eval(m4_index([$1], [\"]) >= 0), [1], [$3],
            [$2])
      m4_cond([m4_index([$1], [\])], [-1], [$2],
              [m4_eval(m4_index([$1], [\\]) >= 0)], [1], [$2],
              [m4_eval(m4_index([$1], [\$]) >= 0)], [1], [$2],
              [m4_eval(m4_index([$1], [\`]) >= 0)], [1], [$3],
              [m4_eval(m4_index([$1], [\"]) >= 0)], [1], [$3],
              [$2])

Macro: m4_default(expr-1, expr-2) ¶
Macro: m4_default_quoted(expr-1, expr-2) ¶
Macro: m4_default_nblank(expr-1, [expr-2]) ¶
Macro: m4_default_nblank_quoted(expr-1, [expr-2]) ¶

  If expr-1 contains text, use it. Otherwise, select expr-2. `m4_default` expands the result, while `m4_default_quoted` does not. Useful for providing a fixed default if the expression that results in expr-1 would otherwise be empty. The difference between `m4_default` and `m4_default_nblank` is whether an argument consisting of just blanks (space, tab, newline) is significant. When using the expanding versions, note that an argument may contain text but still expand to an empty string.

      m4_define([active], [ACTIVE])dnl
      m4_define([empty], [])dnl
      m4_define([demo1], [m4_default([$1], [$2])])dnl
      m4_define([demo2], [m4_default_quoted([$1], [$2])])dnl
      m4_define([demo3], [m4_default_nblank([$1], [$2])])dnl
      m4_define([demo4], [m4_default_nblank_quoted([$1], [$2])])dnl
      demo1([active], [default])
      ⇒ACTIVE
      demo1([], [active])
      ⇒ACTIVE
      demo1([empty], [text])
      ⇒
      -demo1([ ], [active])-
      ⇒- -
      demo2([active], [default])
      ⇒active
      demo2([], [active])
      ⇒active
      demo2([empty], [text])
      ⇒empty
      -demo2([ ], [active])-
      ⇒- -
      demo3([active], [default])
      ⇒ACTIVE
      demo3([], [active])
      ⇒ACTIVE
      demo3([empty], [text])
      ⇒
      -demo3([ ], [active])-
      ⇒-ACTIVE-
      demo4([active], [default])
      ⇒active
      demo4([], [active])
      ⇒active
      demo4([empty], [text])
      ⇒empty
      -demo4([ ], [active])-
      ⇒-active-

Macro: m4_define_default(macro, [default-definition]) ¶

  If macro does not already have a definition, then define it to default-definition.

Macro: m4_ifblank(cond, [if-blank], [if-text]) ¶
Macro: m4_ifnblank(cond, [if-text], [if-blank]) ¶

  If cond is empty or consists only of blanks (space, tab, newline), then expand if-blank; otherwise, expand if-text. Two variants exist, in order to make it easier to select the correct logical sense when using only two parameters. Note that this is more efficient than the equivalent behavior of:

      m4_ifval(m4_normalize([cond]), if-text, if-blank)

Macro: m4_ifndef(macro, if-not-defined, [if-defined]) ¶

  This is shorthand for:

      m4_ifdef([macro], [if-defined], [if-not-defined])

Macro: m4_ifset(macro, [if-true], [if-false]) ¶

  If macro is undefined, or is defined as the empty string, expand to if-false. Otherwise, expands to if-true. Similar to:

      m4_ifval(m4_defn([macro]), [if-true], [if-false])

  except that it is not an error if macro is undefined.

Macro: m4_ifval(cond, [if-true], [if-false]) ¶

  Expands to if-true if cond is not empty, otherwise to if-false. This is shorthand for:

      m4_if([cond], [], [if-false], [if-true])

Macro: m4_ifvaln(cond, [if-true], [if-false]) ¶

  Similar to `m4_ifval`, except guarantee that a newline is present after any non-empty expansion. Often followed by `dnl`.

Macro: m4_n(text) ¶

  Expand to text, and add a newline if text is not empty. Often followed by `dnl`.

#### 8.3.5 Looping constructs ¶

The following macros are useful in implementing recursive algorithms in M4, including loop operations. An M4 list is formed by quoting a list of quoted elements; generally the lists are comma-separated, although `m4_foreach_w` is whitespace-separated. For example, the list ‘`[[a], [b,c]]`’ contains two elements: ‘`[a]`’ and ‘`[b,c]`’. It is common to see lists with unquoted elements when those elements are not likely to be macro names, as in ‘`[fputc_unlocked, fgetc_unlocked]`’.

Although not generally recommended, it is possible for quoted lists to have side effects; all side effects are expanded only once, and prior to visiting any list element. On the other hand, the fact that unquoted macros are expanded exactly once means that macros without side effects can be used to generate lists. For example,

    m4_foreach([i], [[1], [2], [3]m4_errprintn([hi])], [i])
    error→hi
    ⇒123
    m4_define([list], [[1], [2], [3]])
    ⇒
    m4_foreach([i], [list], [i])
    ⇒123

Macro: m4_argn(n, [arg]…) ¶

  Extracts argument n (larger than 0) from the remaining arguments. If there are too few arguments, the empty string is used. For any n besides 1, this is more efficient than the similar ‘`m4_car(m4_shiftn([n], [], [arg…]))`’.

Macro: m4_car(arg…) ¶

  Expands to the quoted first arg. Can be used with `m4_cdr` to recursively iterate through a list. Generally, when using quoted lists of quoted elements, `m4_car` should be called without any extra quotes.

Macro: m4_cdr(arg…) ¶

  Expands to a quoted list of all but the first arg, or the empty string if there was only one argument. Generally, when using quoted lists of quoted elements, `m4_cdr` should be called without any extra quotes.

  For example, this is a simple implementation of `m4_map`; note how each iteration checks for the end of recursion, then merely applies the first argument to the first element of the list, then repeats with the rest of the list. (The actual implementation in M4sugar is a bit more involved, to gain some speed and share code with `m4_map_sep`, and also to avoid expanding side effects in ‘`$2`’ twice).

      m4_define([m4_map], [m4_ifval([$2],
        [m4_apply([$1], m4_car($2))[]$0([$1], m4_cdr($2))])])dnl
      m4_map([ m4_eval], [[[1]], [[1+1]], [[10],[16]]])
      ⇒ 1 2 a

Macro: m4_for(var, first, last, [step], expression) ¶

  Loop over the numeric values between first and last including bounds by increments of step. For each iteration, expand expression with the numeric value assigned to var. If step is omitted, it defaults to ‘`1`’ or ‘`-1`’ depending on the order of the limits. If given, step has to match this order. The number of iterations is determined independently from definition of var; iteration cannot be short-circuited or lengthened by modifying var from within expression.

Macro: m4_foreach(var, list, expression) ¶

  Loop over the comma-separated M4 list list, assigning each value to var, and expand expression. The following example outputs two lines:

      m4_foreach([myvar], [[foo], [bar, baz]],
                 [echo myvar
      ])dnl
      ⇒echo foo
      ⇒echo bar, baz

  Note that for some forms of expression, it may be faster to use `m4_map_args`.

Macro: m4_foreach_w(var, list, expression) ¶

  Loop over the white-space-separated list list, assigning each value to var, and expand expression. If var is only referenced once in expression, it is more efficient to use `m4_map_args_w`.

  The deprecated macro `AC_FOREACH` is an alias of `m4_foreach_w`.

Macro: m4_map(macro, list) ¶
Macro: m4_mapall(macro, list) ¶
Macro: m4_map_sep(macro, separator, list) ¶
Macro: m4_mapall_sep(macro, separator, list) ¶

  Loop over the comma separated quoted list of argument descriptions in list, and invoke macro with the arguments. An argument description is in turn a comma-separated quoted list of quoted elements, suitable for `m4_apply`. The macros `m4_map` and `m4_map_sep` ignore empty argument descriptions, while `m4_mapall` and `m4_mapall_sep` invoke macro with no arguments. The macros `m4_map_sep` and `m4_mapall_sep` additionally expand separator between invocations of macro.

  Note that separator is expanded, unlike in `m4_join`. When separating output with commas, this means that the map result can be used as a series of arguments, by using a single-quoted comma as separator, or as a single string, by using a double-quoted comma.

      m4_map([m4_count], [])
      ⇒
      m4_map([ m4_count], [[],
                           [[1]],
                           [[1], [2]]])
      ⇒ 1 2
      m4_mapall([ m4_count], [[],
                              [[1]],
                              [[1], [2]]])
      ⇒ 0 1 2
      m4_map_sep([m4_eval], [,], [[[1+2]],
                                  [[10], [16]]])
      ⇒3,a
      m4_map_sep([m4_echo], [,], [[[a]], [[b]]])
      ⇒a,b
      m4_count(m4_map_sep([m4_echo], [,], [[[a]], [[b]]]))
      ⇒2
      m4_map_sep([m4_echo], [[,]], [[[a]], [[b]]])
      ⇒a,b
      m4_count(m4_map_sep([m4_echo], [[,]], [[[a]], [[b]]]))
      ⇒1

Macro: m4_map_args(macro, arg…) ¶

  Repeatedly invoke macro with each successive arg as its only argument. In the following example, three solutions are presented with the same expansion; the solution using `m4_map_args` is the most efficient.

      m4_define([active], [ACTIVE])dnl
      m4_foreach([var], [[plain], [active]], [ m4_echo(m4_defn([var]))])
      ⇒ plain active
      m4_map([ m4_echo], [[[plain]], [[active]]])
      ⇒ plain active
      m4_map_args([ m4_echo], [plain], [active])
      ⇒ plain active

  In cases where it is useful to operate on additional parameters besides the list elements, the macro `m4_curry` can be used in macro to supply the argument currying necessary to generate the desired argument list. In the following example, `list_add_n` is more efficient than `list_add_x`. On the other hand, using `m4_map_args_sep` can be even more efficient.

      m4_define([list], [[1], [2], [3]])dnl
      m4_define([add], [m4_eval(([$1]) + ([$2]))])dnl
      dnl list_add_n(N, ARG...)
      dnl Output a list consisting of each ARG added to N
      m4_define([list_add_n],
      [m4_shift(m4_map_args([,m4_curry([add], [$1])], m4_shift($@)))])dnl
      list_add_n([1], list)
      ⇒2,3,4
      list_add_n([2], list)
      ⇒3,4,5
      m4_define([list_add_x],
      [m4_shift(m4_foreach([var], m4_dquote(m4_shift($@)),
        [,add([$1],m4_defn([var]))]))])dnl
      list_add_x([1], list)
      ⇒2,3,4

Macro: m4_map_args_pair(macro, [macro-end = macro], arg…) ¶

  For every pair of arguments arg, invoke macro with two arguments. If there is an odd number of arguments, invoke macro-end, which defaults to macro, with the remaining argument.

      m4_map_args_pair([, m4_reverse], [], [1], [2], [3])
      ⇒, 2, 1, 3
      m4_map_args_pair([, m4_reverse], [, m4_dquote], [1], [2], [3])
      ⇒, 2, 1, [3]
      m4_map_args_pair([, m4_reverse], [, m4_dquote], [1], [2], [3], [4])
      ⇒, 2, 1, 4, 3

Macro: m4_map_args_sep([pre], [post], [sep], arg…) ¶

  Expand the sequence `pre[arg]post` for each argument, additionally expanding sep between arguments. One common use of this macro is constructing a macro call, where the opening and closing parentheses are split between pre and post; in particular, `m4_map_args([macro], [arg])` is equivalent to `m4_map_args_sep([macro(], [)], [], [arg])`. This macro provides the most efficient means for iterating over an arbitrary list of arguments, particularly when repeatedly constructing a macro call with more arguments than arg.

Macro: m4_map_args_w(string, [pre], [post], [sep]) ¶

  Expand the sequence `pre[word]post` for each word in the whitespace-separated string, additionally expanding sep between words. This macro provides the most efficient means for iterating over a whitespace-separated string. In particular, `m4_map_args_w([string], [action(], [)])` is more efficient than `m4_foreach_w([var], [string], [action(m4_defn([var]))])`.

Macro: m4_shiftn(count, …) ¶
Macro: m4_shift2(…) ¶
Macro: m4_shift3(…) ¶

  `m4_shiftn` performs count iterations of `m4_shift`, along with validation that enough arguments were passed in to match the shift count, and that the count is positive. `m4_shift2` and `m4_shift3` are specializations of `m4_shiftn`, introduced in Autoconf 2.62, and are more efficient for two and three shifts, respectively.

Macro: m4_stack_foreach(macro, action) ¶
Macro: m4_stack_foreach_lifo(macro, action) ¶

  For each of the `m4_pushdef` definitions of macro, expand action with the single argument of a definition of macro. `m4_stack_foreach` starts with the oldest definition, while `m4_stack_foreach_lifo` starts with the current definition. action should not push or pop definitions of macro, nor is there any guarantee that the current definition of macro matches the argument that was passed to action. The macro `m4_curry` can be used if action needs more than one argument, although in that case it is more efficient to use m4_stack_foreach_sep.

  Due to technical limitations, there are a few low-level m4sugar functions, such as `m4_pushdef`, that cannot be used as the macro argument.

      m4_pushdef([a], [1])m4_pushdef([a], [2])dnl
      m4_stack_foreach([a], [ m4_incr])
      ⇒ 2 3
      m4_stack_foreach_lifo([a], [ m4_curry([m4_substr], [abcd])])
      ⇒ cd bcd

Macro: m4_stack_foreach_sep(macro, [pre], [post], [sep]) ¶
Macro: m4_stack_foreach_sep_lifo(macro, [pre], [post], [sep]) ¶

  Expand the sequence `pre[definition]post` for each `m4_pushdef` definition of macro, additionally expanding sep between definitions. `m4_stack_foreach_sep` visits the oldest definition first, while `m4_stack_foreach_sep_lifo` visits the current definition first. This macro provides the most efficient means for iterating over a pushdef stack. In particular, `m4_stack_foreach([macro], [action])` is short for `m4_stack_foreach_sep([macro], [action(], [)])`.

#### 8.3.6 Evaluation Macros ¶

The following macros give some control over the order of the evaluation by adding or removing levels of quotes.

Macro: m4_apply(macro, list) ¶

  Apply the elements of the quoted, comma-separated list as the arguments to macro. If list is empty, invoke macro without arguments. Note the difference between `m4_indir`, which expects its first argument to be a macro name but can use names that are otherwise invalid, and `m4_apply`, where macro can contain other text, but must end in a valid macro name.

      m4_apply([m4_count], [])
      ⇒0
      m4_apply([m4_count], [[]])
      ⇒1
      m4_apply([m4_count], [[1], [2]])
      ⇒2
      m4_apply([m4_join], [[|], [1], [2]])
      ⇒1|2

Macro: m4_count(arg, …) ¶

  This macro returns the number of arguments it was passed.

Macro: m4_curry(macro, arg…) ¶

  This macro performs argument currying. The expansion of this macro is another macro name that expects exactly one argument; that argument is then appended to the arg list, and then macro is expanded with the resulting argument list.

      m4_curry([m4_curry], [m4_reverse], [1])([2])([3])
      ⇒3, 2, 1

  Unfortunately, due to a limitation in M4 1.4.x, it is not possible to pass the definition of a builtin macro as the argument to the output of `m4_curry`; the empty string is used instead of the builtin token. This behavior is rectified by using M4 1.6 or newer.

Macro: m4_do(arg, …) ¶

  This macro loops over its arguments and expands each arg in sequence. Its main use is for readability; it allows the use of indentation and fewer `dnl` to result in the same expansion. This macro guarantees that no expansion will be concatenated with subsequent text; to achieve full concatenation, use `m4_unquote(m4_join([], arg…))`.

      m4_define([ab],[1])m4_define([bc],[2])m4_define([abc],[3])dnl
      m4_do([a],[b])c
      ⇒abc
      m4_unquote(m4_join([],[a],[b]))c
      ⇒3
      m4_define([a],[A])m4_define([b],[B])m4_define([c],[C])dnl
      m4_define([AB],[4])m4_define([BC],[5])m4_define([ABC],[6])dnl
      m4_do([a],[b])c
      ⇒ABC
      m4_unquote(m4_join([],[a],[b]))c
      ⇒3

Macro: m4_dquote(arg, …) ¶

  Return the arguments as a quoted list of quoted arguments. Conveniently, if there is just one arg, this effectively adds a level of quoting.

Macro: m4_dquote_elt(arg, …) ¶

  Return the arguments as a series of double-quoted arguments. Whereas `m4_dquote` returns a single argument, `m4_dquote_elt` returns as many arguments as it was passed.

Macro: m4_echo(arg, …) ¶

  Return the arguments, with the same level of quoting. Other than discarding whitespace after unquoted commas, this macro is a no-op.

Macro: m4_expand(arg) ¶

  Return the expansion of arg as a quoted string. Whereas `m4_quote` is designed to collect expanded text into a single argument, `m4_expand` is designed to perform one level of expansion on quoted text. One distinction is in the treatment of whitespace following a comma in the original arg. Any time multiple arguments are collected into one with `m4_quote`, the M4 argument collection rules discard the whitespace. However, with `m4_expand`, whitespace is preserved, even after the expansion of macros contained in arg. Additionally, `m4_expand` is able to expand text that would involve an unterminated comment, whereas expanding that same text as the argument to `m4_quote` runs into difficulty in finding the end of the argument. Since manipulating diversions during argument collection is inherently unsafe, `m4_expand` issues an error if arg attempts to change the current diversion (see Diversion support).

      m4_define([active], [ACT, IVE])dnl
      m4_define([active2], [[ACT, IVE]])dnl
      m4_quote(active, active)
      ⇒ACT,IVE,ACT,IVE
      m4_expand([active, active])
      ⇒ACT, IVE, ACT, IVE
      m4_quote(active2, active2)
      ⇒ACT, IVE,ACT, IVE
      m4_expand([active2, active2])
      ⇒ACT, IVE, ACT, IVE
      m4_expand([# m4_echo])
      ⇒# m4_echo
      m4_quote(# m4_echo)
      )
      ⇒# m4_echo)
      ⇒

  Note that `m4_expand` cannot handle an arg that expands to literal unbalanced quotes, but that quadrigraphs can be used when unbalanced output is necessary. Likewise, unbalanced parentheses should be supplied with double quoting or a quadrigraph.

      m4_define([pattern], [[!@<:@]])dnl
      m4_define([bar], [BAR])dnl
      m4_expand([case $foo in
        m4_defn([pattern])@:}@ bar ;;
        *[)] blah ;;
      esac])
      ⇒case $foo in
      ⇒  [![]) BAR ;;
      ⇒  *) blah ;;
      ⇒esac

Macro: m4_ignore(…) ¶

  This macro was introduced in Autoconf 2.62. Expands to nothing, ignoring all of its arguments. By itself, this isn’t very useful. However, it can be used to conditionally ignore an arbitrary number of arguments, by deciding which macro name to apply to a list of arguments.

      dnl foo outputs a message only if [debug] is defined.
      m4_define([foo],
      [m4_ifdef([debug],[AC_MSG_NOTICE],[m4_ignore])([debug message])])

  Note that for earlier versions of Autoconf, the macro `__gnu__` can serve the same purpose, although it is less readable.

Macro: m4_make_list(arg, …) ¶

  This macro exists to aid debugging of M4sugar algorithms. Its net effect is similar to `m4_dquote`—it produces a quoted list of quoted arguments, for each arg. The difference is that this version uses a comma-newline separator instead of just comma, to improve readability of the list; with the result that it is less efficient than `m4_dquote`.

      m4_define([zero],[0])m4_define([one],[1])m4_define([two],[2])dnl
      m4_dquote(zero, [one], [[two]])
      ⇒[0],[one],[[two]]
      m4_make_list(zero, [one], [[two]])
      ⇒[0],
      ⇒[one],
      ⇒[[two]]
      m4_foreach([number], m4_dquote(zero, [one], [[two]]), [ number])
      ⇒ 0 1 two
      m4_foreach([number], m4_make_list(zero, [one], [[two]]), [ number])
      ⇒ 0 1 two

Macro: m4_quote(arg, …) ¶

  Return the arguments as a single entity, i.e., wrap them into a pair of quotes. This effectively collapses multiple arguments into one, although it loses whitespace after unquoted commas in the process.

Macro: m4_reverse(arg, …) ¶

  Outputs each argument with the same level of quoting, but in reverse order, and with space following each comma for readability.

      m4_define([active], [ACT,IVE])
      ⇒
      m4_reverse(active, [active])
      ⇒active, IVE, ACT

Macro: m4_unquote(arg, …) ¶

  This macro was introduced in Autoconf 2.62. Expand each argument, separated by commas. For a single arg, this effectively removes a layer of quoting, and `m4_unquote([arg])` is more efficient than the equivalent `m4_do([arg])`. For multiple arguments, this results in an unquoted list of expansions. This is commonly used with `m4_split`, in order to convert a single quoted list into a series of quoted elements.

The following example aims at emphasizing the difference between several scenarios: not using these macros, using `m4_defn`, using `m4_quote`, using `m4_dquote`, and using `m4_expand`.

    $ cat example.m4
    dnl Overquote, so that quotes are visible.
    m4_define([show], [$[]1 = [$1], $[]@ = [$@]])
    m4_define([a], [A])
    m4_define([mkargs], [1, 2[,] 3])
    m4_define([arg1], [[$1]])
    m4_divert([0])dnl
    show(a, b)
    show([a, b])
    show(m4_quote(a, b))
    show(m4_dquote(a, b))
    show(m4_expand([a, b]))

    arg1(mkargs)
    arg1([mkargs])
    arg1(m4_defn([mkargs]))
    arg1(m4_quote(mkargs))
    arg1(m4_dquote(mkargs))
    arg1(m4_expand([mkargs]))
    $ autom4te -l m4sugar example.m4
    $1 = A, $@ = [A],[b]
    $1 = a, b, $@ = [a, b]
    $1 = A,b, $@ = [A,b]
    $1 = [A],[b], $@ = [[A],[b]]
    $1 = A, b, $@ = [A, b]

    1
    mkargs
    1, 2[,] 3
    1,2, 3
    [1],[2, 3]
    1, 2, 3

#### 8.3.7 String manipulation in M4 ¶

The following macros may be used to manipulate strings in M4. Many of the macros in this section intentionally result in quoted strings as output, rather than subjecting the arguments to further expansions. As a result, if you are manipulating text that contains active M4 characters, the arguments are passed with single quoting rather than double.

Macro: m4_append(macro-name, string, [separator]) ¶
Macro: m4_append_uniq(macro-name, string, [separator] [if-uniq], [if-duplicate]) ¶

  Redefine macro-name to its former contents with separator and string added at the end. If macro-name was undefined before (but not if it was defined but empty), then no separator is added. As of Autoconf 2.62, neither string nor separator are expanded during this macro; instead, they are expanded when macro-name is invoked.

  `m4_append` can be used to grow strings, and `m4_append_uniq` to grow strings without duplicating substrings. Additionally, `m4_append_uniq` takes two optional parameters as of Autoconf 2.62; if-uniq is expanded if string was appended, and if-duplicate is expanded if string was already present. Also, `m4_append_uniq` warns if separator is not empty, but occurs within string, since that can lead to duplicates.

  Note that `m4_append` can scale linearly in the length of the final string, depending on the quality of the underlying M4 implementation, while `m4_append_uniq` has an inherent quadratic scaling factor. If an algorithm can tolerate duplicates in the final string, use the former for speed. If duplicates must be avoided, consider using `m4_set_add` instead (see Set manipulation in M4).

      m4_define([active], [ACTIVE])dnl
      m4_append([sentence], [This is an])dnl
      m4_append([sentence], [ active ])dnl
      m4_append([sentence], [symbol.])dnl
      sentence
      ⇒This is an ACTIVE symbol.
      m4_undefine([active])dnl
      ⇒This is an active symbol.
      m4_append_uniq([list], [one], [, ], [new], [existing])
      ⇒new
      m4_append_uniq([list], [one], [, ], [new], [existing])
      ⇒existing
      m4_append_uniq([list], [two], [, ], [new], [existing])
      ⇒new
      m4_append_uniq([list], [three], [, ], [new], [existing])
      ⇒new
      m4_append_uniq([list], [two], [, ], [new], [existing])
      ⇒existing
      list
      ⇒one, two, three
      m4_dquote(list)
      ⇒[one],[two],[three]
      m4_append([list2], [one], [[, ]])dnl
      m4_append_uniq([list2], [two], [[, ]])dnl
      m4_append([list2], [three], [[, ]])dnl
      list2
      ⇒one, two, three
      m4_dquote(list2)
      ⇒[one, two, three]

Macro: m4_append_uniq_w(macro-name, strings) ¶

  This macro was introduced in Autoconf 2.62. It is similar to `m4_append_uniq`, but treats strings as a whitespace separated list of words to append, and only appends unique words. macro-name is updated with a single space between new words.

      m4_append_uniq_w([numbers], [1 1 2])dnl
      m4_append_uniq_w([numbers], [ 2 3 ])dnl
      numbers
      ⇒1 2 3

Macro: m4_chomp(string) ¶
Macro: m4_chomp_all(string) ¶

  Output string in quotes, but without a trailing newline. The macro `m4_chomp` is slightly faster, and removes at most one newline; the macro `m4_chomp_all` removes all consecutive trailing newlines. Unlike `m4_flatten`, embedded newlines are left intact, and backslash does not influence the result.

Macro: m4_combine([separator], prefix-list, [infix], suffix-1, [suffix-2], …) ¶

  This macro produces a quoted string containing the pairwise combination of every element of the quoted, comma-separated prefix-list, and every element from the suffix arguments. Each pairwise combination is joined with infix in the middle, and successive pairs are joined by separator. No expansion occurs on any of the arguments. No output occurs if either the prefix or suffix list is empty, but the lists can contain empty elements.

      m4_define([a], [oops])dnl
      m4_combine([, ], [[a], [b], [c]], [-], [1], [2], [3])
      ⇒a-1, a-2, a-3, b-1, b-2, b-3, c-1, c-2, c-3
      m4_combine([, ], [[a], [b]], [-])
      ⇒
      m4_combine([, ], [[a], [b]], [-], [])
      ⇒a-, b-
      m4_combine([, ], [], [-], [1], [2])
      ⇒
      m4_combine([, ], [[]], [-], [1], [2])
      ⇒-1, -2

Macro: m4_escape(string) ¶

  Convert all instances of ‘`[`’, ‘`]`’, ‘`#`’, and ‘`$`’ within string into their respective quadrigraphs. The result is still a quoted string.

Macro: m4_flatten(string) ¶

  Flatten string into a single line. Delete all backslash-newline pairs, and replace all remaining newlines with a space. The result is still a quoted string.

Macro: m4_join([separator], args…) ¶
Macro: m4_joinall([separator], args…) ¶

  Concatenate each arg, separated by separator. `joinall` uses every argument, while `join` omits empty arguments so that there are no back-to-back separators in the output. The result is a quoted string.

      m4_define([active], [ACTIVE])dnl
      m4_join([|], [one], [], [active], [two])
      ⇒one|active|two
      m4_joinall([|], [one], [], [active], [two])
      ⇒one||active|two

  Note that if all you intend to do is join args with commas between them, to form a quoted list suitable for `m4_foreach`, it is more efficient to use `m4_dquote`.

Macro: m4_newline([text]) ¶

  This macro was introduced in Autoconf 2.62, and expands to a newline, followed by any text. It is primarily useful for maintaining macro formatting, and ensuring that M4 does not discard leading whitespace during argument collection.

Macro: m4_normalize(string) ¶

  Remove leading and trailing spaces and tabs, sequences of backslash-then-newline, and replace multiple spaces, tabs, and newlines with a single space. This is a combination of `m4_flatten` and `m4_strip`. To determine if string consists only of bytes that would be removed by `m4_normalize`, you can use `m4_ifblank`.

Macro: m4_re_escape(string) ¶

  Backslash-escape all characters in string that are active in regexps.

Macro: m4_split(string, [regexp = ‘`[\t ]+`’]) ¶

  Split string into an M4 list of elements quoted by ‘`[`’ and ‘`]`’, while keeping white space at the beginning and at the end. If regexp is given, use it instead of ‘`[\t ]+`’ for splitting. If string is empty, the result is an empty list.

Macro: m4_strip(string) ¶

  Strip whitespace from string. Sequences of spaces and tabs are reduced to a single space, then leading and trailing spaces are removed. The result is still a quoted string. Note that this does not interfere with newlines; if you want newlines stripped as well, consider `m4_flatten`, or do it all at once with `m4_normalize`. To quickly test if string has only whitespace, use `m4_ifblank`.

Macro: m4_text_box(message, [frame = ‘`-`’]) ¶

  Add a text box around message, using frame as the border character above and below the message. The frame argument must be a single byte, and does not support quadrigraphs. The frame correctly accounts for the subsequent expansion of message. For example:

      m4_define([macro], [abc])dnl
      m4_text_box([macro])
      ⇒## --- ##
      ⇒## abc ##
      ⇒## --- ##

  The message must contain balanced quotes and parentheses, although quadrigraphs can be used to work around this.

Macro: m4_text_wrap(string, [prefix], [prefix1 = prefix], [width = ‘`79`’]) ¶

  Break string into a series of whitespace-separated words, then output those words separated by spaces, and wrapping lines any time the output would exceed width columns. If given, prefix1 begins the first line, and prefix begins all wrapped lines. If prefix1 is longer than prefix, then the first line consists of just prefix1. If prefix is longer than prefix1, padding is inserted so that the first word of string begins at the same indentation as all wrapped lines. Note that using literal tab characters in any of the arguments will interfere with the calculation of width. No expansions occur on prefix, prefix1, or the words of string, although quadrigraphs are recognized.

  For some examples:

      m4_text_wrap([Short string */], [   ], [/* ], [20])
      ⇒/* Short string */
      m4_text_wrap([Much longer string */], [   ], [/* ], [20])
      ⇒/* Much longer
      ⇒   string */
      m4_text_wrap([Short doc.], [          ], [  --short ], [30])
      ⇒  --short Short doc.
      m4_text_wrap([Short doc.], [          ], [  --too-wide ], [30])
      ⇒  --too-wide
      ⇒          Short doc.
      m4_text_wrap([Super long documentation.], [     ],
                   [  --too-wide ], 30)
      ⇒  --too-wide
      ⇒     Super long
      ⇒     documentation.

Macro: m4_tolower(string) ¶
Macro: m4_toupper(string) ¶

  Return string with letters converted to upper or lower case, respectively.

#### 8.3.8 Arithmetic computation in M4 ¶

The following macros facilitate integer arithmetic operations.

Where a parameter is documented as taking an arithmetic expression, you can use anything that can be parsed by `m4_eval`. Any other numeric parameter should consist of an optional sign followed by one or more decimal digits; it is treated as a decimal integer.

Macros that expand to a number do so as either ‘`0`’, or an optional ‘`-`’ followed by a nonzero decimal digit followed by zero or more decimal digits.

Due to `m4` limitations, arithmetic expressions and numeric parameters should use only numbers that fit into a 32-bit signed integer.

Macro: m4_cmp(expr-1, expr-2) ¶

  Compare the arithmetic expressions expr-1 and expr-2, and expand to ‘`-1`’ if expr-1 is smaller, ‘`0`’ if they are equal, and ‘`1`’ if expr-1 is larger.

Macro: m4_list_cmp(list-1, list-2) ¶

  Compare the two M4 lists consisting of comma-separated arithmetic expressions, left to right. Expand to ‘`-1`’ for the first element pairing where the value from list-1 is smaller, ‘`1`’ where the value from list-2 is smaller, or ‘`0`’ if both lists have the same values. If one list is shorter than the other, the remaining elements of the longer list are compared against zero.

      m4_list_cmp([1, 0],       [1])
      ⇒0
      m4_list_cmp([1, [1 * 0]], [1, 0])
      ⇒0
      m4_list_cmp([1, 2],       [1, 0])
      ⇒1
      m4_list_cmp([1, [1+1], 3],[1, 2])
      ⇒1
      m4_list_cmp([1, 2, -3],   [1, 2])
      ⇒-1
      m4_list_cmp([1, 0],       [1, 2])
      ⇒-1
      m4_list_cmp([1],          [1, 2])
      ⇒-1

Macro: m4_max(arg, …) ¶

  This macro was introduced in Autoconf 2.62. Expand to the value of the maximum arithmetic expression among all the arguments.

Macro: m4_min(arg, …) ¶

  This macro was introduced in Autoconf 2.62. Expand to the value of the minimum arithmetic expression among all the arguments.

Macro: m4_sign(expr) ¶

  Expand to ‘`-1`’ if the arithmetic expression expr is negative, ‘`1`’ if it is positive, and ‘`0`’ if it is zero.

Macro: m4_version_compare(version-1, version-2) ¶

  This macro was introduced in Autoconf 2.53, but had a number of usability limitations that were not lifted until Autoconf 2.62. Compare the version strings version-1 and version-2, and expand to ‘`-1`’ if version-1 is smaller, ‘`0`’ if they are the same, or ‘`1`’ version-2 is smaller. Version strings must be a list of elements separated by ‘`.`’, ‘`,`’ or ‘`-`’, where each element is a number along with optional case-insensitive letters designating beta releases. The comparison stops at the leftmost element that contains a difference, although a 0 element compares equal to a missing element.

  It is permissible to include commit identifiers in version, such as an abbreviated SHA1 of the commit, provided there is still a monotonically increasing prefix to allow for accurate version-based comparisons. For example, this paragraph was written when the development snapshot of autoconf claimed to be at version ‘`2.61a-248-dc51`’, or 248 commits after the 2.61a release, with an abbreviated commit identification of ‘`dc51`’.

      m4_version_compare([1.1], [2.0])
      ⇒-1
      m4_version_compare([2.0b], [2.0a])
      ⇒1
      m4_version_compare([1.1.1], [1.1.1a])
      ⇒-1
      m4_version_compare([1.2], [1.1.1a])
      ⇒1
      m4_version_compare([1.0], [1])
      ⇒0
      m4_version_compare([1.1pre], [1.1PRE])
      ⇒0
      m4_version_compare([1.1a], [1,10])
      ⇒-1
      m4_version_compare([2.61a], [2.61a-248-dc51])
      ⇒-1
      m4_version_compare([2.61b], [2.61a-248-dc51])
      ⇒1

Macro: m4_version_prereq(version, [if-new-enough], [if-old = ‘`m4_fatal`’]) ¶

  Compares version against the version of Autoconf currently running. If the running version is at version or newer, expand if-new-enough, but if version is larger than the version currently executing, expand if-old, which defaults to printing an error message and exiting m4sugar with status 63. When given only one argument, this behaves like `AC_PREREQ` (see Dealing with Autoconf versions). Remember that the autoconf philosophy favors feature checks over version checks.

#### 8.3.9 Set manipulation in M4 ¶

Sometimes, it is necessary to track a set of data, where the order does not matter and where there are no duplicates in the set. The following macros facilitate set manipulations. Each set is an opaque object, which can only be accessed via these basic operations. The underlying implementation guarantees linear scaling for set creation, which is more efficient than using the quadratic `m4_append_uniq`. Both set names and values can be arbitrary strings, except for unbalanced quotes. This implementation ties up memory for removed elements until the next operation that must traverse all the elements of a set; and although that may slow down some operations until the memory for removed elements is pruned, it still guarantees linear performance.

Macro: m4_set_add(set, value, [if-uniq], [if-dup]) ¶

  Adds the string value as a member of set set. Expand if-uniq if the element was added, or if-dup if it was previously in the set. Operates in amortized constant time, so that set creation scales linearly.

Macro: m4_set_add_all(set, value…) ¶

  Adds each value to the set set. This is slightly more efficient than repeatedly invoking `m4_set_add`.

Macro: m4_set_contains(set, value, [if-present], [if-absent]) ¶

  Expands if-present if the string value is a member of set, otherwise if-absent.

      m4_set_contains([a], [1], [yes], [no])
      ⇒no
      m4_set_add([a], [1], [added], [dup])
      ⇒added
      m4_set_add([a], [1], [added], [dup])
      ⇒dup
      m4_set_contains([a], [1], [yes], [no])
      ⇒yes
      m4_set_remove([a], [1], [removed], [missing])
      ⇒removed
      m4_set_contains([a], [1], [yes], [no])
      ⇒no
      m4_set_remove([a], [1], [removed], [missing])
      ⇒missing

Macro: m4_set_contents(set, [sep]) ¶
Macro: m4_set_dump(set, [sep]) ¶

  Expands to a single string consisting of all the members of the set set, each separated by sep, which is not expanded. `m4_set_contents` leaves the elements in set but reclaims any memory occupied by removed elements, while `m4_set_dump` is a faster one-shot action that also deletes the set. No provision is made for disambiguating members that contain a non-empty sep as a substring; use `m4_set_empty` to distinguish between an empty set and the set containing only the empty string. The order of the output is unspecified; in the current implementation, part of the speed of `m4_set_dump` results from using a different output order than `m4_set_contents`. These macros scale linearly in the size of the set before memory pruning, and `m4_set_contents([set], [sep])` is faster than `m4_joinall([sep]m4_set_listc([set]))`.

      m4_set_add_all([a], [1], [2], [3])
      ⇒
      m4_set_contents([a], [-])
      ⇒1-2-3
      m4_joinall([-]m4_set_listc([a]))
      ⇒1-2-3
      m4_set_dump([a], [-])
      ⇒3-2-1
      m4_set_contents([a])
      ⇒
      m4_set_add([a], [])
      ⇒
      m4_set_contents([a], [-])
      ⇒

Macro: m4_set_delete(set) ¶

  Delete all elements and memory associated with set. This is linear in the set size, and faster than removing one element at a time.

Macro: m4_set_difference(seta, setb) ¶
Macro: m4_set_intersection(seta, setb) ¶
Macro: m4_set_union(seta, setb) ¶

  Compute the relation between seta and setb, and output the result as a list of quoted arguments without duplicates and with a leading comma. Set difference selects the elements in seta but not setb, intersection selects only elements in both sets, and union selects elements in either set. These actions are linear in the sum of the set sizes. The leading comma is necessary to distinguish between no elements and the empty string as the only element.

      m4_set_add_all([a], [1], [2], [3])
      ⇒
      m4_set_add_all([b], [3], [], [4])
      ⇒
      m4_set_difference([a], [b])
      ⇒,1,2
      m4_set_difference([b], [a])
      ⇒,,4
      m4_set_intersection([a], [b])
      ⇒,3
      m4_set_union([a], [b])
      ⇒,1,2,3,,4

Macro: m4_set_empty(set, [if-empty], [if-elements]) ¶

  Expand if-empty if the set set has no elements, otherwise expand if-elements. This macro operates in constant time. Using this macro can help disambiguate output from `m4_set_contents` or `m4_set_list`.

Macro: m4_set_foreach(set, variable, action) ¶

  For each element in the set set, expand action with the macro variable defined as the set element. Behavior is unspecified if action recursively lists the contents of set (although listing other sets is acceptable), or if it modifies the set in any way other than removing the element currently contained in variable. This macro is faster than the corresponding `m4_foreach([variable], m4_indir([m4_dquote]m4_set_listc([set])), [action])`, although `m4_set_map` might be faster still.

      m4_set_add_all([a]m4_for([i], [1], [5], [], [,i]))
      ⇒
      m4_set_contents([a])
      ⇒12345
      m4_set_foreach([a], [i],
        [m4_if(m4_eval(i&1), [0], [m4_set_remove([a], i, [i])])])
      ⇒24
      m4_set_contents([a])
      ⇒135

Macro: m4_set_list(set) ¶
Macro: m4_set_listc(set) ¶

  Produce a list of arguments, where each argument is a quoted element from the set set. The variant `m4_set_listc` is unambiguous, by adding a leading comma if there are any set elements, whereas the variant `m4_set_list` cannot distinguish between an empty set and a set containing only the empty string. These can be directly used in macros that take multiple arguments, such as `m4_join` or `m4_set_add_all`, or wrapped by `m4_dquote` for macros that take a quoted list, such as `m4_map` or `m4_foreach`. Any memory occupied by removed elements is reclaimed during these macros.

      m4_set_add_all([a], [1], [2], [3])
      ⇒
      m4_set_list([a])
      ⇒1,2,3
      m4_set_list([b])
      ⇒
      m4_set_listc([b])
      ⇒
      m4_count(m4_set_list([b]))
      ⇒1
      m4_set_empty([b], [0], [m4_count(m4_set_list([b]))])
      ⇒0
      m4_set_add([b], [])
      ⇒
      m4_set_list([b])
      ⇒
      m4_set_listc([b])
      ⇒,
      m4_count(m4_set_list([b]))
      ⇒1
      m4_set_empty([b], [0], [m4_count(m4_set_list([b]))])
      ⇒1

Macro: m4_set_map(set, action) ¶

  For each element in the set set, expand action with a single argument of the set element. Behavior is unspecified if action recursively lists the contents of set (although listing other sets is acceptable), or if it modifies the set in any way other than removing the element passed as an argument. This macro is faster than either corresponding counterpart of `m4_map_args([action]m4_set_listc([set]))` or `m4_set_foreach([set], [var], [action(m4_defn([var]))])`. It is possible to use `m4_curry` if more than one argument is needed for action, although it is more efficient to use `m4_set_map_sep` in that case.

Macro: m4_set_map_sep(set, [pre], [post], [sep]) ¶

  For each element in the set set, expand `pre[element]post`, additionally expanding sep between elements. Behavior is unspecified if the expansion recursively lists the contents of set (although listing other sets is acceptable), or if it modifies the set in any way other than removing the element visited by the expansion. This macro provides the most efficient means for non-destructively visiting the elements of a set; in particular, `m4_set_map([set], [action])` is equivalent to `m4_set_map_sep([set], [action(], [)])`.

Macro: m4_set_remove(set, value, [if-present], [if-absent]) ¶

  If value is an element in the set set, then remove it and expand if-present. Otherwise expand if-absent. This macro operates in constant time so that multiple removals will scale linearly rather than quadratically; but when used outside of `m4_set_foreach` or `m4_set_map`, it leaves memory occupied until the set is later compacted by `m4_set_contents` or `m4_set_list`. Several other set operations are then less efficient between the time of element removal and subsequent memory compaction, but still maintain their guaranteed scaling performance.

Macro: m4_set_size(set) ¶

  Expand to the size of the set set. This implementation operates in constant time, and is thus more efficient than `m4_eval(m4_count(m4_set_listc([set])) - 1)`.

#### 8.3.10 Forbidden Patterns ¶

M4sugar provides a means to define suspicious patterns, patterns describing tokens which should not be found in the output. For instance, if an Autoconf `configure` script includes tokens such as ‘`AC_DEFINE`’, or ‘`dnl`’, then most probably something went wrong (typically a macro was not evaluated because of overquotation).

M4sugar forbids all the tokens matching ‘`^_?m4_`’ and ‘`^dnl$`’. Additional layers, such as M4sh and Autoconf, add additional forbidden patterns to the list.

Macro: m4_pattern_forbid(pattern) ¶

  Declare that no token matching pattern must be found in the output. The output file is (temporarily) split into one word per line as part of the `autom4te` post-processing, with each line (and therefore word) then being checked against the Perl regular expression pattern. If the regular expression matches, and `m4_pattern_allow` does not also match, then an error is raised.

  Comments are not checked; this can be a problem if, for instance, you have some macro left unexpanded after an ‘`#include`’. No consensus is currently found in the Autoconf community, as some people consider it should be valid to name macros in comments (which doesn’t make sense to the authors of this documentation: input, such as macros, should be documented by ‘`dnl`’ comments; reserving ‘`#`’-comments to document the output).

  As an example, if you define your own macros that begin with ‘`M_`’ and are composed from capital letters and underscores, the specification of `m4_pattern_forbid([^M_[A-Z_]+])` will ensure all your macros are expanded when not used in comments.

  As an example of a common use of this macro, consider what happens in packages that want to use the `pkg-config` script via the third-party `PKG_CHECK_MODULES` macro. By default, if a developer checks out the development tree but has not yet installed the pkg-config macros locally, they can manage to successfully run `autoconf` on the package, but the resulting `configure` file will likely result in a confusing shell message about a syntax error on the line mentioning the unexpanded `PKG_CHECK_MODULES` macro. On the other hand, if `configure.ac` includes `m4_pattern_forbid([^PKG_])`, the missing pkg-config macros will be detected immediately without allowing `autoconf` to succeed.

Of course, you might encounter exceptions to these generic rules, for instance you might have to refer to ‘`$m4_flags`’.

Macro: m4_pattern_allow(pattern) ¶

  Any token matching pattern is allowed, including if it matches an `m4_pattern_forbid` pattern.

  For example, Gnulib uses `m4_pattern_forbid([^gl_])` to reserve the `gl_` namespace for itself, but also uses `m4_pattern_allow([^gl_ES$])` to avoid a false negative on the valid locale name.

### 8.4 Debugging via autom4te ¶

At times, it is desirable to see what was happening inside m4, to see why output was not matching expectations. However, post-processing done by `autom4te` means that directly using the m4 builtin `m4_traceon` is likely to interfere with operation. Also, frequent diversion changes and the concept of forbidden tokens make it difficult to use `m4_defn` to generate inline comments in the final output.

There are a couple of tools to help with this. One is the use of the `--trace` option provided by `autom4te` (as well as each of the programs that wrap `autom4te`, such as `autoconf`), in order to inspect when a macro is called and with which arguments. For example, when this paragraph was written, the autoconf version could be found by:

    $ autoconf --trace=AC_INIT
    configure.ac:23:AC_INIT:GNU Autoconf:2.63b.95-3963:bug-autoconf@gnu.org
    $ autoconf --trace='AC_INIT:version is $2'
    version is 2.63b.95-3963

Another trick is to print out the expansion of various m4 expressions to standard error or to an independent file, with no further m4 expansion, and without interfering with diversion changes or the post-processing done to standard output. `m4_errprintn` shows a given expression on standard error. For example, if you want to see the expansion of an autoconf primitive or of one of your autoconf macros, you can do it like this:

    $ cat <<\EOF > configure.ac
    AC_INIT
    m4_errprintn([The definition of AC_DEFINE_UNQUOTED:])
    m4_errprintn(m4_defn([AC_DEFINE_UNQUOTED]))
    AC_OUTPUT
    EOF
    $ autoconf
    error→The definition of AC_DEFINE_UNQUOTED:
    error→_AC_DEFINE_Q([], $@)

## 9 Programming in M4sh ¶

M4sh, pronounced “mash”, is aiming at producing portable Bourne shell scripts. This name was coined by Lars J. Aas, who notes that, according to the Webster’s Revised Unabridged Dictionary (1913):

> Mash \Mash\, n. [Akin to G. meisch, maisch, meische, maische, mash, wash, and prob. to AS. miscian to mix. See “Mix”.]

> 1. A mass of mixed ingredients reduced to a soft pulpy state by beating or pressure...
> 2. A mixture of meal or bran and water fed to animals.
> 3. A mess; trouble. [Obs.] –Beau. & Fl.

M4sh reserves the M4 macro namespace ‘`^_AS_`’ for internal use, and the namespace ‘`^AS_`’ for M4sh macros. It also reserves the shell and environment variable namespace ‘`^as_`’, and the here-document delimiter namespace ‘`^_AS[A-Z]`’ in the output file. You should not define your own macros or output shell code that conflicts with these namespaces.

### 9.1 Common Shell Constructs ¶

M4sh provides portable alternatives for some common shell constructs that unfortunately are not portable in practice.

Macro: AS_BOX(text, [char = ‘`-`’]) ¶

  Expand into shell code that will output text surrounded by a box with char in the top and bottom border. text should not contain a newline, but may contain shell expansions valid for unquoted here-documents. char defaults to ‘`-`’, but can be any character except ‘`/`’, ‘`'`’, ‘`"`’, ‘`\`’, ‘`&`’, or ‘```’. This is useful for outputting a comment box into log files to separate distinct phases of script operation.

Macro: AS_CASE(word, [pattern1], [if-matched1], …, [default]) ¶

  Expand into a shell ‘`case`’ statement, where word is matched against one or more patterns. if-matched is run if the corresponding pattern matched word, else default is run. See Prerequisite Macros for why this macro should be used instead of plain ‘`case`’ in code outside of an `AC_DEFUN` macro, when the contents of the ‘`case`’ use `AC_REQUIRE` directly or indirectly. See Limitations of Shell Builtins, for how this macro avoids some portability issues. See Dealing with unbalanced parentheses for how this macro lets you write code with balanced parentheses even if your code must run on obsolescent shells.

Macro: AS_DIRNAME(file-name) ¶

  Output the directory portion of file-name. For example, if `$file` is ‘`/one/two/three`’, the command `dir=`AS_DIRNAME(["$file"])`` sets `dir` to ‘`/one/two`’.

  `AS_DIRNAME` was designed long ago when the `dirname` command was not universally supported. Nowadays one can safely use `dir=`dirname -- "$file"`` instead. This interface may be improved in the future to avoid forks and losing trailing newlines.

Macro: AS_ECHO(word) ¶

  Emit word to the standard output, followed by a newline. The word must be a single shell word (typically a quoted string). Output the shell expansion of word as-is, even if it starts with ‘`-`’ or contains ‘`\`’. Redirections can be placed outside the macro invocation.

  If the shell variable foo could contain ‘`\`’ or leading ‘`-`’. `AS_ECHO(["$foo"])` is more portable than `echo "$foo"`. See Limitations of Shell Builtins.

  Also, `AS_ECHO(["$foo"])` is often easier to read than the ‘`printf '%s\n' "$foo"`’ that it stands for. However, because it employs ‘`'`’ characters, in contexts where ‘`'`’ is not allowed it is better to use `printf` directly. For example, ‘``eval 'foo=${'AS_ESCAPE([[$1]], [`\])'};printf "%s\\n" "$foo")'``’ would not work if `printf` were replaced with `AS_ECHO`.

Macro: AS_ECHO_N(word) ¶

  Act like `AS_ECHO(word)`, except do not output a following newline.

Macro: AS_ESCAPE(string, [chars = ‘``\"$`’]) ¶

  Expands to string, with any characters in chars escaped with a backslash (‘`\`’). chars should be at most four bytes long, and only contain characters from the set ‘``\"$`’; however, characters may be safely listed more than once in chars for the sake of syntax highlighting editors. The current implementation expands string after adding escapes; if string contains macro calls that in turn expand to text needing shell quoting, you can use `AS_ESCAPE(m4_dquote(m4_expand([string])))`.

  The default for chars (‘`\"$``’) is the set of characters needing escapes when string will be used literally within double quotes. One common variant is the set of characters to protect when string will be used literally within back-ticks or an unquoted here-document (‘`\$``’). Another common variant is ‘`""`’, which can be used to form a double-quoted string containing the same expansions that would have occurred if string were expanded in an unquoted here-document; however, when using this variant, care must be taken that string does not use double quotes within complex variable expansions (such as ‘`${foo-`echo "hi"`}`’) that would be broken with improper escapes.

  This macro is often used with `AS_ECHO`. For an example, observe the output generated by the shell code generated from this snippet:

      foo=bar
      AS_ECHO(["AS_ESCAPE(["$foo" = ])AS_ESCAPE(["$foo"], [""])"])
      ⇒"$foo" = "bar"
      m4_define([macro], [a, [\b]])
      AS_ECHO(["AS_ESCAPE([[macro]])"])
      ⇒macro
      AS_ECHO(["AS_ESCAPE([macro])"])
      ⇒a, b
      AS_ECHO(["AS_ESCAPE(m4_dquote(m4_expand([macro])))"])
      ⇒a, \b

  To escape a string that will be placed within single quotes, use:

      m4_bpatsubst([[string]], ['], ['\\''])

Macro: AS_EXECUTABLE_P(file) ¶

  Emit code to probe whether file is a regular file with executable permissions (and not a directory with search permissions). The caller is responsible for quoting file.

Macro: AS_EXIT([status = ‘`$?`’]) ¶

  Emit code to exit the shell with status, defaulting to ‘`$?`’. This macro works around shells that see the exit status of the command prior to `exit` inside a ‘`trap 0`’ handler (see Limitations of Shell Builtins).

Macro: AS_IF(test1, [run-if-true1], …, [run-if-false]) ¶

  Run shell code test1. If test1 exits with a zero status then run shell code run-if-true1, else examine further tests. If no test exits with a zero status, run shell code run-if-false, with simplifications if either run-if-true1 or run-if-false is empty. For example,

      AS_IF([test "x$foo" = xyes], [HANDLE_FOO([yes])],
            [test "x$foo" != xno], [HANDLE_FOO([maybe])],
            [echo foo not specified])

  ensures any required macros of `HANDLE_FOO` are expanded before the first test.

  This macro should be used instead of plain ‘`if`’ in code outside of an `AC_DEFUN` macro, when the contents of the ‘`if`’ use `AC_REQUIRE` directly or indirectly (see Prerequisite Macros).

Macro: AS_MKDIR_P(file-name) ¶

  Make the directory file-name, including intervening directories as necessary. This is equivalent to ‘`mkdir -p -- file-name`’. If creation of file-name fails, exit the script.

  Also see the `AC_PROG_MKDIR_P` macro (see Particular Program Checks).

Macro: AS_SET_STATUS(status) ¶

  Emit shell code to set the value of ‘`$?`’ to status, as efficiently as possible. However, this is not guaranteed to abort a shell running with `set -e` (see Limitations of Shell Builtins). This should also be used at the end of a complex shell function instead of ‘`return`’ (see Shell Functions) to avoid a DJGPP shell bug.

Macro: AS_TR_CPP(expression) ¶

  Transform expression into a valid right-hand side for a C `#define`. For example:

      # This outputs "#define HAVE_CHAR_P 1".
      # Notice the m4 quoting around #, to prevent an m4 comment
      type="char *"
      echo "[#]define AS_TR_CPP([HAVE_$type]) 1"

Macro: AS_TR_SH(expression) ¶

  Transform expression into shell code that generates a valid shell variable name. The result is literal when possible at m4 time, but must be used with `eval` if expression causes shell indirections. For example:

      # This outputs "Have it!".
      header="sys/some file.h"
      eval AS_TR_SH([HAVE_$header])=yes
      if test "x$HAVE_sys_some_file_h" = xyes; then echo "Have it!"; fi

Macro: AS_SET_CATFILE(var, dir, file) ¶

  Set the polymorphic shell variable var to dir/file, but optimizing the common cases (dir or file is ‘`.`’, file is absolute, etc.).

Macro: AS_UNSET(var) ¶

  Unsets the shell variable var, working around bugs in older shells (see Limitations of Shell Builtins). var can be a literal or indirect variable name.

Macro: AS_VERSION_COMPARE(version-1, version-2, [action-if-less], [action-if-equal], [action-if-greater]) ¶

  Compare two strings version-1 and version-2, possibly containing shell variables, as version strings, and expand action-if-less, action-if-equal, or action-if-greater depending upon the result. The algorithm to compare is similar to the one used by strverscmp in glibc (see String/Array Comparison in The GNU C Library).

### 9.2 Support for indirect variable names ¶

Often, it is convenient to write a macro that will emit shell code operating on a shell variable. The simplest case is when the variable name is known. But a more powerful idiom is writing shell code that can work through an indirection, where another variable or command substitution produces the name of the variable to actually manipulate. M4sh supports the notion of polymorphic shell variables, making it easy to write a macro that can deal with either literal or indirect variable names and output shell code appropriate for both use cases. Behavior is undefined if expansion of an indirect variable does not result in a literal variable name.

Macro: AS_LITERAL_IF(expression, [if-literal], [if-not], [if-simple-ref = if-not]) ¶
Macro: AS_LITERAL_WORD_IF(expression, [if-literal], [if-not], [if-simple-ref = if-not]) ¶

  If the expansion of expression is definitely a shell literal, expand if-literal. If the expansion of expression looks like it might contain shell indirections (such as `$var` or ``expr``), then if-not is expanded. Sometimes, it is possible to output optimized code if expression consists only of shell variable expansions (such as `${var}`), in which case if-simple-ref can be provided; but defaulting to if-not should always be safe. `AS_LITERAL_WORD_IF` only expands if-literal if expression looks like a single shell word, containing no whitespace; while `AS_LITERAL_IF` allows whitespace in expression.

  In order to reduce the time spent recognizing whether an expression qualifies as a literal or a simple indirection, the implementation is somewhat conservative: expression must be a single shell word (possibly after stripping whitespace), consisting only of bytes that would have the same meaning whether unquoted or enclosed in double quotes (for example, ‘`a.b`’ results in if-literal, even though it is not a valid shell variable name; while both ‘`'a'`’ and ‘`[$]`’ result in if-not, because they behave differently than ‘`"'a'"`’ and ‘`"[$]"`’). This macro can be used in contexts for recognizing portable file names (such as in the implementation of `AC_LIBSOURCE`), or coupled with some transliterations for forming valid variable names (such as in the implementation of `AS_TR_SH`, which uses an additional `m4_translit` to convert ‘`.`’ to ‘`_`’).

  This example shows how to read the contents of the shell variable `bar`, exercising all three arguments to `AS_LITERAL_IF`. It results in a script that will output the line ‘`hello`’ three times.

      AC_DEFUN([MY_ACTION],
      [AS_LITERAL_IF([$1],
        [AS_ECHO(["$$1"])],
        [AS_VAR_COPY([var], [$1])
         AS_ECHO(["$var"])],
        [AS_ECHO(["$'"$1"\"])])])
      foo=bar bar=hello
      MY_ACTION([bar])
      MY_ACTION([`echo bar`])
      MY_ACTION([$foo])

Macro: AS_VAR_APPEND(var, text) ¶

  Emit shell code to append the shell expansion of text to the end of the current contents of the polymorphic shell variable var, taking advantage of shells that provide the ‘`+=`’ extension for more efficient scaling.

  For situations where the final contents of var are relatively short (less than 256 bytes), it is more efficient to use the simpler code sequence of `var=${var}text` (or its polymorphic equivalent of `AS_VAR_COPY([t], [var])` and `AS_VAR_SET([var], ["$t"text])`). But in the case when the script will be repeatedly appending text into `var`, issues of scaling start to become apparent. A naive implementation requires execution time linear to the length of the current contents of var as well as the length of text for a single append, for an overall quadratic scaling with multiple appends. This macro takes advantage of shells which provide the extension `var+=text`, which can provide amortized constant time for a single append, for an overall linear scaling with multiple appends. Note that unlike `AS_VAR_SET`, this macro requires that text be quoted properly to avoid field splitting and file name expansion.

Macro: AS_VAR_ARITH(var, expression) ¶

  Emit shell code to compute the arithmetic expansion of expression, assigning the result as the contents of the polymorphic shell variable var. The code takes advantage of shells that provide ‘`$(())`’ for fewer forks, but uses `expr` as a fallback. Therefore, the syntax for a valid expression is rather limited: all operators must occur as separate shell arguments and with proper quoting; the only operators supported are ‘`*`’, ‘`/`’, ‘`%`’, binary ‘`+`’, binary ‘`-`’, ‘`>`’, ‘`>=`’, ‘`<`’, ‘`<=`’, ‘`!=`’, ‘`&`’, and ‘`|`’; all variables containing numbers must be expanded prior to the computation; the first shell argument must not start with ‘`-`’; and each number must be an optional ‘`-`’ followed by one or more decimal digits, where the first digit is nonzero if there is more than one digit. In the following example, this snippet will print ‘`(2+3)*4 == 20`’.

      bar=3
      AS_VAR_ARITH([foo], [\( 2 + $bar \) \* 4])
      echo "(2+$bar)*4 == $foo"

Macro: AS_VAR_COPY(dest, source) ¶

  Emit shell code to assign the contents of the polymorphic shell variable source to the polymorphic shell variable dest. For example, executing this M4sh snippet will output ‘`bar hi`’:

      foo=bar bar=hi
      AS_VAR_COPY([a], [foo])
      AS_VAR_COPY([b], [$foo])
      echo "$a $b"

  When it is necessary to access the contents of an indirect variable inside a shell double-quoted context, the recommended idiom is to first copy the contents into a temporary literal shell variable.

      for header in stdint_h inttypes_h ; do
        AS_VAR_COPY([var], [ac_cv_header_$header])
        AS_ECHO(["$header detected: $var"])
      done

Macro: AS_VAR_IF(var, [word], [if-equal], [if-not-equal]) ¶

  Output a shell conditional statement. If the contents of the polymorphic shell variable var match the string word, execute if-equal; otherwise execute if-not-equal. word must be a single shell word (typically a quoted string). Avoids shell bugs if an interrupt signal arrives while a command substitution in var is being expanded.

Macro: AS_VAR_PUSHDEF(m4-name, value) ¶
Macro: AS_VAR_POPDEF(m4-name) ¶

  A common M4sh idiom involves composing shell variable names from an m4 argument (for example, writing a macro that uses a cache variable). value can be an arbitrary string, which will be transliterated into a valid shell name by `AS_TR_SH`. In order to access the composed variable name based on value, it is easier to declare a temporary m4 macro m4-name with `AS_VAR_PUSHDEF`, then use that macro as the argument to subsequent `AS_VAR` macros as a polymorphic variable name, and finally free the temporary macro with `AS_VAR_POPDEF`. These macros are often followed with `dnl`, to avoid excess newlines in the output.

  Here is an involved example, that shows the power of writing macros that can handle composed shell variable names:

      m4_define([MY_CHECK_HEADER],
      [AS_VAR_PUSHDEF([my_Header], [ac_cv_header_$1])dnl
      AS_VAR_IF([my_Header], [yes], [echo "header $1 detected"])dnl
      AS_VAR_POPDEF([my_Header])dnl
      ])
      MY_CHECK_HEADER([stdint.h])
      for header in inttypes.h stdlib.h ; do
        MY_CHECK_HEADER([$header])
      done

  In the above example, `MY_CHECK_HEADER` can operate on polymorphic variable names. In the first invocation, the m4 argument is `stdint.h`, which transliterates into a literal `stdint_h`. As a result, the temporary macro `my_Header` expands to the literal shell name ‘`ac_cv_header_stdint_h`’. In the second invocation, the m4 argument to `MY_CHECK_HEADER` is `$header`, and the temporary macro `my_Header` expands to the indirect shell name ‘`$as_my_Header`’. During the shell execution of the for loop, when ‘`$header`’ contains ‘`inttypes.h`’, then ‘`$as_my_Header`’ contains ‘`ac_cv_header_inttypes_h`’. If this script is then run on a platform where all three headers have been previously detected, the output of the script will include:

      header stdint.h detected
      header inttypes.h detected
      header stdlib.h detected

Macro: AS_VAR_SET(var, [value]) ¶

  Emit shell code to assign the contents of the polymorphic shell variable var to the shell expansion of value. value is not subject to field splitting or file name expansion, so if command substitution is used, it may be done with ‘``""``’ rather than using an intermediate variable (see Shell Substitutions). However, value does undergo rescanning for additional macro names; behavior is unspecified if late expansion results in any shell meta-characters.

Macro: AS_VAR_SET_IF(var, [if-set], [if-undef]) ¶

  Emit a shell conditional statement, which executes if-set if the polymorphic shell variable `var` is set to any value, and if-undef otherwise.

Macro: AS_VAR_TEST_SET(var) ¶

  Emit a shell statement that results in a successful exit status only if the polymorphic shell variable `var` is set.

### 9.3 Initialization Macros ¶

Macro: AS_BOURNE_COMPATIBLE ¶

  Set up the shell to be more compatible with the Bourne shell as standardized by POSIX, if possible. This may involve setting environment variables, or setting options, or similar implementation-specific actions. This macro is deprecated, since `AS_INIT` already invokes it.

Macro: AS_INIT ¶

  Initialize the M4sh environment. This macro calls `m4_init`, then outputs the `#! /bin/sh` line, a notice about where the output was generated from, and code to sanitize the environment for the rest of the script. Among other initializations, this sets `SHELL` to the shell chosen to run the script (see CONFIG_SHELL), and `LC_ALL` to ensure the C locale. Finally, it changes the current diversion to `BODY`. `AS_INIT` is called automatically by `AC_INIT` and `AT_INIT`, so shell code in `configure`, `config.status`, and `testsuite` all benefit from a sanitized shell environment.

Macro: AS_INIT_GENERATED(file, [comment]) ¶

  Emit shell code to start the creation of a subsidiary shell script in file, including changing file to be executable. This macro populates the child script with information learned from the parent (thus, the emitted code is equivalent in effect, but more efficient, than the code output by `AS_INIT`, `AS_BOURNE_COMPATIBLE`, and `AS_SHELL_SANITIZE`). If present, comment is output near the beginning of the child, prior to the shell initialization code, and is subject to parameter expansion, command substitution, and backslash quote removal. The parent script should check the exit status after this macro, in case file could not be properly created (for example, if the disk was full). If successfully created, the parent script can then proceed to append additional M4sh constructs into the child script.

  Note that the child script starts life without a log file open, so if the parent script uses logging (see AS_MESSAGE_LOG_FD), you must temporarily disable any attempts to use the log file until after emitting code to open a log within the child. On the other hand, if the parent script has `AS_MESSAGE_FD` redirected somewhere besides ‘`1`’, then the child script already has code that copies stdout to that descriptor. Currently, the suggested idiom for writing a M4sh shell script from within another script is:

      AS_INIT_GENERATED([file], [[# My child script.
      ]]) || { AS_ECHO(["Failed to create child script"]); AS_EXIT; }
      m4_pushdef([AS_MESSAGE_LOG_FD])dnl
      cat >> "file" <<\__EOF__
      # Code to initialize AS_MESSAGE_LOG_FD
      m4_popdef([AS_MESSAGE_LOG_FD])dnl
      # Additional code
      __EOF__

  This, however, may change in the future as the M4sh interface is stabilized further.

  Also, be aware that use of `LINENO` within the child script may report line numbers relative to their location in the parent script, even when using `AS_LINENO_PREPARE`, if the parent script was unable to locate a shell with working `LINENO` support.

Macro: AS_LINENO_PREPARE ¶

  Find a shell that supports the special variable `LINENO`, which contains the number of the currently executing line. This macro is automatically invoked by `AC_INIT` in configure scripts.

Macro: AS_ME_PREPARE ¶

  Set up variable `as_me` to be the basename of the currently executing script. This macro is automatically invoked by `AC_INIT` in configure scripts.

Macro: AS_TMPDIR(prefix, [dir = ‘`${TMPDIR:=/tmp}`’]) ¶

  Create, as safely as possible, a temporary sub-directory within dir with a name starting with prefix. prefix should be 2–4 characters, to make it slightly easier to identify the owner of the directory. If dir is omitted, then the value of `TMPDIR` will be used (defaulting to ‘`/tmp`’). On success, the name of the newly created directory is stored in the shell variable `tmp`. On error, the script is aborted.

  Typically, this macro is coupled with some exit traps to delete the created directory and its contents on exit or interrupt. However, there is a slight window between when the directory is created and when the name is actually known to the shell, so an interrupt at the right moment might leave the temporary directory behind. Hence it is important to use a prefix that makes it easier to determine if a leftover temporary directory from an interrupted script is safe to delete.

  If you set `TMPDIR=$tmp` after invoking this macro, you should reset `TMPDIR` before deleting the created directory, to avoid breaking commands that rely on `$TMPDIR`.

  The use of the output variable ‘`$tmp`’ rather than something in the ‘`as_`’ namespace is historical; it has the unfortunate consequence that reusing this otherwise common name for any other purpose inside your script has the potential to break any cleanup traps designed to remove the temporary directory.

Macro: AS_SHELL_SANITIZE ¶

  Initialize the shell suitably for `configure` scripts. This has the effect of `AS_BOURNE_COMPATIBLE`, and sets some other environment variables for predictable results from configuration tests. For example, it sets `LC_ALL` to change to the default C locale. See Special Shell Variables. This macro is deprecated, since `AS_INIT` already invokes it.

### 9.4 File Descriptor Macros ¶

The following macros define file descriptors used to output messages (or input values) from `configure` scripts. For example:

    AS_ECHO(["$wombats found"]) >&AS_MESSAGE_LOG_FD
    AS_ECHO_N(['Enter desired kangaroo count: ']) >&AS_MESSAGE_FD
    read kangaroos <&AS_ORIGINAL_STDIN_FD

However doing so is seldom needed, because Autoconf provides higher level macros as described below.

Macro: AS_MESSAGE_FD ¶

  The file descriptor for ‘`checking for...`’ messages and results. By default, `AS_INIT` sets this to ‘`1`’ for standalone M4sh clients. However, `AC_INIT` shuffles things around to another file descriptor, in order to allow the `-q` option of `configure` to choose whether messages should go to the script’s standard output or be discarded.

  If you want to display some messages, consider using one of the printing macros (see Printing Messages) instead. Copies of messages output via these macros are also recorded in `config.log`.

Macro: AS_MESSAGE_LOG_FD ¶

  This must either be empty, or expand to a file descriptor for log messages. By default, `AS_INIT` sets this macro to the empty string for standalone M4sh clients, thus disabling logging. However, `AC_INIT` shuffles things around so that both `configure` and `config.status` use `config.log` for log messages. Macros that run tools, like `AC_COMPILE_IFELSE` (see Running the Compiler), redirect all output to this descriptor. You may want to do so if you develop such a low-level macro.

Macro: AS_ORIGINAL_STDIN_FD ¶

  This must expand to a file descriptor for the original standard input. By default, `AS_INIT` sets this macro to ‘`0`’ for standalone M4sh clients. However, `AC_INIT` shuffles things around for safety.

  When `configure` runs, it may accidentally execute an interactive command that has the same name as the non-interactive meant to be used or checked. If the standard input was the terminal, such interactive programs would cause `configure` to stop, pending some user input. Therefore `configure` redirects its standard input from `/dev/null` during its initialization. This is not normally a problem, since `configure` normally does not need user input.

  In the extreme case where your `configure` script really needs to obtain some values from the original standard input, you can read them explicitly from `AS_ORIGINAL_STDIN_FD`.

## 10 Writing Autoconf Macros ¶

When you write a feature test that could be applicable to more than one software package, the best thing to do is encapsulate it in a new macro. Here are some instructions and guidelines for writing Autoconf macros. You should also familiarize yourself with M4sugar (see Programming in M4) and M4sh (see Programming in M4sh).

### 10.1 Macro Definitions ¶

Macro: AC_DEFUN(name, [body]) ¶

  Autoconf macros are defined using the `AC_DEFUN` macro, which is similar to the M4 builtin `m4_define` macro; this creates a macro named name and with body as its expansion. In addition to defining a macro, `AC_DEFUN` adds to it some code that is used to constrain the order in which macros are called, while avoiding redundant output (see Prerequisite Macros).

An Autoconf macro definition looks like this:

    AC_DEFUN(macro-name, macro-body)

You can refer to any arguments passed to the macro as ‘`$1`’, ‘`$2`’, etc. See How to define new macros in GNU M4, for more complete information on writing M4 macros.

Most macros fall in one of two general categories. The first category includes macros which take arguments, in order to generate output parameterized by those arguments. Macros in this category are designed to be directly expanded, often multiple times, and should not be used as the argument to `AC_REQUIRE`. The other category includes macros which are shorthand for a fixed block of text, and therefore do not take arguments. For this category of macros, directly expanding the macro multiple times results in redundant output, so it is more common to use the macro as the argument to `AC_REQUIRE`, or to declare the macro with `AC_DEFUN_ONCE` (see One-Shot Macros).

Be sure to properly quote both the macro-body and the macro-name to avoid any problems if the macro happens to have been previously defined.

Each macro should have a header comment that gives its prototype, and a brief description. When arguments have default values, display them in the prototype. For example:

    # AC_MSG_ERROR(ERROR, [EXIT-STATUS = 1])
    # --------------------------------------
    m4_define([AC_MSG_ERROR],
      [{ AS_MESSAGE([error: $1], [2])
         exit m4_default([$2], [1]); }])

Comments about the macro should be left in the header comment. Most other comments make their way into `configure`, so just keep using ‘`#`’ to introduce comments.

If you have some special comments about pure M4 code, comments that make no sense in `configure` and in the header comment, then use the builtin `dnl`: it causes M4 to discard the text through the next newline.

Keep in mind that `dnl` is rarely needed to introduce comments; `dnl` is more useful to get rid of the newlines following macros that produce no output, such as `AC_REQUIRE`.

Public third-party macros need to use `AC_DEFUN`, and not `m4_define`, in order to be found by `aclocal` (see Extending aclocal in GNU Automake). Additionally, if it is ever determined that a macro should be made obsolete, it is easy to convert from `AC_DEFUN` to `AU_DEFUN` in order to have `autoupdate` assist the user in choosing a better alternative, but there is no corresponding way to make `m4_define` issue an upgrade notice (see AU_DEFUN).

There is another subtle, but important, difference between using `m4_define` and `AC_DEFUN`: only the former is unaffected by `AC_REQUIRE`. When writing a file, it is always safe to replace a block of text with a `m4_define` macro that will expand to the same text. But replacing a block of text with an `AC_DEFUN` macro with the same content does not necessarily give the same results, because it changes the location where any embedded but unsatisfied `AC_REQUIRE` invocations within the block will be expanded. For an example of this, see Expanded Before Required.

### 10.2 Macro Names ¶

All of the public Autoconf macros have all-uppercase names in the namespace ‘`^AC_`’ to prevent them from accidentally conflicting with other text; Autoconf also reserves the namespace ‘`^_AC_`’ for internal macros. All shell variables that they use for internal purposes have mostly-lowercase names starting with ‘`ac_`’. Autoconf also uses here-document delimiters in the namespace ‘`^_AC[A-Z]`’. During `configure`, files produced by Autoconf make heavy use of the file system namespace ‘`^conf`’.

Since Autoconf is built on top of M4sugar (see Programming in M4sugar) and M4sh (see Programming in M4sh), you must also be aware of those namespaces (‘`^_?\(m4\|AS\)_`’). And since `configure.ac` is also designed to be scanned by Autoheader, Autoscan, Autoupdate, and Automake, you should be aware of the ‘`^_?A[HNUM]_`’ namespaces. In general, you should not use the namespace of a package that does not own the macro or shell code you are writing.

To ensure that your macros don’t conflict with present or future Autoconf macros, you should prefix your own macro names and any shell variables they use with some other sequence. Possibilities include your initials, or an abbreviation for the name of your organization or software package. Historically, people have not always followed the rule of using a namespace appropriate for their package, and this has made it difficult for determining the origin of a macro (and where to report bugs about that macro), as well as difficult for the true namespace owner to add new macros without interference from pre-existing uses of third-party macros. Perhaps the best example of this confusion is the `AM_GNU_GETTEXT` macro, which belongs, not to Automake, but to Gettext.

Most of the Autoconf macros’ names follow a structured naming convention that indicates the kind of feature check by the name. The macro names consist of several words, separated by underscores, going from most general to most specific. The names of their cache variables use the same convention (see Cache Variable Names, for more information on them).

The first word of the name after the namespace initials (such as ‘`AC_`’) usually tells the category of the feature being tested. Here are the categories used in Autoconf for specific test macros, the kind of macro that you are more likely to write. They are also used for cache variables, in all-lowercase. Use them where applicable; where they’re not, invent your own categories.

`C`

  C language builtin features.

`DECL`

  Declarations of C variables in header files.

`FUNC`

  Functions in libraries.

`GROUP`

  POSIX group owners of files.

`HEADER`

  Header files.

`LIB`

  C libraries.

`PROG`

  The base names of programs.

`MEMBER`

  Members of aggregates.

`SYS`

  Operating system features.

`TYPE`

  C builtin or declared types.

`VAR`

  C variables in libraries.

After the category comes the name of the particular feature being tested. Any further words in the macro name indicate particular aspects of the feature. For example, `AC_PROG_MAKE_SET` checks whether `make` sets a variable to its own name.

An internal macro should have a name that starts with an underscore; Autoconf internals should therefore start with ‘`_AC_`’. Additionally, a macro that is an internal subroutine of another macro should have a name that starts with an underscore and the name of that other macro, followed by one or more words saying what the internal macro does. For example, `AC_PATH_X` has internal macros `_AC_PATH_X_XMKMF` and `_AC_PATH_X_DIRECT`.

### 10.3 Dependencies Between Macros ¶

Some Autoconf macros depend on other macros having been called first in order to work correctly. Autoconf provides a way to ensure that certain macros are called if needed and a way to warn the user if macros are called in an order that might cause incorrect operation.

#### 10.3.1 Prerequisite Macros ¶

A macro that you write might need to use values that have previously been computed by other macros. For example, `AC_DECL_YYTEXT` examines the output of `flex` or `lex`, so it depends on `AC_PROG_LEX` having been called first to set the shell variable `LEX`.

Rather than forcing the user of the macros to keep track of the dependencies between them, you can use the `AC_REQUIRE` macro to do it automatically. `AC_REQUIRE` can ensure that a macro is only called if it is needed, and only called once.

Macro: AC_REQUIRE(macro-name) ¶

  If the M4 macro macro-name has not already been called, call it (without any arguments). Make sure to quote macro-name with square brackets. macro-name must have been defined using `AC_DEFUN` or else contain a call to `AC_PROVIDE` to indicate that it has been called.

  `AC_REQUIRE` must be used inside a macro defined by `AC_DEFUN`; it must not be called from the top level. Also, it does not make sense to require a macro that takes parameters.

`AC_REQUIRE` is often misunderstood. It really implements dependencies between macros in the sense that if one macro depends upon another, the latter is expanded before the body of the former. To be more precise, the required macro is expanded before the outermost defined macro in the current expansion stack. In particular, ‘`AC_REQUIRE([FOO])`’ is not replaced with the body of `FOO`. For instance, this definition of macros:

    AC_DEFUN([TRAVOLTA],
    [test "$body_temperature_in_Celsius" -gt 38 &&
      dance_floor=occupied])
    AC_DEFUN([NEWTON_JOHN],
    [test "x$hair_style" = xcurly &&
      dance_floor=occupied])

    AC_DEFUN([RESERVE_DANCE_FLOOR],
    [if test "x`date +%A`" = xSaturday; then
      AC_REQUIRE([TRAVOLTA])
      AC_REQUIRE([NEWTON_JOHN])
    fi])

with this `configure.ac`

    AC_INIT([Dance Manager], [1.0], [bug-dance@example.org])
    RESERVE_DANCE_FLOOR
    if test "x$dance_floor" = xoccupied; then
      AC_MSG_ERROR([cannot pick up here, let's move])
    fi

does not leave you with a better chance to meet a kindred soul on days other than Saturday, since the call to `RESERVE_DANCE_FLOOR` expands to:

    test "$body_temperature_in_Celsius" -gt 38 &&
      dance_floor=occupied
    test "x$hair_style" = xcurly &&
      dance_floor=occupied
    if test "x`date +%A`" = xSaturday; then

    fi

This behavior was chosen on purpose: (i) it prevents messages in required macros from interrupting the messages in the requiring macros; (ii) it avoids bad surprises when shell conditionals are used, as in:

    if ...; then
      AC_REQUIRE([SOME_CHECK])
    fi
    ...
    SOME_CHECK

However, this implementation can lead to another class of problems. Consider the case where an outer macro first expands, then indirectly requires, an inner macro:

    AC_DEFUN([TESTA], [[echo in A
    if test -n "$SEEN_A" ; then echo duplicate ; fi
    SEEN_A=:]])
    AC_DEFUN([TESTB], [AC_REQUIRE([TESTA])[echo in B
    if test -z "$SEEN_A" ; then echo bug ; fi]])
    AC_DEFUN([TESTC], [AC_REQUIRE([TESTB])[echo in C]])
    AC_DEFUN([OUTER], [[echo in OUTER]
    TESTA
    TESTC])
    OUTER

Prior to Autoconf 2.64, the implementation of `AC_REQUIRE` recognized that `TESTB` needed to be hoisted prior to the expansion of `OUTER`, but because `TESTA` had already been directly expanded, it failed to hoist `TESTA`. Therefore, the expansion of `TESTB` occurs prior to its prerequisites, leading to the following output:

    in B
    bug
    in OUTER
    in A
    in C

Newer Autoconf is smart enough to recognize this situation, and hoists `TESTA` even though it has already been expanded, but issues a syntax warning in the process. This is because the hoisted expansion of `TESTA` defeats the purpose of using `AC_REQUIRE` to avoid redundant code, and causes its own set of problems if the hoisted macro is not idempotent:

    in A
    in B
    in OUTER
    in A
    duplicate
    in C

The bug is not in Autoconf, but in the macro definitions. If you ever pass a particular macro name to `AC_REQUIRE`, then you are implying that the macro only needs to be expanded once. But to enforce this, either the macro must be declared with `AC_DEFUN_ONCE` (although this only helps in Autoconf 2.64 or newer), or all uses of that macro should be through `AC_REQUIRE`; directly expanding the macro defeats the point of using `AC_REQUIRE` to eliminate redundant expansion. In the example, this rule of thumb was violated because `TESTB` requires `TESTA` while `OUTER` directly expands it. One way of fixing the bug is to factor `TESTA` into two macros, the portion designed for direct and repeated use (here, named `TESTA`), and the portion designed for one-shot output and used only inside `AC_REQUIRE` (here, named `TESTA_PREREQ`). Then, by fixing all clients to use the correct calling convention according to their needs:

    AC_DEFUN([TESTA], [AC_REQUIRE([TESTA_PREREQ])[echo in A]])
    AC_DEFUN([TESTA_PREREQ], [[echo in A_PREREQ
    if test -n "$SEEN_A" ; then echo duplicate ; fi
    SEEN_A=:]])
    AC_DEFUN([TESTB], [AC_REQUIRE([TESTA_PREREQ])[echo in B
    if test -z "$SEEN_A" ; then echo bug ; fi]])
    AC_DEFUN([TESTC], [AC_REQUIRE([TESTB])[echo in C]])
    AC_DEFUN([OUTER], [[echo in OUTER]
    TESTA
    TESTC])
    OUTER

the resulting output will then obey all dependency rules and avoid any syntax warnings, whether the script is built with old or new Autoconf versions:

    in A_PREREQ
    in B
    in OUTER
    in A
    in C

You can use the helper macros `AS_IF` and `AS_CASE` in top-level code to enforce expansion of required macros outside of shell conditional constructs; these helpers are not needed in the bodies of macros defined by `AC_DEFUN`. You are furthermore encouraged, although not required, to put all `AC_REQUIRE` calls at the beginning of a macro. You can use `dnl` to avoid the empty lines they leave.

Autoconf will normally warn if an `AC_REQUIRE` call refers to a macro that has not been defined. However, the `aclocal` tool relies on parsing an incomplete set of input files to trace which macros have been required, in order to then pull in additional files that provide those macros; for this particular use case, pre-defining the macro `m4_require_silent_probe` will avoid the warnings.

#### 10.3.2 Suggested Ordering ¶

Some macros should be run before another macro if both are called, but neither requires that the other be called. For example, a macro that changes the behavior of the C compiler should be called before any macros that run the C compiler. Many of these dependencies are noted in the documentation.

Autoconf provides the `AC_BEFORE` macro to warn users when macros with this kind of dependency appear out of order in a `configure.ac` file. The warning occurs when creating `configure` from `configure.ac`, not when running `configure`.

For example, `AC_PROG_CPP` checks whether the C compiler can run the C preprocessor when given the `-E` option. It should therefore be called after any macros that change which C compiler is being used, such as `AC_PROG_CC`. So `AC_PROG_CC` contains:

    AC_BEFORE([$0], [AC_PROG_CPP])dnl

This warns the user if a call to `AC_PROG_CPP` has already occurred when `AC_PROG_CC` is called.

Macro: AC_BEFORE(this-macro-name, called-macro-name) ¶

  Make M4 print a warning message to the standard error output if called-macro-name has already been called. this-macro-name should be the name of the macro that is calling `AC_BEFORE`. The macro called-macro-name must have been defined using `AC_DEFUN` or else contain a call to `AC_PROVIDE` to indicate that it has been called.

#### 10.3.3 One-Shot Macros ¶

Some macros should be called only once, either because calling them multiple time is unsafe, or because it is bad style. For instance Autoconf ensures that `AC_CANONICAL_BUILD` and cousins (see Getting the Canonical System Type) are evaluated only once, because it makes no sense to run these expensive checks more than once. Such one-shot macros can be defined using `AC_DEFUN_ONCE`.

Macro: AC_DEFUN_ONCE(macro-name, macro-body) ¶

  Declare macro macro-name like `AC_DEFUN` would (see Macro Definitions), but add additional logic that guarantees that only the first use of the macro (whether by direct expansion or `AC_REQUIRE`) causes an expansion of macro-body; the expansion will occur before the start of any enclosing macro defined by `AC_DEFUN`. Subsequent expansions are silently ignored. Generally, it does not make sense for macro-body to use parameters such as `$1`.

Prior to Autoconf 2.64, a macro defined by `AC_DEFUN_ONCE` would emit a warning if it was directly expanded a second time, so for portability, it is better to use `AC_REQUIRE` than direct invocation of macro-name inside a macro defined by `AC_DEFUN` (see Prerequisite Macros).

### 10.4 Obsoleting Macros ¶

Configuration and portability technology has evolved over the years. Often better ways of solving a particular problem are developed, or ad-hoc approaches are systematized. This process has occurred in many parts of Autoconf. One result is that some of the macros are now considered obsolete; they still work, but are no longer considered the best thing to do, hence they should be replaced with more modern macros. Ideally, `autoupdate` should replace the old macro calls with their modern implementation.

Autoconf provides a simple means to obsolete a macro.

Macro: AU_DEFUN(old-macro, implementation, [message], [silent]) ¶

  Define old-macro as implementation, just like `AC_DEFUN`, but also declare old-macro to be obsolete. When `autoupdate` is run, occurrences of old-macro will be replaced by the text of implementation in the updated `configure.ac` file.

  If a simple textual replacement is not enough to finish the job of updating a `configure.ac` to modern style, provide instructions for whatever additional manual work is required as message. These instructions will be printed by `autoupdate`, and embedded in the updated `configure.ac` file, next to the text of implementation.

  Normally, `autoconf` will also issue a warning (in the “obsolete” category) when it expands old-macro. This warning does not include message; it only advises the maintainer to run `autoupdate`. If it is inappropriate to issue this warning, set the silent argument to the word `silent`. One might want to use a silent `AU_DEFUN` when old-macro is used in a widely-distributed third-party macro. If that macro’s maintainers are aware of the need to update their code, it’s unnecessary to nag all of the transitive users of old-macro as well. This capability was added to `AU_DEFUN` in Autoconf 2.70; older versions of autoconf will ignore the silent argument and issue the warning anyway.

  Caution: If implementation contains M4 or M4sugar macros, they will be evaluated when `autoupdate` is run, not emitted verbatim like the rest of implementation. This cannot be avoided with extra quotation, because then old-macro will not work when it is called normally. See the definition of `AC_FOREACH` in `general.m4` for a workaround.

Macro: AU_ALIAS(old-name, new-name, [silent]) ¶

  A shorthand version of `AU_DEFUN`, to be used when a macro has simply been renamed. `autoupdate` will replace calls to old-name with calls to new-name, keeping any arguments intact. No instructions for additional manual work will be printed.

  The silent argument works the same as the silent argument to `AU_DEFUN`. It was added to `AU_ALIAS` in Autoconf 2.70.

  Caution: `AU_ALIAS` cannot be used when new-name is an M4 or M4sugar macro. See above.

### 10.5 Coding Style ¶

The Autoconf macros follow a strict coding style. You are encouraged to follow this style, especially if you intend to distribute your macro, either by contributing it to Autoconf itself or the Autoconf Macro Archive, or by other means.

The first requirement is to pay great attention to the quotation. For more details, see The Autoconf Language, and M4 Quotation.

Do not try to invent new interfaces. It is likely that there is a macro in Autoconf that resembles the macro you are defining: try to stick to this existing interface (order of arguments, default values, etc.). We are conscious that some of these interfaces are not perfect; nevertheless, when harmless, homogeneity should be preferred over creativity.

Be careful about clashes both between M4 symbols and between shell variables.

If you stick to the suggested M4 naming scheme (see Macro Names), you are unlikely to generate conflicts. Nevertheless, when you need to set a special value, avoid using a regular macro name; rather, use an “impossible” name. For instance, up to version 2.13, the macro `AC_SUBST` used to remember what symbol macros were already defined by setting `AC_SUBST_symbol`, which is a regular macro name. But since there is a macro named `AC_SUBST_FILE`, it was just impossible to ‘`AC_SUBST(FILE)`’! In this case, `AC_SUBST(symbol)` or `_AC_SUBST(symbol)` should have been used (yes, with the parentheses).

No Autoconf macro should ever enter the user-variable name space; i.e., except for the variables that are the actual result of running the macro, all shell variables should start with `ac_`. In addition, small macros or any macro that is likely to be embedded in other macros should be careful not to use obvious names.

Do not use `dnl` to introduce comments: most of the comments you are likely to write are either header comments which are not output anyway, or comments that should make their way into `configure`. There are exceptional cases where you do want to comment special M4 constructs, in which case `dnl` is right, but keep in mind that it is unlikely.

M4 ignores the leading blanks and newlines before each argument. Use this feature to indent in such a way that arguments are (more or less) aligned with the opening parenthesis of the macro being called. For instance, instead of

    AC_CACHE_CHECK(for EMX OS/2 environment,
    ac_cv_emxos2,
    [AC_COMPILE_IFELSE([AC_LANG_PROGRAM(, [return __EMX__;])],
    [ac_cv_emxos2=yes], [ac_cv_emxos2=no])])

write

    AC_CACHE_CHECK([for EMX OS/2 environment], [ac_cv_emxos2],
    [AC_COMPILE_IFELSE([AC_LANG_PROGRAM([], [return __EMX__;])],
                       [ac_cv_emxos2=yes],
                       [ac_cv_emxos2=no])])

or even

    AC_CACHE_CHECK([for EMX OS/2 environment],
                   [ac_cv_emxos2],
                   [AC_COMPILE_IFELSE([AC_LANG_PROGRAM([],
                                                       [return __EMX__;])],
                                      [ac_cv_emxos2=yes],
                                      [ac_cv_emxos2=no])])

When using `AC_RUN_IFELSE` or any macro that cannot work when cross-compiling, provide a pessimistic value (typically ‘`no`’).

Feel free to use various tricks to prevent auxiliary tools, such as syntax-highlighting editors, from behaving improperly. For instance, instead of:

    m4_bpatsubst([$1], [$"])

use

    m4_bpatsubst([$1], [$""])

so that Emacsen do not open an endless “string” at the first quote. For the same reasons, avoid:

    test $[#] != 0

and use:

    test $[@%:@] != 0

Otherwise, the closing bracket would be hidden inside a ‘`#`’-comment, breaking the bracket-matching highlighting from Emacsen. Note the preferred style to escape from M4: ‘`$[1]`’, ‘`$[@]`’, etc. Do not escape when it is unnecessary. Common examples of useless quotation are ‘`[$]$1`’ (write ‘`$$1`’), ‘`[$]var`’ (use ‘`$var`’), etc.

When using `sed`, don’t use `-e` except for indenting purposes. With the `s` and `y` commands, the preferred separator is ‘`/`’ unless ‘`/`’ itself might appear in the pattern or replacement, in which case you should use ‘`|`’, or optionally ‘`,`’ if you know the pattern and replacement cannot contain a file name. If none of these characters will do, choose a printable character that cannot appear in the pattern or replacement. Characters from the set ‘`"#$&'()*;<=>?`|~`’ are good choices if the pattern or replacement might contain a file name, since they have special meaning to the shell and are less likely to occur in file names.

See Macro Definitions, for details on how to define a macro. If a macro doesn’t use `AC_REQUIRE`, is expected to never be the object of an `AC_REQUIRE` directive, and macros required by other macros inside arguments do not need to be expanded before this macro, then use `m4_define`. In case of doubt, use `AC_DEFUN`. Also take into account that public third-party macros need to use `AC_DEFUN` in order to be found by `aclocal` (see Extending aclocal in GNU Automake). All the `AC_REQUIRE` statements should be at the beginning of the macro, and each statement should be followed by `dnl`.

You should not rely on the number of arguments: instead of checking whether an argument is missing, test that it is not empty. It provides both a simpler and a more predictable interface to the user, and saves room for further arguments.

Unless the macro is short, try to leave the closing ‘`])`’ at the beginning of a line, followed by a comment that repeats the name of the macro being defined. This introduces an additional newline in `configure`; normally, that is not a problem, but if you want to remove it you can use ‘`[]dnl`’ on the last line. You can similarly use ‘`[]dnl`’ after a macro call to remove its newline. ‘`[]dnl`’ is recommended instead of ‘`dnl`’ to ensure that M4 does not interpret the ‘`dnl`’ as being attached to the preceding text or macro output. For example, instead of:

    AC_DEFUN([AC_PATH_X],
    [AC_MSG_CHECKING([for X])
    AC_REQUIRE_CPP()
    # ...omitted...
      AC_MSG_RESULT([libraries $x_libraries, headers $x_includes])
    fi])

you would write:

    AC_DEFUN([AC_PATH_X],
    [AC_REQUIRE_CPP()[]dnl
    AC_MSG_CHECKING([for X])
    # ...omitted...
      AC_MSG_RESULT([libraries $x_libraries, headers $x_includes])
    fi[]dnl
    ])# AC_PATH_X

If the macro is long, try to split it into logical chunks. Typically, macros that check for a bug in a function and prepare its `AC_LIBOBJ` replacement should have an auxiliary macro to perform this setup. Do not hesitate to introduce auxiliary macros to factor your code.

In order to highlight the recommended coding style, here is a macro written the old way:

    dnl Check for EMX on OS/2.
    dnl _AC_EMXOS2
    AC_DEFUN(_AC_EMXOS2,
    [AC_CACHE_CHECK(for EMX OS/2 environment, ac_cv_emxos2,
    [AC_COMPILE_IFELSE([AC_LANG_PROGRAM(, return __EMX__;)],
    ac_cv_emxos2=yes, ac_cv_emxos2=no)])
    test "x$ac_cv_emxos2" = xyes && EMXOS2=yes])

and the new way:

    # _AC_EMXOS2
    # ----------
    # Check for EMX on OS/2.
    m4_define([_AC_EMXOS2],
    [AC_CACHE_CHECK([for EMX OS/2 environment], [ac_cv_emxos2],
    [AC_COMPILE_IFELSE([AC_LANG_PROGRAM([], [return __EMX__;])],
                       [ac_cv_emxos2=yes],
                       [ac_cv_emxos2=no])])
    test "x$ac_cv_emxos2" = xyes && EMXOS2=yes[]dnl
    ])# _AC_EMXOS2

## 11 Portable Shell Programming ¶

When writing your own checks, there are some shell-script programming techniques you should avoid in order to make your code portable. The Bourne shell and upward-compatible shells like the Korn shell and Bash have evolved over the years, and many features added to the original System7 shell are now supported on all interesting porting targets. However, the following discussion between Russ Allbery and Robert Lipe is worth reading:

Russ Allbery:

> The GNU assumption that `/bin/sh` is the one and only shell leads to a permanent deadlock. Vendors don’t want to break users’ existing shell scripts, and there are some corner cases in the Bourne shell that are not completely compatible with a POSIX shell. Thus, vendors who have taken this route will never (OK…“never say never”) replace the Bourne shell (as `/bin/sh`) with a POSIX shell.

Robert Lipe:

> This is exactly the problem. While most (at least most System V’s) do have a Bourne shell that accepts shell functions most vendor `/bin/sh` programs are not the POSIX shell.

> So while most modern systems do have a shell somewhere that meets the POSIX standard, the challenge is to find it.

For this reason, part of the job of M4sh (see Programming in M4sh) is to find such a shell. But to prevent trouble, if you’re not using M4sh you should not take advantage of features that were added after Unix version 7, circa 1977 (see Systemology); you should not use aliases, negated character classes, or even `unset`. `#` comments, while not in Unix version 7, were retrofitted in the original Bourne shell and can be assumed to be part of the least common denominator.

On the other hand, if you’re using M4sh you can assume that the shell has the features that were added in SVR2 (circa 1984), including shell functions, `return`, `unset`, and I/O redirection for builtins. For more information, refer to https://www.in-ulm.de/~mascheck/bourne/. However, some pitfalls have to be avoided for portable use of these constructs; these will be documented in the rest of this chapter. See in particular Shell Functions and Limitations of Shell Builtins.

The set of external programs you should run in a `configure` script is fairly small. See Utilities in Makefiles in The GNU Coding Standards, for the list. This restriction allows users to start out with a fairly small set of programs and build the rest, avoiding too many interdependencies between packages.

Some of these external utilities have a portable subset of features; see Limitations of Usual Tools.

There are other sources of documentation about shells. The specification for the POSIX Shell Command Language, though more generous than the restrictive shell subset described above, is fairly portable nowadays. Also please see the Shell FAQs.

### 11.1 Systemology ¶

This section aims at presenting some systems and pointers to documentation. It may help you addressing particular problems reported by users.

POSIX-conforming systems are derived from the Unix operating system.

The Rosetta Stone for Unix contains a table correlating the features of various POSIX-conforming systems. Unix History is a simplified diagram of how many Unix systems were derived from each other.

The Heirloom Project provides some variants of traditional implementations of Unix utilities.

Darwin ¶

  Darwin is a partially proprietary operating system maintained by Apple Computer and used by most of their products. It is also known as macOS, iOS, etc. depending on the exact variant. Older versions were called “Mac OS X.”

  By default the file system will be case insensitive, albeit case preserving. This can cause nasty problems: for instance, the installation attempt for a package having an `INSTALL` file can result in ‘`make install`’ reporting that nothing is to be done!

  Darwin does support case-sensitive file systems, but they must be formatted specially as such, and Apple discourages use of a case-sensitive volume for the base operating system. To build software that expects case-sensitive filenames, it is best to create a separate disk volume or disk image formatted as case sensitive; this can be done using the `diskutil` command or the Disk Utility application.

QNX ¶

  QNX is a realtime operating system running on Intel architecture meant to be scalable from the small embedded systems to the hundred processor super-computer. More information is available on the QNX home page.

Unix version 7 ¶

  Officially this was called the “Seventh Edition” of “the UNIX time-sharing system” but we use the more-common name “Unix version 7”. Documentation is available in the Unix Seventh Edition Manual. Previous versions of Unix are called “Unix version 6”, etc., but they were not as widely used.

### 11.2 Shellology ¶

There are several families of shells, most prominently the Bourne family and the C shell family which are deeply incompatible. If you want to write portable shell scripts, avoid members of the C shell family. The the Shell difference FAQ includes a small history of POSIX shells, and a comparison between several of them.

Below we describe some of the members of the Bourne shell family.

Ash ¶

  Ash is often used on GNU/Linux and BSD systems as a light-weight Bourne-compatible shell. Ash 0.2 has some bugs that are fixed in the 0.3.x series, but portable shell scripts should work around them, since version 0.2 is still shipped with many GNU/Linux distributions.

  To be compatible with Ash 0.2:

  - don’t use ‘`$?`’ after expanding empty or unset variables, or at the start of an `eval`:

        foo=
        false
        $foo
        echo "Do not use it: $?"
        false
        eval 'echo "Do not use it: $?"'

  - don’t use command substitution within variable expansion:

        cat ${FOO=`bar`}

  - beware that single builtin substitutions are not performed by a subshell, hence their effect applies to the current shell! See Shell Substitutions, item “Command Substitution”.
Bash ¶

  To detect whether you are running Bash, test whether `BASH_VERSION` is set. To require POSIX compatibility, run ‘`set -o posix`’. See Bash POSIX Mode in The GNU Bash Reference Manual, for details.

Bash 2.05 and later ¶

  Versions 2.05 and later of Bash use a different format for the output of the `set` builtin, designed to make evaluating its output easier. However, this output is not compatible with earlier versions of Bash (or with many other shells, probably). So if you use Bash 2.05 or higher to execute `configure`, you’ll need to use Bash 2.05 for all other build tasks as well.

Ksh ¶

  The Korn shell is compatible with the Bourne family and it mostly conforms to POSIX. It has two major variants commonly called ‘`ksh88`’ and ‘`ksh93`’, named after the years of initial release. It is usually called `ksh`, but is called `sh` on some hosts if you set your path appropriately.

  On Solaris 11, `/bin/sh` and `/usr/bin/ksh` are both ‘`ksh93`’. On Solaris 10 and earlier, `/bin/sh` is a pre-POSIX Bourne shell and the Korn shell is found elsewhere: `/usr/bin/ksh` is ‘`ksh88`’ on Solaris 10, `/usr/xpg4/bin/sh` is a POSIX-compliant variant of ‘`ksh88`’ on Solaris 10 and later, and `/usr/dt/bin/dtksh` is ‘`ksh93`’. Variants that are not standard may be parts of optional packages. There is no extra charge for these packages, but they are not part of a minimal OS install and therefore some installations may not have it.

Pdksh ¶

  A public-domain clone of the Korn shell called `pdksh` is widely available: it has most of the ‘`ksh88`’ features along with a few of its own. It usually sets `KSH_VERSION`, except if invoked as `/bin/sh` on OpenBSD, and similarly to Bash you can require POSIX compatibility by running ‘`set -o posix`’. Unfortunately, with `pdksh` 5.2.14 (the latest stable version as of 2025) POSIX mode is buggy and causes `pdksh` to depart from POSIX in at least one respect, see Shell Substitutions.

Zsh ¶

  To detect whether you are running `zsh`, test whether `ZSH_VERSION` is set. By default `zsh` is not compatible with the Bourne shell: you must execute ‘`emulate sh`’, and for `zsh` versions before 3.1.6-dev-18 you must also set `NULLCMD` to ‘`:`’. See Compatibility in The Z Shell Manual, for details.

  The default Mac OS X `sh` was originally Zsh; it was changed to Bash in Mac OS X 10.2 (2002) and changed back to Zsh in macOS 10.15 (2019).

### 11.3 Invoking the Shell ¶

The Korn shell (up to at least version M-12/28/93d) has a bug when invoked on a file whose name does not contain a slash. It first searches for the file’s name in `PATH`, and if found it executes that rather than the original file. For example, assuming there is a binary executable `/usr/bin/script` in your `PATH`, the last command in the following example fails because the Korn shell finds `/usr/bin/script` and refuses to execute it as a shell script:

    $ touch xxyzzyz script
    $ ksh xxyzzyz
    $ ksh ./script
    $ ksh script
    ksh: script: cannot execute

Bash 2.03 has a bug when invoked with the `-c` option: if the option-argument ends in backslash-newline, Bash incorrectly reports a syntax error. The problem does not occur if a character follows the backslash:

    $ $ bash -c 'echo foo \
    > '
    bash: -c: line 2: syntax error: unexpected end of file
    $ bash -c 'echo foo \
    >  '
    foo

See Backslash-Newline Before Empty Lines, for how this can cause problems in makefiles.

### 11.4 Here-Documents ¶

Because unquoted here-documents are subject to parameter expansion and command substitution, the characters ‘`$`’ and ‘```’ are special in unquoted here-documents and should be escaped by ‘`\`’ if you want them as-is. Also, ‘`\`’ is special if it precedes ‘`$`’, ‘```’, newline or ‘`\`’ itself, so ‘`\`’ should be doubled if it appears before these characters and you want it as-is.

Using command substitutions in a here-document that is fed to a shell function is not portable. For example, with Solaris 10 `/bin/sh`:

    $ kitty () { cat; }
    $ kitty <<EOF
    > `echo ok`
    > EOF
    /tmp/sh199886: cannot open
    $ echo $?
    1

Some shells mishandle large here-documents: for example, Solaris 10 `dtksh` and the UnixWare 7.1.1 POSIX shell, which are derived from Korn shell version M-12/28/93d, mishandle braced variable expansion that crosses a 1024- or 4096-byte buffer boundary within a here-document. Only the part of the variable name after the boundary is used. For example, `${variable}` could be replaced by the expansion of `${ble}`. If the end of the variable name is aligned with the block boundary, the shell reports an error, as if you used `${}`. Instead of `${variable-default}`, the shell may expand `${riable-default}`, or even `${fault}`. This bug can often be worked around by omitting the braces: `$variable`. The bug was fixed in ‘`ksh93g`’ (1998-04-30) but as of 2006 many operating systems were still shipping older versions with the bug.

Empty here-documents are not portable either; with the following code, `zsh` up to at least version 4.3.10 creates a file with a single newline, whereas other shells create an empty file:

    cat >file <<EOF
    EOF

Many shells (including the Bourne shell) implement here-documents inefficiently. In particular, some shells can be extremely inefficient when a single statement contains many here-documents. For instance if your `configure.ac` includes something like:

    AS_IF([<cross_compiling>],
      [assume this and that],
      [check this
       check that
       check something else
       ...
       on and on forever
       ...])

A shell parses the whole `if`/`fi` construct generated by `AS_IF`, creating temporary files for each here-document in it. Some shells create links for such here-documents on every `fork`, so that the clean-up code they had installed correctly removes them. It is creating the links that can take the shell forever.

Moving the tests out of the `if`/`fi`, or creating multiple `if`/`fi` constructs, would improve the performance significantly. Anyway, this kind of construct is not exactly the typical use of Autoconf. In fact, it’s even not recommended, because M4 macros can’t look into shell conditionals, so we may fail to expand a macro when it was expanded before in a conditional path, and the condition turned out to be false at runtime, and we end up not executing the macro at all.

Be careful with the use of ‘`<<-`’ to unindent here-documents. The behavior is only portable for stripping leading `TAB`s, and things can silently break if an overzealous editor converts to using leading spaces (not all shells are nice enough to warn about unterminated here-documents).

    $ printf 'cat <<-x\n\t1\n\t 2\n\tx\n' | bash && echo done
    1
     2
    done
    $ printf 'cat <<-x\n 1\n  2\n x\n' | bash-3.2 && echo done
     1
      2
     x
    done

### 11.5 File Descriptors ¶

Most shells, if not all (including Bash, Zsh, Ash), output traces on stderr, even for subshells. This might result in undesirable content if you meant to capture the standard-error output of the inner command:

    $ ash -x -c '(eval "echo foo >&2") 2>stderr'
    $ cat stderr
    + eval echo foo >&2
    + echo foo
    foo
    $ bash -x -c '(eval "echo foo >&2") 2>stderr'
    $ cat stderr
    + eval 'echo foo >&2'
    ++ echo foo
    foo
    $ zsh -x -c '(eval "echo foo >&2") 2>stderr'
    # Traces on startup files deleted here.
    $ cat stderr
    +zsh:1> eval echo foo >&2
    +zsh:1> echo foo
    foo

One workaround is to grep out uninteresting lines, hoping not to remove good ones.

If you intend to redirect both standard error and standard output, redirect standard output first. This works better with AIX, since its shell mishandles tracing if standard error is redirected first:

    $ sh -x -c ': 2>err >out'
    + :
    + 2> err $ cat err
    1> out

Don’t try to redirect the standard error of a command substitution. It must be done inside the command substitution. When running ‘`: `cd /zorglub` 2>/dev/null`’ expect the error message to escape, while ‘`: `cd /zorglub 2>/dev/null``’ works properly.

On the other hand, some shells, such as Solaris or FreeBSD `/bin/sh`, warn about missing programs before performing redirections. Therefore, to silently check whether a program exists, it is necessary to perform redirections on a subshell or brace group:

    $ /bin/sh -c 'nosuch 2>/dev/null'
    nosuch: not found
    $ /bin/sh -c '(nosuch) 2>/dev/null'
    $ /bin/sh -c '{ nosuch; } 2>/dev/null'
    $ bash -c 'nosuch 2>/dev/null'

FreeBSD 6.2 sh may mix the trace output lines from the statements in a shell pipeline.

It is worth noting that Zsh (but not Ash nor Bash) makes it possible in assignments though: ‘`foo=`cd /zorglub` 2>/dev/null`’.

Some shells, like `ash`, don’t recognize bi-directional redirection (‘`<>`’). And even on shells that recognize it, it is not portable to use on fifos: POSIX does not require read-write support for named pipes, and Cygwin does not support it:

    $ mkfifo fifo
    $ exec 5<>fifo
    $ echo hi >&5
    bash: echo: write error: Communication error on send

Furthermore, versions of `dash` before 0.5.6 mistakenly truncate regular files when using ‘`<>`’:

    $ echo a > file
    $ bash -c ': 1<>file'; cat file
    a
    $ dash -c ': 1<>file'; cat file
    $ rm a

Solaris 10 `/bin/sh` executes redirected compound commands in a subshell, while other shells don’t:

    $ /bin/sh -c 'foo=0; { foo=1; } 2>/dev/null; echo $foo'
    0
    $ ksh -c 'foo=0; { foo=1; } 2>/dev/null; echo $foo'
    1
    $ bash -c 'foo=0; { foo=1; } 2>/dev/null; echo $foo'
    1

Solaris 10 `sh` will try to optimize away a `:` command (even if it is redirected) in a loop after the first iteration, or in a shell function after the first call:

    $ for i in 1 2 3 ; do : >x$i; done
    $ ls x*
    x1
    $ f () { : >$1; }; f y1; f y2; f y3;
    $ ls y*
    y1

As a workaround, `echo` or `eval` can be used.

When running a subsidiary program be careful if descriptor 0 is not open for reading or decriptors 1 and 2 are not open for writing, as the subsidiary program might not behave as expected. In particular, don’t rely on file descriptors 0, 1, and 2 remaining closed in a subsidiary program. If any of these descriptors is closed, the operating system may open an unspecified file for the descriptor in the newly created process.

If you want a file descriptor above 2 to be inherited into a child process, then you must use redirections specific to that command or a containing subshell or command group, rather than relying on `exec` in the shell. In `ksh` as well as HP-UX `sh`, file descriptors above 2 which are opened using ‘`exec n>file`’ are closed by a subsequent ‘`exec`’ (such as that involved in the fork-and-exec which runs a program or script):

    $ echo 'echo hello >&5' >k
    $ /bin/sh -c 'exec 5>t; ksh ./k; exec 5>&-; cat t
    hello
    $ bash -c 'exec 5>t; ksh ./k; exec 5>&-; cat t
    hello
    $ ksh -c 'exec 5>t; ksh ./k; exec 5>&-; cat t
    ./k[1]: 5: cannot open [Bad file number]
    $ ksh -c '(ksh ./k) 5>t; cat t'
    hello
    $ ksh -c '{ ksh ./k; } 5>t; cat t'
    hello
    $ ksh -c '5>t ksh ./k; cat t
    hello

Don’t rely on duplicating a closed file descriptor to cause an error. With Solaris 10 `/bin/sh`, failed duplication is silently ignored, which can cause unintended leaks to the original file descriptor. In this example, observe the leak to standard output:

    $ bash -c 'echo hi >&3' 3>&-; echo $?
    bash: 3: Bad file descriptor
    1
    $ /bin/sh -c 'echo hi >&3' 3>&-; echo $?
    hi
    0

Fortunately, an attempt to close an already closed file descriptor will portably succeed. Likewise, it is safe to use either style of ‘`n<&-`’ or ‘`n>&-`’ for closing a file descriptor, even if it doesn’t match the read/write mode that the file descriptor was opened with.

DOS variants cannot rename or remove open files, such as in ‘`mv foo bar >foo`’ or ‘`rm foo >foo`’, even though this is perfectly portable among POSIX hosts.

A few ancient systems reserved some file descriptors. By convention, file descriptor 3 was opened to `/dev/tty` when you logged into Eighth Edition (1985) through Tenth Edition Unix (1989). File descriptor 4 had a special use on the Stardent/Kubota Titan (circa 1990), though we don’t now remember what it was. Both these systems are obsolete, so it’s now safe to treat file descriptors 3 and 4 like any other file descriptors.

On the other hand, you can’t portably use multi-digit file descriptors. `dash` and Solaris `ksh` don’t understand any file descriptor larger than ‘`9`’:

    $ bash -c 'exec 10>&-'; echo $?
    0
    $ ksh -c 'exec 9>&-'; echo $?
    0
    $ ksh -c 'exec 10>&-'; echo $?
    ksh[1]: exec: 10: not found
    127
    $ dash -c 'exec 9>&-'; echo $?
    0
    $ dash -c 'exec 10>&-'; echo $?
    exec: 1: 10: not found
    2

### 11.6 Signal Handling ¶

Portable handling of signals within the shell is another major source of headaches. This is worsened by the fact that various different, mutually incompatible approaches are possible in this area, each with its distinctive merits and demerits. A detailed description of these possible approaches, as well as of their pros and cons, can be found in this article.

Solaris 10 `/bin/sh` automatically traps most signals by default; the shell still exits with error upon termination by one of those signals, but in such a case the exit status might be somewhat unexpected (even if allowed by POSIX, strictly speaking):

    $ bash -c 'kill -1 $$'; echo $? # Will exit 128 + (signal number).
    Hangup
    129
    $ /bin/ksh -c 'kill -15 $$'; echo $? # Likewise.
    Terminated
    143
    $ for sig in 1 2 3 15; do
    >   echo $sig:
    >   /bin/sh -c "kill -$s \$\$"; echo $?
    > done
    signal 1:
    Hangup
    129
    signal 2:
    208
    signal 3:
    208
    signal 15:
    208

This gets even worse if one is using the POSIX “wait” interface to get details about the shell process terminations: it will result in the shell having exited normally, rather than by receiving a signal.

    $ cat > foo.c <<'END'
    #include <stdio.h>    /* for printf */
    #include <stdlib.h>   /* for system */
    #include <sys/wait.h> /* for WIF* macros */
    int main(void)
    {
      int status = system ("kill -15 $$");
      printf ("Terminated by signal: %s\n",
              WIFSIGNALED (status) ? "yes" : "no");
      printf ("Exited normally: %s\n",
              WIFEXITED (status) ? "yes" : "no");
      return 0;
    }
    END
    $ cc -o foo foo.c
    $ ./a.out # On GNU/Linux
    Terminated by signal: no
    Exited normally: yes
    $ ./a.out # On Solaris 10
    Terminated by signal: yes
    Exited normally: no

Various shells seem to handle `SIGQUIT` specially: they ignore it even if it is not blocked, and even if the shell is not running interactively (in fact, even if the shell has no attached tty); among these shells are at least Bash (from version 2 onward), Zsh 4.3.12, Solaris 10 `/bin/ksh` and `/usr/xpg4/bin/sh`, and AT&T `ksh93` (2011). Still, `SIGQUIT` seems to be trappable quite portably within all these shells. OTOH, some other shells doesn’t special-case the handling of `SIGQUIT`; among these shells are at least `pdksh` 5.2.14, Solaris 10 and NetBSD 5.1 `/bin/sh`, and the Almquist Shell 0.5.5.1.

Some shells (especially Korn shells and derivatives) might try to propagate to themselves a signal that has killed a child process; this is not a bug, but a conscious design choice (although its overall value might be debatable). The exact details of how this is attained vary from shell to shell. For example, upon running `perl -e 'kill 2, $$'`, after the perl process has been interrupted, AT&T `ksh93` (2011) will proceed to send itself a `SIGINT`, while Solaris 10 `/bin/ksh` and `/usr/xpg4/bin/sh` will proceed to exit with status 130 (i.e., 128 + 2). In any case, if there is an active trap associated with `SIGINT`, those shells will correctly execute it.

Some Korn shells, when a child process die due receiving a signal with signal number n, can leave in ‘`$?`’ an exit status of 256+n instead of the more common 128+n. Observe the difference between AT&T `ksh93` (2011) and `bash` 4.1.5 on Debian:

    $ /bin/ksh -c 'sh -c "kill -1 \$\$"; echo $?'
    /bin/ksh: line 1: 7837: Hangup
    257
    $ /bin/bash -c 'sh -c "kill -1 \$\$"; echo $?'
    /bin/bash: line 1:  7861 Hangup        (sh -c "kill -1 \$\$")
    129

This `ksh` behavior is allowed by POSIX, if implemented with due care; see this Austin Group discussion for more background. However, if it is not implemented with proper care, such a behavior might cause problems in some corner cases. To see why, assume we have a “wrapper” script like this:

    #!/bin/sh
    # Ignore some signals in the shell only, not in its child processes.
    trap : 1 2 13 15
    wrapped_command "$@"
    ret=$?
    other_command
    exit $ret

If `wrapped_command` is interrupted by a `SIGHUP` (which has signal number 1), `ret` will be set to 257. Unless the `exit` shell builtin is smart enough to understand that such a value can only have originated from a signal, and adjust the final wait status of the shell appropriately, the value 257 will just get truncated to 1 by the closing `exit` call, so that a caller of the script will have no way to determine that termination by a signal was involved. Observe the different behavior of AT&T `ksh93` (2011) and `bash` 4.1.5 on Debian:

    $ cat foo.sh
    #!/bin/sh
    sh -c 'kill -1 $$'
    ret=$?
    echo $ret
    exit $ret
    $ /bin/ksh foo.sh; echo $?
    foo.sh: line 2: 12479: Hangup
    257
    1
    $ /bin/bash foo.sh; echo $?
    foo.sh: line 2: 12487 Hangup        (sh -c 'kill -1 $$')
    129
    129

### 11.7 File System Conventions ¶

Autoconf uses shell-script processing extensively, so the file names that it processes should not contain characters that are special to the shell. Special characters include space, tab, newline, NUL, and the following:

    " # $ & ' ( ) * ; < = > ? [ \ ` |

Also, file names should not begin with ‘`~`’ or ‘`-`’, and should contain neither ‘`-`’ immediately after ‘`/`’ nor ‘`~`’ immediately after ‘`:`’. On POSIX-like platforms, directory names should not contain ‘`:`’, as this runs afoul of ‘`:`’ used as the path separator.

These restrictions apply not only to the files that you distribute, but also to the absolute file names of your source, build, and destination directories.

On some POSIX-like platforms, ‘`!`’ and ‘`^`’ are special too, so they should be avoided.

POSIX lets implementations treat leading `//` specially, but requires leading `///` and beyond to be equivalent to `/`. Most Unix variants treat `//` like `/`. However, some treat `//` as a “super-root” that can provide access to files that are not otherwise reachable from `/`. The super-root tradition began with Apollo Domain/OS, which died out long ago, but unfortunately Cygwin has revived it.

While `autoconf` and friends are usually run on some POSIX variety, they can be used on other systems, most notably DOS variants. This impacts several assumptions regarding file names.

For example, the following code:

    case $foo_dir in
      /*) # Absolute
         ;;
      *)
         foo_dir=$dots$foo_dir ;;
    esac

fails to properly detect absolute file names on those systems, because they can use a drivespec, and usually use a backslash as directory separator. If you want to be portable to DOS variants (at the price of rejecting valid but oddball POSIX file names like `a:\b`), you can check for absolute file names like this:

    case $foo_dir in
      [\\/]* | ?:[\\/]* ) # Absolute
         ;;
      *)
         foo_dir=$dots$foo_dir ;;
    esac

Make sure you quote the brackets if appropriate and keep the backslash as first character. See Limitations of Shell Builtins.

Also, because the colon is used as part of a drivespec, these systems don’t use it as path separator. When creating or accessing paths, you can use the `PATH_SEPARATOR` output variable instead. `configure` sets this to the appropriate value for the build system (‘`:`’ or ‘`;`’) when it starts up.

File names need extra care as well. While DOS variants that are POSIXy enough to run `autoconf` (such as DJGPP) are usually able to handle long file names properly, there are still limitations that can seriously break packages. Several of these issues can be easily detected by the doschk package.

A short overview follows; problems are marked with SFN/LFN to indicate where they apply: SFN means the issues are only relevant to plain DOS, not to DOS under Microsoft Windows variants, while LFN identifies problems that exist even under Microsoft Windows variants.

No multiple dots (SFN)

  DOS cannot handle multiple dots in file names. This is an especially important thing to remember when building a portable configure script, as `autoconf` uses a .in suffix for template files.

  This is perfectly OK on POSIX variants:

      AC_CONFIG_HEADERS([config.h])
      AC_CONFIG_FILES([source.c foo.bar])
      AC_OUTPUT

  but it causes problems on DOS, as it requires ‘`config.h.in`’, ‘`source.c.in`’ and ‘`foo.bar.in`’. To make your package more portable to DOS-based environments, you should use this instead:

      AC_CONFIG_HEADERS([config.h:config.hin])
      AC_CONFIG_FILES([source.c:source.cin foo.bar:foobar.in])
      AC_OUTPUT

No leading dot (SFN)

  DOS cannot handle file names that start with a dot. This is usually not important for `autoconf`.

Case insensitivity (LFN)

  DOS is case insensitive, so you cannot, for example, have both a file called ‘`INSTALL`’ and a directory called ‘`install`’. This also affects `make`; if there’s a file called ‘`INSTALL`’ in the directory, ‘`make install`’ does nothing (unless the ‘`install`’ target is marked as PHONY).

The 8+3 limit (SFN)

  Because the DOS file system only stores the first 8 characters of the file name and the first 3 of the extension, those must be unique. That means that `foobar-part1.c`, `foobar-part2.c` and `foobar-prettybird.c` all resolve to the same file name (`FOOBAR-P.C`). The same goes for `foo.bar` and `foo.bartender`.

  The 8+3 limit is not usually a problem under Microsoft Windows, as it uses numeric tails in the short version of file names to make them unique. However, a registry setting can turn this behavior off. While this makes it possible to share file trees containing long file names between SFN and LFN environments, it also means the above problem applies there as well.

Invalid characters (LFN)

  Some characters are invalid in DOS file names, and should therefore be avoided. In a LFN environment, these are ‘`/`’, ‘`\`’, ‘`?`’, ‘`*`’, ‘`:`’, ‘`<`’, ‘`>`’, ‘`|`’ and ‘`"`’. In a SFN environment, other characters are also invalid. These include ‘`+`’, ‘`,`’, ‘`[`’ and ‘`]`’.

Invalid names (LFN)

  Some DOS file names are reserved, and cause problems if you try to use files with those names. These names include `CON`, `AUX`, `COM1`, `COM2`, `COM3`, `COM4`, `LPT1`, `LPT2`, `LPT3`, `NUL`, and `PRN`. File names are case insensitive, so even names like `aux/config.guess` are disallowed.

### 11.8 Shell Pattern Matching ¶

Portable patterns should avoid the following constructs:

- A bracket expression like ‘`[a-z]`’ outside the C locale. This pattern may match characters that are not lower-case letters. Outside the C locale, use ‘`[[:lower:]]`’ instead, or to match just lower-case ASCII letters use ‘`[abcdefghijklmnopqrstuvwxyz]`’.
- A bracket expression starting with ‘`^`’. For example, ‘`[!-aeiou]`’ is portable, but ‘`[^-aeiou]`’ is not.
- When matching file names, a pattern containing ‘`*`’, ‘`?`’ or ‘`[...]`’ that might match the file names ‘`.`’ or ‘`..`’. For example ‘`.*`’ might match ‘`.`’ and ‘`..`’, but it might not.
- Any of the following characters, unless quoted or escaped or in a bracket expression:

      ! # % = [ ] ^ { } ~

  An unescaped ‘`[`’ that does not introduce a bracket expression is problematic because, for example, ‘`a*[`’ might match the file name ‘`abc[`’, but it might not.

  Curly braces are problematic because POSIX allows but does not require brace expansion. For example, the shell command ‘`echo a{b,c,d}e`’ outputs ‘`abe ace ade`’ with Bash, but ‘`a{b,c,d}e`’ with Dash.

- Backlashes in bracket expressions, unless doubled. For example, ‘`[\\^]`’ is portable, but ‘`[\^]`’ is not.
- An unescaped backslash at pattern end.
- A NUL byte. More generally, a NUL byte should never appear anywhere in a portable shell script.

### 11.9 Shell Substitutions ¶

Contrary to a persistent urban legend, the Bourne shell does not systematically split variables and back-quoted expressions, in particular on the right-hand side of assignments and in the argument of `case`. For instance, the following code:

    case "$given_srcdir" in
    .)  top_srcdir="`printf '%s\n' "$dots" | sed 's|/$||'`" ;;
    *)  top_srcdir="$dots$given_srcdir" ;;
    esac

is more readable when written as:

    case $given_srcdir in
    .)  top_srcdir=`printf '%s\n' "$dots" | sed 's|/$||'` ;;
    *)  top_srcdir=$dots$given_srcdir ;;
    esac

and in fact it is even more portable: in the first case of the first attempt, the computation of `top_srcdir` is not portable, since not all shells properly understand `"`…"…"…`"`, for example Solaris 10 `ksh`:

    $ foo="`echo " bar" | sed 's, ,,'`"
    ksh: : cannot execute
    ksh: bar | sed 's, ,,': cannot execute

POSIX does not specify behavior for this sequence. On the other hand, behavior for `"`…\"…\"…`"` is specified by POSIX, but in practice, not all shells understand it the same way: pdksh 5.2.14 prints spurious quotes when in POSIX mode:

    $ echo "`echo \"hello\"`"
    hello
    $ set -o posix
    $ echo "`echo \"hello\"`"
    "hello"

There is just no portable way to use double-quoted strings inside double-quoted back-quoted expressions (pfew!).

Bash 4.1 has a bug where quoted empty strings adjacent to unquoted parameter expansions are elided during word splitting. Meanwhile, zsh does not perform word splitting except when in Bourne compatibility mode. In the example below, the correct behavior is to have five arguments to the function, and exactly two spaces on either side of the middle ‘`-`’, since word splitting collapses multiple spaces in ‘`$f`’ but leaves empty arguments intact.

    $ bash -c 'n() { echo "$#$@"; }; f="  -  "; n - ""$f"" -'
    3- - -
    $ ksh -c 'n() { echo "$#$@"; }; f="  -  "; n - ""$f"" -'
    5-  -  -
    $ zsh -c 'n() { echo "$#$@"; }; f="  -  "; n - ""$f"" -'
    3-   -   -
    $ zsh -c 'emulate sh;
    > n() { echo "$#$@"; }; f="  -  "; n - ""$f"" -'
    5-  -  -

You can work around this by doing manual word splitting, such as using ‘`"$str" $list`’ rather than ‘`"$str"$list`’.

There are also portability pitfalls with particular expansions:

`$@` ¶

  Autoconf macros often use the `set` command to update ‘`$@`’, so if you are writing shell code intended for `configure` you should not assume that the value of ‘`$@`’ persists for any length of time.

  You may see usages like ‘`${1+"$@"}`’ in older shell scripts designed to work around a portability problem in ancient shells. Unfortunately this runs afoul of bugs in more-recent shells, and nowadays it is better to use plain ‘`"$@"`’ instead.

  The portability problem with ancient shells was significant. When there are no positional arguments ‘`"$@"`’ should be discarded, but the original Unix version 7 Bourne shell mistakenly treated it as equivalent to ‘`""`’ instead, and many ancient shells followed its lead.

  For many years shell scripts worked around this portability problem by using ‘`${1+"$@"}`’ instead of ‘`"$@"`’, and you may see this usage in older scripts. Unfortunately, ‘`${1+"$@"}`’ does not work with `ksh93` M 93t+ (2009) as shipped in AIX 7.2 (2015), as this shell drops a trailing empty argument:

      $ set a b c ""
      $ set ${1+"$@"}
      $ echo $#
      3

  Also, ‘`${1+"$@"}`’ does not work with Zsh 4.2.6 (2005) and earlier, as shipped in Mac OS X releases before 10.5, as this old Zsh incorrectly word splits the result:

      zsh $ emulate sh
      zsh $ for i in "$@"; do echo $i; done
      Hello World
      !
      zsh $ for i in ${1+"$@"}; do echo $i; done
      Hello
      World
      !

  To work around these problems Autoconf does two things. First, in the shell code that it generates Autoconf avoids ‘`"$@"`’ if it is possible that there may be no positional arguments. You can use this workaround in your own code, too, if you want it to be portable to ancient shells. For example, instead of:

      cat conftest.c "$@"

  you can use this:

      case $# in
        0) cat conftest.c;;
        *) cat conftest.c "$@";;
      esac

  Second, Autoconf-generated `configure` scripts work around most of the old Zsh problem by using Zsh’s “global aliases” to convert ‘`${1+"$@"}`’ into ‘`"$@"`’ by itself:

      test ${ZSH_VERSION+y} && alias -g '${1+"$@"}'='"$@"'

  This workaround is for the benefit of any instances of ‘`${1+"$@"}`’ in user-written code appearing in `configure` scripts. However, it is not a complete solution, as Zsh recognizes the alias only when a shell word matches it exactly, which means older Zsh still mishandles more-complicated cases like ‘`"foo"${1+"$@"}`’.

`${10}` ¶

  The 10th, 11th, … positional parameters can be accessed only after a `shift`. The 7th Edition shell reported an error if given `${10}`, and Solaris 10 `/bin/sh` still acts that way:

      $ set 1 2 3 4 5 6 7 8 9 10
      $ echo ${10}
      bad substitution

  Conversely, not all shells obey the POSIX rule that when braces are omitted, multiple digits beyond a ‘`$`’ imply the single-digit positional parameter expansion concatenated with the remaining literal digits. To work around the issue, you must use braces.

      $ bash -c 'set a b c d e f g h i j; echo $10 ${1}0'
      a0 a0
      $ dash -c 'set a b c d e f g h i j; echo $10 ${1}0'
      j a0

`${var-value}` ¶
`${var:-value}`
`${var=value}`
`${var:=value}`
`${var?value}`
`${var:?value}`
`${var+value}`
`${var:+value}`

  When using ‘`${var-value}`’ or similar notations that modify a parameter expansion, POSIX requires that value must be a single shell word, which can contain quoted strings but cannot contain unquoted spaces. If this requirement is not met Solaris 10 `/bin/sh` sometimes complains, and anyway the behavior is not portable.

      $ /bin/sh -c 'echo ${a-b c}'
      /bin/sh: bad substitution
      $ /bin/sh -c 'echo ${a-'\''b c'\''}'
      b c
      $ /bin/sh -c 'echo "${a-b c}"'
      b c
      $ /bin/sh -c 'cat <<EOF
      ${a-b c}
      EOF
      b c

  Most shells treat the special parameters `*` and `@` as being unset if there are no positional parameters. However, some shells treat them as being set to the empty string. POSIX does not clearly specify either behavior.

      $ bash -c 'echo "* is ${*-unset}."'
      * is unset.
      $ dash -c 'echo "* is ${*-unset}."'
      * is .

  According to POSIX, if an expansion occurs inside double quotes, then the use of unquoted double quotes within value is unspecified, and any single quotes become literal characters; in that case, escaping must be done with backslash. Likewise, the use of unquoted here-documents is a case where double quotes have unspecified results:

      $ /bin/sh -c 'echo "${a-"b  c"}"'
      /bin/sh: bad substitution
      $ ksh -c 'echo "${a-"b  c"}"'
      b c
      $ bash -c 'echo "${a-"b  c"}"'
      b  c
      $ /bin/sh -c 'a=; echo ${a+'\''b  c'\''}'
      b  c
      $ /bin/sh -c 'a=; echo "${a+'\''b  c'\''}"'
      'b  c'
      $ /bin/sh -c 'a=; echo "${a+\"b  c\"}"'
      "b  c"
      $ /bin/sh -c 'a=; echo "${a+b  c}"'
      b  c
      $ /bin/sh -c 'cat <<EOF
      ${a-"b  c"}
      EOF'
      "b  c"
      $ /bin/sh -c 'cat <<EOF
      ${a-'b  c'}
      EOF'
      'b  c'
      $ bash -c 'cat <<EOF
      ${a-"b  c"}
      EOF'
      b  c
      $ bash -c 'cat <<EOF
      ${a-'b  c'}
      EOF'
      'b  c'

  Perhaps the easiest way to work around quoting issues in a manner portable to all shells is to place the results in a temporary variable, then use ‘`$t`’ as the value, rather than trying to inline the expression needing quoting.

      $ /bin/sh -c 't="b  c\"'\''}\\"; echo "${a-$t}"'
      b  c"'}\
      $ ksh -c 't="b  c\"'\''}\\"; echo "${a-$t}"'
      b  c"'}\
      $ bash -c 't="b  c\"'\''}\\"; echo "${a-$t}"'
      b  c"'}\

`${var=value}` ¶

  When using ‘`${var=value}`’ to assign a default value to var, remember that even though the assignment to var does not undergo file name expansion, the result of the variable expansion does unless the expansion occurred within double quotes. In particular, when using `:` followed by unquoted variable expansion for the side effect of setting a default value, if the final value of ‘`$var`’ contains any globbing characters (either from value or from prior contents), the shell has to spend time performing file name expansion and field splitting even though those results will not be used. Therefore, it is a good idea to consider double quotes when performing default initialization; while remembering how this impacts any quoting characters appearing in value.

      $ time bash -c ': "${a=/usr/bin/*}"; echo "$a"'
      /usr/bin/*

      real	0m0.005s
      user	0m0.002s
      sys	0m0.003s
      $ time bash -c ': ${a=/usr/bin/*}; echo "$a"'
      /usr/bin/*

      real	0m0.039s
      user	0m0.026s
      sys	0m0.009s
      $ time bash -c 'a=/usr/bin/*; : ${a=noglob}; echo "$a"'
      /usr/bin/*

      real	0m0.031s
      user	0m0.020s
      sys	0m0.010s

      $ time bash -c 'a=/usr/bin/*; : "${a=noglob}"; echo "$a"'
      /usr/bin/*

      real	0m0.006s
      user	0m0.002s
      sys	0m0.003s

  As with ‘`+`’ and ‘`-`’, value must be a single shell word, otherwise some shells, such as Solaris 10 `/bin/sh` or on Digital Unix V 5.0, die because of a “bad substitution”. Meanwhile, POSIX requires that with ‘`=`’, quote removal happens prior to the assignment, and the expansion be the final contents of var without quoting (and thus subject to field splitting), in contrast to the behavior with ‘`-`’ passing the quoting through to the final expansion. However, `bash` 4.1 does not obey this rule.

      $ ksh -c 'echo ${var-a\ \ b}'
      a  b
      $ ksh -c 'echo ${var=a\ \ b}'
      a b
      $ bash -c 'echo ${var=a\ \ b}'
      a  b

  Finally, POSIX states that when mixing ‘`${a=b}`’ with regular commands, it is unspecified whether the assignments affect the parent shell environment. It is best to perform assignments independently from commands, to avoid the problems demonstrated in this example running on Solaris 10:

      $ cmd='x= y=${x:=b} sh -c "echo +\$x+\$y+";printf "%s\\n" -$x-'
      $ bash -c "$cmd"
      +b+b+
      -b-
      $ /bin/sh -c "$cmd"
      ++b+
      --
      $ ksh -c "$cmd"
      +b+b+
      --

`${var=value}` ¶

  Solaris 10 `/bin/sh` has a frightening bug in its handling of literal assignments. Imagine you need set a variable to a string containing ‘`}`’. This ‘`}`’ character confuses Solaris 10 `/bin/sh` when the affected variable was already set. This bug can be exercised by running:

      $ unset foo
      $ foo=${foo='}'}
      $ echo $foo
      }
      $ foo=${foo='}'   # no error; this hints to what the bug is
      $ echo $foo
      }
      $ foo=${foo='}'}
      $ echo $foo
      }}
       ^ ugh!

  It seems that ‘`}`’ is interpreted as matching ‘`${`’, even though it is enclosed in single quotes. The problem doesn’t happen using double quotes, or when using a temporary variable holding the problematic string.

`${var=expanded-value}` ¶

  On shells so old that they are no longer relevant, the command

      # Set the shell variable to a default value
      # if it is not already set.
      : ${var="$default"}

  misbehaved badly in some cases. Older scripts worked around the bugs by using one of following two lines, the latter of which was more portable:

      var=${var="$default"}
      test ${var+y} || var=$default

  However, these workarounds are no longer needed.

`${#var}` ¶
`${var%word}`
`${var%%word}`
`${var#word}`
`${var##word}`

  POSIX requires support for these usages, but they do not work with many traditional shells, e.g., Solaris 10 `/bin/sh`.

  Also, `pdksh` 5.2.14 mishandles some word forms. For example if ‘`$1`’ is ‘`a/b`’ and ‘`$2`’ is ‘`a`’, then ‘`${1#$2}`’ should yield ‘`/b`’, but with `pdksh` it yields the empty string.

``commands`` ¶

  POSIX requires shells to trim all trailing newlines from command output before substituting it, so assignments like ‘`dir=`printf '%s\n' "$file" | tr a A``’ do not work as expected if ‘`$file`’ ends in a newline.

  While in general it makes no sense, do not substitute a single builtin with side effects, because Ash 0.2, trying to optimize, does not fork a subshell to perform the command. For instance, if you wanted to check that `cd` is silent, do not use ‘`test -z "`cd /`"`’ because the following can happen:

      $ pwd
      /tmp
      $ test -z "`cd /`" && pwd
      /

  The result of ‘`foo=`exit 1``’ is left as an exercise to the reader.

  The MSYS shell leaves a stray byte in the expansion of a double-quoted command substitution of a native program, if the end of the substitution is not aligned with the end of the double quote. This may be worked around by inserting another pair of quotes:

      $ echo "`printf 'foo\r\n'` bar" > broken
      $ echo "`printf 'foo\r\n'`"" bar" | cmp - broken
      - broken differ: char 4, line 1

  Upon interrupt or SIGTERM, some shells may abort a command substitution, replace it with a null string, and wrongly evaluate the enclosing command before entering the trap or ending the script. This can lead to spurious errors:

      $ sh -c 'if test `sleep 5; echo hi` = hi; then echo yes; fi'
      $ ^C
      sh: test: hi: unexpected operator/operand

  You can avoid this by assigning the command substitution to a temporary variable:

      $ sh -c 'res=`sleep 5; echo hi`
               if test "x$res" = xhi; then echo yes; fi'
      $ ^C

`$(commands)` ¶

  This construct is meant to replace ‘``commands``’, and it has most of the problems listed under ``commands``.

  This construct can be nested while this is impossible to do portably with back quotes. Although it is almost universally supported, unfortunately Solaris 10 and earlier releases lack it:

      $ showrev -c /bin/sh | grep version
      Command version: SunOS 5.10 Generic 142251-02 Sep 2010
      $ echo $(echo blah)
      syntax error: `(' unexpected

  If you do use ‘`$(commands)`’, make sure that the commands do not start with a parenthesis, as that would cause confusion with a different notation ‘`$((expression))`’ that in modern shells is an arithmetic expression not a command. To avoid the confusion, insert a space between the two opening parentheses.

  Avoid commands that contain unbalanced parentheses in here-documents, comments, or case statement patterns, as many shells mishandle them. For example, Bash 3.1, ‘`ksh88`’, `pdksh` 5.2.14, and Zsh 4.2.6 all mishandle the following valid command:

      echo $(case x in x) echo hello;; esac)

`$((expression))` ¶

  Arithmetic expansion is not portable as some shells (most notably Solaris 10 `/bin/sh`) don’t support it.

  Among shells that do support ‘`$(( ))`’, not all of them obey the POSIX rule that octal and hexadecimal constants must be recognized:

      $ bash -c 'echo $(( 010 + 0x10 ))'
      24
      $ zsh -c 'echo $(( 010 + 0x10 ))'
      26
      $ zsh -c 'emulate sh; echo $(( 010 + 0x10 ))'
      24
      $ pdksh -c 'echo $(( 010 + 0x10 ))'
      pdksh:  010 + 0x10 : bad number `0x10'
      $ pdksh -c 'echo $(( 010 ))'
      10

  When it is available, using arithmetic expansion provides a noticeable speedup in script execution; but testing for support requires `eval` to avoid syntax errors. The following construct is used by `AS_VAR_ARITH` to provide arithmetic computation when all arguments are decimal integers without leading zeros, and all operators are properly quoted and appear as distinct arguments:

      if ( eval 'test $(( 1 + 1 )) = 2' ) 2>/dev/null; then
        eval 'func_arith ()
        {
          func_arith_result=$(( $* ))
        }'
      else
        func_arith ()
        {
          func_arith_result=`expr "$@"`
        }
      fi
      func_arith 1 + 1
      foo=$func_arith_result

`^` ¶

  Always quote ‘`^`’, otherwise traditional shells such as `/bin/sh` on Solaris 10 treat this like ‘`|`’.

### 11.10 Assignments ¶

When setting several variables in a row, be aware that the order of the evaluation is undefined. For instance ‘`foo=1 foo=2; echo $foo`’ gives ‘`1`’ with Solaris 10 `/bin/sh`, but ‘`2`’ with Bash. You must use ‘`;`’ to enforce the order: ‘`foo=1; foo=2; echo $foo`’.

Don’t rely on the following to find `subdir/program`:

    PATH=subdir$PATH_SEPARATOR$PATH program

as this does not work with Zsh 3.0.6. Use something like this instead:

    (PATH=subdir$PATH_SEPARATOR$PATH; export PATH; exec program)

Don’t rely on the exit status of an assignment: Ash 0.2 does not change the status and propagates that of the last statement:

    $ false || foo=bar; echo $?
    1
    $ false || foo=`:`; echo $?
    0

and to make things even worse, QNX 4.25 just sets the exit status to 0 in any case:

    $ foo=`exit 1`; echo $?
    0

To assign default values, follow this algorithm:

1. If the default value is a literal and does not contain any closing brace, use:

       : "${var='my literal'}"

2. If the default value contains no closing brace, has to be expanded, and the variable being initialized is not intended to be IFS-split (i.e., it’s not a list), then use:

       : ${var="$default"}

3. If the default value contains no closing brace, has to be expanded, and the variable being initialized is intended to be IFS-split (i.e., it’s a list), then use:

       var=${var="$default"}

4. If the default value contains a closing brace, then use:

       test ${var+y} || var="has a '}'"

In most cases ‘`var=${var="$default"}`’ is fine, but in case of doubt, just use the last form. See Shell Substitutions, items ‘`${var:-value}`’ and ‘`${var=value}`’ for the rationale.

### 11.11 Parentheses in Shell Scripts ¶

Beware of two opening parentheses in a row, as many shell implementations treat them specially, and POSIX says that a portable script cannot use ‘`((`’ outside the ‘`$((`’ form used for shell arithmetic. In traditional shells, ‘`((cat))`’ behaves like ‘`(cat)`’; but many shells, including Bash and the Korn shell, treat ‘`((cat))`’ as an arithmetic expression equivalent to ‘`let "cat"`’, and may or may not report an error when they detect that ‘`cat`’ is not a number. As another example, ‘`pdksh`’ 5.2.14 does not treat the following code as a traditional shell would:

    if ((true) || false); then
      echo ok
    fi

To work around this problem, insert a space between the two opening parentheses. There is a similar problem and workaround with ‘`$((`’; see Shell Substitutions.

### 11.12 Special Shell Variables ¶

Some shell variables should not be used, since they can have a deep influence on the behavior of the shell. In order to recover a sane behavior from the shell, some variables should be unset; M4sh takes care of this and provides fallback values, whenever needed, to cater for a very old `/bin/sh` that does not support `unset`. (see Portable Shell Programming).

As a general rule, shell variable names containing a lower-case letter are safe; you can define and use these variables without worrying about their effect on the underlying system, and without worrying about whether the shell changes them unexpectedly. (The exception is the shell variable `status`, as described below.)

Here is a list of names that are known to cause trouble. This list is not exhaustive, but you should be safe if you avoid the name `status` and names containing only upper-case letters and underscores.

`?`

  Not all shells correctly reset ‘`$?`’ after conditionals (see Limitations of Shell Builtins). Not all shells manage ‘`$?`’ correctly in shell functions (see Shell Functions) or in traps (see Limitations of Shell Builtins). Not all shells reset ‘`$?`’ to zero after an empty command.

      $ bash -c 'false; $empty; echo $?'
      0
      $ zsh -c 'false; $empty; echo $?'
      1

`_` ¶

  Many shells reserve ‘`$_`’ for various purposes, e.g., the name of the last command executed.

`CDPATH` ¶

  When this variable is set it specifies a list of directories to search when invoking `cd` with a relative file name that did not start with ‘`./`’ or ‘`../`’. POSIX says that if a nonempty directory name from `CDPATH` is used successfully, `cd` prints the resulting absolute file name. Unfortunately this output can break idioms like ‘`abs=`cd src && pwd``’ because `abs` receives the name twice. Also, many shells do not conform to this part of POSIX; for example, `zsh` prints the result only if a directory name other than `.` was chosen from `CDPATH`.

  In practice the shells that have this problem also support `unset`, so you can work around the problem as follows:

      (unset CDPATH) >/dev/null 2>&1 && unset CDPATH

  You can also avoid output by ensuring that your directory name is absolute or anchored at ‘`./`’, as in ‘`abs=`cd ./src && pwd``’.

  Configure scripts use M4sh, which automatically unsets `CDPATH` if possible, so you need not worry about this problem in those scripts.

`CLICOLOR_FORCE` ¶

  When this variable is set, some implementations of tools like `ls` attempt to add color to their output via terminal escape sequences, even when the output is not directed to a terminal, and can thus cause spurious failures in scripts. Configure scripts use M4sh, which automatically unsets this variable.

`DUALCASE` ¶

  In the MKS shell, case statements and file name generation are case-insensitive unless `DUALCASE` is nonzero. Autoconf-generated scripts export this variable when they start up.

`ENV` ¶
`MAIL`
`MAILPATH`
`PS1`
`PS2`
`PS4`

  These variables should not matter for shell scripts, since they are supposed to affect only interactive shells. However, at least one shell (the pre-3.0 UWIN Korn shell) gets confused about whether it is interactive, which means that (for example) a `PS1` with a side effect can unexpectedly modify ‘`$?`’. To work around this bug, M4sh scripts (including `configure` scripts) do something like this:

      (unset ENV) >/dev/null 2>&1 && unset ENV MAIL MAILPATH
      PS1='$ '
      PS2='> '
      PS4='+ '

  (actually, there is some complication due to bugs in `unset`; see Limitations of Shell Builtins).

`FPATH` ¶

  The Korn shell uses `FPATH` to find shell functions, so avoid `FPATH` in portable scripts. `FPATH` is consulted after `PATH`, but you still need to be wary of tests that use `PATH` to find whether a command exists, since they might report the wrong result if `FPATH` is also set.

`GREP_OPTIONS` ¶

  When this variable is set, some implementations of `grep` honor these options, even if the options include direction to enable colored output via terminal escape sequences, and the result can cause spurious failures when the output is not directed to a terminal. Configure scripts use M4sh, which automatically unsets this variable.

`HOME` ¶

  Strange things can happen if `HOME` is unset or empty. For example, plain `cd` (with no arguments) has unspecified behavior. Also, `HOME` should be absolute, not relative.

`IFS` ¶

  Long ago, shell scripts inherited `IFS` from the environment, but this caused many problems so modern shells ignore any environment settings for `IFS`.

  Don’t set the first character of `IFS` to backslash. Indeed, Bourne shells use the first character (backslash) when joining the components in ‘`"$@"`’ and some shells then reinterpret (!) the backslash escapes, so you can end up with backspace and other strange characters.

  The proper value for `IFS` (in regular code, not when performing splits) is ‘``SPC``TAB``RET``’. The first character is especially important, as it is used to join the arguments in ‘`$*`’; however, note that traditional shells, but also bash-2.04, fail to adhere to this and join with a space anyway.

  M4sh guarantees that `IFS` will have the default value at the beginning of a script, and many macros within autoconf rely on this setting. It is okay to use blocks of shell code that temporarily change the value of `IFS` in order to split on another character, but remember to restore it before expanding further macros.

  Unsetting `IFS` instead of resetting it to the default sequence is not suggested, since code that tries to save and restore the variable’s value will incorrectly reset it to an empty value, thus disabling field splitting:

      unset IFS
      # default separators used for field splitting

      save_IFS=$IFS
      IFS=:
      # ...
      IFS=$save_IFS
      # no field splitting performed

`LANG` ¶
`LC_ALL`
`LC_COLLATE`
`LC_CTYPE`
`LC_MESSAGES`
`LC_MONETARY`
`LC_NUMERIC`
`LC_TIME`

  You should set all these variables to ‘`C`’ because so much configuration code assumes the C locale and POSIX requires that locale environment variables be set to ‘`C`’ if the C locale is desired; `configure` scripts and M4sh do that for you. Export these variables after setting them.

`LANGUAGE` ¶

  `LANGUAGE` is not specified by POSIX, but it is a GNU extension that overrides `LC_ALL` in some cases, so you (or M4sh) should set it too.

`LC_ADDRESS` ¶
`LC_IDENTIFICATION`
`LC_MEASUREMENT`
`LC_NAME`
`LC_PAPER`
`LC_TELEPHONE`

  These locale environment variables are GNU extensions. They are treated like their POSIX brethren (`LC_COLLATE`, etc.) as described above.

`LINENO` ¶

  Most modern shells provide the current line number in `LINENO`. Its value is the line number of the beginning of the current command. M4sh, and hence Autoconf, attempts to execute `configure` with a shell that supports `LINENO`. If no such shell is available, it attempts to implement `LINENO` with a Sed prepass that replaces each instance of the string `$LINENO` (not followed by an alphanumeric character) with the line’s number. In M4sh scripts you should execute `AS_LINENO_PREPARE` so that these workarounds are included in your script; configure scripts do this automatically in `AC_INIT`.

  You should not rely on `LINENO` within `eval` or shell functions, as the behavior differs in practice. The presence of a quoted newline within simple commands can alter which line number is used as the starting point for `$LINENO` substitutions within that command. Also, the possibility of the Sed prepass means that you should not rely on `$LINENO` when quoted, when in here-documents, or when line continuations are used. Subshells should be OK, though. In the following example, lines 1, 9, and 14 are portable, but the other instances of `$LINENO` do not have deterministic values:

      $ cat lineno
      echo 1. $LINENO
      echo "2. $LINENO
      3. $LINENO"
      cat <<EOF
      5. $LINENO
      6. $LINENO
      7. \$LINENO
      EOF
      ( echo 9. $LINENO )
      eval 'echo 10. $LINENO'
      eval 'echo 11. $LINENO
      echo 12. $LINENO'
      echo 13. '$LINENO'
      echo 14. $LINENO '
      15.' $LINENO
      f () { echo $1 $LINENO;
      echo $1 $LINENO }
      f 18.
      echo 19. \
      $LINENO

      $ bash-3.2 ./lineno
      1. 1
      2. 3
      3. 3
      5. 4
      6. 4
      7. $LINENO
      9. 9
      10. 10
      11. 12
      12. 13
      13. $LINENO
      14. 14
      15. 14
      18. 16
      18. 17
      19. 19

      $ zsh-4.3.4 ./lineno
      1. 1
      2. 2
      3. 2
      5. 4
      6. 4
      7. $LINENO
      9. 9
      10. 1
      11. 1
      12. 2
      13. $LINENO
      14. 14
      15. 14
      18. 0
      18. 1
      19. 19

      $ pdksh-5.2.14 ./lineno
      1. 1
      2. 2
      3. 2
      5. 4
      6. 4
      7. $LINENO
      9. 9
      10. 0
      11. 0
      12. 0
      13. $LINENO
      14. 14
      15. 14
      18. 16
      18. 17
      19. 19

      $ sed '=' <lineno |
      >   sed '
      >     N
      >     s,$,-,
      >     t loop
      >     :loop
      >     s,^\([0-9]*\)\(.*\)[$]LINENO\([^a-zA-Z0-9_]\),\1\2\1\3,
      >     t loop
      >     s,-$,,
      >     s,^[0-9]*\n,,
      >   ' |
      >   sh
      1. 1
      2. 2
      3. 3
      5. 5
      6. 6
      7. \7
      9. 9
      10. 10
      11. 11
      12. 12
      13. 13
      14. 14
      15. 15
      18. 16
      18. 17
      19. 20

  In particular, note that `config.status` (and any other subsidiary script created by `AS_INIT_GENERATED`) might report line numbers relative to the parent script as a result of the potential Sed pass.

`NULLCMD` ¶

  When executing the command ‘`>foo`’, `zsh` executes ‘`$NULLCMD >foo`’ unless it is operating in Bourne shell compatibility mode and the `zsh` version is newer than 3.1.6-dev-18. If you are using an older `zsh` and forget to set `NULLCMD`, your script might be suspended waiting for data on its standard input.

`OLDPWD` ¶

  POSIX requires that `cd` must update the `OLDPWD` environment variable, if set, to point to the name of the previous directory, but Solaris 10 `/bin/sh` does not support this.

`options` ¶

  For `zsh` 4.3.10, `options` is treated as an associative array even after `emulate sh`, so it should not be used.

`PATH_SEPARATOR` ¶

  On DJGPP systems, the `PATH_SEPARATOR` environment variable can be set to either ‘`:`’ or ‘`;`’ to control the path separator Bash uses to set up certain environment variables (such as `PATH`). You can set this variable to ‘`;`’ if you want `configure` to use ‘`;`’ as a separator; this might be useful if you plan to use non-POSIX shells to execute files. See File System Conventions, for more information about `PATH_SEPARATOR`.

`POSIXLY_CORRECT` ¶

  In the GNU environment, exporting `POSIXLY_CORRECT` with any value (even empty) causes programs to try harder to conform to POSIX. Autoconf does not directly manipulate this variable, but `bash` ties the shell variable `POSIXLY_CORRECT` to whether the script is running in POSIX mode. Therefore, take care when exporting or unsetting this variable, so as not to change whether `bash` is in POSIX mode.

      $ bash --posix -c 'set -o | grep posix
      > unset POSIXLY_CORRECT
      > set -o | grep posix'
      posix           on
      posix           off

`PWD` ¶

  POSIX requires that `cd` must update the `PWD` environment variable to point to the name of the current directory, but traditional shells do not support this. This can cause confusion if one shell instance maintains `PWD` but a subsidiary and different shell does not know about `PWD` and executes `cd`; in this case `PWD` points to the wrong directory. Use ‘``pwd``’ rather than ‘`$PWD`’.

`RANDOM` ¶

  Many shells provide `RANDOM`, a variable that returns a different integer each time it is used. It is common practice to use `$RANDOM` as part of a file name, but code shouldn’t rely on `$RANDOM` expanding to a nonempty string.

`status` ¶

  This variable is an alias to ‘`$?`’ for `zsh` (at least 3.1.6), hence read-only. Do not use it.

### 11.13 Shell Functions ¶

Nowadays, it is difficult to find a shell that does not support shell functions at all. However, some differences should be expected.

When declaring a shell function, you must include whitespace between the ‘`)`’ after the function name and the start of the compound expression, to avoid upsetting `ksh`. While it is possible to use any compound command, most scripts use ‘`{…}`’.

    $ /bin/sh -c 'a(){ echo hi;}; a'
    hi
    $ ksh -c 'a(){ echo hi;}; a'
    ksh: syntax error at line 1: `}' unexpected
    $ ksh -c 'a() { echo hi;}; a'
    hi

Inside a shell function, you should not rely on the error status of a subshell if the last command of that subshell was `exit` or `trap`, as this triggers bugs in zsh 4.x; while Autoconf tries to find a shell that does not exhibit the bug, zsh might be the only shell present on the user’s machine.

Likewise, the state of ‘`$?`’ is not reliable when entering a shell function. This has the effect that using a function as the first command in a `trap` handler can cause problems.

    $ bash -c 'foo() { echo $?; }; trap foo 0; (exit 2); exit 2'; echo $?
    2
    2
    $ ash -c 'foo() { echo $?; }; trap foo 0; (exit 2); exit 2'; echo $?
    0
    2

DJGPP bash 2.04 has a bug in that `return` from a shell function which also used a command substitution causes a segmentation fault. To work around the issue, you can use `return` from a subshell, or ‘`AS_SET_STATUS`’ as last command in the execution flow of the function (see Common Shell Constructs).

Not all shells treat shell functions as simple commands impacted by ‘`set -e`’, for example with Solaris 10 `/bin/sh`:

    $ bash -c 'f() { return 1; }; set -e; f; echo oops'
    $ /bin/sh -c 'f() { return 1; }; set -e; f; echo oops'
    oops

Shell variables and functions may share the same namespace, for example with Solaris 10 `/bin/sh`:

    $ f () { :; }; f=; f
    f: not found

For this reason, Autoconf (actually M4sh, see Programming in M4sh) uses the prefix ‘`as_fn_`’ for its functions.

Handling of positional parameters and shell options varies among shells. For example, Korn shells reset and restore trace output (‘`set -x`’) and other options upon function entry and exit.

It is not portable to pass temporary environment variables to shell functions. Solaris 10 `/bin/sh` does not see the variable. Meanwhile, not all shells follow the POSIX rule that the assignment must affect the current environment in the same manner as special built-ins.

    $ /bin/sh -c 'func() { echo $a;}; a=1 func; echo $a'
    ⇒
    ⇒
    $ ash -c 'func() { echo $a;}; a=1 func; echo $a'
    ⇒1
    ⇒
    $ bash -c 'set -o posix; func() { echo $a;}; a=1 func; echo $a'
    ⇒1
    ⇒1

Some ancient Bourne shell variants with function support did not reset ‘`$i, i >= 0`’, upon function exit, so effectively the arguments of the script were lost after the first function invocation. It is probably not worth worrying about these shells any more.

With AIX sh, a `trap` on 0 installed in a shell function triggers at function exit rather than at script exit. See Limitations of Shell Builtins.

### 11.14 Limitations of Shell Builtins ¶

No, no, we are serious: some shells do have limitations! :)

You should always keep in mind that any builtin or command may support options, and therefore differ in behavior with arguments starting with a dash. For instance, even the innocent ‘`echo "$word"`’ can give unexpected results when `word` starts with a dash. To avoid this problem, use ‘`printf '%s\n' "$word"`’. Many of these limitations can be worked around using M4sh (see Programming in M4sh).

`.`

  Use `.` only with regular files (use ‘`test -f`’). Bash 2.03, for instance, chokes on ‘`. /dev/null`’. Remember that `.` uses `PATH` if its argument contains no slashes. Also, some shells, including bash 3.2, implicitly append the current directory to this `PATH` search, even though POSIX forbids it. So if you want to use `.` on a file `foo` in the current directory, you must use ‘`. ./foo`’.

  Not all shells gracefully handle syntax errors within a sourced file. On one extreme, some non-interactive shells abort the entire script. On the other, `zsh` 4.3.10 has a bug where it fails to react to the syntax error.

      $ echo 'fi' > syntax
      $ bash -c '. ./syntax; echo $?'
      ./syntax: line 1: syntax error near unexpected token `fi'
      ./syntax: line 1: `fi'
      2
      $ ash -c '. ./syntax; echo $?'
      ./syntax: 1: Syntax error: "fi" unexpected
      $ zsh -c '. ./syntax; echo $?'
      ./syntax:1: parse error near `fi'
      0

`!`

  The Unix version 7 shell did not support negating the exit status of commands with `!`, and this feature is still absent from some shells (e.g., Solaris 10 `/bin/sh`). Other shells, such as FreeBSD `/bin/sh` or `ash`, have bugs when using `!`:

      $ sh -c '! : | :'; echo $?
      1
      $ ash -c '! : | :'; echo $?
      0
      $ sh -c '! { :; }'; echo $?
      1
      $ ash -c '! { :; }'; echo $?
      {: not found
      Syntax error: "}" unexpected
      2

  Shell code like this:

      if ! cmp file1 file2 >/dev/null 2>&1; then
        echo files differ or trouble
      fi

  is therefore not portable in practice. Typically it is easy to rewrite such code, e.g.:

      cmp file1 file2 >/dev/null 2>&1 ||
        echo files differ or trouble

  In M4sh, the `AS_IF` macro provides an easy way to write these kinds of conditionals:

      AS_IF([cmp -s file file.new], [],
        [echo files differ or trouble])

  This kind of rewriting is needed in code outside macro definitions that calls other macros. See Common Shell Constructs. It is also useful inside macro definitions, where the then and else branches might contain macro arguments.

  More generally, one can always rewrite ‘`! command`’ as:

      AS_IF([command], [(exit 1)])

`&&` and `||`

  If an AND-OR list is not inside `AC_DEFUN`, and it contains calls to Autoconf macros, it should be rewritten using `AS_IF`. See Common Shell Constructs. The operators `&&` and `||` have equal precedence and are left associative, so instead of:

      # This is dangerous outside AC_DEFUN.
      cmp a b >/dev/null 2>&1 &&
        AS_ECHO([files are same]) >$tmpfile ||
          AC_MSG_NOTICE([files differ, or echo failed])

  you can use:

      # This is OK outside AC_DEFUN.
      AS_IF([AS_IF([cmp a b >/dev/null 2>&1],
               [AS_ECHO([files are same]) >$tmpfile],
               [false])],
        [AC_MSG_NOTICE([files differ, or echo failed])])

`{...}`

  Bash 3.2 (and earlier versions) sometimes does not properly set ‘`$?`’ when failing to write redirected output of a compound command. This problem is most commonly observed with ‘`{…}`’; it does not occur with ‘`(…)`’. For example:

      $ bash -c '{ echo foo; } >/bad; echo $?'
      bash: line 1: /bad: Permission denied
      0
      $ bash -c 'while :; do echo; done >/bad; echo $?'
      bash: line 1: /bad: Permission denied
      0

  To work around the bug, prepend ‘`:;`’:

      $ bash -c ':;{ echo foo; } >/bad; echo $?'
      bash: line 1: /bad: Permission denied
      1

  POSIX requires a syntax error if a brace list has no contents. However, not all shells obey this rule; and on shells where empty lists are permitted, the effect on ‘`$?`’ is inconsistent. To avoid problems, ensure that a brace list is never empty.

      $ bash -c 'false; { }; echo $?' || echo $?
      bash: line 1: syntax error near unexpected token `}'
      bash: line 1: `false; { }; echo $?'
      2
      $ zsh -c 'false; { }; echo $?' || echo $?
      1
      $ pdksh -c 'false; { }; echo $?' || echo $?
      0

`break`

  The use of ‘`break 2`’ etc. is safe.

`case`

  If a `case` command is not inside `AC_DEFUN`, and it contains calls to Autoconf macros, it should be rewritten using `AS_CASE`. See Common Shell Constructs. Instead of:

      # This is dangerous outside AC_DEFUN.
      case $filename in
        *.[ch]) AC_MSG_NOTICE([C source file]);;
      esac

  use:

      # This is OK outside AC_DEFUN.
      AS_CASE([$filename],
        [[*.[ch]]], [AC_MSG_NOTICE([C source file])])

  You don’t need to quote the argument; no splitting is performed.

  You don’t need the final ‘`;;`’, but you should use it.

  POSIX requires support for `case` patterns with opening parentheses like this:

      case $file_name in
        (*.c) echo "C source code";;
      esac

  but the `(` in this example is not portable to a few obsolescent Bourne shell implementations, which is a pity for those of us using tools that rely on balanced parentheses. For instance, with Solaris 10 `/bin/sh`:

      $ case foo in (foo) echo foo;; esac
      error→syntax error: `(' unexpected

  The leading ‘`(`’ can be omitted safely. Unfortunately, there are contexts where unbalanced parentheses cause other problems, such as when using a syntax-highlighting editor that searches for the balancing counterpart, or more importantly, when using a case statement as an underquoted argument to an Autoconf macro. See Dealing with unbalanced parentheses, for trade-offs involved in various styles of dealing with unbalanced ‘`)`’.

  Zsh handles pattern fragments derived from parameter expansions or command substitutions as though quoted:

      $ pat=\?; case aa in ?$pat) echo match;; esac
      $ pat=\?; case a? in ?$pat) echo match;; esac
      match

  Because of a bug in its `fnmatch`, Bash fails to properly handle backslashes in character classes:

      bash-2.02$ case /tmp in [/\\]*) echo OK;; esac
      bash-2.02$

  This is extremely unfortunate, since you are likely to use this code to handle POSIX or MS-DOS absolute file names. To work around this bug, always put the backslash first:

      bash-2.02$ case '\TMP' in [\\/]*) echo OK;; esac
      OK
      bash-2.02$ case /tmp in [\\/]*) echo OK;; esac
      OK

  Many Bourne shells cannot handle closing brackets in character classes correctly.

  Some shells also have problems with backslash escaping in case you do not want to match the backslash: both a backslash and the escaped character match this pattern. To work around this, specify the character class in a variable, so that quote removal does not apply afterwards, and the special characters don’t have to be backslash-escaped:

      $ case '\' in [\<]) echo OK;; esac
      OK
      $ scanset='[<]'; case '\' in $scanset) echo OK;; esac
      $

  Even with this, Solaris `ksh` matches a backslash if the set contains any of the characters ‘`|`’, ‘`&`’, ‘`(`’, or ‘`)`’.

  Some shells, such as Ash 0.3.8, are confused by an empty `case`/`esac`:

      ash-0.3.8 $ case foo in esac;
      error→Syntax error: ";" unexpected (expecting ")")

  POSIX requires `case` to give an exit status of 0 if no cases match. However, `/bin/sh` in Solaris 10 does not obey this rule. Meanwhile, it is unclear whether a case that matches, but contains no statements, must also change the exit status to 0. The M4sh macro `AS_CASE` works around these inconsistencies.

      $ bash -c 'case `false` in ?) ;; esac; echo $?'
      0
      $ /bin/sh -c 'case `false` in ?) ;; esac; echo $?'
      255

`cd`

  POSIX requires that `cd` must support the `-L` (“logical”) and `-P` (“physical”) options, with `-L` being the default. However, traditional shells do not support these options, and their `cd` command has the `-P` behavior.

  Portable scripts should assume neither option is supported, and should assume neither behavior is the default. This can be a bit tricky, since the POSIX default behavior means that, for example, ‘`ls ..`’ and ‘`cd ..`’ may refer to different directories if the current logical directory is a symbolic link. It is safe to use `cd dir` if dir contains no `..` components. Also, Autoconf-generated scripts check for this problem when computing variables like `ac_top_srcdir` (see Performing Configuration Actions), so it is safe to `cd` to these variables.

  POSIX specifies an `-e` option that affects the exit status when the `-P` option is in effect. Portable scripts should avoid it for the same reason they avoid `-P`.

  POSIX says that a successful `cd` updates the values of the `OLDPWD` and `PWD` variables, and that `cd -` acts like `cd "$OLDPWD"`. Older shells, such as Solaris 10 `/bin/sh`, do not support this and portable scripts should not rely on it.

  If `HOME` is unset or empty, the behavior of `cd` with no arguments is implementation-defined, so portable scripts should avoid it in that case.

  POSIX 1003.1-2024 states that `cd` must fail if given an explicit empty argument. However, earlier POSIX editions did not specify the behavior and some shells do nothing, some change to the first entry in `CDPATH`, some change to `HOME`, and some exit the shell rather than returning an error. Unfortunately, this means that if ‘`$var`’ is empty, then ‘`cd "$var"`’ is less predictable than ‘`cd $var`’ (at least the latter is well-behaved in all shells at changing to `HOME`, although this is probably not what you wanted in a script). You should check that a directory name was supplied before trying to change locations.

  If `cd` fails, POSIX requires the invoking shell to behave as if `cd` exited with status 1, which means the invoking shell can continue. However, Solaris 10 `/bin/sh` aborts the containing command if interactive, and exits otherwise.

  See Special Shell Variables, for portability problems involving `cd` and the `CDPATH` environment variable. Also please see the discussion of the `pwd` command.

`echo`

  The simple `echo` is probably the most surprising source of portability troubles. It is not possible to use ‘`echo`’ portably unless both options and escape sequences are omitted.

  Do not use options, as some shells support them and others do not. For example, POSIX says that the behavior of ‘`echo -n foo`’ is implementation-defined. On some platforms the output is ‘`foo`’ without a trailing newline, on others it is ‘`-n foo`’ with a trailing newline, and POSIX allows even other behavior.

  Do not use backslashes in the arguments, as there is no consensus on their handling. For ‘`echo '\n' | wc -l`’, the `sh` of Solaris 10 outputs 2, but Bash and Zsh (in `sh` emulation mode) output 1. The problem is truly `echo`: all the shells understand ‘`'\n'`’ as the string composed of a backslash and an ‘`n`’.

  Because of these problems, do not pass a string containing arbitrary characters to `echo`. For example, ‘`echo "$foo"`’ is safe only if you know that foo’s value cannot contain backslashes and cannot start with ‘`-`’.

  Normally, `printf` is safer and easier to use than `echo` and `echo -n`. Thus, you should use `printf '%s\n'` instead of `echo`, and similarly use `printf %s` instead of `echo -n`.

  Older scripts, written before `printf` was portable, sometimes used a here-document as a safer alternative to `echo`, like this:

      cat <<EOF
      $foo
      EOF

  However, this usage is problematic, as even some modern shells have hard-to-reproduce bugs when dealing with here-documents.

`eval`

  The `eval` command is useful in limited circumstances, e.g., using commands like ‘`eval table_$key=\$value`’ and ‘`eval value=table_$key`’ to simulate a hash table when the key is known to be alphanumeric.

  You should also be wary of common bugs in `eval` implementations. In some shell implementations (e.g., older `ash`, OpenBSD 3.8 `sh`, `pdksh` v5.2.14 99/07/13.2, and `zsh` 4.2.5), the arguments of ‘`eval`’ are evaluated in a context where ‘`$?`’ is 0, so they exhibit behavior like this:

      $ false; eval 'echo $?'
      0

  The correct behavior here is to output a nonzero value, but portable scripts should not rely on this.

  You should not rely on `LINENO` within `eval`. See Special Shell Variables.

  Note that, even though these bugs are easily avoided, `eval` is tricky to use on arbitrary arguments. It is obviously unwise to use ‘`eval $cmd`’ if the string value of ‘`cmd`’ was derived from an untrustworthy source. But even if the string value is valid, ‘`eval $cmd`’ might not work as intended, since it causes field splitting and file name expansion to occur twice, once for the `eval` and once for the command itself. It is therefore safer to use ‘`eval "$cmd"`’. For example, if cmd has the value ‘`cat test?.c`’, ‘`eval $cmd`’ might expand to the equivalent of ‘`cat test;.c`’ if there happens to be a file named `test;.c` in the current directory; and this in turn mistakenly attempts to invoke `cat` on the file `test` and then execute the command `.c`. To avoid this problem, use ‘`eval "$cmd"`’ rather than ‘`eval $cmd`’.

  However, suppose that you want to output the text of the evaluated command just before executing it. Assuming the previous example, ‘`printf '%s\n' "Executing: $cmd"`’ outputs ‘`Executing: cat test?.c`’, but this output doesn’t show the user that ‘`test;.c`’ is the actual name of the copied file. Conversely, ‘`printf 'Executing:'; eval "printf ' %s' $cmd"; printf '\n'`’ works on this example, but it fails with ‘`cmd='cat foo >bar'`’, since it mistakenly replaces the contents of `bar` by the string ‘`cat foo`’. No simple, general, and portable solution to this problem is known.

`exec`

  POSIX describes several categories of shell built-ins. Special built-ins (such as `exit`) must impact the environment of the current shell, and need not be available through `exec`. All other built-ins are regular, and must not propagate variable assignments to the environment of the current shell. However, the group of regular built-ins is further distinguished by commands that do not require a `PATH` search (such as `cd`), in contrast to built-ins that are offered as a more efficient version of something that must still be found in a `PATH` search (such as `echo`). POSIX is not clear on whether `exec` must work with the list of 17 utilities that are invoked without a `PATH` search, and many platforms lack an executable for some of those built-ins:

      $ sh -c 'exec cd /tmp'
      sh: line 0: exec: cd: not found

  All other built-ins that provide utilities specified by POSIX must have a counterpart executable that exists on `PATH`, although POSIX allows `exec` to use the built-in instead of the executable. For example, contrast `bash` 3.2 and `pdksh` 5.2.14:

      $ bash -c 'pwd --version' | head -n1
      bash: line 0: pwd: --: invalid option
      pwd: usage: pwd [-LP]
      $ bash -c 'exec pwd --version' | head -n1
      pwd (GNU coreutils) 6.10
      $ pdksh -c 'exec pwd --version' | head -n1
      pdksh: pwd: --: unknown option

  When it is desired to avoid a regular shell built-in, the workaround is to use some other forwarding command, such as `env` or `nice`, that will ensure a path search:

      $ pdksh -c 'exec true --version' | head -n1
      $ pdksh -c 'nice true --version' | head -n1
      true (GNU coreutils) 6.10
      $ pdksh -c 'env true --version' | head -n1
      true (GNU coreutils) 6.10

`exit`

  The default value of `exit` is supposed to be `$?`; unfortunately, some shells, such as the DJGPP port of Bash 2.04, just perform ‘`exit 0`’.

      bash-2.04$ foo=`exit 1` || echo fail
      fail
      bash-2.04$ foo=`(exit 1)` || echo fail
      fail
      bash-2.04$ foo=`(exit 1); exit` || echo fail
      bash-2.04$

  Using ‘`exit $?`’ restores the expected behavior.

  Some shell scripts, such as those generated by `autoconf`, use a trap to clean up before exiting. If the last shell command exited with nonzero status, the trap also exits with nonzero status so that the invoker can tell that an error occurred.

  Unfortunately, in some shells, such as Solaris 10 `/bin/sh`, an exit trap ignores the `exit` command’s argument. In these shells, a trap cannot determine whether it was invoked by plain `exit` or by `exit 1`. Instead of calling `exit` directly, use the `AC_MSG_ERROR` macro that has a workaround for this problem.

`export`

  The builtin `export` dubs a shell variable environment variable. Each update of exported variables corresponds to an update of the environment variables. Conversely, each environment variable received by the shell when it is launched should be imported as a shell variable marked as exported.

  Alas, some older shells, such as Solaris 10 `/bin/sh`, forget to `export` the environment variables they receive. As a result, two variables coexist: the environment variable and the shell variable. The following code demonstrates this failure:

      #!/bin/sh
      echo $FOO
      FOO=bar
      echo $FOO
      exec /bin/sh $0

  when run with ‘`FOO=foo`’ in the environment, these shells print alternately ‘`foo`’ and ‘`bar`’, although they should print only ‘`foo`’ and then a sequence of ‘`bar`’s.

  Therefore you should `export` again each environment variable that you update; the export can occur before or after the assignment.

  POSIX is not clear on whether the `export` of an undefined variable causes the variable to be defined with the value of an empty string, or merely marks any future definition of a variable by that name for export. Various shells behave differently in this regard:

      $ sh -c 'export foo; env | grep foo'
      $ ash -c 'export foo; env | grep foo'
      foo=

  POSIX requires `export` to honor assignments made as arguments, but older shells do not support this, including `/bin/sh` in Solaris 10. Portable scripts should separate assignments and exports into different statements.

      $ bash -c 'export foo=bar; echo $foo'
      bar
      $ /bin/sh -c 'export foo=bar; echo $foo'
      /bin/sh: foo=bar: is not an identifier
      $ /bin/sh -c 'export foo; foo=bar; echo $foo'
      bar

  POSIX requires `export` to work with any arbitrary value for the contents of the variable being exported, as long as the total size of the environment combined with arguments doesn’t exceed `ARG_MAX` when executing a child process. However, some shells have extensions that involve interpreting some environment values specially, regardless of the variable name. We currently know of one case: all versions of Bash released prior to 27 September 2014 interpret an environment variable with an initial content substring of `() {` as an exported function definition (this is the “Shellshock” remote execution bug, CVE-2014-6271 and friends, where it was possible to exploit the function parser to cause remote code execution on child bash startup; newer versions of Bash use special environment variable names instead of values to implement the same feature).

  There may be entries inherited into the environment that are not valid as shell variable names; POSIX states that processes should be tolerant of these names. Some shells such as `dash` do this by removing those names from the environment at startup, while others such as `bash` hide the entry from shell access but still pass it on to child processes. While you can set such names using `env` for a direct child process, you cannot rely on them being preserved through an intermediate pass through the shell.

`false`

  Don’t expect `false` to exit with status 1: in native Solaris `/bin/false` exits with status 255.

`for`

  To loop over positional arguments, use:

      for arg
      do
        printf '%s\n' "$arg"
      done

  You may not leave the `do` on the same line as `for`, since some shells improperly grok:

      for arg; do
        printf '%s\n' "$arg"
      done

  If you want to explicitly refer to the positional arguments, use:

      for arg in "$@"; do
        printf '%s\n' "$arg"
      done

  POSIX requires support for a `for` loop with no list after `in`. However, Solaris 10 `/bin/sh` treats that as a syntax error. It is possible to work around this by providing any shell word that expands to nothing, or by ignoring an obvious sentinel.

      $ /bin/sh -c 'for a in $empty; do echo hi; done'
      $ /bin/sh -c 'for a in ; do echo hi; done'
      /bin/sh: syntax error at line 1: `;' unexpected

  This syntax problem is most frequently encountered in code that goes through several layers of expansion, such as an m4 macro or makefile variable used as a list body, where the first layer of expansion (m4 or make) can end up expanding to nothing in the version handed to the shell. In the makefile context, one common workaround is to use a shell variable rather than a make variable as the source of the list.

      $ cat Makefile
      list =
      bad:
      	@for arg in $(list); do \
      	  printf '%s\n' $$arg; \
      	done
      good:
      	@list='$(list)'; \
      	for arg in $$list; do \
      	  printf '%s\n' $$arg; \
      	done
      $ make bad 2&>1 | head -n1
      sh: syntax error at line 1: `;' unexpected
      $ make bad list='a b'
      a
      b
      $ make good
      $ make good list='a b'
      a
      b

  In Solaris 10 `/bin/sh`, when the list of arguments of a `for` loop starts with unquoted tokens looking like variable assignments, the loop is not executed on those tokens:

      $ /bin/sh -c 'for v in a=b c=d x e=f; do echo $v; done'
      x
      e=f

  Thankfully, quoting the assignment-like tokens, or starting the list with other tokens (including unquoted variable expansion that results in an assignment-like result), avoids the problem, so it is easy to work around:

      $ /bin/sh -c 'for v in "a=b"; do echo $v; done'
      a=b
      $ /bin/sh -c 'x=a=b; for v in $x c=d; do echo $v; done'
      a=b
      c=d

`if`

  If an `if` command is not inside `AC_DEFUN`, and it contains calls to Autoconf macros, it should be rewritten using `AS_IF`. See Common Shell Constructs.

  Using `if ! …` is not portable. See `!` notes.

  Some very old shells did not reset the exit status from an `if` with no `else`:

      $ if (exit 42); then true; fi; echo $?
      42

  whereas a proper shell should have printed ‘`0`’. Although this is no longer a portability problem, as any shell that supports functions gets it correct, it explains why some makefiles have lengthy constructs:

      if test -f "$file"; then
        install "$file" "$dest"
      else
        :
      fi

`printf`

  A format string starting with a ‘`-`’ can cause problems. Bash interprets it as an option and gives an error. And ‘`--`’ to mark the end of options is not good in the NetBSD Almquist shell (e.g., 0.4.6) which takes that literally as the format string. Putting the ‘`-`’ in a ‘`%c`’ or ‘`%s`’ is probably easiest:

      printf %s -foo

  AIX 7.2 `sh` mishandles octal escapes in multi-byte locales by treating them as characters instead of bytes. For example, in a locale using the UTF-8 encoding, ‘`printf '\351'`’ outputs the two bytes C3, A9 (the UTF-8 encoding for U+00E9) instead of the desired single byte E9. To work around the bug, use the C locale.

  Bash 2.03 mishandles an escape sequence that happens to evaluate to ‘`%`’:

      $ printf '\045'
      bash: printf: `%': missing format character

  Large outputs may cause trouble. On Solaris 10, for example, `/usr/bin/printf` is buggy, so when using `/bin/sh` the command ‘`printf %010000x 123`’ normally dumps core.

  Since `printf` is not always a shell builtin, there is a potential speed penalty for using `printf '%s\n'` as a replacement for an `echo` that does not interpret ‘`\`’ or leading ‘`-`’. With Solaris `ksh`, it is possible to use `print -r --` for this role instead.

  See Limitations of Shell Builtins, for a discussion of portable alternatives to both `printf` and `echo`.

`pwd`

  With modern shells, plain `pwd` outputs a “logical” directory name, some of whose components may be symbolic links. These directory names are in contrast to “physical” directory names, whose components are all directories.

  POSIX requires that `pwd` must support the `-L` (“logical”) and `-P` (“physical”) options, with `-L` being the default. However, traditional shells do not support these options, and their `pwd` command has the `-P` behavior.

  Portable scripts should assume neither option is supported, and should assume neither behavior is the default. Also, on many hosts ‘`/bin/pwd`’ is equivalent to ‘`pwd -P`’, but POSIX does not require this behavior and portable scripts should not rely on it.

  Typically it’s best to use plain `pwd`. On modern hosts this outputs logical directory names, which have the following advantages:

  - Logical names are what the user specified.
  - Physical names may not be portable from one installation host to another due to network file system gymnastics.
  - On modern hosts ‘`pwd -P`’ may fail due to lack of permissions to some parent directory, but plain `pwd` cannot fail for this reason.

  Also please see the discussion of the `cd` command.

`read`

  No options are portable, not even support `-r` (Solaris 10 `/bin/sh` for example).

`set`

  With the FreeBSD 6.0 shell, the `set` command (without any options) does not sort its output.

  The `set` builtin faces the usual problem with arguments starting with a dash. Modern shells understand `--` to specify the end of the options (any argument after `--` is a parameter, even ‘`-x`’ for instance), but some ancient shells stop option processing only after a non-option argument is found. Therefore, use ‘`dummy`’ or simply ‘`x`’ to end the option processing, and use `shift` to pop it out:

      set x $my_list; shift

  Avoid ‘`set -`’, e.g., ‘`set - $my_list`’. POSIX no longer requires support for this command, and in traditional shells ‘`set - $my_list`’ resets the `-v` and `-x` options, which makes scripts harder to debug.

  Some nonstandard shells do not recognize more than one option (e.g., ‘`set -e -x`’ assigns ‘`-x`’ to the command line). It is better to combine them:

      set -ex

  The `-e` option has historically been under-specified, with enough ambiguities to cause numerous differences across various shell implementations; see for example this overview, or this link, documenting a change to POSIX 2008 to match `ksh88` behavior. Note that mixing `set -e` and shell functions is asking for surprises:

      set -e
      doit()
      {
        rm file
        echo one
      }
      doit || echo two

  According to the recommendation, ‘`one`’ should always be output regardless of whether the `rm` failed, because it occurs within the body of the shell function ‘`doit`’ invoked on the left side of ‘`||`’, where the effects of ‘`set -e`’ are not enforced. Likewise, ‘`two`’ should never be printed, since the failure of `rm` does not abort the function, such that the status of ‘`doit`’ is 0.

  The BSD shell has had several problems with the `-e` option. Older versions of the BSD shell (circa 1990) mishandled ‘`&&`’, ‘`||`’, ‘`if`’, and ‘`case`’ when `-e` was in effect, causing the shell to exit unexpectedly in some cases. This was particularly a problem with makefiles, and led to circumlocutions like ‘`sh -c 'test -f file || touch file'`’, where the seemingly-unnecessary ‘`sh -c '…'`’ wrapper works around the bug (see Failure in Make Rules).

  Even relatively-recent versions of the BSD shell (e.g., OpenBSD 3.4) wrongly exit with `-e` if the last command within a compound statement fails and is guarded by an ‘`&&`’ only. For example:

      #! /bin/sh
      set -e
      foo=''
      test -n "$foo" && exit 1
      echo one
      if :; then
        test -n "$foo" && exit 1
        echo two
        test -n "$foo" && exit 1
      fi
      echo three

  does not print ‘`three`’. One workaround is to change the last instance of ‘`test -n "$foo" && exit 1`’ to be ‘`if test -n "$foo"; then exit 1; fi`’ instead. Another possibility is to warn BSD users not to use ‘`sh -e`’.

  When ‘`set -e`’ is in effect, a failed command substitution in Solaris 10 `/bin/sh` cannot be ignored, even with ‘`||`’.

      $ /bin/sh -c 'set -e; foo=`false` || echo foo; echo bar'
      $ bash -c 'set -e; foo=`false` || echo foo; echo bar'
      foo
      bar

  Moreover, a command substitution, successful or not, causes this shell to exit from a failing outer command even in presence of an ‘`&&`’ list:

      $ bash -c 'set -e; false `true` && echo notreached; echo ok'
      ok
      $ sh -c 'set -e; false `true` && echo notreached; echo ok'
      $

  Job control is not provided by all shells, so the use of ‘`set -m`’ or ‘`set -b`’ must be done with care. When using `zsh` in native mode, asynchronous notification (‘`set -b`’) is enabled by default, and using ‘`emulate sh`’ to switch to POSIX mode does not clear this setting (although asynchronous notification has no impact unless job monitoring is also enabled). Also, `zsh` 4.3.10 and earlier have a bug where job control can be manipulated in interactive shells, but not in subshells or scripts. Furthermore, some shells, like `pdksh`, fail to treat subshells as interactive, even though the parent shell was.

      $ echo $ZSH_VERSION
      4.3.10
      $ set -m; echo $?
      0
      $ zsh -c 'set -m; echo $?'
      set: can't change option: -m
      $ (set -m); echo $?
      set: can't change option: -m
      1
      $ pdksh -ci 'echo $-; (echo $-)'
      cim
      c

  Use of `set -n` (typically via `sh -n script`) to validate a script is not foolproof. Modern `ksh93` tries to be helpful by informing you about better syntax, but switching the script to use the suggested syntax in order to silence the warnings would render the script no longer portable to older shells:

      $ ksh -nc '``'
      ksh: warning: line 1: `...` obsolete, use $(...)
      0

  Autoconf itself uses `sh -n` within its testsuite to check that correct scripts were generated, but only after first probing for other shell features (such as `test ${BASH_VERSION+y}`) that indicate a reasonably fast and working implementation.

`shift`

  Not only is `shift`ing a bad idea when there is nothing left to shift, but in addition it is not portable: the shell of MIPS RISC/OS 4.52 refuses to do it.

  Don’t use ‘`shift 2`’ etc.; while it in the SVR1 shell (1983), it is also absent in many pre-POSIX shells.

`source`

  This command is not portable, as POSIX does not require it; use `.` instead.

`test`

  The `test` program is the way to perform many file and string tests. It is often invoked by the alternate name ‘`[`’, but using that name in Autoconf code is asking for trouble since it is an M4 quote character.

  The `-a`, `-o`, ‘`(`’, and ‘`)`’ operands are not present in all implementations, and have been marked obsolete by POSIX 2008. This is because there are inherent ambiguities in using them. For example, ‘`test "$1" -a "$2"`’ looks like a binary operator to check whether two strings are both non-empty, but if ‘`$1`’ is the literal ‘`!`’, then some implementations of `test` treat it as a negation of the unary operator `-a`.

  Thus, portable uses of `test` should never have more than four arguments, and scripts should use shell constructs like ‘`&&`’ and ‘`||`’ instead. If you combine ‘`&&`’ and ‘`||`’ in the same statement, keep in mind that they have equal precedence, so it is often better to parenthesize even when this is redundant. For example:

      # Not portable:
      test "X$a" = "X$b" -a \
        '(' "X$c" != "X$d" -o "X$e" = "X$f" ')'

      # Portable:
      test "X$a" = "X$b" &&
        { test "X$c" != "X$d" || test "X$e" = "X$f"; }

  `test` does not process options like most other commands do; for example, it does not recognize the `--` argument as marking the end of options.

  It is safe to use ‘`!`’ as a `test` operator. For example, ‘`if test ! -d foo; …`’ is portable even though ‘`if ! test -d foo; …`’ is not.

`test` (files)

  To enable `configure` scripts to support cross-compilation, they shouldn’t do anything that tests features of the build system instead of the host system. But occasionally you may find it necessary to check whether some arbitrary file exists. To do so, use ‘`test -f`’, ‘`test -r`’, or ‘`test -x`’. Do not use ‘`test -e`’, because Solaris 10 `/bin/sh` lacks it.

  To test for symbolic links on systems that have them, use ‘`test -h`’ rather than ‘`test -L`’; either form conforms to POSIX, but `-h` has been around longer.

  The commands ‘`test A -ot B`’ and ‘`test A -nt B`’ are not reliable on macOS `sh` through at least macOS Sequoia 15.1.1 (2024), where ‘`test`’ ignores the subsecond part of file timestamps. To work around this bug, arrange for the timestamps to be at least one second apart.

  For historical reasons, POSIX reluctantly allows implementations of ‘`test -x`’ that will succeed for the root user, even if no execute permissions are present. Furthermore, shells do not all agree on whether Access Control Lists should affect ‘`test -r`’, ‘`test -w`’, and ‘`test -x`’; some shells base test results strictly on the current user id compared to file owner and mode, as if by `stat(2)`; while other shells base test results on whether the current user has the given right, even if that right is only granted by an ACL, as if by `faccessat(2)`. Furthermore, there is a classic time of check to time of use race between any use of `test` followed by operating on the just-checked file. Therefore, it is a good idea to write scripts that actually attempt an operation, and are prepared for the resulting failure if permission is denied, rather than trying to avoid an operation based solely on whether `test` guessed that it might not be permitted.

`test` (strings)

  POSIX says that ‘`test "string"`’ succeeds if string is not null, but this usage is not portable to traditional platforms like Solaris 10 `/bin/sh`, which mishandle strings like ‘`!`’ and ‘`-n`’. However, it is portable to test if a variable is set to a non-empty value, by using ‘`test ${var+y}`’, since all known implementations properly distinguish between no arguments and a known-safe string of ‘`y`’.

  POSIX also says that ‘`test ! "string"`’, ‘`test -n "string"`’ and ‘`test -z "string"`’ work with any string, but some shells (such as Solaris 10) get confused if string looks like an operator:

      $ test -n =
      test: argument expected
      $ test ! -n
      test: argument expected
      $ test -z ")"; echo $?
      0

  Similarly, POSIX says that both ‘`test "string1" = "string2"`’ and ‘`test "string1" != "string2"`’ work for any pairs of strings, but in practice this is not true for troublesome strings that look like operators or parentheses, or that begin with ‘`-`’.

  It is best to protect such strings with a leading ‘`X`’, e.g., ‘`test "Xstring" != X`’ rather than ‘`test -n "string"`’ or ‘`test ! "string"`’.

  It is common to find variations of the following idiom:

      test -n "`echo $ac_feature | sed 's/[a-zA-Z0-9_-]//g'`" &&
        action

  to take an action when a token matches a given pattern. Such constructs should be avoided by using:

      AS_CASE([$ac_feature],
        [[*[!-a-zA-Z0-9_]*]], [action])

  If the pattern is a complicated regular expression that cannot be expressed as a shell pattern, use something like this instead:

      expr "X$ac_feature" : 'X.*[^-a-zA-Z0-9_]' >/dev/null &&
        action

  ‘`expr "Xfoo" : "Xbar"`’ is more robust than ‘`echo "Xfoo" | grep "^Xbar"`’, because it avoids problems when ‘`foo`’ contains backslashes.

`trap`

  It is safe to trap at least the signals 1, 2, 13, and 15. You can also trap 0, i.e., have the `trap` run when the script ends (either via an explicit `exit`, or the end of the script). The trap for 0 should be installed outside of a shell function, or AIX 7.3 `/bin/sh` will invoke the trap at the end of this function.

  POSIX says that ‘`trap - 1 2 13 15`’ resets the traps for the specified signals to their default values, but many common shells (e.g., Solaris 10 `/bin/sh`) misinterpret this and attempt to execute a “command” named `-` when the specified conditions arise. POSIX 2008 also added a requirement to support ‘`trap 1 2 13 15`’ to reset traps, as this is supported by a larger set of shells, but there are still shells like `dash` that mistakenly try to execute `1` instead of resetting the traps. Therefore, there is no portable workaround, except for ‘`trap - 0`’, for which ‘`trap '' 0`’ is a portable substitute.

  Although POSIX is not absolutely clear on this point, it is widely admitted that when entering the trap ‘`$?`’ should be set to the exit status of the last command run before the trap. The ambiguity can be summarized as: “when the trap is launched by an `exit`, what is the last command run: that before `exit`, or `exit` itself?”

  Bash considers `exit` to be the last command, while Zsh and Solaris 10 `/bin/sh` consider that when the trap is run it is still in the `exit`, hence it is the previous exit status that the trap receives:

      $ cat trap.sh
      trap 'echo $?' 0
      (exit 42); exit 0
      $ zsh trap.sh
      42
      $ bash trap.sh
      0

  The portable solution is then simple: when you want to ‘`exit 42`’, run ‘`(exit 42); exit 42`’, the first `exit` being used to set the exit status to 42 for Zsh, and the second to trigger the trap and pass 42 as exit status for Bash. In M4sh, this is covered by using `AS_EXIT`.

  The shell in FreeBSD 4.0 has the following bug: ‘`$?`’ is reset to 0 by empty lines if the code is inside `trap`.

      $ trap 'false

      echo $?' 0
      $ exit
      0

  Fortunately, this bug only affects `trap`.

  Several shells fail to execute an exit trap that is defined inside a subshell, when the last command of that subshell is not a builtin. A workaround is to use ‘`exit $?`’ as the shell builtin.

      $ bash -c '(trap "echo hi" 0; /bin/true)'
      hi
      $ /bin/sh -c '(trap "echo hi" 0; /bin/true)'
      $ /bin/sh -c '(trap "echo hi" 0; /bin/true; exit $?)'
      hi

  Likewise, older implementations of `bash` failed to preserve ‘`$?`’ across an exit trap consisting of a single cleanup command.

      $ bash -c 'trap "/bin/true" 0; exit 2'; echo $?
      2
      $ bash-2.05b -c 'trap "/bin/true" 0; exit 2'; echo $?
      0
      $ bash-2.05b -c 'trap ":; /bin/true" 0; exit 2'; echo $?
      2

  Be aware that a trap can be called from any number of places in your script, and therefore the trap handler should not make assumptions about shell state. For some examples, if your script temporarily modifies `IFS`, then the trap should include an initialization back to its typical value of space-tab-newline (autoconf does this for generated `configure` files). Likewise, if your script changes the current working directory at some point after the trap is installed, then your trap cannot assume which directory it is in, and should begin by changing directories to an absolute path if that is important to the cleanup efforts (autotest does this for generated `testsuite` files).

`true`

  Don’t worry: as far as we know `true` is portable. Nevertheless, it’s not always a builtin (e.g., Bash 1.x), and the portable shell community tends to prefer using `:`. This has a funny side effect: when asked whether `false` is more portable than `true` Alexandre Oliva answered:

  > In a sense, yes, because if it doesn’t exist, the shell will produce an exit status of failure, which is correct for `false`, but not for `true`.

  Remember that even though ‘`:`’ ignores its arguments, it still takes time to compute those arguments. It is a good idea to use double quotes around any arguments to ‘`:`’ to avoid time spent in field splitting and file name expansion.

`unset`

  In some nonconforming shells (e.g., Solaris 10 `/bin/ksh` and `/usr/xpg4/bin/sh`, NetBSD 5.99.43 sh, or Bash 2.05a), `unset FOO` fails when `FOO` is not set. This can interfere with `set -e` operation. You can use

      FOO=; unset FOO

  if you are not sure that `FOO` is set.

  A few ancient shells lack `unset` entirely. For some variables such as `PS1`, you can use a neutralizing value instead:

      PS1='$ '

  Usually, shells that do not support `unset` need less effort to make the environment sane, so for example is not a problem if you cannot unset `CDPATH` on those shells. However, Bash 2.01 mishandles `unset MAIL` and `unset MAILPATH` in some cases and dumps core. So, you should do something like

      ( (unset MAIL) || exit 1) >/dev/null 2>&1 && unset MAIL || :

  See Special Shell Variables, for some neutralizing values. Also, see Limitations of Builtins, for the case of environment variables.

`wait`

  The exit status of `wait` is not always reliable.

### 11.15 Limitations of Usual Tools ¶

The small set of tools you can expect to find on any machine can still include some limitations you should be aware of.

`awk`

  Don’t leave white space before the opening parenthesis in a user function call. POSIX does not allow this and GNU Awk rejects it:

      $ gawk 'function die () { print "Aaaaarg!"  }
              BEGIN { die () }'
      gawk: cmd. line:2:         BEGIN { die () }
      gawk: cmd. line:2:                      ^ parse error
      $ gawk 'function die () { print "Aaaaarg!"  }
              BEGIN { die() }'
      Aaaaarg!

  POSIX says that if a program contains only ‘`BEGIN`’ actions, and contains no instances of `getline`, then the program merely executes the actions without reading input. However, traditional Awk implementations (such as Solaris 10 `awk`) read and discard input in this case. Portable scripts can redirect input from `/dev/null` to work around the problem. For example:

      awk 'BEGIN {print "hello world"}' </dev/null

  POSIX says that in an ‘`END`’ action, ‘`$NF`’ (and presumably, ‘`$1`’) retain their value from the last record read, if no intervening ‘`getline`’ occurred. However, some implementations (such as Solaris 10 ‘`/usr/bin/awk`’, ‘`nawk`’, or Darwin ‘`awk`’) reset these variables. A workaround is to use an intermediate variable prior to the ‘`END`’ block. For example:

      $ cat end.awk
      { tmp = $1 }
      END { print "a", $1, $NF, "b", tmp }
      $ echo 1 | awk -f end.awk
      a   b 1
      $ echo 1 | gawk -f end.awk
      a 1 1 b 1

  If you want your program to be deterministic, don’t depend on `for` on arrays:

      $ cat for.awk
      END {
        arr["foo"] = 1
        arr["bar"] = 1
        for (i in arr)
          print i
      }
      $ gawk -f for.awk </dev/null
      foo
      bar
      $ nawk -f for.awk </dev/null
      bar
      foo

  Some Awk implementations, such as HP-UX 11.0’s native one, mishandle anchors:

      $ echo xfoo | $AWK '/foo|^bar/ { print }'
      $ echo bar | $AWK '/foo|^bar/ { print }'
      bar
      $ echo xfoo | $AWK '/^bar|foo/ { print }'
      xfoo
      $ echo bar | $AWK '/^bar|foo/ { print }'
      bar

  Either do not depend on such patterns (i.e., use ‘`/^(.*foo|bar)/`’, or use a simple test to reject such implementations.

  On ‘`ia64-hp-hpux11.23`’, Awk mishandles `printf` conversions after `%u`:

      $ awk 'BEGIN { printf "%u %d\n", 0, -1 }'
      0 0

  AIX version 5.2 has an arbitrary limit of 399 on the length of regular expressions and literal strings in an Awk program.

  Traditional Awk implementations derived from Unix version 7, such as Solaris `/bin/awk`, have many limitations and do not conform to POSIX. Nowadays `AC_PROG_AWK` (see Particular Program Checks) finds you an Awk that doesn’t have these problems, but if for some reason you prefer not to use `AC_PROG_AWK` you may need to address them. For more detailed descriptions, see `awk` language history in GNU Awk User’s Guide.

  Traditional Awk does not support multidimensional arrays or user-defined functions.

  Traditional Awk does not support the `-v` option. You can use assignments after the program instead, e.g., `$AWK '{print v $1}' v=x`; however, don’t forget that such assignments are not evaluated until they are encountered (e.g., after any `BEGIN` action).

  Traditional Awk does not support the keywords `delete` or `do`.

  Traditional Awk does not support the expressions `a?b:c`, `!a`, `a^b`, or `a^=b`.

  Traditional Awk does not support the predefined `CONVFMT` or `ENVIRON` variables.

  Traditional Awk supports only the predefined functions `exp`, `index`, `int`, `length`, `log`, `split`, `sprintf`, `sqrt`, and `substr`.

  Traditional Awk `getline` is not at all compatible with POSIX; avoid it.

  Traditional Awk has `for (i in a) …` but no other uses of the `in` keyword. For example, it lacks `if (i in a) …`.

  In code portable to both traditional and modern Awk, `FS` must be a string containing just one ordinary character, and similarly for the field-separator argument to `split`.

  Traditional Awk has a limit of 99 fields in a record and splits the input even if you don’t refer to any field in the script. To circumvent this problem, set ‘`FS`’ to an unusual character and use `split`.

  Traditional Awk has a limit of at most 99 bytes in a number formatted by `OFMT`; for example, `OFMT="%.300e"; print 0.1;` typically dumps core.

  The original version of Awk had a limit of at most 99 bytes per `split` field, 99 bytes per `substr` substring, and 99 bytes per run of non-special characters in a `printf` format, but these bugs have been fixed on all practical hosts that we know of.

  HP-UX 11.00 Awk requires that input files have a line length of at most 3070 bytes.

`basename`

  Long ago some hosts lacked a working `basename`, and portable scripts needed to use `expr` instead. Nowadays it is safe to use `basename`. For example:

      base=`basename -- "$file"`

`cat`

  Don’t rely on any option.

`cc`

  The command ‘`cc -c foo.c`’ traditionally produces an object file named `foo.o`. Most compilers allow `-c` to be combined with `-o` to specify a different object file name, but POSIX does not require this combination and a few compilers lack support for it. See C Compiler Characteristics, for how GNU Make tests for this feature with `AC_PROG_CC_C_O`.

  When a compilation such as ‘`cc -o foo foo.c`’ fails, some compilers (such as CDS on Reliant Unix) leave a `foo.o`.

  HP-UX `cc` doesn’t accept `.S` files to preprocess and assemble. ‘`cc -c foo.S`’ appears to succeed, but in fact does nothing.

  The default executable, produced by ‘`cc foo.c`’, can be

  - `a.out` – usual POSIX convention.
  - `b.out` – i960 compilers (including `gcc`).
  - `a.exe` – DJGPP port of `gcc`.
  - `a_out.exe` – GNV `cc` wrapper for DEC C on OpenVMS.
  - `foo.exe` – various MS-DOS compilers.

  The C compiler’s traditional name is `cc`, but other names like `gcc` are common. POSIX 1003.1-2024 specifies the name `c17`, but older POSIX editions specified `c99` or `c89`, future POSIX standards will likely specify other commands, and anyway these standard names are rarely used in practice. Typically the C compiler is invoked from makefiles that use ‘`$(CC)`’, so the value of the ‘`CC`’ make variable selects the compiler name.

`chgrp`
`chown`

  It is not portable to change a file’s group to a group that the owner does not belong to.

`chmod`

  Avoid usages like ‘`chmod -w file`’; use ‘`chmod a-w file`’ instead, for two reasons. First, plain `-w` does not necessarily make the file unwritable, since it does not affect mode bits that correspond to bits in the file mode creation mask. Second, POSIX says that the `-w` might be interpreted as an implementation-specific option, not as a mode; POSIX suggests using ‘`chmod -- -w file`’ to avoid this confusion, but unfortunately ‘`--`’ does not work on some older hosts.

`cmp`

  `cmp` performs a raw data comparison of two files, while `diff` compares two text files. Therefore, if you might compare DOS files, even if only checking whether two files are different, use `diff` to avoid spurious differences due to differences of newline encoding.

`cp`

  The `-i`, `-f`, `-p` and `-R` options are widely used. POSIX also specifies `-H`, `-L`, and `-P`. Avoid other options in portable scripts.

  Traditionally, file timestamps had 1-second resolution, and ‘`cp -p`’ copied the timestamps exactly. However, many modern file systems have timestamps with 1-nanosecond resolution. Unfortunately, some older ‘`cp -p`’ implementations truncate timestamps when copying files, which can cause the destination file to appear to be older than the source. The exact amount of truncation depends on the resolution of the system calls that `cp` uses. Traditionally this was `utime`, which has 1-second resolution. Since GNU Core Utilities 6.12 (2008), GNU `cp` can set timestamps to the full nanosecond resolution, using the system calls `futimens` and `utimensat` when they are available. As of 2024, though, some platforms (such as AIX 7.3) still have ‘`cp -p`’ that supports only 1-second resolution, and some operating systems and file systems have similar limitations even when running GNU `cp`.

  Although ‘`cp -p`’ always tries to copy ownerships, whether it actually does so is a system dependent policy decision implemented by the kernel. If the kernel allows it then it happens. If the kernel does not allow it then it does not happen. It is not something `cp` itself has control over.

  In Unix System V any user can chown files to any other user, and System V also has a non-sticky `/tmp`. That probably derives from the heritage of System V in a business environment without hostile users. BSD changed this to be a more secure model where only root can `chown` files and a sticky `/tmp` is used. That undoubtedly derives from the heritage of BSD in a campus environment.

  GNU/Linux and Solaris by default follow BSD, but can be configured to allow a System V style `chown`. On the other hand, HP-UX follows System V, but can be configured to use the modern security model and disallow `chown`. Since it is an administrator-configurable parameter you can’t use the name of the kernel as an indicator of the behavior.

`date`

  When most versions of `date` do not recognize a ‘`%`’ conversion specification, they quietly pass it through, and exit with success:

      $ date --version | head -n 1
      date (GNU coreutils) 9.5
      $ date +'%H:%M %Q'
      17:25 %Q

  However, this behavior is not required by POSIX.

`dirname`

  Long ago some hosts lacked a working `dirname` and portable scripts needed to use use `AS_DIRNAME` (see Programming in M4sh). Nowadays `dirname` suffices and the following are equivalent:

      dir=`dirname -- "$file"`
      dir=`AS_DIRNAME(["$file"])`

`egrep`

  Although POSIX stopped requiring `egrep` in 2001, a few traditional hosts (notably Solaris 11) do not support the POSIX replacement `grep -E`. Also, some traditional implementations do not work on long input lines. To work around these problems, invoke `AC_PROG_EGREP` and then use `$EGREP`.

  Portable extended regular expressions should use ‘`\`’ only to escape characters in the string ‘`$()*+.?[\^{|`’. For example, ‘`\}`’ is not portable, even though it typically matches ‘`}`’.

  The empty alternative is not portable. Use ‘`?`’ instead. For instance with Digital Unix v5.0:

      > printf 'foo\n|foo\n' | $EGREP '^(|foo|bar)$'
      |foo
      > printf 'bar\nbar|\n' | $EGREP '^(foo|bar|)$'
      bar|
      > printf 'foo\nfoo|\n|bar\nbar\n' | $EGREP '^(foo||bar)$'
      foo
      |bar

  For more information about what can appear in portable extended regular expressions, see Problematic Expressions in GNU Grep.

  `$EGREP` also suffers the limitations of `grep` (see Limitations of Usual Tools).

`expr`

  Not all implementations obey the POSIX rule that ‘`--`’ separates options from arguments; likewise, not all implementations provide the extension to POSIX that the first argument can be treated as part of a valid expression rather than an invalid option if it begins with ‘`-`’. When performing arithmetic, use ‘`expr 0 + $var`’ if ‘`$var`’ might be a negative number, to keep `expr` from interpreting it as an option.

  No `expr` keyword starts with ‘`X`’, so use ‘`expr X"word" : 'Xregex'`’ to keep `expr` from misinterpreting word.

  Don’t use `length`, `substr`, `match` and `index`.

`expr` (‘`|`’) ¶

  You can use ‘`|`’. Although POSIX does require that ‘`expr ''`’ return the empty string, it does not specify the result when you ‘`|`’ together the empty string (or zero) with the empty string. For example:

      expr '' \| ''

  POSIX 1003.2-1992 returns the empty string for this case, but traditional Unix returns ‘`0`’ (Solaris is one such example). In POSIX 1003.1-2001, the specification was changed to match traditional Unix’s behavior (which is bizarre, but it’s too late to fix this). Please note that the same problem does arise when the empty string results from a computation, as in:

      expr bar : foo \| foo : bar

  Avoid this portability problem by avoiding the empty string.

`expr` (‘`:`’)

  Portable `expr` regular expressions should use ‘`\`’ to escape only characters in the string ‘`$()*.123456789[\^{}`’. For example, alternation, ‘`\|`’, is common but POSIX does not require its support, so it should be avoided in portable scripts. Similarly, ‘`\+`’ and ‘`\?`’ should be avoided.

  Portable `expr` regular expressions should not begin with ‘`^`’. Patterns are automatically anchored so leading ‘`^`’ is not needed anyway.

  On the other hand, the behavior of the ‘`$`’ anchor is not portable on multi-line strings. POSIX is ambiguous whether the anchor applies to each line, as was done in older versions of the GNU Core Utilities, or whether it applies only to the end of the overall string, as in Coreutils 6.0 and most other implementations.

      $ baz='foo
      > bar'
      $ expr "X$baz" : 'X\(foo\)$'

      $ expr-5.97 "X$baz" : 'X\(foo\)$'
      foo

  The POSIX standard is ambiguous as to whether ‘`expr 'a' : '\(b\)'`’ outputs ‘`0`’ or the empty string. In practice, it outputs the empty string on most platforms, but portable scripts should not assume this. For instance, the QNX 4.25 native `expr` returns ‘`0`’.

  One might think that a way to get a uniform behavior would be to use the empty string as a default value:

      expr a : '\(b\)' \| ''

  Unfortunately this behaves exactly as the original expression; see the `expr` (‘`|`’) entry for more information.

  Some ancient `expr` implementations (e.g., Solaris 10 `/usr/ucb/expr`) have a silly length limit that causes `expr` to fail if the matched substring is longer than 120 bytes. In this case, you might want to fall back on ‘`printf|sed`’ if `expr` fails. Nowadays this is of practical importance only for the rare installer who mistakenly puts `/usr/ucb` before `/usr/bin` in `PATH` on Solaris 10.

  On Mac OS X 10.4, `expr` mishandles the pattern ‘`[^-]`’ in some cases. For example, the command

      expr Xpowerpc-apple-darwin8.1.0 : 'X[^-]*-[^-]*-\(.*\)'

  outputs ‘`apple-darwin8.1.0`’ rather than the correct ‘`darwin8.1.0`’. This particular case can be worked around by substituting ‘`[^--]`’ for ‘`[^-]`’.

  Don’t leave, there is some more!

  The QNX 4.25 `expr`, in addition of preferring ‘`0`’ to the empty string, has a funny behavior in its exit status: it’s always 1 when parentheses are used!

      $ val=`expr 'a' : 'a'`; echo "$?: $val"
      0: 1
      $ val=`expr 'a' : 'b'`; echo "$?: $val"
      1: 0

      $ val=`expr 'a' : '\(a\)'`; echo "?: $val"
      1: a
      $ val=`expr 'a' : '\(b\)'`; echo "?: $val"
      1: 0

  In practice this can be a big problem if you are ready to catch failures of `expr` programs with some other method (such as using `sed`), since you may get twice the result. For instance

      $ expr 'a' : '\(a\)' || echo 'a' | sed 's/^\(a\)$/\1/'

  outputs ‘`a`’ on most hosts, but ‘`aa`’ on QNX 4.25. A simple workaround consists of testing `expr` and using a variable set to `expr` or to `false` according to the result.

  On HP-UX 11, `expr` supports only a single sub-expression.

      $ expr 'Xfoo' : 'X\(f\(oo\)*\)$'
      expr: More than one '\(' was used.

`fgrep`

  Although POSIX stopped requiring `fgrep` in 2001, a few traditional hosts (notably Solaris 11) do not support the POSIX replacement `grep -F`. Also, some traditional implementations do not work on long input lines. To work around these problems, invoke `AC_PROG_FGREP` and then use `$FGREP`.

`find`

  Many operands of GNU `find` are not standardized by POSIX and are missing on many platforms. These nonportable operands include `-follow`, `-maxdepth`, `-mindepth`, `-printf`, and `,`. See the POSIX spec for `find` for `find` operands that should be portable nowadays.

  The replacement of ‘`{}`’ is guaranteed only if the argument is exactly ‘`{}`’, not if ‘`{}`’ only a part of a larger argument. For instance, on AIX 7.3:

      $ touch foo
      $ find . -name foo -exec echo '{}-{}' \;
      {}-{}

  while GNU `find` reports ‘`./foo-./foo`’. POSIX allows either behavior.

  AIX 7.3 ‘`find ... -exec command +`’ incorrectly fails if command is not executed. For example, ‘`find . -name '*.tmp' -exec rm {} +`’ incorrectly outputs a diagnostic and fails if no file names end in ‘`.tmp`’. To work around this problem you can use a circumlocution like ‘`find . -name '*.tmp' -exec rm -f {} + -o -exec true ';'`’.

`grep`

  Portable scripts can rely on the `grep` options `-c`, `-l`, `-n`, and `-v`, but should avoid other options. For example, don’t use `-w`, as POSIX does not require it. Also, portable scripts should not combine `-c` with `-l`, as POSIX does not allow this.

  Some of the options required by POSIX are not portable in practice. Don’t use ‘`grep -q`’ to suppress output, because traditional `grep` implementations (e.g., Solaris 10) do not support `-q`. Don’t use ‘`grep -s`’ to suppress output either, because POSIX says `-s` does not suppress output, only some error messages; also, the `-s` option of traditional `grep` behaved like `-q` does in most modern implementations. Instead, redirect the standard output and standard error (in case the file doesn’t exist) of `grep` to `/dev/null`. Check the exit status of `grep` to determine whether it found a match.

  The QNX4 implementation fails to count lines with `grep -c '$'`, but works with `grep -c '^'`. Other alternatives for counting lines are to use `sed -n '$='` or `wc -l`.

  Some traditional `grep` implementations do not work on long input lines. On AIX the default `grep` silently truncates long lines on the input before matching.

  Also, Solaris 11 `grep` does not support `-e`. To work around this, invoke `AC_PROG_GREP` and then use `$GREP`.

  Another possible workaround for the multiple `-e` problem is to separate the patterns by newlines, for example:

      grep 'foo
      bar' in.txt

  except that this fails with traditional `grep` implementations and with OpenBSD 3.8 `grep`.

  Traditional `grep` implementations (e.g., Solaris 11) do not support the `-E` or `-F` options. To work around these problems, invoke `AC_PROG_EGREP` and then use `$EGREP`, and similarly for `AC_PROG_FGREP` and `$FGREP`. Even if you are willing to require support for POSIX `grep`, your script should not use both `-E` and `-F`, since POSIX does not allow this combination.

  Portable `grep` regular expressions should use ‘`\`’ only to escape characters in the string ‘`$()*.123456789[\^{}`’. For example, alternation, ‘`\|`’, is common but POSIX does not require its support in basic regular expressions, so it should be avoided in portable scripts. Solaris and HP-UX `grep` do not support it. Similarly, the following escape sequences should also be avoided: ‘`\<`’, ‘`\>`’, ‘`\+`’, ‘`\?`’, ‘`\``’, ‘`\'`’, ‘`\B`’, ‘`\b`’, ‘`\S`’, ‘`\s`’, ‘`\W`’, and ‘`\w`’. For more information about what can appear in portable regular expressions, see Problematic Expressions in GNU Grep.

  POSIX does not specify the behavior of `grep` on binary files. An example where this matters is using BSD `grep` to search text that includes embedded ANSI escape sequences for colored output to terminals (‘`\033[m`’ is the sequence to restore normal output); the behavior depends on whether input is seekable:

      $ printf 'esc\033[mape\n' > sample
      $ grep . sample
      Binary file sample matches
      $ cat sample | grep .
      escape

`join`

  On NetBSD, `join -a 1 file1 file2` mistakenly behaves like `join -a 1 -a 2 1 file1 file2`, resulting in a usage warning; the workaround is to use `join -a1 file1 file2` instead.

  On some circa-2020 BSD-based systems `join` mishandles inputs with missing fields. For example, an empty line is not treated as containing an empty join field. As a workaround, input lines should always have a join field.

  On platforms with the BusyBox tools, the `join` command is entirely missing. As a workaround, you can simulate special cases of the `join` command using an `awk` script. For an example, see https://lists.gnu.org/r/bug-gnulib/2021-04/msg00054.html.

`ln`

  The `-f` option is portable nowadays.

  Symbolic links are not available on some systems; use ‘`$(LN_S)`’ as a portable substitute.

  For versions of the DJGPP before 2.04, `ln` emulates symbolic links to executables by generating a stub that in turn calls the real program. This feature also works with nonexistent files like in the POSIX spec. So ‘`ln -s file link`’ generates `link.exe`, which attempts to call `file.exe` if run. But this feature only works for executables, so ‘`cp -p`’ is used instead for these systems. DJGPP versions 2.04 and later have full support for symbolic links.

`ls`

  The portable options are `-acdilrtu`. Current practice is for `-l` to output both owner and group, even though ancient versions of `ls` omitted the group.

  On ancient hosts, ‘`ls foo`’ sent the diagnostic ‘`foo not found`’ to standard output if `foo` did not exist. Hence a shell command like ‘`sources=`ls *.c 2>/dev/null``’ did not always work, since it was equivalent to ‘`sources='*.c not found'`’ in the absence of ‘`.c`’ files. This is no longer a practical problem, since current `ls` implementations send diagnostics to standard error.

  The behavior of `ls` on a directory that is being concurrently modified is not always predictable, because of a data race where cached information returned by `readdir` does not match the current directory state. In fact, Mac OS X 10.5 had an intermittent bug where `readdir`, and thus `ls`, sometimes lists a file more than once if other files were added or removed from the directory immediately prior to the `ls` call. Since `ls` already sorts its output, the duplicate entries can be avoided by piping the results through `uniq`.

`mkdir`

  Combining the `-m` and `-p` options, as in ‘`mkdir -m go-w -p dir`’, often leads to trouble. FreeBSD `mkdir` incorrectly attempts to change the permissions of dir even if it already exists. HP-UX 11.23 `mkdir` often assigns the wrong permissions to any newly-created parents of dir.

  POSIX does not clearly specify whether ‘`mkdir -p foo`’ should succeed when `foo` is a symbolic link to an already-existing directory. The GNU `mkdir` succeeds, but Solaris 10 `mkdir` fails.

  Traditional `mkdir -p` implementations suffer from race conditions. For example, if you invoke `mkdir -p a/b` and `mkdir -p a/c` at the same time, both processes might detect that `a` is missing, one might create `a`, then the other might try to create `a` and fail with a `File exists` diagnostic. Solaris 10 `mkdir` is vulnerable, and other traditional Unix systems are probably vulnerable too. This possible race is harmful in parallel builds when several Make rules call `mkdir -p` to construct directories. You may use `install-sh -d` as a safe replacement, for example by setting ‘`MKDIR_P='/path/to/install-sh -d'`’ in the environment of `configure`, assuming the package distributes `install-sh`.

`mkfifo`
`mknod`

  The GNU Coding Standards state that `mknod` is safe to use on platforms where it has been tested to exist; but it is generally portable only for creating named FIFOs, since device numbers are platform-specific. Autotest uses `mkfifo` to implement parallel testsuites. POSIX states that behavior is unspecified when opening a named FIFO for both reading and writing; on at least Cygwin, this results in failure on any attempt to read or write to that file descriptor.

`mktemp`

  Shell scripts can use temporary files safely with `mktemp`, but it does not exist on all systems. A portable way to create a safe temporary file name is to create a temporary directory with mode 700 and use a file inside this directory. Both methods prevent attackers from gaining control, though `mktemp` is far less likely to fail gratuitously under attack.

  Here is sample code to create a new temporary directory ‘`$dir`’ safely:

      # Create a temporary directory $dir in $TMPDIR (default /tmp).
      # Use mktemp if possible; otherwise fall back on mkdir,
      # with $RANDOM to make collisions less likely.
      : "${TMPDIR:=/tmp}"
      {
        dir=`
          (umask 077 && mktemp -d "$TMPDIR/fooXXXXXX") 2>/dev/null
        ` &&
        test -d "$dir"
      } || {
        dir=$TMPDIR/foo$$-$RANDOM
        (umask 077 && mkdir "$dir")
      } || exit $?

`mv`

  The only portable options are `-f` and `-i`.

  Moving individual files between file systems is portable (it was in Unix version 6), but it is not always atomic: when doing ‘`mv new existing`’, there’s a critical section where neither the old nor the new version of `existing` actually exists.

  On some systems moving files from `/tmp` can sometimes cause undesirable (but perfectly valid) warnings, even if you created these files. This is because `/tmp` belongs to a group that ordinary users are not members of, and files created in `/tmp` inherit the group of `/tmp`. When the file is copied, `mv` issues a diagnostic without failing:

      $ touch /tmp/foo
      $ mv /tmp/foo .
      error→mv: ./foo: set owner/group (was: 100/0): Operation not permitted
      $ echo $?
      0
      $ ls foo
      foo

  This annoying behavior conforms to POSIX, unfortunately.

  Moving directories across mount points is not portable, use `cp` and `rm`.

  DOS variants cannot rename or remove open files, and do not support commands like ‘`mv foo bar >foo`’, even though this is perfectly portable among POSIX hosts.

`od`

  In Mac OS X versions prior to 10.4.3, `od` does not support the standard POSIX options `-A`, `-j`, `-N`, or `-t`, or the XSI option, `-s`. The only supported POSIX option is `-v`, and the only supported XSI options are those in `-bcdox`. The BSD `hexdump` program can be used instead.

  In some versions of some operating systems derived from Solaris 11, `od` prints decimal byte values padded with zeros rather than with spaces:

      $ printf '#!' | od -A n -t d1 -N 2
               035 033

  instead of

      $ printf '#!' | od -A n -t d1 -N 2
                35  33

  We have observed this on both OpenIndiana and OmniOS; Illumos may also be affected. As a workaround, you can use octal output (option `-t o1`).

`rm`

  The `-f` and `-r` options are portable.

  It is not portable to invoke `rm` without options or operands. On the other hand, POSIX now requires `rm -f` to silently succeed when there are no operands (useful for constructs like `rm -rf $filelist` without first checking if ‘`$filelist`’ was empty). But this was not always portable; at least NetBSD `rm` built before 2008 would fail with a diagnostic.

  A file might not be removed even if its parent directory is writable and searchable. Many POSIX hosts cannot remove a mount point, a named stream, a working directory, or a last link to a file that is being executed.

  DOS variants cannot rename or remove open files, and do not support commands like ‘`rm foo >foo`’, even though this is perfectly portable among POSIX hosts.

`rmdir`

  Just as with `rm`, some platforms refuse to remove a working directory.

`sed`

  The portable options are `-e`, `-f`, and `-n`. POSIX standardized `-E` in 2024 but some older implementations lack it. Although GNU `sed` supports other options like `-i`, these can be missing or have different meanings elsewhere.

  Patterns should not include the separator (unless escaped), even as part of a character class. Even when escaped, patterns should not include separators that are also used as `sed` metacharacters. For example, GNU sed 4.0.9 rejects ‘`s,x\{1\,\},,`’, while sed 4.1 strips the backslash before the comma before evaluating the basic regular expression.

  Avoid empty patterns, such as the parenthesized empty pattern in ‘`\(\)`’ or the empty pattern followed by an interval expression in ‘`\{2\}`’. POSIX does not require support for empty patterns.

  Comments in Sed scripts should not contain ‘`n`’ immediately after the leading ‘`#`’. Although POSIX.1-2024 says this is equivalent to the `-n` option, earlier POSIX editions said that the equivalence occurs only if the comment is the first line of the script, and many `sed` implementations are confused about this. It is more portable to use `-n`.

  HP-UX sed has a limit of 99 commands (not counting ‘`:`’ commands) and 48 labels, which cannot be circumvented by using more than one script file. It can execute up to 19 reads with the ‘`r`’ command per cycle. Solaris 10 `/usr/ucb/sed` rejects usages that exceed a limit of about 6000 bytes for the internal representation of commands.

  Some `sed` implementations have a buffer limited to 4000 bytes, and this limits the size of input lines, output lines, and internal buffers that can be processed portably. Likewise, not all `sed` implementations can handle embedded `NUL` or a missing trailing newline.

  Ranges within a bracket expression of a regular expression are only well-defined in the ‘`C`’ (or ‘`POSIX`’) locale. Meanwhile, support for character classes like ‘`[[:upper:]]`’ is not yet universal, so if you cannot guarantee the setting of `LC_ALL`, it is better to spell out a range ‘`[ABCDEFGHIJKLMNOPQRSTUVWXYZ]`’ than to rely on ‘`[A-Z]`’.

  Additionally, POSIX states that regular expressions are only well-defined on characters. Unfortunately, there exist platforms such as Mac OS X 10.5 where not all 8-bit byte values are valid characters, even though that platform has a single-byte ‘`C`’ locale. Although this practice was disallowed by recent releases of POSIX, it means that in the ‘`C`’ locale not all bytes will be matched by the regular expression ‘`.`’:

      $ printf '\200\n' | LC_ALL=C sed -n /./p | wc -l
      0
      $ printf '\200\n' | LC_ALL=en_US.ISO8859-1 sed -n /./p | wc -l
      1

  Anchors (‘`^`’ and ‘`$`’) inside groups are not portable.

  Some `sed` implementations, e.g., Solaris 11.4, restrict the special role of the asterisk ‘`*`’ to one-character regular expressions and back-references, and the special role of interval expressions ‘`\{m\}`’, ‘`\{m,\}`’, or ‘`\{m,n\}`’ to one-character regular expressions. This may lead to unexpected behavior:

      $ echo '1*23*4' | /usr/bin/sed 's/\(.\)*/x/g'
      x2x4
      $ echo '1*23*4' | /usr/xpg4/bin/sed 's/\(.\)*/x/g'
      x

  In the normal case when `-E` is not used, portable `sed` regular expressions should use ‘`\`’ only to escape characters in the string ‘`$*.123456789[\^n`’. For example, POSIX.1-2024 says it is implementation-defined whether ‘`\|`’ means alternation or simply matches ‘`|`’, so it should be avoided in portable scripts. Solaris `sed` does not support alternation; e.g., ‘`sed '/a\|b/d'`’ deletes only lines that contain the literal string ‘`a|b`’. Similarly, ‘`\+`’ and ‘`\?`’ should be avoided.

  Portable `sed` replacement strings should use ‘`\`’ only to escape the delimiter character, newline, and characters in the string ‘`&123456789\`’. For example, although in GNU `sed` the command ‘`s/AB/\n/`’ replaces the first ‘`AB`’ with a newline, in OpenBSD and Solaris `sed` it acts like ‘`s/AB/n/`’. To be portable, escape a newline instead:

      # This is portable; "sed 's/AB/\n/'" is not.
      sed 's/AB/\
      /'

  The `-e` option is mostly portable. However, its argument cannot be empty, as this fails on AIX 7.3. Some people prefer to use ‘`-e`’:

      sed -e 'command-1' \
          -e 'command-2'

  as opposed to the equivalent:

      sed '
        command-1
        command-2
      '

  The following usage is sometimes equivalent:

      sed 'command-1;command-2'

  but POSIX says that this use of a semicolon has undefined effect if command-1’s verb is ‘`{`’, ‘`a`’, ‘`b`’, ‘`c`’, ‘`i`’, ‘`r`’, ‘`t`’, ‘`w`’ or ‘`:`’, or if command-1 is an ‘`s`’ with the ‘`w`’ option, so you should use semicolon only with simple scripts that do not use these constructs.

  Avoid redundant ‘`;`’, as some `sed` implementations, such as NetBSD 1.4.2’s, incorrectly try to interpret the second ‘`;`’ as a command:

      $ echo a | sed 's/x/x/;;s/x/x/'
      sed: 1: "s/x/x/;;s/x/x/": invalid command code ;

  POSIX requires each `-e` and `-f` option to specify a syntactically complete script. Although GNU `sed` also allows `-e` and `-f` options to specify script fragments that it assembles into a full script, this is not portable. For example, the `sed` programs on Solaris 11, HP-UX 11, and AIX do not allow script fragments:

      $ sed -e 'i\' -e ouch
      Unrecognized command: ouch

  In practice, however, this technique of joining fragments through `-e` works for multiple `sed` functions within ‘`{`’ and ‘`}`’, even if that is not specified by POSIX:

      $ echo a | sed -n -e '/a/{' -e s/a/b/ -e p -e '}'
      b

  Commands should not be followed by white space. Although trailing white space often works, it can be dicey in some situations and it is simpler to avoid it entirely.

  Commands inside { } brackets are further restricted. POSIX.1-2004 says that they cannot be preceded by addresses, ‘`!`’, or ‘`;`’, and that each command must be followed immediately by a newline, without any intervening blanks or semicolons. The closing bracket must be alone on a line, other than white space preceding or following it. Although these restrictions were lifted in POSIX.1-2008, it is more portable to respect them.

  Contrary to yet another urban legend, you may portably use ‘`&`’ in the replacement part of the `s` command to mean “what was matched”. All descendants of Unix version 7 `sed` (at least; we don’t have first hand experience with older `sed` implementations) have supported it.

  POSIX requires that you must not have any white space between ‘`!`’ and the following command. It is OK to have blanks between the address and the ‘`!`’. For instance, on Solaris:

      $ echo "foo" | sed -n '/bar/ ! p'
      error→Unrecognized command: /bar/ ! p
      $ echo "foo" | sed -n '/bar/! p'
      error→Unrecognized command: /bar/! p
      $ echo "foo" | sed -n '/bar/ !p'
      foo

  POSIX also says that you should not combine ‘`!`’ and ‘`;`’. If you use ‘`!`’, it is best to put it on a command that is delimited by newlines rather than ‘`;`’.

  POSIX requires that the ‘`b`’, ‘`t`’, ‘`r`’, and ‘`w`’ commands be followed by exactly one space before their argument. On the other hand, no white space is allowed between ‘`:`’ and the subsequent label. Branch labels should contain at most 8 bytes, each of which should be an ASCII graphical character. Do not put trailing white space after a branch label.

  If a sed script is specified on the command line and ends in an ‘`a`’, ‘`c`’, or ‘`i`’ command, the last line of inserted text should be followed by a newline. Otherwise some `sed` implementations (e.g., OpenBSD 3.9) do not append a newline to the inserted text.

  Many `sed` implementations (e.g., Mac OS X 10.4, OpenBSD 3.9, Solaris 11 `/usr/ucb/sed`) strip leading white space from the text of ‘`a`’, ‘`c`’, and ‘`i`’ commands. Prepend a backslash to work around this incompatibility with POSIX:

      $ echo flushleft | sed 'a\
      >    indented
      > '
      flushleft
      indented
      $ echo foo | sed 'a\
      > \   indented
      > '
      flushleft
         indented

  POSIX requires that with a missing regular expression, the last regular expression from either an address specification or substitution command is used. However, busybox 1.6.1 complains when using a substitution command with a replacement containing a back-reference to a missing regular expression; the workaround is repeating the regular expression.

      $ echo abc | busybox sed '/a\(b\)c/ s//\1/'
      sed: No previous regexp.
      $ echo abc | busybox sed '/a\(b\)c/ s/a\(b\)c/\1/'
      b

  Portable scripts should be aware of the inconsistencies and options for handling word boundaries, as these are not specified by POSIX.

                      \<      \b      [[:<:]]
      Solaris 11      yes     no      no
      Solaris XPG4    yes     no      error
      NetBSD 5.1      no      no      yes
      FreeBSD 9.1     no      no      yes
      GNU             yes     yes     error
      busybox         yes     yes     error

`sed` (‘`t`’)

  There are two things one should remember about ‘`t`’ in `sed`. First, ‘`t`’ jumps if some substitution succeeded, not only the immediately preceding substitution. Therefore, always use a fake ‘`t clear`’ followed by a ‘`:clear`’ on the next line, to reset the ‘`t`’ flag where needed.

  Second, do not rely on `sed` to clear the flag at each new cycle.

  For example, the following script replaces all instances of “keep me” with “kept”, and replaces the contents of all lines that did not contain “keep me” with “deleted”.

      t clear
      :clear
      s/keep me/kept/g
      t end
      s/.*/deleted/g
      :end

`sed` (‘`w`’)

  When a script contains multiple commands to write lines to the same output file, BusyBox `sed` mistakenly opens a separate output stream for each command. This can cause one of the commands to “win” and the others to “lose”, in the sense that their output is discarded. For example:

      sed -n -e '
        /a/w xxx
        /b/w xxx
      ' <<EOF
      a
      b
      EOF

  This might output only ‘`a`’ to `xxx`; the ‘`b`’ is lost. To avoid the problem, a portable script should contain at most one ‘`w`’ or ‘`s/.../.../w`’ command per output file.

`sleep`

  Using `sleep` is generally portable. However, remember that adding a `sleep` to work around timestamp issues, with a minimum granularity of one second, doesn’t scale well for parallel builds on modern machines with sub-second process completion.

`sort`

  Remember that sort order is influenced by the current locale. Inside `configure`, the C locale is in effect, but in Makefile snippets, you may need to specify `LC_ALL=C sort`.

`tar`

  There are multiple file formats for `tar`; if you use Automake, the macro `AM_INIT_AUTOMAKE` has some options controlling which level of portability to use.

`touch`

  If you specify the desired timestamp (e.g., with the `-r` option), older `touch` implementations use the `utime` or `utimes` system call, which can result in the same kind of timestamp truncation problems that ‘`cp -p`’ has.

`tr`

  Many `tr` implementations do not support multi-byte locales well. For example, Solaris 10 `tr` rejects character classes in multi-byte locales. Also, ranges have well-defined behavior only in the ‘`C`’ (or ‘`POSIX`’) locale, so if you cannot guarantee the setting of `LC_ALL` it is better to spell out a range ‘`[ABCDEFGHIJKLMNOPQRSTUVWXYZ]`’ than to rely on ‘`[A-Z]`’.

  Not all versions of `tr` handle all backslash character escapes. For example, Solaris 10 `/usr/ucb/tr` falls over, even though Solaris contains more modern `tr` in other locations. Using octal escapes is more portable for carriage returns, since ‘`\015`’ is the same for both ASCII and EBCDIC, and since use of literal carriage returns in scripts causes a number of other problems. But for other characters, like newline, using octal escapes ties the operation to ASCII, so it is better to use literal characters.

      $ { echo moon; echo light; } | /usr/ucb/tr -d '\n' ; echo
      moo
      light
      $ { echo moon; echo light; } | /usr/bin/tr -d '\n' ; echo
      moonlight
      $ { echo moon; echo light; } | /usr/ucb/tr -d '\012' ; echo
      moonlight
      $ nl='
      '; { echo moon; echo light; } | /usr/ucb/tr -d "$nl" ; echo
      moonlight

  Not all versions of `tr` recognize direct ranges of characters: at least Solaris `/usr/bin/tr` still fails to do so. But you can use `/usr/xpg4/bin/tr` instead, or add brackets (which in POSIX transliterate to themselves).

      $ echo "Hazy Fantazy" | LC_ALL=C /usr/bin/tr a-z A-Z
      HAZy FAntAZy
      $ echo "Hazy Fantazy" | LC_ALL=C /usr/bin/tr '[a-z]' '[A-Z]'
      HAZY FANTAZY
      $ echo "Hazy Fantazy" | LC_ALL=C /usr/xpg4/bin/tr a-z A-Z
      HAZY FANTAZY

  When providing two arguments, be sure the second string is at least as long as the first.

      $ echo abc | /usr/xpg4/bin/tr bc d
      adc
      $ echo abc | coreutils/tr bc d
      add

  On platforms with the BusyBox tools, `tr` does not support the `[x*n]` option syntax.

      $ echo abc | tr 'abcd' '[A*4]'
      [A*
      $ echo abc | coreutils/tr 'abcd' '[A*4]'
      AAA
      $ echo xyz | tr 'a-z' '[A*]'
      ]]]
      $ echo xyz | coreutils/tr 'a-z' '[A*]'
      AAA

  POSIX requires `tr` to operate on binary files. But at least Solaris `/usr/ucb/tr` and `/usr/bin/tr` silently discard `NUL` in the input prior to doing any translation. When using `tr` to process a binary file that may contain `NUL` bytes, it is necessary to use `/usr/xpg4/bin/tr` instead, or `/usr/xpg6/bin/tr` if that is available.

      $ printf 'a\0b' | /usr/ucb/tr x x | od -An -tx1
       61 62
      $ printf 'a\0b' | /usr/bin/tr x x | od -An -tx1
       61 62
      $ printf 'a\0b' | /usr/xpg4/bin/tr x x | od -An -tx1
       61 00 62

  Solaris `/usr/ucb/tr` additionally fails to handle ‘`\0`’ as the octal escape for `NUL`.

      $ printf 'abc' | /usr/ucb/tr 'bc' '\0d' | od -An -tx1
       61 62 63
      $ printf 'abc' | /usr/bin/tr 'bc' '\0d' | od -An -tx1
       61 00 64
      $ printf 'abc' | /usr/xpg4/bin/tr 'bc' '\0d' | od -An -tx1
       61 00 64

## 12 Portable Make Programming ¶

Writing portable makefiles is an art. Since a makefile’s commands are executed by the shell, you must consider the shell portability issues already mentioned. However, other issues are specific to `make` itself.

### 12.1 `$<` in Ordinary Make Rules ¶

POSIX says that the ‘`$<`’ construct in makefiles can be used only in inference rules and in the ‘`.DEFAULT`’ rule; its meaning in ordinary rules is unspecified. Solaris `make` for instance replaces it with the empty string. OpenBSD (3.0 and later) `make` diagnoses these uses and errors out.

### 12.2 Failure in Make Rules ¶

Unless errors are being ignored (e.g., because a makefile command line is preceded by a ‘`-`’ prefix), POSIX 2008 requires that `make` must invoke each command with the equivalent of a ‘`sh -e -c`’ subshell, which causes the subshell to exit immediately if a subsidiary simple-command fails, with some complicated exceptions. Historically not all `make` implementations followed this rule. For example, the command ‘`touch T; rm -f U`’ may attempt to remove `U` even if the `touch` fails, although this is not permitted with POSIX make. One way to work around failures in simple commands is to reword them so that they always succeed, e.g., ‘`touch T || :; rm -f U`’. However, even this approach can run into common bugs in BSD implementations of the `-e` option of `sh` and `set` (see Limitations of Shell Builtins), so if you are worried about porting to buggy BSD shells it may be simpler to migrate complicated `make` actions into separate scripts.

### 12.3 Makefile Command Line Prefixes ¶

Makefile command lines can be preceded by zero or more of the command line prefixes ‘`-`’, ‘`@`’, and ‘`+`’, which modify how `make` processes the command. Although POSIX says these are the only command line prefixes, some `make` implementations, such as Solaris `make`, support the additional prefixes ‘`!`’ and ‘`?`’. Portable makefiles should therefore avoid using these two characters at the start of a makefile command line. For example:

    mishandled-by-Solaris-make:; ! grep FIXME foo.c
    portable-to-Solaris-make:; :;! grep FIXME foo.c

### 12.4 Special Characters in Make Macro Names ¶

POSIX limits macro names to nonempty strings containing only ASCII letters and digits, ‘`.`’, and ‘`_`’. Many `make` implementations allow a wider variety of characters, but portable makefiles should avoid them. It is portable to start a name with a special character, e.g., ‘`$(.FOO)`’.

Some ancient `make` implementations don’t support leading underscores in macro names. An example is NEWS-OS 4.2R.

    $ cat Makefile
    _am_include = #
    _am_quote =
    all:; @echo this is test
    $ make
    Make: Must be a separator on rules line 2.  Stop.
    $ cat Makefile2
    am_include = #
    am_quote =
    all:; @echo this is test
    $ make -f Makefile2
    this is test

However, this problem is no longer of practical concern.

### 12.5 Backslash-Newline Before Empty Lines ¶

On some versions of HP-UX, `make` reads multiple newlines following a backslash, continuing to the next non-empty line. For example,

    FOO = one \

    BAR = two

    test:
            : FOO is "$(FOO)"
            : BAR is "$(BAR)"

shows `FOO` equal to `one BAR = two`. Other implementations sensibly let a backslash continue only to the immediately following line.

### 12.6 Backslash-Newline in Make Comments ¶

According to POSIX, Make comments start with `#` and continue until an unescaped newline is reached.

    $ cat Makefile
    # A = foo \
          bar \
          baz

    all:
            @echo ok
    $ make   # GNU make
    ok

However this is not always the case. Some implementations discard everything from `#` through the end of the line, ignoring any trailing backslash.

    $ pmake  # BSD make
    "Makefile", line 3: Need an operator
    Fatal errors encountered -- cannot continue

Therefore, if you want to comment out a multi-line definition, prefix each line with `#`, not only the first.

    # A = foo \
    #     bar \
    #     baz

### 12.7 `make macro=value` and Submakes ¶

A command-line variable definition such as `foo=bar` overrides any definition of `foo` in a makefile. Some `make` implementations (such as GNU `make`) propagate this override to subsidiary invocations of `make`. Some other implementations do not pass the substitution along to submakes.

    $ cat Makefile
    foo = foo
    one:
            @printf '%s\n' $(foo)
            $(MAKE) two
    two:
            @printf '%s\n' $(foo)
    $ make foo=bar            # GNU make 3.79.1
    bar
    make two
    make[1]: Entering directory `/home/adl'
    bar
    make[1]: Leaving directory `/home/adl'
    $ pmake foo=bar           # BSD make
    bar
    pmake two
    foo

You have a few possibilities if you do want the `foo=bar` override to propagate to submakes. One is to use the `-e` option, which causes all environment variables to have precedence over the makefile macro definitions, and declare foo as an environment variable:

    $ env foo=bar make -e

The `-e` option is propagated to submakes automatically, and since the environment is inherited between `make` invocations, the `foo` macro is overridden in submakes as expected.

This syntax (`foo=bar make -e`) is portable only when used outside of a makefile, for instance from a script or from the command line. When run inside a `make` rule, GNU `make` 3.80 and prior versions forget to propagate the `-e` option to submakes.

Moreover, using `-e` could have unexpected side effects if your environment contains some other macros usually defined by the makefile. (See also the note about `make -e` and `SHELL` below.)

If you can foresee all macros that a user might want to override, then you can propagate them to submakes manually, from your makefile:

    foo = foo
    one:
            @printf '%s\n' $(foo)
            $(MAKE) foo=$(foo) two
    two:
            @printf '%s\n' $(foo)

Another way to propagate a variable to submakes in a portable way is to expand an extra variable in every invocation of ‘`$(MAKE)`’ within your makefile:

    foo = foo
    one:
            @printf '%s\n' $(foo)
            $(MAKE) $(SUBMAKEFLAGS) two
    two:
            @printf '%s\n' $(foo)

Users must be aware that this technique is in use to take advantage of it, e.g. with `make foo=bar SUBMAKEFLAGS='foo=bar'`, but it allows any macro to be overridden. Makefiles generated by `automake` use this technique, expanding `$(AM_MAKEFLAGS)` on the command lines of submakes (see Automake in GNU Automake).

### 12.8 The Make Macro MAKEFLAGS ¶

POSIX requires `make` to use `MAKEFLAGS` to affect the current and recursive invocations of make, but allows implementations several formats for the variable. It is tricky to parse `$MAKEFLAGS` to determine whether `-s` for silent execution or `-k` for continued execution are in effect. For example, you cannot assume that the first space-separated word in `$MAKEFLAGS` contains single-letter options, since in the Cygwin version of GNU `make` it is either `--unix` or `--win32` with the second word containing single-letter options.

    $ cat Makefile
    all:
            @printf 'MAKEFLAGS = %s\n' '$(MAKEFLAGS)'
    $ make
    MAKEFLAGS = --unix
    $ make -k
    MAKEFLAGS = --unix -k

### 12.9 The Make Macro `SHELL` ¶

Many `make` implementations use the `$(SHELL)` macro to spawn shell processes and execute Make rules. This is a builtin macro with a default value upplied by `make`; the default can be overridden by a makefile or by a command-line argument, though not by the environment.

Other `make` implementations use other ways to spawn shell processes, and the POSIX standard for `make` says that portable makefiles should neither define nor use the `$(SHELL)` macro.

Despite this prohibition, in practice it does not hurt to define and then possibly use `SHELL` in your makefiles and in some cases it can help your builds use a better shell to spawn shell processes. So it’s a good idea to define `SHELL` in your makefiles. If you use Autoconf, you can use its standard output variable `SHELL` as follows:

    SHELL = @SHELL@

If you use Automake, this is done for you.

Do not force `SHELL = /bin/sh` because that is not correct everywhere. Remember, `/bin/sh` is not POSIX compliant on some systems, such as Solaris 10. Additionally, DJGPP lacks `/bin/sh`, and when its GNU `make` port sees such a setting it enters a special emulation mode where features like pipes and redirections are emulated on top of DOS’s `command.com`. Unfortunately this emulation is incomplete; for instance it does not handle command substitutions. Using `@SHELL@` means that your makefile will benefit from the same improved shell, such as `bash` or `ksh`, that was discovered during `configure`, so that you aren’t fighting two different sets of shell bugs between the two contexts.

Do not rely on whether `make`’s `SHELL` settings are exported to subprocesses, as implementations differ:

    $ cat Makefile
    all:
            @printf '%s\n' '$(SHELL)'
            @printenv SHELL
    $ env SHELL=/bin/sh make -e SHELL=/bin/ksh  # BSD make, AIX make
    /bin/ksh
    /bin/ksh
    $ env SHELL=/bin/sh make -e SHELL=/bin/ksh  # GNU make
    /bin/ksh
    sh

### 12.10 Parallel Make ¶

Support for parallel execution in `make` implementation varies. Generally, using GNU make is your best bet.

When NetBSD or FreeBSD `make` are run in parallel mode, they will reuse the same shell for multiple commands within one recipe. This can have various unexpected consequences. For example, changes of directories or variables persist between recipes, so that:

    all:
            @var=value; cd /; pwd; echo $$var; echo $$$$
            @pwd; echo $$var; echo $$$$

may output the following with `make -j1`, at least on NetBSD up to 5.1 and FreeBSD up to 8.2:

    /
    value
    32235
    /
    value
    32235

while without `-j1`, or with `-B`, the output looks less surprising:

    /
    value
    32238
    /tmp

    32239

Another consequence is that, if one command in a recipe uses `exit 0` to indicate a successful exit, the shell will be gone and the remaining commands of this recipe will not be executed.

The BSD `make` implementations, when run in parallel mode, will also pass the `Makefile` recipes to the shell through its standard input, thus making it unusable from the recipes:

    $ cat Makefile
    read:
            @read line; echo LINE: $$line
    $ echo foo | make read
    LINE: foo
    $ echo foo | make -j1 read # NetBSD 5.1 and FreeBSD 8.2
    LINE:

Moreover, when FreeBSD `make` (up at least to 8.2) is run in parallel mode, it implements the `@` and `-` “recipe modifiers” by dynamically modifying the active shell flags. This behavior has the effects of potentially clobbering the exit status of recipes silenced with the `@` modifier if they also unset the `errexit` shell flag, and of mangling the output in unexpected ways:

    $ cat Makefile
    a:
            @echo $$-; set +e; false
    b:
            -echo $$-; false; echo set -
    $ make a; echo status: $?
    ehBc
    *** Error code 1
    status: 1
    $ make -j1 a; echo status: $?
    ehB
    status: 0
    $ make b
    echo $-; echo set -
    hBc
    set -
    $ make -j1 b
    echo $-; echo hvB

You can avoid all these issues by using the `-B` option to enable compatibility semantics. However, that will effectively also disable all parallelism as that will cause prerequisites to be updated in the order they are listed in a rule.

Some make implementations (among them, FreeBSD `make`, NetBSD `make`, and Solaris `dmake`), when invoked with a `-jN` option, connect the standard output and standard error of all their child processes to pipes or temporary regular files. This can lead to subtly different semantics in the behavior of the spawned processes. For example, even if the `make` standard output is connected to a tty, the recipe command will not be:

    $ cat Makefile
    all:
            @test -t 1 && echo "Is a tty" || echo "Is not a tty"
    $ make -j 2 # FreeBSD 8.2 make
    Is not a tty
    $ make -j 2 # NetBSD 5.1 make
    --- all ---
    Is not a tty
    $ dmake -j 2 # Solaris 10 dmake
    hostname --> 1 job
    hostname --> Job output
    Is not a tty

On the other hand:

    $ make -j 2 # GNU make, Heirloom make
    Is a tty

The above examples also show additional status output produced in parallel mode for targets being updated by Solaris `dmake` and NetBSD `make` (but not by FreeBSD `make`).

Furthermore, parallel runs of those `make` implementations will route standard error from commands that they spawn into their own standard output, and may remove leading whitespace from output lines.

### 12.11 Comments in Make Rules ¶

Do not try to put comments (lines beginning with ‘`#`’) in a rule, as they end the rule. It is OK for a rule line to start with a tab followed by ‘`#`’, as a comment passed to a shell that does nothing.

To use the ‘`#`’ character in a command, put it in a rule not a macro, as the character cannot portably appear in macros (see Comments in Make Macros). So for example, assuming the output variable `COMMENT_CHAR` stands for ‘`#`’, the following replaces ‘`@COMMENT_CHAR@`’ by ‘`#`’ in a generated header:

    foo.h: foo.h.in
            sed -e 's|@''COMMENT_CHAR''@|@COMMENT_CHAR@|g' \
              '$(srcdir)/foo.h.in' > $@

The funny shell quoting avoids a substitution at `config.status` run time of the left-hand side of the `sed` ‘`s`’ command.

### 12.12 Newlines in Make Rules ¶

In shell scripts, newlines can be used inside string literals. But in the shell statements of `Makefile` rules, this is not possible: a newline not preceded by a backslash separates commands, whereas a newline preceded by a backslash becomes part of the shell statement. So, how can a newline be used in a string literal?

The trick is to set up a shell variable `nl` that contains a newline. For example, the following uses a multi-line ‘`sed`’ expression that appends an empty line after every line of a file:

    output: input
            eval "$$(printf 'nl="\n"\n')"; \
            sed "a\\$$nl" input >$@

### 12.13 Comments in Make Macros ¶

In macro definitions, text from ‘`#`’ until line end is ignored, which has the unfortunate effect of disallowing ‘`#`’ even in quotes. Thus, the following does not work:

    CPPFLAGS = "-DCOMMENT_CHAR='#'"

as ‘`CPPFLAGS`’ is expanded to ‘`"-DCOMMENT_CHAR='`’.

GNU `make`, when not in POSIX mode, lets you put ‘`#`’ into a macro value by escaping it with a backslash, i.e., ‘`\#`’. However, this usage is not portable. See Comments in Make Rules, for a portable alternative.

Even without quoting involved, comments can have surprising effects, because the whitespace before them is part of the variable value:

    foo = bar # trailing comment
    print: ; @echo "$(foo)."

prints ‘`bar .`’, which is usually not intended, and can expose `make` bugs as described below.

### 12.14 Trailing whitespace in Make Macros ¶

GNU `make` 3.80 mistreats trailing whitespace in macro substitutions and appends another spurious suffix:

    empty =
    foo = bar $(empty)
    print: ; @echo $(foo:=.test)

prints ‘`bar.test .test`’.

BSD and Solaris `make` implementations do not honor trailing whitespace in macro definitions as POSIX requires:

    foo = bar # Note the space after "bar".
    print: ; @echo $(foo)t

prints ‘`bart`’ instead of ‘`bar t`’. To work around this, you can use a helper macro as in the previous example.

### 12.15 Command-line Macros and whitespace ¶

Some `make` implementations may strip trailing whitespace off of macros set on the command line in addition to leading whitespace. Further, some may strip leading whitespace off of macros set from environment variables:

    $ echo 'print: ; @echo "x$(foo)x$(bar)x"' |
      foo=' f f ' make -f - bar=' b b '
    x f f xb b x  # AIX, BSD, GNU make
    xf f xb b x   # HP-UX
    x f f xb bx   # Solaris make

### 12.16 The `obj/` Subdirectory and Make ¶

Never name one of your subdirectories `obj/` if you don’t like surprises.

If an `obj/` directory exists, BSD `make` enters it before reading the makefile. Hence the makefile in the current directory is not read.

    $ cat Makefile
    all:
            echo Hello
    $ cat obj/Makefile
    all:
            echo World
    $ make      # GNU make
    echo Hello
    Hello
    $ pmake     # BSD make
    echo World
    World

### 12.17 Exit Status of `make -k` ¶

Do not rely on the exit status of `make -k`. Some implementations reflect whether they encountered an error in their exit status; other implementations always succeed.

    $ cat Makefile
    all:
            false
    $ make -k; echo exit status: $?    # GNU make
    false
    make: *** [all] Error 1
    exit status: 2
    $ pmake -k; echo exit status: $?   # BSD make
    false
    *** Error code 1 (continuing)
    exit status: 0

### 12.18 `VPATH` and Make ¶

POSIX does not specify the semantics of `VPATH`. Typically, `make` supports `VPATH`, but its implementation is not consistent.

Autoconf and Automake support makefiles whose usages of `VPATH` are portable to recent-enough popular implementations of `make`, but to keep the resulting makefiles portable, a package’s makefile prototypes must take the following issues into account. These issues are complicated and are often poorly understood, and installers who use `VPATH` should expect to find many bugs in this area. If you use `VPATH`, the simplest way to avoid these portability bugs is to stick with GNU `make`, since it is the most commonly-used `make` among Autoconf users.

Here are some known issues with some `VPATH` implementations.

#### 12.18.1 Variables listed in `VPATH` ¶

Do not set `VPATH` to the value of another variable, for example ‘`VPATH = $(srcdir)`’, because some ancient versions of `make` do not do variable substitutions on the value of `VPATH`. For example, use this

    srcdir = @srcdir@
    VPATH = @srcdir@

rather than ‘`VPATH = $(srcdir)`’. Note that with GNU Automake, there is no need to set this yourself.

#### 12.18.2 `VPATH` and Double-colon Rules ¶

With ancient versions of Sun `make`, any assignment to `VPATH` causes `make` to execute only the first set of double-colon rules. However, this problem is no longer of practical concern.

#### 12.18.3 `$<` Not Supported in Explicit Rules ¶

Using `$<` in explicit rules is not portable. The prerequisite file must be named explicitly in the rule. If you want to find the prerequisite via a `VPATH` search, you have to code the whole thing manually. See Build Directories.

#### 12.18.4 Automatic Rule Rewriting ¶

Some `make` implementations, such as Solaris, search for prerequisites in `VPATH` and then rewrite each occurrence as a plain word in the rule. For instance:

    # This isn't portable to GNU make.
    VPATH = ../pkg/src
    f.c: if.c
            cp if.c f.c

executes `cp ../pkg/src/if.c f.c` if `if.c` is found in `../pkg/src`.

However, this rule leads to real problems in practice. For example, if the source directory contains an ordinary file named `test` that is used in a dependency, Solaris `make` rewrites commands like ‘`if test -r foo; …`’ to ‘`if ../pkg/src/test -r foo; …`’, which is typically undesirable. In fact, `make` is completely unaware of shell syntax used in the rules, so the VPATH rewrite can potentially apply to any whitespace-separated word in a rule, including shell variables, functions, and keywords.

    $ mkdir build
    $ cd build
    $ cat > Makefile <<'END'
    VPATH = ..
    all: arg func for echo
            func () { for arg in "$$@"; do echo $$arg; done; }; \
            func "hello world"
    END
    $ touch ../arg ../func ../for ../echo
    $ make
    ../func () { ../for ../arg in "$@"; do ../echo $arg; done; }; \
    ../func "hello world"
    sh: syntax error at line 1: `do' unexpected
    *** Error code 2

To avoid this problem, portable makefiles should never mention a source file or dependency whose name is that of a shell keyword like `for` or `until`, a shell command like `cat` or `gcc` or `test`, or a shell function or variable used in the corresponding `Makefile` recipe.

Because of these problems GNU `make` and many other `make` implementations do not rewrite commands, so portable makefiles should search `VPATH` manually. It is tempting to write this:

    # This isn't portable to Solaris make.
    VPATH = ../pkg/src
    f.c: if.c
            cp `test -f if.c || echo $(VPATH)/`if.c f.c

However, the “prerequisite rewriting” still applies here. So if `if.c` is in `../pkg/src`, Solaris `make` executes

    cp `test -f ../pkg/src/if.c || echo ../pkg/src/`if.c f.c

which reduces to

    cp if.c f.c

and thus fails. Oops.

A simple workaround, and good practice anyway, is to use ‘`$?`’ and ‘`$@`’ when possible:

    VPATH = ../pkg/src
    f.c: if.c
            cp $? $@

but this does not generalize well to commands with multiple prerequisites. A more general workaround is to rewrite the rule so that the prerequisite `if.c` never appears as a plain word. For example, these three rules would be safe, assuming `if.c` is in `../pkg/src` and the other files are in the working directory:

    VPATH = ../pkg/src
    f.c: if.c f1.c
            cat `test -f ./if.c || echo $(VPATH)/`if.c f1.c >$@
    g.c: if.c g1.c
            cat `test -f 'if.c' || echo $(VPATH)/`if.c g1.c >$@
    h.c: if.c h1.c
            cat `test -f "if.c" || echo $(VPATH)/`if.c h1.c >$@

Things get worse when your prerequisites are in a macro.

    VPATH = ../pkg/src
    HEADERS = f.h g.h h.h
    install-HEADERS: $(HEADERS)
            for i in $(HEADERS); do \
              $(INSTALL) -m 644 \
                `test -f $$i || echo $(VPATH)/`$$i \
                $(DESTDIR)$(includedir)/$$i; \
            done

The above `install-HEADERS` rule is not Solaris-proof because `for i in $(HEADERS);` is expanded to `for i in f.h g.h h.h;` where `f.h` and `g.h` are plain words and are hence subject to `VPATH` adjustments.

If the three files are in `../pkg/src`, the rule is run as:

    for i in ../pkg/src/f.h ../pkg/src/g.h h.h; do \
      install -m 644 \
         `test -f $i || echo ../pkg/src/`$i \
         /usr/local/include/$i; \
    done

where the two first `install` calls fail. For instance, consider the `f.h` installation:

    install -m 644 \
      `test -f ../pkg/src/f.h || \
        echo ../pkg/src/ \
      `../pkg/src/f.h \
      /usr/local/include/../pkg/src/f.h;

It reduces to:

    install -m 644 \
      ../pkg/src/f.h \
      /usr/local/include/../pkg/src/f.h;

Note that the manual `VPATH` search did not cause any problems here; however this command installs `f.h` in an incorrect directory.

Trying to quote `$(HEADERS)` in some way, as we did for `foo.c` a few makefiles ago, does not help:

    install-HEADERS: $(HEADERS)
            headers='$(HEADERS)'; \
            for i in $$headers; do \
              $(INSTALL) -m 644 \
                `test -f $$i || echo $(VPATH)/`$$i \
                $(DESTDIR)$(includedir)/$$i; \
            done

Now, `headers='$(HEADERS)'` macro-expands to:

    headers='f.h g.h h.h'

but `g.h` is still a plain word. (As an aside, the idiom `headers='$(HEADERS)'; for i in $$headers;` is a good idea if `$(HEADERS)` can be empty, because some shells diagnose a syntax error on `for i in;`.)

One workaround is to strip this unwanted `../pkg/src/` prefix manually:

    VPATH = ../pkg/src
    HEADERS = f.h g.h h.h
    install-HEADERS: $(HEADERS)
            headers='$(HEADERS)'; \
            for i in $$headers; do \
              i=`expr "$$i" : '$(VPATH)/\(.*\)'`;
              $(INSTALL) -m 644 \
                `test -f $$i || echo $(VPATH)/`$$i \
                $(DESTDIR)$(includedir)/$$i; \
            done

Automake does something similar.

#### 12.18.5 Make Target Lookup ¶

GNU `make` uses a complex algorithm to decide when it should use files found via a `VPATH` search. See How Directory Searches are Performed in The GNU Make Manual.

If a target needs to be rebuilt, GNU `make` discards the file name found during the `VPATH` search for this target, and builds the file locally using the file name given in the makefile. If a target does not need to be rebuilt, GNU `make` uses the file name found during the `VPATH` search.

Other `make` implementations, like NetBSD `make`, are easier to describe: the file name found during the `VPATH` search is used whether the target needs to be rebuilt or not. Therefore new files are created locally, but existing files are updated at their `VPATH` location.

OpenBSD and FreeBSD `make`, however, never perform a `VPATH` search for a dependency that has an explicit rule. This is extremely annoying.

When attempting a `VPATH` build for an autoconfiscated package (e.g., `mkdir build && cd build && ../configure`), this means GNU `make` builds everything locally in the `build` directory, while BSD `make` builds new files locally and updates existing files in the source directory.

    $ cat Makefile
    VPATH = ..
    all: foo.x bar.x
    foo.x bar.x: newer.x
            @echo Building $@
    $ touch ../bar.x
    $ touch ../newer.x
    $ make        # GNU make
    Building foo.x
    Building bar.x
    $ pmake       # NetBSD make
    Building foo.x
    Building ../bar.x
    $ fmake       # FreeBSD make, OpenBSD make
    Building foo.x
    Building bar.x
    $ make        # GNU make
    Building foo.x
    $ pmake       # NetBSD make
    Building foo.x
    $ fmake       # FreeBSD make, OpenBSD make
    Building foo.x
    Building bar.x

Note how NetBSD `make` updates `../bar.x` in its VPATH location, and how FreeBSD and OpenBSD `make` always update `bar.x`, even when `../bar.x` is up to date.

Another point worth mentioning is that once GNU `make` has decided to ignore a `VPATH` file name (e.g., it ignored `../bar.x` in the above example) it continues to ignore it when the target occurs as a prerequisite of another rule.

The following example shows that GNU `make` does not look up `bar.x` in `VPATH` before performing the `.x.y` rule, because it ignored the `VPATH` result of `bar.x` while running the `bar.x: newer.x` rule.

    $ cat Makefile
    VPATH = ..
    all: bar.y
    bar.x: newer.x
            @echo Building $@
    .SUFFIXES: .x .y
    .x.y:
            cp $< $@
    $ touch ../bar.x
    $ touch ../newer.x
    $ make        # GNU make
    Building bar.x
    cp bar.x bar.y
    cp: cannot stat 'bar.x': No such file or directory
    make: *** [bar.y] Error 1
    $ pmake       # NetBSD make
    Building ../bar.x
    cp ../bar.x bar.y
    $ rm bar.y
    $ fmake       # FreeBSD make, OpenBSD make
    echo Building bar.x
    cp bar.x bar.y
    cp: cannot stat 'bar.x': No such file or directory
    *** Error code 1

Note that if you drop away the command from the `bar.x: newer.x` rule, GNU `make` magically starts to work: it knows that `bar.x` hasn’t been updated, therefore it doesn’t discard the result from `VPATH` (`../bar.x`) in succeeding uses. FreeBSD and OpenBSD still don’t work, though.

    $ cat Makefile
    VPATH = ..
    all: bar.y
    bar.x: newer.x
    .SUFFIXES: .x .y
    .x.y:
            cp $< $@
    $ touch ../bar.x
    $ touch ../newer.x
    $ make        # GNU make
    cp ../bar.x bar.y
    $ rm bar.y
    $ pmake       # NetBSD make
    cp ../bar.x bar.y
    $ rm bar.y
    $ fmake       # FreeBSD make, OpenBSD make
    cp bar.x bar.y
    cp: cannot stat 'bar.x': No such file or directory
    *** Error code 1

It seems the sole solution that would please every `make` implementation is to never rely on `VPATH` searches for targets. In other words, `VPATH` should be reserved to sources that are not built.

### 12.19 Single Suffix Rules and Separated Dependencies ¶

A Single Suffix Rule is basically a usual suffix (inference) rule (‘`.from.to:`’), but which destination suffix is empty (‘`.from:`’).

Separated dependencies simply refers to listing the prerequisite of a target, without defining a rule. Usually one can list on the one hand side, the rules, and on the other hand side, the dependencies.

Solaris `make` does not support separated dependencies for targets defined by single suffix rules:

    $ cat Makefile
    .SUFFIXES: .in
    foo: foo.in
    .in:
            cp $< $@
    $ touch foo.in
    $ make
    $ ls
    Makefile  foo.in

while GNU Make does:

    $ gmake
    cp foo.in foo
    $ ls
    Makefile  foo       foo.in

Note it works without the ‘`foo: foo.in`’ dependency.

    $ cat Makefile
    .SUFFIXES: .in
    .in:
            cp $< $@
    $ make foo
    cp foo.in foo

and it works with double suffix inference rules:

    $ cat Makefile
    foo.out: foo.in
    .SUFFIXES: .in .out
    .in.out:
            cp $< $@
    $ make
    cp foo.in foo.out

As a result, in such a case, you have to write target rules.

### 12.20 Timestamp Resolution and Make ¶

Traditionally, file timestamps had 1-second resolution, and `make` used those timestamps to determine whether one file was newer than the other. However, many modern file systems have timestamps with 1-nanosecond resolution. Some `make` implementations look at the entire timestamp; others ignore the fractional part, which can lead to incorrect results. Normally this is not a problem, but in some extreme cases you may need to use tricks like ‘`sleep 1`’ to work around timestamp truncation bugs.

Traditionally, commands like ‘`cp -p`’ and ‘`touch -r`’ did not copy file timestamps to their full resolutions (see Limitations of Usual Tools). Hence you should be wary of rules like this:

    dest: src
            cp -p src dest

as `dest` often appears to be older than `src` after the timestamp is truncated, and this can cause `make` to do needless rework the next time it is invoked. To work around this problem, you can use a timestamp file, e.g.:

    dest-stamp: src
            cp -p src dest
            echo >dest-stamp

Apart from timestamp resolution, there are also differences in handling equal timestamps. Although POSIX suggests that `make` update a target with the same timestamp as its newest prerequisite, only HP-UX `make` is known to do so; GNU and other `make` implementations do not update, a behavior that POSIX also allows.

The HP-UX behavior can cause spurious rebuilds and this in turn can cause `make` to fail if it tries to rebuild generated files in a possibly read-only source tree with tools not present on the build machine. Conversely, if a source file is updated immediately after running `make` in a file system with low-resolution timestamps, the GNU behavior can cause a later `make` to neglect rebuilds and quietly generate incorrect results. To avoid both these problems, use GNU `make` on a platform with high-resolution timestamps.

## 13 Portable C and C++ Programming ¶

C and C++ programs often use low-level features of the underlying system, and therefore are often more difficult to make portable to other platforms.

Several standards have been developed to help make your programs more portable. If you write programs with these standards in mind, you can have greater confidence that your programs work on a wide variety of systems. Language Standards Supported by GCC for a list of C-related standards. Many programs also assume the POSIX standard.

The first widely used C variant was K&R C, which predates any C standard. K&R C compilers are no longer of practical interest, though, and Autoconf assumes at least C89, the first C standard, which is sometimes called “C90” due to a delay in standardization. C has since gone through the standards C99, C11, C17, and C23, and Autoconf is compatible with all these standards.

Program portability is a huge topic, and this section can only briefly introduce common pitfalls. See Portability between System Types in The GNU Coding Standards, for more information.

### 13.1 Varieties of Unportability ¶

Autoconf tests and ordinary programs often need to test what is allowed on a system, and therefore they may need to deliberately exceed the boundaries of what the standards allow, if only to see whether an optional feature is present. When you write such a program, you should keep in mind the difference between constraints, unspecified behavior, and undefined behavior.

In C, a constraint is a rule that the compiler must enforce. An example constraint is that C programs must not declare a bit-field with negative width. Tests can therefore reliably assume that programs with negative-width bit-fields are rejected by a compiler that conforms to the standard.

Unspecified behavior is valid behavior, where the standard allows multiple possibilities. For example, the order of evaluation of function arguments is unspecified. Some unspecified behavior is implementation-defined, i.e., documented by the implementation, but since Autoconf tests cannot read the documentation they cannot distinguish between implementation-defined and other unspecified behavior. It is common for Autoconf tests to probe implementations to determine otherwise-unspecified behavior.

Undefined behavior is invalid behavior, where the standard allows the implementation to do anything it pleases. For example, dereferencing a null pointer leads to undefined behavior. If possible, test programs should avoid undefined behavior, since a program with undefined behavior might succeed on a test that should fail.

The above rules apply to programs that are intended to conform to the standard. However, strictly-conforming programs are quite rare, since the standards are so limiting. A major goal of Autoconf is to support programs that use implementation features not described by the standard, and it is fairly common for test programs to violate the above rules, if the programs work well enough in practice.

### 13.2 Integer Overflow ¶

Although some traditional C programs assume that signed integer overflow wraps around reliably using two’s complement arithmetic, the C standard says that program behavior is undefined on overflow, and these C programs may not work on many modern implementations.

#### 13.2.1 Basics of Integer Overflow ¶

In languages like C, integer overflow wraps around for unsigned integer types that are at least as wide as `unsigned int`; e.g., `UINT_MAX + 1` yields zero. This is guaranteed by the C standard and is portable in practice, unless you specify aggressive, nonstandard optimization options suitable only for special applications.

In contrast, the C standard says that signed integer overflow leads to undefined behavior where a program can do anything, including dumping core or overrunning a buffer. The misbehavior can even precede the overflow. Such an overflow can occur during addition, subtraction, multiplication, division, and left shift. It can even occur for unsigned types like `unsigned short int` that are narrower than `int`, as values of these types are widened to `int` before computation.

Despite this requirement of the standard, some C programs assume that signed integer overflow silently wraps around modulo a power of two, using two’s complement arithmetic, so long as you convert the resulting value to a signed integer type. These programs can have problems, especially when optimization is enabled. If you assume a GCC-like compiler, you can work around the problems by compiling with GCC’s `-fwrapv` option; however, this is not portable.

For historical reasons C17 and earlier also allowed implementations with ones’ complement or signed magnitude arithmetic, but C23 requires two’s complement and it is safe to assume two’s complement nowadays.

Also, overflow can occur when converting an out-of-range value to a signed integer type. Here a standard implementation must define what happens, and this can include raising an exception. Although practical implementations typically wrap around silently in this case, a few debugging implementations trap instead.

#### 13.2.2 Examples of Code Assuming Wraparound Overflow ¶

There was long a tension between what the C standard requires for signed integer overflow, and what traditional C programs commonly assumed. The standard allows aggressive optimizations based on assumptions that overflow never occurs, but traditionally many C programs relied on overflow wrapping around. Although these programs did not conform to the standard, they formerly worked in practice because traditionally compilers did not optimize in such a way that would break the programs. Nowadays, though, compilers do perform these optimizations, so portable programs can no longer assume reliable wraparound on signed integer overflow.

The C Standard says that if a program has signed integer overflow its behavior is undefined, and the undefined behavior can even precede the overflow. To take an extreme example:

    if (password == expected_password)
      allow_superuser_privileges ();
    else if (counter++ == INT_MAX)
      abort ();
    else
      printf ("%d password mismatches\n", counter);

If the `int` variable `counter` equals `INT_MAX`, `counter++` must overflow and the behavior is undefined, so the C standard allows the compiler to optimize away the test against `INT_MAX` and the `abort` call. Worse, if an earlier bug in the program lets the compiler deduce that `counter == INT_MAX` or that `counter` previously overflowed, the C standard allows the compiler to optimize away the password test and generate code that allows superuser privileges unconditionally.

Here is an example derived from the 7th Edition Unix implementation of `atoi` (1979-01-10):

    char *p;
    int f, n;
    ...
    while (*p >= '0' && *p <= '9')
      n = n * 10 + *p++ - '0';
    return (f ? -n : n);

Even if the input string is in range, on most modern machines this has signed overflow when computing the most negative integer (the `-n` overflows) or a value near an extreme integer (the `+` overflows).

Here is another example, derived from the 7th Edition implementation of `rand` (1979-01-10). Here the programmer expects both multiplication and addition to wrap on overflow:

    static long int randx = 1;
    ...
    randx = randx * 1103515245 + 12345;
    return (randx >> 16) & 077777;

In the following example, derived from the GNU C Library 2.15 implementation of `mktime` (2012-03-21), the code assumes wraparound arithmetic in `+` to detect signed overflow:

    time_t t, t1, t2;
    int sec_requested, sec_adjustment;
    ...
    t1 = t + sec_requested;
    t2 = t1 + sec_adjustment;
    if (((t1 < t) != (sec_requested < 0))
        | ((t2 < t1) != (sec_adjustment < 0)))
      return -1;

Although some of these examples will likely behave as if signed integer overflow wraps around reliably, other examples are likely to misbehave when optimization is enabled. All these examples should be avoided in portable code because signed integer overflow is not reliable on modern systems, and it’s not worth worrying about which of these examples happen to work on most platforms and which do not.

#### 13.2.3 Optimizations That Break Wraparound Arithmetic ¶

Compilers sometimes generate code that is incompatible with wraparound integer arithmetic. A simple example is an algebraic simplification: a compiler might translate `(i * 2000) / 1000` to `i * 2` because it assumes that `i * 2000` does not overflow. The translation is not equivalent to the original when overflow occurs: e.g., in the typical case of 32-bit signed two’s complement wraparound `int`, if `i` has type `int` and value `1073742`, the original expression returns −2147483 but the optimized version returns the mathematically correct value 2147484.

More subtly, loop induction optimizations often exploit the undefined behavior of signed overflow. Consider the following contrived function `sumc`:

    int
    sumc (int lo, int hi)
    {
      int sum = 0;
      for (int i = lo; i <= hi; i++)
        sum ^= i * 53;
      return sum;
    }

To avoid multiplying by 53 each time through the loop, an optimizing compiler might internally transform `sumc` to the equivalent of the following:

    int
    transformed_sumc (int lo, int hi)
    {
      int sum = 0;
      int hic = hi * 53;
      for (int ic = lo * 53; ic <= hic; ic += 53)
        sum ^= ic;
      return sum;
    }

This transformation is allowed by the C standard, but it is invalid for wraparound arithmetic when `INT_MAX / 53 < hi`, because then the overflow in computing expressions like `hi * 53` can cause the expression `i <= hi` to yield a different value from the transformed expression `ic <= hic`.

For this reason, compilers that use loop induction and similar techniques often do not support reliable wraparound arithmetic when a loop induction variable like `ic` is involved. Since loop induction variables are generated by the compiler, and are not visible in the source code, it is not always trivial to say whether the problem affects your code.

Hardly any code actually depends on wraparound arithmetic in cases like these, so in practice these loop induction optimizations are almost always useful. However, edge cases in this area can cause problems. For example:

    for (int j = 1; 0 < j; j *= 2)
      test (j);

Here, the loop attempts to iterate through all powers of 2 that `int` can represent, but the C standard allows a compiler to optimize away the comparison and generate an infinite loop, under the argument that behavior is undefined on overflow. As of this writing this optimization is done on some platforms by GCC with `-O2`, so this code is not portable in practice.

#### 13.2.4 Practical Advice for Signed Overflow Issues ¶

Ideally the safest approach is to avoid signed integer overflow entirely. For example, instead of multiplying two signed integers, you can convert them to double-width integers, multiply the wider values, then test whether the result is in the narrower range. Or you can use more-complicated code employing unsigned integers of the same width.

Rewriting code in this way will be inconvenient, though, especially if the signed values might be negative and no wider type is available. Using unsigned arithmetic to check for overflow is particularly painful to do portably and efficiently when dealing with an integer type like `uid_t` whose width and signedness vary from platform to platform. Also, this approach may hurt performance.

Hence it is often useful to maintain code that needs wraparound on overflow, instead of rewriting the code. The rest of this section attempts to give practical advice for this situation.

To detect integer overflow portably when attempting operations like `sum = a + b`, you can use the C23 `<stdckdint.h>` macros `ckd_add`, `ckd_sub`, and `ckd_mul`. The following code adds two integers with overflow wrapping around reliably in the sum:

    #include <stdckdint.h>
    ...
    /* Set sum = a + b, with wraparound.  */
    if (ckd_add (&sum, a, b))
      /* 'sum' has just the low order bits.  */;
    else
      /* 'sum' is the correct answer.  */;

To be portable to pre-C23 platforms you can use Gnulib’s `stdckdint` module, which emulates this part of C23 (see Gnulib). Invoking the `stdckdint` macros typically costs just one machine instruction for the arithmetic and another instruction for the rare branch on overflow.

If your code uses a signed loop index, make sure that the index cannot overflow, along with all signed expressions derived from the index. Here is a contrived example of problematic code with two instances of overflow.

    for (int i = INT_MAX - 10; i <= INT_MAX; i++)
      if (i + 1 < 0)
        {
          report_overflow ();
          break;
        }

Because of the two overflows, a compiler might optimize away or transform the two comparisons in a way that is incompatible with the wraparound assumption.

If your code is intended to be compiled only by GCC and assumes wraparound behavior, and you want to insulate it against any GCC optimizations that would fail to support that behavior, you should use GCC’s `-fwrapv` option, which causes signed overflow to wrap around reliably (except for division and remainder, as discussed in the next section).

If you need to write portable code and therefore cannot assume that signed integer overflow wraps around reliably, you should consider debugging with a GCC option that causes signed overflow to raise an exception. These options include `-fsanitize=undefined` and `-ftrapv`.

#### 13.2.5 Signed Integer Division and Integer Overflow ¶

Overflow in signed integer division is not always harmless: for example, on CPUs of the i386 family, dividing `INT_MIN` by `-1` yields a SIGFPE signal which by default terminates the program. Worse, taking the remainder of these two values typically yields the same signal on these CPUs, behavior that the C standard allows.

### 13.3 Preprocessor Arithmetic ¶

In C99 and later, preprocessor arithmetic, used for `#if` expressions, must be evaluated as if all signed values are of type `intmax_t` and all unsigned values of type `uintmax_t`. Many compilers are buggy in this area, though. For example, as of 2007, Sun C mishandles `#if LLONG_MIN < 0` on a platform with 32-bit `long int` and 64-bit `long long int`. Also, some older preprocessors mishandle constants ending in `LL`. To work around these problems, you can compute the value of expressions like `LONG_MAX < LLONG_MAX` at `configure`-time rather than at `#if`-time.

### 13.4 Properties of Null Pointers ¶

Most modern hosts reliably fail when you attempt to dereference a null pointer.

On almost all modern hosts, null pointers use an all-bits-zero internal representation, so you can reliably use `memset` with 0 to set all the pointers in an array to null values.

If `p` is a null pointer to an object type, the C expression `p + 0` always evaluates to `p` on modern hosts, even though the standard says that it has undefined behavior.

### 13.5 Buffer Overruns and Subscript Errors ¶

Buffer overruns and subscript errors are the most common dangerous errors in C programs. They result in undefined behavior because storing outside an array typically modifies storage that is used by some other object, and most modern systems lack runtime checks to catch these errors. Programs should not rely on buffer overruns being caught.

There is one exception to the usual rule that a portable program cannot address outside an array. In C, it is valid to compute the address just past an object, e.g., `&a[N]` where `a` has `N` elements, so long as you do not dereference the resulting pointer. But it is not valid to compute the address just before an object, e.g., `&a[-1]`; nor is it valid to compute two past the end, e.g., `&a[N+1]`. On most platforms `&a[-1] < &a[0] && &a[N] < &a[N+1]`, but this is not reliable in general, and it is usually easy enough to avoid the potential portability problem, e.g., by allocating an extra unused array element at the start or end.

Valgrind can catch many overruns. GCC users might also consider using the `-fsanitize=` options to catch overruns. See Program Instrumentation Options in Using the GNU Compiler Collection (GCC).

Buffer overruns are usually caused by off-by-one errors, but there are more subtle ways to get them.

Using `int` values to index into an array or compute array sizes causes problems on typical 64-bit hosts where an array index might be 2^{31} or larger. Index values of type `size_t` avoid this problem, but cannot be negative. Index values of type `ptrdiff_t` are signed, and are wide enough in practice.

If you add or multiply two numbers to calculate an array size, e.g., `malloc (x * sizeof y + z)`, havoc ensues if the addition or multiplication overflows.

Many implementations of the `alloca` function silently misbehave and can generate buffer overflows if given sizes that are too large. The size limits are implementation dependent, but are at least 4000 bytes on all platforms that we know about.

The standard functions `asctime`, `asctime_r`, `ctime`, `ctime_r`, and `gets` are prone to buffer overflows, and portable code should not use them unless the inputs are known to be within certain limits. The time-related functions can overflow their buffers if given timestamps out of range (e.g., a year less than -999 or greater than 9999). Time-related buffer overflows cannot happen with recent-enough versions of the GNU C library, but are possible with other implementations. The `gets` function is the worst, since it almost invariably overflows its buffer when presented with an input line larger than the buffer.

### 13.6 Volatile Objects ¶

The keyword `volatile` is often misunderstood in portable code. Its use inhibits some memory-access optimizations, but programmers often wish that it had a different meaning than it actually does.

`volatile` was designed for code that accesses special objects like memory-mapped device registers whose contents spontaneously change. Such code is inherently low-level, and it is difficult to specify portably what `volatile` means in these cases. The C standard says, “What constitutes an access to an object that has volatile-qualified type is implementation-defined,” so in theory each implementation is supposed to fill in the gap by documenting what `volatile` means for that implementation. In practice, though, this documentation is usually absent or incomplete.

One area of confusion is the distinction between objects defined with volatile types, and volatile lvalues. From the C standard’s point of view, an object defined with a volatile type has externally visible behavior. You can think of such objects as having little oscilloscope probes attached to them, so that the user can observe some properties of accesses to them, just as the user can observe data written to output files. However, accesses via volatile lvalues to ordinary objects are merely side effects (i.e., changes to the state of the execution environment), and the implementation is not required to document their visibility any further. For example:

    /* Declare and access a volatile object.
       Accesses to X are "visible" to users.  */
    static int volatile x;
    x = 1;

    /* Access two ordinary objects via a volatile lvalue.
       Although each read and write is a side effect,
       the accesses are not directly "visible" to users.  */
    int y = 0;
    int *z = malloc (sizeof *z);
    *z = 7;
    int volatile *p;
    p = &y;
    *p = *p + 1;
    p = z;
    *p = *p + 1;

Programmers often wish that `volatile` meant “Perform the memory access here and now, without merging several memory accesses, without changing the memory word size, and without reordering.” But the C standard does not require this. For objects defined with a volatile type, accesses must be done before the next sequence point; but otherwise merging, reordering, and word-size change is allowed.

Even when accessing objects defined with a volatile type, the C standard allows only extremely limited signal handlers: in C23 the behavior is undefined if a signal handler refers to any non-local object that is not a lock-free atomic object and that is not `constexpr` (other than by writing to a `sig_atomic_t volatile` object), or calls any standard library function other than from a small set that includes `abort`, `_Exit`, `quick_exit`, some `<stdatomic.h>` functions, and `signal`. Hence C compilers need not worry about a signal handler disturbing ordinary computation. POSIX allows some additional behavior in a portable signal handler, but is still quite restrictive. See When is a Volatile Object Accessed? in Using the GNU Compiler Collection (GCC), for some restrictions imposed by GCC. See Defining Signal Handlers in The GNU C Library, for some restrictions imposed by the GNU C library. Restrictions differ on other platforms.

If possible, it is best to use a signal handler that fits within the limits imposed by the C and POSIX standards.

If this is not practical, you can try the following rules of thumb. A signal handler should access only volatile lvalues, preferably lvalues that refer to objects defined with a volatile type, and should not assume that the accessed objects have an internally consistent state if they are larger than a machine word. Furthermore, installers should employ compilers and compiler options that are commonly used for building operating system kernels, because kernels often need more from `volatile` than the C Standard requires, and installers who compile an application in a similar environment can sometimes benefit from the extra constraints imposed by kernels on compilers. Admittedly we are hand-waving somewhat here, as there are few guarantees in this area; the rules of thumb may help to fix some bugs but there is a good chance that they will not fix them all.

For `volatile`, C++ has the same problems that C does. Multithreaded applications have even more problems with `volatile`, but they are beyond the scope of this section.

The bottom line is that using `volatile` typically hurts performance but should not hurt correctness. In some cases its use does help correctness, but these cases are often so poorly understood that all too often adding `volatile` to a data structure merely alleviates some symptoms of a bug while not fixing the bug in general.

### 13.7 Floating Point Portability ¶

Almost all modern systems use IEEE-754 floating point, and it is safe to assume IEEE-754 in most portable code these days. For more information, please see David Goldberg’s classic paper What Every Computer Scientist Should Know About Floating-Point Arithmetic.

### 13.8 Exiting Portably ¶

A C or C++ program can exit with status N by returning N from the `main` function. Portable programs are supposed to exit either with status 0 or `EXIT_SUCCESS` to succeed, or with status `EXIT_FAILURE` to fail, but in practice it is portable to fail by exiting with status 1, and test programs that assume POSIX can fail by exiting with status values from 1 through 255.

A program can also exit with status N by passing N to the `exit` function, and a program can fail by calling the `abort` function. If a program is specialized to just some platforms, it can fail by calling functions specific to those platforms, e.g., `_exit` (POSIX). However, like other functions, an exit function should be declared, typically by including a header. For example, if a C program calls `exit`, it should include `stdlib.h` either directly or via the default includes (see Default Includes).

A program can fail due to undefined behavior such as dereferencing a null pointer, but this is not recommended as undefined behavior allows an implementation to do whatever it pleases and this includes exiting successfully.

## 14 Manual Configuration ¶

A few kinds of features can’t be guessed automatically by running test programs. For example, the details of the object-file format, or special options that need to be passed to the compiler or linker. Autoconf provides a uniform method for handling unguessable features, by giving each operating system a canonical system type, also known as a canonical name or target triplet.

If you use any of the macros described in this chapter, you must distribute the helper scripts `config.guess` and `config.sub` along with your source code. Some Autoconf macros use these macros internally, so you may need to distribute these scripts even if you do not use any of these macros yourself. See Configure Input: Source Code, Macros, and Auxiliary Files, for information about the `AC_CONFIG_AUX_DIR` macro which you can use to control in which directory `configure` looks for helper scripts, and where to get the scripts from.

### 14.1 Specifying target triplets ¶

Autoconf-generated `configure` scripts can make decisions based on a canonical name for the system type, or target triplet, which has the form: ‘`cpu-vendor-os`’, where os can be ‘`system`’ or ‘`kernel-system`’

`configure` can usually guess the canonical name for the type of system it’s running on. To do so it runs a script called `config.guess`, which infers the name using the `uname` command or symbols predefined by the C preprocessor.

Alternately, the user can specify the system type with command line arguments to `configure` (see Specifying a System Type. Doing so is necessary when cross-compiling. In the most complex case of cross-compiling, three system types are involved. The options to specify them are:

`--build=build-type`

  the type of system on which the package is being configured and compiled. It defaults to the result of running `config.guess`. Specifying a build-type that differs from host-type enables cross-compilation mode.

`--host=host-type`

  the type of system on which the package runs. By default it is the same as the build machine. The tools that get used to build and manipulate binaries will, by default, all be prefixed with `host-type-`, such as `host-type-gcc`, `host-type-g++`, `host-type-ar`, and `host-type-nm`. If the binaries produced by these tools can be executed by the build system, the configure script will make use of it in `AC_RUN_IFELSE` invocations; otherwise, cross-compilation mode is enabled. Specifying a host-type that differs from build-type, when build-type was also explicitly specified, equally enables cross-compilation mode.

`--target=target-type`

  the type of system for which any compiler tools in the package produce code (rarely needed). By default, it is the same as host.

If you mean to override the result of `config.guess` but still produce binaries for the build machine, use `--build`, not `--host`.

So, for example, to produce binaries for 64-bit MinGW, use a command like this:

    ./configure --host=x86_64-w64-mingw64

If your system has the ability to execute MinGW binaries but you don’t want to make use of this feature and instead prefer cross-compilation guesses, use a command like this:

    ./configure --build=x86_64-pc-linux-gnu --host=x86_64-w64-mingw64

Note that if you do not specify `--host`, `configure` fails if it can’t run the code generated by the specified compiler. For example, configuring as follows fails:

    ./configure CC=x86_64-w64-mingw64-gcc

When cross-compiling, `configure` will warn about any tools (compilers, linkers, assemblers) whose name is not prefixed with the host type. This is an aid to users performing cross-compilation. Continuing the example above, if a cross-compiler named `cc` is used with a native `pkg-config`, then libraries found by `pkg-config` will likely cause subtle build failures; but using the names `x86_64-w64-mingw64-gcc` and `x86_64-w64-mingw64-pkg-config` avoids any confusion. Avoiding the warning is as simple as creating the correct symlinks naming the cross tools.

`configure` recognizes short aliases for some system types; for example, ‘`mingw64`’ can be used instead of ‘`x86_64-pc-mingw64`’. `configure` runs a script called `config.sub` to canonicalize system type aliases.

This section deliberately omits the description of the obsolete interface; see Hosts and Cross-Compilation.

### 14.2 Getting the Canonical System Type ¶

The following macros make the system type available to `configure` scripts.

The variables ‘`build_alias`’, ‘`host_alias`’, and ‘`target_alias`’ are always exactly the arguments of `--build`, `--host`, and `--target`; in particular, they are left empty if the user did not use them, even if the corresponding `AC_CANONICAL` macro was run. Any configure script may use these variables anywhere. These are the variables that should be used when in interaction with the user.

If you need to recognize some special environments based on their system type, run the following macros to get canonical system names. These variables are not set before the macro call.

Macro: AC_CANONICAL_BUILD ¶

  Compute the canonical build-system type variable, `build`, and its three individual parts `build_cpu`, `build_vendor`, and `build_os`.

  If `--build` was specified, then `build` is the canonicalization of `build_alias` by `config.sub`, otherwise it is determined by the shell script `config.guess`.

Macro: AC_CANONICAL_HOST ¶

  Compute the canonical host-system type variable, `host`, and its three individual parts `host_cpu`, `host_vendor`, and `host_os`.

  If `--host` was specified, then `host` is the canonicalization of `host_alias` by `config.sub`, otherwise it defaults to `build`.

Macro: AC_CANONICAL_TARGET ¶

  Compute the canonical target-system type variable, `target`, and its three individual parts `target_cpu`, `target_vendor`, and `target_os`.

  If `--target` was specified, then `target` is the canonicalization of `target_alias` by `config.sub`, otherwise it defaults to `host`.

Note that there can be artifacts due to the backward compatibility code. See Hosts and Cross-Compilation, for more.

### 14.3 Using the System Type ¶

In `configure.ac` the system type is generally used by one or more `case` statements to select system-specifics. Shell wildcards can be used to match a group of system types.

For example, an extra assembler code object file could be chosen, giving access to a CPU cycle counter register. `$(CYCLE_OBJ)` in the following would be used in a makefile to add the object to a program or library.

    AS_CASE([$host],
      [aarch64*-*-*], [CYCLE_OBJ=pmccntr.o],
      [i?86-*-*],     [CYCLE_OBJ=rdtsc.o],
      [CYCLE_OBJ=""])
    AC_SUBST([CYCLE_OBJ])

`AC_CONFIG_LINKS` (see Creating Configuration Links) is another good way to select variant source files, for example optimized code for some CPUs. The configured CPU type doesn’t always indicate exact CPU types, so some runtime capability checks may be necessary too.

    AS_CASE([$host],
      [aarch64*-*-*], [AC_CONFIG_LINKS([dither.c:aarch64/dither.c])],
      [powerpc*-*-*], [AC_CONFIG_LINKS([dither.c:powerpc/dither.c])],
      [AC_CONFIG_LINKS([dither.c:generic/dither.c])])

The host system type can also be used to find cross-compilation tools with `AC_CHECK_TOOL` (see Generic Program and File Checks).

The above examples all show ‘`$host`’, since this is where the code is going to run. Only rarely is it necessary to test ‘`$build`’ (which is where the build is being done).

Whenever you’re tempted to use ‘`$host`’ it’s worth considering whether some sort of probe would be better. New system types come along periodically or previously missing features are added. Well-written probes can adapt themselves to such things, but hard-coded lists of names can’t. Here are some guidelines,

- Availability of libraries and library functions should always be checked by probing.
- Variant behavior of system calls is best identified with runtime tests if possible, but bug workarounds or obscure difficulties might have to be driven from ‘`$host`’.
- Assembler code is inevitably highly CPU-specific and is best selected according to ‘`$host_cpu`’.
- Assembler variations like underscore prefix on globals or ELF versus COFF type directives are however best determined by probing, perhaps even examining the compiler output.

‘`$target`’ is for use by a package creating a compiler or similar. For ordinary packages it’s meaningless and should not be used. It indicates what the created compiler should generate code for, if it can cross-compile. ‘`$target`’ generally selects various hard-coded CPU and system conventions, since usually the compiler or tools under construction themselves determine how the target works.

## 15 Site Configuration ¶

`configure` scripts support several kinds of local configuration decisions. There are ways for users to specify where external software packages are, include or exclude optional features, install programs under modified names, and set default values for `configure` options.

### 15.1 Controlling Help Output ¶

Users consult ‘`configure --help`’ to learn of configuration decisions specific to your package. By default, `configure` breaks this output into sections for each type of option; within each section, help strings appear in the order `configure.ac` defines them:

    Optional Features:
      ...
      --enable-bar            include bar

    Optional Packages:
      ...
      --with-foo              use foo

Macro: AC_PRESERVE_HELP_ORDER ¶

  Request an alternate `--help` format, in which options of all types appear together, in the order defined. Call this macro before any `AC_ARG_ENABLE` or `AC_ARG_WITH`.

      Optional Features and Packages:
        ...
        --enable-bar            include bar
        --with-foo              use foo

### 15.2 Working With External Software ¶

Some packages require, or can optionally use, other software packages that are already installed. The user can give `configure` command line options to specify which such external software to use. The options have one of these forms:

    --with-package[=arg]
    --without-package

For example, `--with-gnu-ld` means work with the GNU linker instead of some other linker. `--with-x` means work with The X Window System.

The user can give an argument by following the package name with ‘`=`’ and the argument. Giving an argument of ‘`no`’ is for packages that are used by default; it says to not use the package. An argument that is neither ‘`yes`’ nor ‘`no`’ could include a name or number of a version of the other package, to specify more precisely which other package this program is supposed to work with. If no argument is given, it defaults to ‘`yes`’. `--without-package` is equivalent to `--with-package=no`.

Normally `configure` scripts complain about `--with-package` options that they do not support. See Controlling Checking of `configure` Options, for details, and for how to override the defaults.

For each external software package that may be used, `configure.ac` should call `AC_ARG_WITH` to detect whether the `configure` user asked to use it. Whether each package is used or not by default, and which arguments are valid, is up to you.

Macro: AC_ARG_WITH(package, help-string, [action-if-given], [action-if-not-given]) ¶

  If the user gave `configure` the option `--with-package` or `--without-package`, run shell commands action-if-given. If neither option was given, run shell commands action-if-not-given. The name package indicates another software package that this program should work with. It should consist only of alphanumeric characters, dashes, plus signs, and dots.

  The option’s argument is available to the shell commands action-if-given in the shell variable `withval`, which is actually just the value of the shell variable named `with_package`, with any non-alphanumeric characters in package changed into ‘`_`’. You may use that variable instead, if you wish.

  Note that action-if-not-given is not expanded until the point that `AC_ARG_WITH` was expanded. If you need the value of `with_package` set to a default value by the time argument parsing is completed, use `m4_divert_text` to the `DEFAULTS` diversion (see m4_divert_text) (if done as an argument to `AC_ARG_WITH`, also provide non-diverted text to avoid a shell syntax error).

  The argument help-string is a description of the option that looks like this:

        --with-readline         support fancy command line editing

  help-string may be more than one line long, if more detail is needed. Just make sure the columns line up in ‘`configure --help`’. Avoid tabs in the help string. The easiest way to provide the proper leading whitespace is to format your help-string with the macro `AS_HELP_STRING` (see Making Your Help Strings Look Pretty).

  The following example shows how to use the `AC_ARG_WITH` macro in a common situation. You want to let the user decide whether to enable support for an external library (e.g., the readline library); if the user specified neither `--with-readline` nor `--without-readline`, you want to enable support for readline only if the library is available on the system.

      AC_ARG_WITH([readline],
        [AS_HELP_STRING([--with-readline],
          [support fancy command line editing @<:@default=check@:>@])],
        [],
        [: m4_divert_text([DEFAULTS], [with_readline=check])])

      LIBREADLINE=
      AS_IF([test "x$with_readline" != xno],
        [AC_CHECK_LIB([readline], [main],
          [AC_SUBST([LIBREADLINE], ["-lreadline -lncurses"])
           AC_DEFINE([HAVE_LIBREADLINE], [1],
                     [Define if you have libreadline])
          ],
          [if test "x$with_readline" != xcheck; then
             AC_MSG_FAILURE(
               [--with-readline was given, but test for readline failed])
           fi
          ], -lncurses)])

  The next example shows how to use `AC_ARG_WITH` to give the user the possibility to enable support for the readline library, in case it is still experimental and not well tested, and is therefore disabled by default.

      AC_ARG_WITH([readline],
        [AS_HELP_STRING([--with-readline],
          [enable experimental support for readline])],
        [],
        [with_readline=no])

      LIBREADLINE=
      AS_IF([test "x$with_readline" != xno],
        [AC_CHECK_LIB([readline], [main],
          [AC_SUBST([LIBREADLINE], ["-lreadline -lncurses"])
           AC_DEFINE([HAVE_LIBREADLINE], [1],
                     [Define if you have libreadline])
          ],
          [AC_MSG_FAILURE(
             [--with-readline was given, but test for readline failed])],
          [-lncurses])])

  The last example shows how to use `AC_ARG_WITH` to give the user the possibility to disable support for the readline library, given that it is an important feature and that it should be enabled by default.

      AC_ARG_WITH([readline],
        [AS_HELP_STRING([--without-readline],
          [disable support for readline])],
        [],
        [with_readline=yes])

      LIBREADLINE=
      AS_IF([test "x$with_readline" != xno],
        [AC_CHECK_LIB([readline], [main],
          [AC_SUBST([LIBREADLINE], ["-lreadline -lncurses"])
           AC_DEFINE([HAVE_LIBREADLINE], [1],
                     [Define if you have libreadline])
          ],
          [AC_MSG_FAILURE(
             [readline test failed (--without-readline to disable)])],
          [-lncurses])])

  These three examples can be easily adapted to the case where `AC_ARG_ENABLE` should be preferred to `AC_ARG_WITH` (see Choosing Package Options).

### 15.3 Choosing Package Options ¶

If a software package has optional compile-time features, the user can give `configure` command line options to specify whether to compile them. The options have one of these forms:

    --enable-feature[=arg]
    --disable-feature

These options allow users to choose which optional features to build and install. `--enable-feature` options should never make a feature behave differently or cause one feature to replace another. They should only cause parts of the program to be built rather than left out.

The user can give an argument by following the feature name with ‘`=`’ and the argument. Giving an argument of ‘`no`’ requests that the feature not be made available. A feature with an argument looks like `--enable-debug=stabs`. If no argument is given, it defaults to ‘`yes`’. `--disable-feature` is equivalent to `--enable-feature=no`.

Normally `configure` scripts complain about `--enable-package` options that they do not support. See Controlling Checking of `configure` Options, for details, and for how to override the defaults.

For each optional feature, `configure.ac` should call `AC_ARG_ENABLE` to detect whether the `configure` user asked to include it. Whether each feature is included or not by default, and which arguments are valid, is up to you.

Macro: AC_ARG_ENABLE(feature, help-string, [action-if-given], [action-if-not-given]) ¶

  If the user gave `configure` the option `--enable-feature` or `--disable-feature`, run shell commands action-if-given. If neither option was given, run shell commands action-if-not-given. The name feature indicates an optional user-level facility. It should consist only of alphanumeric characters, dashes, plus signs, and dots.

  The option’s argument is available to the shell commands action-if-given in the shell variable `enableval`, which is actually just the value of the shell variable named `enable_feature`, with any non-alphanumeric characters in feature changed into ‘`_`’. You may use that variable instead, if you wish. The help-string argument is like that of `AC_ARG_WITH` (see Working With External Software).

  Note that action-if-not-given is not expanded until the point that `AC_ARG_ENABLE` was expanded. If you need the value of `enable_feature` set to a default value by the time argument parsing is completed, use `m4_divert_text` to the `DEFAULTS` diversion (see m4_divert_text) (if done as an argument to `AC_ARG_ENABLE`, also provide non-diverted text to avoid a shell syntax error).

  You should format your help-string with the macro `AS_HELP_STRING` (see Making Your Help Strings Look Pretty).

  See the examples suggested with the definition of `AC_ARG_WITH` (see Working With External Software) to get an idea of possible applications of `AC_ARG_ENABLE`.

### 15.4 Making Your Help Strings Look Pretty ¶

Properly formatting the ‘`help strings`’ which are used in `AC_ARG_WITH` (see Working With External Software) and `AC_ARG_ENABLE` (see Choosing Package Options) can be challenging. Specifically, you want your own ‘`help strings`’ to line up in the appropriate columns of ‘`configure --help`’ just like the standard Autoconf ‘`help strings`’ do. This is the purpose of the `AS_HELP_STRING` macro.

Macro: AS_HELP_STRING(left-hand-side, right-hand-side [indent-column = ‘`26`’], [wrap-column = ‘`79`’]) ¶

  Expands into a help string that looks pretty when the user executes ‘`configure --help`’. It is typically used in `AC_ARG_WITH` (see Working With External Software) or `AC_ARG_ENABLE` (see Choosing Package Options). The following example makes this clearer.

      AC_ARG_WITH([foo],
        [AS_HELP_STRING([--with-foo],
           [use foo (default is no)])],
        [use_foo=$withval],
        [use_foo=no])

  Then the last few lines of ‘`configure --help`’ appear like this:

      --enable and --with options recognized:
        --with-foo              use foo (default is no)

  Macro expansion is performed on the first argument. However, the second argument of `AS_HELP_STRING` is treated as a whitespace separated list of text to be reformatted, and is not subject to macro expansion. Since it is not expanded, it should not be double quoted. See The Autoconf Language, for a more detailed explanation.

  The `AS_HELP_STRING` macro is particularly helpful when the left-hand-side and/or right-hand-side are composed of macro arguments, as shown in the following example. Be aware that left-hand-side may not expand to unbalanced quotes, although quadrigraphs can be used.

      AC_DEFUN([MY_ARG_WITH],
        [AC_ARG_WITH(m4_translit([[$1]], [_], [-]),
           [AS_HELP_STRING([--with-m4_translit([$1], [_], [-])],
                           [use $1 (default is $2)])],
           [use_[]$1=$withval],
           [use_[]$1=$2])])
      MY_ARG_WITH([a_b], [no])

  Here, the last few lines of ‘`configure --help`’ will include:

      --enable and --with options recognized:
        --with-a-b              use a_b (default is no)

  The parameters indent-column and wrap-column were introduced in Autoconf 2.62. Generally, they should not be specified; they exist for fine-tuning of the wrapping.

      AS_HELP_STRING([--option], [description of option])
      ⇒  --option                description of option
      AS_HELP_STRING([--option], [description of option], [15], [30])
      ⇒  --option     description of
      ⇒               option

### 15.5 Controlling Checking of `configure` Options ¶

The `configure` script checks its command-line options against a list of known options, like `--help` or `--config-cache`. An unknown option ordinarily indicates a mistake by the user and `configure` halts with an error. However, by default unknown `--with-package` and `--enable-feature` options elicit only a warning, to support configuring entire source trees.

Source trees often contain multiple packages with a top-level `configure` script that uses the `AC_CONFIG_SUBDIRS` macro (see Configuring Other Packages in Subdirectories). Because the packages generally support different `--with-package` and `--enable-feature` options, the GNU Coding Standards say they must accept unrecognized options without halting. Even a warning message is undesirable here, so `AC_CONFIG_SUBDIRS` automatically disables the warnings.

This default behavior may be modified in two ways. First, the installer can invoke `configure --disable-option-checking` to disable these warnings, or invoke `configure --enable-option-checking=fatal` options to turn them into fatal errors, respectively. Second, the maintainer can use `AC_DISABLE_OPTION_CHECKING`.

Macro: AC_DISABLE_OPTION_CHECKING ¶

  By default, disable warnings related to any unrecognized `--with-package` or `--enable-feature` options. This is implied by `AC_CONFIG_SUBDIRS`.

  The installer can override this behavior by passing `--enable-option-checking` (enable warnings) or `--enable-option-checking=fatal` (enable errors) to `configure`.

### 15.6 Configuring Site Details ¶

Some software packages require complex site-specific information. Some examples are host names to use for certain services, company names, and email addresses to contact. Since some configuration scripts generated by Metaconfig ask for such information interactively, people sometimes wonder how to get that information in Autoconf-generated configuration scripts, which aren’t interactive.

Such site configuration information should be put in a file that is edited only by users, not by programs. The location of the file can either be based on the `prefix` variable, or be a standard location such as the user’s home directory. It could even be specified by an environment variable. The programs should examine that file at runtime, rather than at compile time. Runtime configuration is more convenient for users and makes the configuration process simpler than getting the information while configuring. See Variables for Installation Directories in The GNU Coding Standards, for more information on where to put data files.

### 15.7 Transforming Program Names When Installing ¶

Autoconf supports changing the names of programs when installing them. In order to use these transformations, `configure.ac` must call the macro `AC_ARG_PROGRAM`.

Macro: AC_ARG_PROGRAM ¶

  Place in output variable `program_transform_name` a sequence of `sed` commands for changing the names of installed programs.

  If any of the options described below are given to `configure`, program names are transformed accordingly. Otherwise, if `AC_CANONICAL_TARGET` has been called and a `--target` value is given, the target type followed by a dash is used as a prefix. Otherwise, no program name transformation is done.

#### 15.7.1 Transformation Options ¶

You can specify name transformations by giving `configure` these command line options:

`--program-prefix=prefix`

  prepend prefix to the names;

`--program-suffix=suffix`

  append suffix to the names;

`--program-transform-name=expression`

  perform `sed` substitution expression on the names.

#### 15.7.2 Transformation Examples ¶

These transformations are useful with programs that can be part of a cross-compilation development environment. For example, a cross-assembler running on x86-64 configured with `--target=aarch64-linux-gnu` is normally installed as `aarch64-linux-gnu-as`, rather than `as`, which could be confused with a native x86-64 assembler.

You can force a program name to begin with `g`, if you don’t want GNU programs installed on your system to shadow other programs with the same name. For example, if you configure GNU `diff` with `--program-prefix=g`, then when you run ‘`make install`’ it is installed as `/usr/local/bin/gdiff`.

As a more sophisticated example, you could use

    --program-transform-name='s/^/g/; s/^gg/g/; s/^gless/less/'

to prepend ‘`g`’ to most of the program names in a source tree, excepting those like `gdb` that already have one and those like `less` and `lesskey` that aren’t GNU programs. (That is assuming that you have a source tree containing those programs that is set up to use this feature.)

One way to install multiple versions of some programs simultaneously is to append a version number to the name of one or both. For example, if you want to keep Autoconf version 1 around for awhile, you can configure Autoconf version 2 using `--program-suffix=2` to install the programs as `/usr/local/bin/autoconf2`, `/usr/local/bin/autoheader2`, etc. Nevertheless, pay attention that only the binaries are renamed, therefore you’d have problems with the library files which might overlap.

#### 15.7.3 Transformation Rules ¶

Here is how to use the variable `program_transform_name` in a `Makefile.in`:

    PROGRAMS = cp ls rm
    transform = @program_transform_name@
    install:
            for p in $(PROGRAMS); do \
              $(INSTALL_PROGRAM) $$p $(DESTDIR)$(bindir)/`echo $$p | \
                                                  sed '$(transform)'`; \
            done

    uninstall:
            for p in $(PROGRAMS); do \
              rm -f $(DESTDIR)$(bindir)/`echo $$p | sed '$(transform)'`; \
            done

It is guaranteed that `program_transform_name` is never empty, and that there are no useless separators. Therefore you may safely embed `program_transform_name` within a sed program using ‘`;`’:

    transform = @program_transform_name@
    transform_exe = s/$(EXEEXT)$$//;$(transform);s/$$/$(EXEEXT)/

Whether to do the transformations on documentation files (Texinfo or `man`) is a tricky question; there seems to be no perfect answer, due to the several reasons for name transforming. Documentation is not usually particular to a specific architecture, and Texinfo files do not conflict with system documentation. But they might conflict with earlier versions of the same files, and `man` pages sometimes do conflict with system documentation. As a compromise, it is probably best to do name transformations on `man` pages but not on Texinfo manuals.

### 15.8 Setting Site Defaults ¶

Autoconf-generated `configure` scripts allow your site to provide default values for some configuration values. You do this by creating site- and system-wide initialization files.

If the environment variable `CONFIG_SITE` is set, `configure` uses its value as a space-separated list of shell scripts to read; it is recommended that these be absolute file names. Otherwise, it reads the shell script `prefix/share/config.site` if it exists, then `prefix/etc/config.site` if it exists. Thus, settings in machine-specific files override those in machine-independent ones in case of conflict.

Site files can be arbitrary shell scripts, but only certain kinds of code are really appropriate to be in them. Because `configure` reads any cache file after it has read any site files, a site file can define a default cache file to be shared between all Autoconf-generated `configure` scripts run on that system (see Cache Files). If you set a default cache file in a site file, it is a good idea to also set the output variable `CC` in that site file, because the cache file is only valid for a particular compiler, but many systems have several available.

You can examine or override the value set by a command line option to `configure` in a site file; options set shell variables that have the same names as the options, with any dashes turned into underscores. The exceptions are that `--without-` and `--disable-` options are like giving the corresponding `--with-` or `--enable-` option and the value ‘`no`’. Thus, `--cache-file=localcache` sets the variable `cache_file` to the value ‘`localcache`’; `--enable-warnings=no` or `--disable-warnings` sets the variable `enable_warnings` to the value ‘`no`’; `--prefix=/usr` sets the variable `prefix` to the value ‘`/usr`’; etc.

Site files are also good places to set default values for other output variables, such as `CFLAGS`, if you need to give them non-default values: anything you would normally do, repetitively, on the command line. If you use non-default values for prefix or exec_prefix (wherever you locate the site file), you can set them in the site file if you specify it with the `CONFIG_SITE` environment variable.

You can set some cache values in the site file itself. Doing this is useful if you are cross-compiling, where it is impossible to check features that require running a test program. You could “prime the cache” by setting those values correctly for that system in `prefix/etc/config.site`. To find out the names of the cache variables you need to set, see the documentation of the respective Autoconf macro. If the variables or their semantics are undocumented, you may need to look for shell variables with ‘`_cv_`’ in their names in the affected `configure` scripts, or in the Autoconf M4 source code for those macros; but in that case, their name or semantics may change in a future Autoconf version.

The cache file is careful to not override any variables set in the site files. Similarly, you should not override command-line options in the site files. Your code should check that variables such as `prefix` and `cache_file` have their default values (as set near the top of `configure`) before changing them.

Here is a sample file `/usr/share/local/gnu/share/config.site`. The command ‘`configure --prefix=/usr/share/local/gnu`’ would read this file (if `CONFIG_SITE` is not set to a different file).

    # /usr/share/local/gnu/share/config.site for configure
    #
    # Change some defaults.
    test "$prefix" = NONE && prefix=/usr/share/local/gnu
    test "$exec_prefix" = NONE && exec_prefix=/usr/local/gnu
    test "$sharedstatedir" = '${prefix}/com' && sharedstatedir=/var
    test "$localstatedir" = '${prefix}/var' && localstatedir=/var
    test "$runstatedir" = '${localstatedir}/run' && runstatedir=/run

    # Give Autoconf 2.x generated configure scripts a shared default
    # cache file for feature test results, architecture-specific.
    if test "$cache_file" = /dev/null; then
      cache_file="$prefix/var/config.cache"
      # A cache file is only valid for one C compiler.
      CC=gcc
    fi

Another use of `config.site` is for priming the directory variables in a manner consistent with the Filesystem Hierarchy Standard (FHS). Once the following file is installed at `/usr/share/config.site`, a user can execute simply `./configure --prefix=/usr` to get all the directories chosen in the locations recommended by FHS.

    # /usr/share/config.site for FHS defaults when installing below /usr,
    # and the respective settings were not changed on the command line.
    if test "$prefix" = /usr; then
      test "$sysconfdir" = '${prefix}/etc' && sysconfdir=/etc
      test "$sharedstatedir" = '${prefix}/com' && sharedstatedir=/var
      test "$localstatedir" = '${prefix}/var' && localstatedir=/var
    fi

Likewise, on platforms where 64-bit libraries are built by default, then installed in `/usr/local/lib64` instead of `/usr/local/lib`, it is appropriate to install `/usr/local/share/config.site`:

    # /usr/local/share/config.site for platforms that prefer
    # the directory /usr/local/lib64 over /usr/local/lib.
    test "$libdir" = '${exec_prefix}/lib' && libdir='${exec_prefix}/lib64'

## 16 Running `configure` Scripts ¶

Below are instructions on how to configure a package that uses a `configure` script, suitable for inclusion as an `INSTALL` file in the package. A plain-text version of `INSTALL` which you may use comes with Autoconf.

### 16.1 Basic Installation ¶

The following shell commands:

    test -f configure || ./bootstrap
    ./configure
    make
    make install

should configure, build, and install this package. The first line, which bootstraps, is intended for developers; when building from distribution tarballs it does nothing and can be skipped. A package might name the bootstrapping script differently; if the name is `autogen.sh`, for example, the first line should say `./autogen.sh` instead of `./bootstrap`.

The following more-detailed instructions are generic; see the `README` file for instructions specific to this package. More recommendations for GNU packages can be found in Makefile Conventions in GNU Coding Standards.

Many packages have scripts meant for developers instead of ordinary builders, as they may use developer tools that are less commonly installed, or they may access the network, which has privacy implications. These scripts attempt to bootstrap by building the `configure` script and related files, possibly using developer tools or the network. Because the output of bootstrapping is system-independent, it is normally run by a package developer so that its output can be put into the distribution tarball and ordinary builders and users need not bootstrap. Some packages have commands like `./autopull.sh` and `./autogen.sh` that you can run instead of `./bootstrap`, for more fine-grained control over bootstrapping.

The `configure` script attempts to guess correct values for various system-dependent variables used during compilation. It uses those values to create a `Makefile` in each directory of the package. It may also create one or more `.h` files containing system-dependent definitions. Finally, it creates a script `config.status` that you can run in the future to recreate the current configuration, and a file `config.log` containing output useful for debugging `configure`.

It can also use an optional file (typically called `config.cache` and enabled with `--cache-file=config.cache` or simply `-C`) that saves the results of its tests to speed up reconfiguring. Caching is disabled by default to prevent problems with accidental use of stale cache files.

If you need to do unusual things to compile the package, please try to figure out how `configure` could check whether to do them, and mail diffs or instructions to the address given in the `README` so they can be considered for the next release. If you are using the cache, and at some point `config.cache` contains results you don’t want to keep, you may remove or edit it.

The `autoconf` program generates `configure` from the file `configure.ac`. Normally you should edit `configure.ac` instead of editing `configure` directly.

The simplest way to compile this package is:

1. `cd` to the directory containing the package’s source code.
2. If this is a developer checkout and file `configure` does not yet exist, run the bootstrapping script (typically `./bootstrap` or `./autogen.sh`) to bootstrap and create the file. You may need special developer tools and network access to bootstrap, and the network access may have privacy implications.
3. Type ‘`./configure`’ to configure the package for your system. This might take a while. While running, `configure` prints messages telling which features it is checking for.
4. Type ‘`make`’ to compile the package.
5. Optionally, type ‘`make check`’ to run any self-tests that come with the package, generally using the just-built uninstalled binaries.
6. Type ‘`make install`’ to install the programs and any data files and documentation. When installing into a prefix owned by root, it is recommended that the package be configured and built as a regular user, and only the ‘`make install`’ phase executed with root privileges.
7. Optionally, type ‘`make installcheck`’ to repeat any self-tests, but this time using the binaries in their final installed location. This target does not install anything. Running this target as a regular user, particularly if the prior ‘`make install`’ required root privileges, verifies that the installation completed correctly.
8. You can remove the program binaries and object files from the source code directory by typing ‘`make clean`’. To also remove the files that `configure` created (so you can compile the package for a different kind of computer), type ‘`make distclean`’. There is also a ‘`make maintainer-clean`’ target, but that is intended mainly for the package’s developers. If you use it, you may have to bootstrap again.
9. If the package follows the GNU Coding Standards, you can type ‘`make uninstall`’ to remove the installed files.

### 16.2 Installation Prerequisites ¶

Installation requires a POSIX-like environment with a shell and at least the following standard utilities:

> `awk cat cp diff echo expr false ls mkdir mv printf pwd rm rmdir sed sort test tr`

This package’s installation may need other standard utilities such as `grep`, `make`, `sleep` and `touch`, along with compilers like `gcc`.

### 16.3 Compilers and Options ¶

Some systems require unusual options for compilation or linking that the `configure` script does not know about. Run ‘`./configure --help`’ for details on some of the pertinent environment variables.

You can give `configure` initial values for configuration parameters by setting variables in the command line or in the environment. Here is an example:

    ./configure CC=gcc CFLAGS=-g LIBS=-lposix

Defining Variables and Preset Output Variables for more details.

### 16.4 Compiling For Multiple Architectures ¶

You can compile the package for more than one kind of computer at the same time, by placing the object files for each system in their own directory. To do this, you can use GNU `make`. `cd` to the directory where you want the object files and executables to go and run the `configure` script. `configure` automatically checks for the source code in the directory that `configure` is in and in `..`. This is known as a VPATH build.

With a non-GNU `make`, it is safer to compile the package for one system at a time in the source code directory. After you have installed the package for one system, use ‘`make distclean`’ before reconfiguring for another system.

Some platforms, notably macOS, support “fat” or “universal” binaries, where a single binary can execute on different architectures. On these platforms you can configure and compile just once, with options specific to that platform.

### 16.5 Installation Names ¶

By default, ‘`make install`’ installs the package’s commands under `/usr/local/bin`, include files under `/usr/local/include`, etc. You can specify an installation prefix other than `/usr/local` by giving `configure` the option `--prefix=prefix`, where prefix must be an absolute file name.

You can specify separate installation prefixes for architecture-specific files and architecture-independent files. If you pass the option `--exec-prefix=prefix` to `configure`, the package uses prefix as the prefix for installing programs and libraries. Documentation and other data files still use the regular prefix.

In addition, if you use an unusual directory layout you can give options like `--bindir=dir` to specify different values for particular kinds of files. Run ‘`configure --help`’ for a list of the directories you can set and what kinds of files go in them. In general, the default for these options is expressed in terms of ‘`${prefix}`’, so that specifying just `--prefix` will affect all of the other directory specifications that were not explicitly provided.

The most portable way to affect installation locations is to pass the correct locations to `configure`; however, many packages provide one or both of the following shortcuts of passing variable assignments to the ‘`make install`’ command line to change installation locations without having to reconfigure or recompile.

The first method involves providing an override variable for each affected directory. For example, ‘`make install prefix=/alternate/directory`’ will choose an alternate location for all directory configuration variables that were expressed in terms of ‘`${prefix}`’. Any directories that were specified during `configure`, but not in terms of ‘`${prefix}`’, must each be overridden at install time for the entire installation to be relocated. The approach of makefile variable overrides for each directory variable is required by the GNU Coding Standards, and ideally causes no recompilation. However, some platforms have known limitations with the semantics of shared libraries that end up requiring recompilation when using this method, particularly noticeable in packages that use GNU Libtool.

The second method involves providing the ‘`DESTDIR`’ variable. For example, ‘`make install DESTDIR=/alternate/directory`’ will prepend ‘`/alternate/directory`’ before all installation names. The approach of ‘`DESTDIR`’ overrides is not required by the GNU Coding Standards, and does not work on platforms that have drive letters. On the other hand, it does better at avoiding recompilation issues, and works well even when some directory options were not specified in terms of ‘`${prefix}`’ at `configure` time.

### 16.6 Optional Features ¶

If the package supports it, you can cause programs to be installed with an extra prefix or suffix on their names by giving `configure` the option `--program-prefix=PREFIX` or `--program-suffix=SUFFIX`.

Some packages pay attention to `--enable-feature` and `--disable-feature` options to `configure`, where feature indicates an optional part of the package. They may also pay attention to `--with-package` and `--without-package` options, where package is something like ‘`gnu-ld`’. ‘`./configure --help`’ should mention the `--enable-...` and `--with-...` options that the package recognizes.

Some packages offer the ability to configure how verbose the execution of `make` will be. For these packages, running ‘`./configure --enable-silent-rules`’ sets the default to minimal output, which can be overridden with `make V=1`; while running ‘`./configure --disable-silent-rules`’ sets the default to verbose, which can be overridden with `make V=0`.

### 16.7 Specifying a System Type ¶

By default `configure` builds for the current system. To create binaries that can run on a different system type, specify a `--host=type` option along with compiler variables that specify how to generate object code for type. For example, to create binaries intended to run on a 64-bit ARM processor:

    ./configure --host=aarch64-linux-gnu \
       CC=aarch64-linux-gnu-gcc \
       CXX=aarch64-linux-gnu-g++

If done on a machine that can execute these binaries (e.g., via `qemu-aarch64`, `$QEMU_LD_PREFIX`, and Linux’s `binfmt_misc` capability), the build behaves like a native build. Otherwise it is a cross-build: `configure` will make cross-compilation guesses instead of running test programs, and `make check` will not work.

A system type can either be a short name like ‘`mingw64`’, or a canonical name like ‘`x86_64-pc-linux-gnu`’. Canonical names have the form cpu-company-system where system is either os or kernel-os. To canonicalize and validate a system type, you can run the command `config.sub`, which is often squirreled away in a subdirectory like `build-aux`. For example:

    $ build-aux/config.sub arm64-linux
    aarch64-unknown-linux-gnu
    $ build-aux/config.sub riscv-lnx
    Invalid configuration 'riscv-lnx': OS 'lnx' not recognized

You can look at the `config.sub` file to see which types are recognized. If the file is absent, this package does not need the system type.

If `configure` fails with the diagnostic “cannot guess build type”. `config.sub` did not recognize your system’s type. In this case, first fetch the newest versions of these files from the GNU config package. If that fixes things, please report it to the maintainers of the package containing `configure`. Otherwise, you can try the configure option `--build=type` where type comes close to your system type; also, please report the problem to config-patches@gnu.org.

For more details about configuring system types, see Manual Configuration.

### 16.8 Sharing Defaults ¶

If you want to set default values for `configure` scripts to share, you can create a site shell script called `config.site` that gives default values for variables like `CC`, `cache_file`, and `prefix`. `configure` looks for `prefix/share/config.site` if it exists, then `prefix/etc/config.site` if it exists. Or, you can set the `CONFIG_SITE` environment variable to the location of the site script. A warning: not all `configure` scripts look for a site script.

### 16.9 Defining Variables ¶

Variables not defined in a site shell script can be set in the environment passed to `configure`. However, some packages may run configure again during the build, and the customized values of these variables may be lost. In order to avoid this problem, you should set them in the `configure` command line, using ‘`VAR=value`’. For example:

    ./configure CC=/usr/local2/bin/gcc

causes the specified `gcc` to be used as the C compiler (unless it is overridden in the site shell script).

Unfortunately, this technique does not work for `CONFIG_SHELL` due to an Autoconf limitation. Until the limitation is lifted, you can use this workaround:

    CONFIG_SHELL=/bin/bash ./configure CONFIG_SHELL=/bin/bash

### 16.10 `configure` Invocation ¶

`configure` recognizes the following options to control how it operates.

`--help`
`-h`

  Print a summary of all of the options to `configure`, and exit.

`--help=short`
`--help=recursive`

  Print a summary of the options unique to this package’s `configure`, and exit. The `short` variant lists options used only in the top level, while the `recursive` variant lists options also present in any nested packages.

`--version`
`-V`

  Print the version of Autoconf used to generate the `configure` script, and exit.

`--cache-file=file` ¶

  Enable the cache: use and save the results of the tests in file, traditionally `config.cache`. file defaults to `/dev/null` to disable caching.

`--config-cache`
`-C`

  Alias for `--cache-file=config.cache`.

`--srcdir=dir`

  Look for the package’s source code in directory dir. Usually `configure` can determine that directory automatically.

`--prefix=dir`

  Use dir as the installation prefix. Installation Names for more details, including other options available for fine-tuning the installation locations.

`--host=type`

  Build binaries for system type. See Specifying a System Type.

`--enable-feature`
`--disable-feature`

  Enable or disable the optional feature. See Optional Features.

`--with-package`
`--without-package`

  Use or omit package when building. See Optional Features.

`--quiet`
`--silent`
`-q`

  Do not print messages saying which checks are being made. To suppress all normal output, redirect it to `/dev/null` (any error messages will still be shown).

`--no-create`
`-n`

  Run the configure checks, but stop before creating any output files.

`configure` also recognizes several environment variables, and accepts some other, less widely useful, options. Run ‘`configure --help`’ for more details.

## 17 config.status Invocation ¶

The `configure` script creates a file named `config.status`, which actually configures, instantiates, the template files. It also records the configuration options that were specified when the package was last configured in case reconfiguring is needed.

Synopsis:

    ./config.status [option]... [tag]...

It configures each tag; if none are specified, all the templates are instantiated. A tag refers to a file or other tag associated with a configuration action, as specified by an `AC_CONFIG_ITEMS` macro (see Performing Configuration Actions). The files must be specified without their dependencies, as in

    ./config.status foobar

not

    ./config.status foobar:foo.in:bar.in

The supported options are:

`--help`
`-h`

  Print a summary of the command line options, the list of the template files, and exit.

`--version`
`-V`

  Print the version number of Autoconf and the configuration settings, and exit.

`--config`

  Print the configuration settings in reusable way, quoted for the shell, and exit. For example, for a debugging build that otherwise reuses the configuration from a different build directory build-dir of a package in src-dir, you could use the following:

      args=`build-dir/config.status --config`
      eval src-dir/configure "$args" CFLAGS=-g --srcdir=src-dir

  Note that it may be necessary to override a `--srcdir` setting that was saved in the configuration, if the arguments are used in a different build directory.

`--silent`
`--quiet`
`-q`

  Do not print progress messages.

`--debug`
`-d`

  Don’t remove the temporary files.

`--file=file[:template]`

  Require that file be instantiated as if ‘`AC_CONFIG_FILES(file:template)`’ was used. Both file and template may be ‘`-`’ in which case the standard output and/or standard input, respectively, is used. If a template file name is relative, it is first looked for in the build tree, and then in the source tree. See Performing Configuration Actions, for more details.

  This option and the following ones provide one way for separately distributed packages to share the values computed by `configure`. Doing so can be useful if some of the packages need a superset of the features that one of them, perhaps a common library, does. These options allow a `config.status` file to create files other than the ones that its `configure.ac` specifies, so it can be used for a different package, or for extracting a subset of values. For example,

      echo '@CC@' | ./config.status --file=-

  provides the value of `@CC@` on standard output.

`--header=file[:template]`

  Same as `--file` above, but with ‘`AC_CONFIG_HEADERS`’.

`--recheck`

  Ask `config.status` to update itself and exit (no instantiation). This option is useful if you change `configure`, so that the results of some tests might be different from the previous run. The `--recheck` option reruns `configure` with the same arguments you used before, plus the `--no-create` option, which prevents `configure` from running `config.status` and creating `Makefile` and other files, and the `--no-recursion` option, which prevents `configure` from running other `configure` scripts in subdirectories. (This is so other Make rules can run `config.status` when it changes; see Automatic Remaking, for an example).

`config.status` checks several optional environment variables that can alter its behavior:

Variable: CONFIG_SHELL ¶

  The shell with which to run `configure`. It must be Bourne-compatible, and the absolute name of the shell should be passed. The default is a shell that supports `LINENO` if available, and `/bin/sh` otherwise.

Variable: CONFIG_STATUS ¶

  The file name to use for the shell script that records the configuration. The default is `./config.status`. This variable is useful when one package uses parts of another and the `configure` scripts shouldn’t be merged because they are maintained separately.

You can use `./config.status` in your makefiles. For example, in the dependencies given above (see Automatic Remaking), `config.status` is run twice when `configure.ac` has changed. If that bothers you, you can make each run only regenerate the files for that rule:

    config.h: stamp-h
    stamp-h: config.h.in config.status
            ./config.status config.h
            echo > stamp-h

    Makefile: Makefile.in config.status
            ./config.status Makefile

The calling convention of `config.status` has changed; see Obsolete `config.status` Invocation, for details.

## 18 Obsolete Constructs ¶

Autoconf changes, and throughout the years some constructs have been obsoleted. Most of the changes involve the macros, but in some cases the tools themselves, or even some concepts, are now considered obsolete.

You may completely skip this chapter if you are new to Autoconf. Its intention is mainly to help maintainers updating their packages by understanding how to move to more modern constructs.

### 18.1 Obsolete `config.status` Invocation ¶

`config.status` now supports arguments to specify the files to instantiate; see config.status Invocation, for more details. Before, environment variables had to be used.

Variable: CONFIG_COMMANDS ¶

  The tags of the commands to execute. The default is the arguments given to `AC_OUTPUT` and `AC_CONFIG_COMMANDS` in `configure.ac`.

Variable: CONFIG_FILES ¶

  The files in which to perform ‘`@variable@`’ substitutions. The default is the arguments given to `AC_OUTPUT` and `AC_CONFIG_FILES` in `configure.ac`.

Variable: CONFIG_HEADERS ¶

  The files in which to substitute C `#define` statements. The default is the arguments given to `AC_CONFIG_HEADERS`; if that macro was not called, `config.status` ignores this variable.

Variable: CONFIG_LINKS ¶

  The symbolic links to establish. The default is the arguments given to `AC_CONFIG_LINKS`; if that macro was not called, `config.status` ignores this variable.

In config.status Invocation, using this old interface, the example would be:

    config.h: stamp-h
    stamp-h: config.h.in config.status
            CONFIG_COMMANDS= CONFIG_LINKS= CONFIG_FILES= \
              CONFIG_HEADERS=config.h ./config.status
            echo > stamp-h

    Makefile: Makefile.in config.status
            CONFIG_COMMANDS= CONFIG_LINKS= CONFIG_HEADERS= \
              CONFIG_FILES=Makefile ./config.status

(If `configure.ac` does not call `AC_CONFIG_HEADERS`, there is no need to set `CONFIG_HEADERS` in the `make` rules. Equally for `CONFIG_COMMANDS`, etc.)

### 18.2 `acconfig.h` ¶

In order to produce `config.h.in`, `autoheader` needs to build or to find templates for each symbol. Modern releases of Autoconf use `AH_VERBATIM` and `AH_TEMPLATE` (see Autoheader Macros), but in older releases a file, `acconfig.h`, contained the list of needed templates. `autoheader` copied comments and `#define` and `#undef` statements from `acconfig.h` in the current directory, if present. This file used to be mandatory if you `AC_DEFINE` any additional symbols.

Modern releases of Autoconf also provide `AH_TOP` and `AH_BOTTOM` if you need to prepend/append some information to `config.h.in`. Ancient versions of Autoconf had a similar feature: if `./acconfig.h` contains the string ‘`@TOP@`’, `autoheader` copies the lines before the line containing ‘`@TOP@`’ into the top of the file that it generates. Similarly, if `./acconfig.h` contains the string ‘`@BOTTOM@`’, `autoheader` copies the lines after that line to the end of the file it generates. Either or both of those strings may be omitted. An even older alternate way to produce the same effect in ancient versions of Autoconf is to create the files `file.top` (typically `config.h.top`) and/or `file.bot` in the current directory. If they exist, `autoheader` copies them to the beginning and end, respectively, of its output.

In former versions of Autoconf, the files used in preparing a software package for distribution were:

    configure.ac --.   .------> autoconf* -----> configure
                   +---+
    [aclocal.m4] --+   `---.
    [acsite.m4] ---'       |
                           +--> [autoheader*] -> [config.h.in]
    [acconfig.h] ----.     |
                     +-----'
    [config.h.top] --+
    [config.h.bot] --'

Using only the `AH_` macros, `configure.ac` should be self-contained, and should not depend upon `acconfig.h` etc.

### 18.3 Using `autoupdate` to Modernize `configure.ac` ¶

The `autoupdate` program updates a `configure.ac` file that calls Autoconf macros by their old names to use the current macro names. In version 2 of Autoconf, most of the macros were renamed to use a more uniform and descriptive naming scheme. See Macro Names, for a description of the new scheme. Although the old names still work (see Obsolete Macros, for a list of the old macros and the corresponding new names), you can make your `configure.ac` files more readable and make it easier to use the current Autoconf documentation if you update them to use the new macro names.

If given no arguments, `autoupdate` updates `configure.ac`, backing up the original version with the suffix `~` (or the value of the environment variable `SIMPLE_BACKUP_SUFFIX`, if that is set). If you give `autoupdate` an argument, it reads that file instead of `configure.ac` and writes the updated file to the standard output.

`autoupdate` accepts the following options:

`--help`
`-h`

  Print a summary of the command line options and exit.

`--version`
`-V`

  Print the version number of Autoconf and exit.

`--verbose`
`-v`

  Report processing steps.

`--debug`
`-d`

  Don’t remove the temporary files.

`--force`
`-f`

  Force the update even if the file has not changed. Disregard the cache.

`--include=dir`
`-I dir`

  Also look for input files in dir. Multiple invocations accumulate. Directories are browsed from last to first.

`--prepend-include=dir`
`-B dir`

  Prepend directory dir to the search path. This is used to include the language-specific files before any third-party macros.

### 18.4 Obsolete Macros ¶

Several macros are obsoleted in Autoconf, for various reasons (typically they failed to quote properly, couldn’t be extended for more recent issues, etc.). They are still supported, but deprecated: their use should be avoided.

During the jump from Autoconf version 1 to version 2, most of the macros were renamed to use a more uniform and descriptive naming scheme, but their signature did not change. See Macro Names, for a description of the new naming scheme. Below, if there is just the mapping from old names to new names for these macros, the reader is invited to refer to the definition of the new macro for the signature and the description.

Macro: AC_AIX ¶

  This macro is a platform-specific subset of `AC_USE_SYSTEM_EXTENSIONS` (see AC_USE_SYSTEM_EXTENSIONS).

Macro: AC_ALLOCA ¶

  Replaced by `AC_FUNC_ALLOCA` (see AC_FUNC_ALLOCA).

Macro: AC_ARG_ARRAY ¶

  Removed because of limited usefulness.

Macro: AC_C_CROSS ¶

  This macro is obsolete; it does nothing.

Macro: AC_C_LONG_DOUBLE ¶

  If the C compiler supports a working `long double` type with more range or precision than the `double` type, define `HAVE_LONG_DOUBLE`.

  You should use `AC_TYPE_LONG_DOUBLE` or `AC_TYPE_LONG_DOUBLE_WIDER` instead. See Particular Type Checks.

Macro: AC_CANONICAL_SYSTEM ¶

  Determine the system type and set output variables to the names of the canonical system types. See Getting the Canonical System Type, for details about the variables this macro sets.

  The user is encouraged to use either `AC_CANONICAL_BUILD`, or `AC_CANONICAL_HOST`, or `AC_CANONICAL_TARGET`, depending on the needs. Using `AC_CANONICAL_TARGET` is enough to run the two other macros (see Getting the Canonical System Type).

Macro: AC_CHAR_UNSIGNED ¶

  Replaced by `AC_C_CHAR_UNSIGNED` (see AC_C_CHAR_UNSIGNED).

Macro: AC_CHECK_TYPE(type, default) ¶

  Autoconf, up to 2.13, used to provide this version of `AC_CHECK_TYPE`, deprecated because of its flaws. First, although it is a member of the `CHECK` clan, it does more than just checking. Secondly, missing types are defined using `#define`, not `typedef`, and this can lead to problems in the case of pointer types.

  This use of `AC_CHECK_TYPE` is obsolete and discouraged; see Generic Type Checks, for the description of the current macro.

  If the type type is not defined, define it to be the C (or C++) builtin type default, e.g., ‘`short int`’ or ‘`unsigned int`’.

  This macro is equivalent to:

      AC_CHECK_TYPE([type], [],
        [AC_DEFINE_UNQUOTED([type], [default],
           [Define to 'default'
            if <sys/types.h> does not define.])])

  In order to keep backward compatibility, the two versions of `AC_CHECK_TYPE` are implemented, selected using these heuristics:

  1. If there are three or four arguments, the modern version is used.
  2. If the second argument appears to be a C or C++ type, then the obsolete version is used. This happens if the argument is a C or C++ builtin type or a C identifier ending in ‘`_t`’, optionally followed by one of ‘`[(*` ’ and then by a string of zero or more characters taken from the set ‘`[]()* _a-zA-Z0-9`’.
  3. If the second argument is spelled with the alphabet of valid C and C++ types, the user is warned and the modern version is used.
  4. Otherwise, the modern version is used.

  You are encouraged either to use a valid builtin type, or to use the equivalent modern code (see above), or better yet, to use `AC_CHECK_TYPES` together with

      #ifndef HAVE_LOFF_T
      typedef loff_t off_t;
      #endif

Macro: AC_CHECKING(feature-description) ¶

  Same as

      AC_MSG_NOTICE([checking feature-description...]

  See AC_MSG_NOTICE.

Macro: AC_COMPILE_CHECK(echo-text, includes, function-body, action-if-true, [action-if-false]) ¶

  This is an obsolete version of `AC_TRY_COMPILE` itself replaced by `AC_COMPILE_IFELSE` (see Running the Compiler), with the addition that it prints ‘`checking for echo-text`’ to the standard output first, if echo-text is non-empty. Use `AC_MSG_CHECKING` and `AC_MSG_RESULT` instead to print messages (see Printing Messages).

Macro: AC_CONST ¶

  Replaced by `AC_C_CONST` (see AC_C_CONST).

Macro: AC_CROSS_CHECK ¶

  Same as `AC_C_CROSS`, which is obsolete too, and does nothing `:-)`.

Macro: AC_CYGWIN ¶

  Check for the Cygwin environment in which case the shell variable `CYGWIN` is set to ‘`yes`’. Don’t use this macro, the dignified means to check the nature of the host is using `AC_CANONICAL_HOST` (see Getting the Canonical System Type). As a matter of fact this macro is defined as:

      AC_REQUIRE([AC_CANONICAL_HOST])[]dnl
      case $host_os in
        *cygwin* ) CYGWIN=yes;;
               * ) CYGWIN=no;;
      esac

  Beware that the variable `CYGWIN` has a special meaning when running Cygwin, and should not be changed. That’s yet another reason not to use this macro.

Macro: AC_DECL_SYS_SIGLIST ¶

  Same as:

      AC_CHECK_DECLS([sys_siglist], [], [],
      [#include <signal.h>
      /* NetBSD declares sys_siglist in unistd.h.  */
      #ifdef HAVE_UNISTD_H
      # include <unistd.h>
      #endif
      ])

  See AC_CHECK_DECLS.

Macro: AC_DECL_YYTEXT ¶

  Does nothing, now integrated in `AC_PROG_LEX` (see AC_PROG_LEX).

Macro: AC_DIAGNOSE(category, message) ¶

  Replaced by `m4_warn` (see m4_warn).

Macro: AC_DIR_HEADER ¶

  Like calling `AC_FUNC_CLOSEDIR_VOID` (see AC_FUNC_CLOSEDIR_VOID) and `AC_HEADER_DIRENT` (see AC_HEADER_DIRENT), but defines a different set of C preprocessor macros to indicate which header file is found:

  Header | Old Symbol | New Symbol
  `dirent.h` | `DIRENT` | `HAVE_DIRENT_H`
  `sys/ndir.h` | `SYSNDIR` | `HAVE_SYS_NDIR_H`
  `sys/dir.h` | `SYSDIR` | `HAVE_SYS_DIR_H`
  `ndir.h` | `NDIR` | `HAVE_NDIR_H`

Macro: AC_DYNIX_SEQ ¶

  If on DYNIX/ptx, add `-lseq` to output variable `LIBS`. This macro used to be defined as

      AC_CHECK_LIB([seq], [getmntent], [LIBS="-lseq $LIBS"])

  now it is just `AC_FUNC_GETMNTENT` (see AC_FUNC_GETMNTENT).

Macro: AC_EXEEXT ¶

  Defined the output variable `EXEEXT` based on the output of the compiler, which is now done automatically. Typically set to empty string if POSIX and ‘`.exe`’ if a DOS variant.

Macro: AC_EMXOS2 ¶

  Similar to `AC_CYGWIN` but checks for the EMX environment on OS/2 and sets `EMXOS2`. Don’t use this macro, the dignified means to check the nature of the host is using `AC_CANONICAL_HOST` (see Getting the Canonical System Type).

Macro: AC_ENABLE(feature, action-if-given, [action-if-not-given]) ¶

  This is an obsolete version of `AC_ARG_ENABLE` that does not support providing a help string (see AC_ARG_ENABLE).

Macro: AC_ERROR ¶

  Replaced by `AC_MSG_ERROR` (see AC_MSG_ERROR).

Macro: AC_FATAL(message) ¶

  Replaced by `m4_fatal` (see m4_fatal).

Macro: AC_FIND_X ¶

  Replaced by `AC_PATH_X` (see AC_PATH_X).

Macro: AC_FIND_XTRA ¶

  Replaced by `AC_PATH_XTRA` (see AC_PATH_XTRA).

Macro: AC_FOREACH ¶

  Replaced by `m4_foreach_w` (see m4_foreach_w).

Macro: AC_FUNC_CHECK ¶

  Replaced by `AC_CHECK_FUNC` (see AC_CHECK_FUNC).

Macro: AC_FUNC_SETVBUF_REVERSED ¶

  Do nothing. Formerly, this macro checked whether `setvbuf` takes the buffering type as its second argument and the buffer pointer as the third, instead of the other way around, and defined `SETVBUF_REVERSED`. However, the last systems to have the problem were those based on SVR2, which became obsolete in 1987, and the macro is no longer needed.

Macro: AC_FUNC_WAIT3 ¶

  If `wait3` is found and fills in the contents of its third argument (a ‘`struct rusage *`’), which HP-UX does not do, define `HAVE_WAIT3`.

  These days portable programs should use `waitpid`, not `wait3`, as `wait3` has been removed from POSIX.

Macro: AC_GCC_TRADITIONAL ¶

  Replaced by `AC_PROG_GCC_TRADITIONAL` (see AC_PROG_GCC_TRADITIONAL), which is itself obsolete.

Macro: AC_GETGROUPS_T ¶

  Replaced by `AC_TYPE_GETGROUPS` (see AC_TYPE_GETGROUPS).

Macro: AC_GETLOADAVG ¶

  Replaced by `AC_FUNC_GETLOADAVG` (see AC_FUNC_GETLOADAVG).

Macro: AC_GNU_SOURCE ¶

  This macro is a platform-specific subset of `AC_USE_SYSTEM_EXTENSIONS` (see AC_USE_SYSTEM_EXTENSIONS).

Macro: AC_HAVE_FUNCS ¶

  Replaced by `AC_CHECK_FUNCS` (see AC_CHECK_FUNCS).

Macro: AC_HAVE_HEADERS ¶

  Replaced by `AC_CHECK_HEADERS` (see AC_CHECK_HEADERS).

Macro: AC_HAVE_LIBRARY(library, [action-if-found], [action-if-not-found], [other-libraries]) ¶

  This macro is equivalent to calling `AC_CHECK_LIB` with a function argument of `main`. In addition, library can be written as any of ‘`foo`’, `-lfoo`, or ‘`libfoo.a`’. In all of those cases, the compiler is passed `-lfoo`. However, library cannot be a shell variable; it must be a literal name. See AC_CHECK_LIB.

Macro: AC_HAVE_POUNDBANG ¶

  Replaced by `AC_SYS_INTERPRETER` (see AC_SYS_INTERPRETER).

Macro: AC_HEADER_CHECK ¶

  Replaced by `AC_CHECK_HEADER` (see AC_CHECK_HEADER).

Macro: AC_HEADER_EGREP ¶

  Replaced by `AC_EGREP_HEADER` (see AC_EGREP_HEADER).

Macro: AC_HEADER_TIME ¶

  This macro used to check whether it was possible to include `time.h` and `sys/time.h` in the same source file, defining `TIME_WITH_SYS_TIME` if so.

  Nowadays, it is equivalent to ‘`AC_CHECK_HEADERS([sys/time.h])`’, although it does still define `TIME_WITH_SYS_TIME` for compatibility’s sake. `time.h` is universally present, and the systems on which `sys/time.h` conflicted with `time.h` are obsolete.

Macro: AC_HELP_STRING ¶

  Replaced by `AS_HELP_STRING` (see AS_HELP_STRING).

Macro: AC_INIT(unique-file-in-source-dir) ¶

  Formerly `AC_INIT` used to have a single argument, and was equivalent to:

      AC_INIT
      AC_CONFIG_SRCDIR(unique-file-in-source-dir)

  See AC_INIT and AC_CONFIG_SRCDIR.

Macro: AC_INLINE ¶

  Replaced by `AC_C_INLINE` (see AC_C_INLINE).

Macro: AC_INT_16_BITS ¶

  If the C type `int` is 16 bits wide, define `INT_16_BITS`. Use ‘`AC_CHECK_SIZEOF(int)`’ instead (see AC_CHECK_SIZEOF).

Macro: AC_IRIX_SUN ¶

  If on IRIX (Silicon Graphics Unix), add `-lsun` to output `LIBS`. If you were using it to get `getmntent`, use `AC_FUNC_GETMNTENT` instead. If you used it for the NIS versions of the password and group functions, use ‘`AC_CHECK_LIB(sun, getpwnam)`’. Up to Autoconf 2.13, it used to be

      AC_CHECK_LIB([sun], [getmntent], [LIBS="-lsun $LIBS"])

  now it is defined as

      AC_FUNC_GETMNTENT
      AC_CHECK_LIB([sun], [getpwnam])

  See AC_FUNC_GETMNTENT and AC_CHECK_LIB.

Macro: AC_ISC_POSIX ¶

  This macro adds `-lcposix` to output variable `LIBS` if necessary for POSIX facilities. Sun dropped support for the obsolete INTERACTIVE Systems Corporation Unix on 2006-07-23. New programs need not use this macro. It is implemented as `AC_SEARCH_LIBS([strerror], [cposix])` (see AC_SEARCH_LIBS).

Macro: AC_LANG_C ¶

  Same as ‘`AC_LANG([C])`’ (see AC_LANG).

Macro: AC_LANG_CPLUSPLUS ¶

  Same as ‘`AC_LANG([C++])`’ (see AC_LANG).

Macro: AC_LANG_FORTRAN77 ¶

  Same as ‘`AC_LANG([Fortran 77])`’ (see AC_LANG).

Macro: AC_LANG_RESTORE ¶

  Select the language that is saved on the top of the stack, as set by `AC_LANG_SAVE`, remove it from the stack, and call `AC_LANG(language)`. See Language Choice, for the preferred way to change languages.

Macro: AC_LANG_SAVE ¶

  Remember the current language (as set by `AC_LANG`) on a stack. The current language does not change. `AC_LANG_PUSH` is preferred (see AC_LANG_PUSH).

Macro: AC_LINK_FILES(source…, dest…) ¶

  This is an obsolete version of `AC_CONFIG_LINKS` (see AC_CONFIG_LINKS. An updated version of:

      AC_LINK_FILES(config/$machine.h config/$obj_format.h,
                    host.h            object.h)

  is:

      AC_CONFIG_LINKS([host.h:config/$machine.h
                      object.h:config/$obj_format.h])

Macro: AC_LN_S ¶

  Replaced by `AC_PROG_LN_S` (see AC_PROG_LN_S).

Macro: AC_LONG_64_BITS ¶

  Define `LONG_64_BITS` if the C type `long int` is 64 bits wide. Use the generic macro ‘`AC_CHECK_SIZEOF([long int])`’ instead (see AC_CHECK_SIZEOF).

Macro: AC_LONG_DOUBLE ¶

  If the C compiler supports a working `long double` type with more range or precision than the `double` type, define `HAVE_LONG_DOUBLE`.

  You should use `AC_TYPE_LONG_DOUBLE` or `AC_TYPE_LONG_DOUBLE_WIDER` instead. See Particular Type Checks.

Macro: AC_LONG_FILE_NAMES ¶

  Replaced by

      AC_SYS_LONG_FILE_NAMES

  See AC_SYS_LONG_FILE_NAMES.

Macro: AC_MAJOR_HEADER ¶

  Replaced by `AC_HEADER_MAJOR` (see AC_HEADER_MAJOR).

Macro: AC_MEMORY_H ¶

  Used to define `NEED_MEMORY_H` if the `mem` functions were defined in `memory.h`. Today it is equivalent to ‘`AC_CHECK_HEADERS([memory.h])`’ (see AC_CHECK_HEADERS). Adjust your code to get the `mem` functions from `string.h` instead.

Macro: AC_MINGW32 ¶

  Similar to `AC_CYGWIN` but checks for the MinGW compiler environment and sets `MINGW32`. Don’t use this macro, the dignified means to check the nature of the host is using `AC_CANONICAL_HOST` (see Getting the Canonical System Type).

Macro: AC_MINIX ¶

  This macro is a platform-specific subset of `AC_USE_SYSTEM_EXTENSIONS` (see AC_USE_SYSTEM_EXTENSIONS).

Macro: AC_MINUS_C_MINUS_O ¶

  Replaced by `AC_PROG_CC_C_O` (see AC_PROG_CC_C_O).

Macro: AC_MMAP ¶

  Replaced by `AC_FUNC_MMAP` (see AC_FUNC_MMAP).

Macro: AC_MODE_T ¶

  Replaced by `AC_TYPE_MODE_T` (see AC_TYPE_MODE_T).

Macro: AC_OBJEXT ¶

  Defined the output variable `OBJEXT` based on the output of the compiler, after .c files have been excluded. Typically set to ‘`o`’ if POSIX, ‘`obj`’ if a DOS variant. Now the compiler checking macros handle this automatically.

Macro: AC_OBSOLETE(this-macro-name, [suggestion]) ¶

  Make M4 print a message to the standard error output warning that this-macro-name is obsolete, and giving the file and line number where it was called. this-macro-name should be the name of the macro that is calling `AC_OBSOLETE`. If suggestion is given, it is printed at the end of the warning message; for example, it can be a suggestion for what to use instead of this-macro-name.

  For instance

      AC_OBSOLETE([$0], [; use AC_CHECK_HEADERS(unistd.h) instead])dnl

  You are encouraged to use `AU_DEFUN` instead, since it gives better services to the user (see AU_DEFUN).

Macro: AC_OFF_T ¶

  Replaced by `AC_TYPE_OFF_T` (see AC_TYPE_OFF_T).

Macro: AC_OUTPUT([file]…, [extra-cmds], [init-cmds]) ¶

  The use of `AC_OUTPUT` with arguments is deprecated. This obsoleted interface is equivalent to:

      AC_CONFIG_FILES(file...)
      AC_CONFIG_COMMANDS([default],
                         extra-cmds, init-cmds)
      AC_OUTPUT

  See AC_CONFIG_FILES, AC_CONFIG_COMMANDS, and AC_OUTPUT.

Macro: AC_OUTPUT_COMMANDS(extra-cmds, [init-cmds]) ¶

  Specify additional shell commands to run at the end of `config.status`, and shell commands to initialize any variables from `configure`. This macro may be called multiple times. It is obsolete, replaced by `AC_CONFIG_COMMANDS` (see AC_CONFIG_COMMANDS).

  Here is an unrealistic example:

      fubar=27
      AC_OUTPUT_COMMANDS(
        [AS_ECHO(["this is extra $fubar, and so on."])],
        [fubar=$fubar])
      AC_OUTPUT_COMMANDS(
        [AS_ECHO(["this is another, extra, bit"])],
        [AS_ECHO(["init bit"])])

  Aside from the fact that `AC_CONFIG_COMMANDS` requires an additional key, an important difference is that `AC_OUTPUT_COMMANDS` is quoting its arguments twice, unlike `AC_CONFIG_COMMANDS`. This means that `AC_CONFIG_COMMANDS` can safely be given macro calls as arguments:

      AC_CONFIG_COMMANDS(foo, [my_FOO()])

  Conversely, where one level of quoting was enough for literal strings with `AC_OUTPUT_COMMANDS`, you need two with `AC_CONFIG_COMMANDS`. The following lines are equivalent:

      AC_OUTPUT_COMMANDS([echo "Square brackets: []"])
      AC_CONFIG_COMMANDS([default], [[echo "Square brackets: []"]])

Macro: AC_PID_T ¶

  Replaced by `AC_TYPE_PID_T` (see AC_TYPE_PID_T).

Macro: AC_PREFIX ¶

  Replaced by `AC_PREFIX_PROGRAM` (see AC_PREFIX_PROGRAM).

Macro: AC_PROG_CC_C89 ¶

  Now done by `AC_PROG_CC` (see AC_PROG_CC).

Macro: AC_PROG_CC_C99 ¶

  Now done by `AC_PROG_CC` (see AC_PROG_CC).

Macro: AC_PROG_CC_STDC ¶

  Now done by `AC_PROG_CC` (see AC_PROG_CC).

Macro: AC_PROG_GCC_TRADITIONAL ¶

  Used to put GCC into “traditional” (pre-ISO C) compilation mode, on systems with headers that did not work correctly with a standard-compliant compiler. GCC has not supported traditional compilation in many years, and all of the systems that required this are long obsolete themselves. This macro is now a compatibility synonym for `AC_PROG_CC` (see AC_PROG_CC).

Macro: AC_PROGRAMS_CHECK ¶

  Replaced by `AC_CHECK_PROGS` (see AC_CHECK_PROGS).

Macro: AC_PROGRAMS_PATH ¶

  Replaced by `AC_PATH_PROGS` (see AC_PATH_PROGS).

Macro: AC_PROGRAM_CHECK ¶

  Replaced by `AC_CHECK_PROG` (see AC_CHECK_PROG).

Macro: AC_PROGRAM_EGREP ¶

  Replaced by `AC_EGREP_CPP` (see AC_EGREP_CPP).

Macro: AC_PROGRAM_PATH ¶

  Replaced by `AC_PATH_PROG` (see AC_PATH_PROG).

Macro: AC_REMOTE_TAPE ¶

  Removed because of limited usefulness.

Macro: AC_RESTARTABLE_SYSCALLS ¶

  This macro was renamed `AC_SYS_RESTARTABLE_SYSCALLS`. However, these days portable programs should use `sigaction` with `SA_RESTART` if they want restartable system calls. They should not rely on `HAVE_RESTARTABLE_SYSCALLS`, since nowadays whether a system call is restartable is a dynamic issue, not a configuration-time issue.

Macro: AC_RETSIGTYPE ¶

  Replaced by `AC_TYPE_SIGNAL` (see AC_TYPE_SIGNAL), which itself is obsolete.

Macro: AC_RSH ¶

  Removed because of limited usefulness.

Macro: AC_SCO_INTL ¶

  Equivalent to the obsolescent macro `AC_FUNC_STRFTIME`. See AC_FUNC_STRFTIME.

Macro: AC_SETVBUF_REVERSED ¶

  Replaced by

      AC_FUNC_SETVBUF_REVERSED

  See AC_FUNC_SETVBUF_REVERSED.

Macro: AC_SET_MAKE ¶

  Replaced by `AC_PROG_MAKE_SET` (see AC_PROG_MAKE_SET).

Macro: AC_SIZEOF_TYPE ¶

  Replaced by `AC_CHECK_SIZEOF` (see AC_CHECK_SIZEOF).

Macro: AC_SIZE_T ¶

  Replaced by `AC_TYPE_SIZE_T` (see AC_TYPE_SIZE_T).

Macro: AC_STAT_MACROS_BROKEN ¶

  Replaced by `AC_HEADER_STAT` (see AC_HEADER_STAT).

Macro: AC_STDC_HEADERS ¶

  Replaced by `AC_HEADER_STDC` (see AC_HEADER_STDC), which is itself obsolete. Nowadays it is safe to assume the facilities of C89 exist.

Macro: AC_STRCOLL ¶

  Replaced by `AC_FUNC_STRCOLL` (see AC_FUNC_STRCOLL).

Macro: AC_STRUCT_ST_BLKSIZE ¶

  If `struct stat` contains an `st_blksize` member, define `HAVE_STRUCT_STAT_ST_BLKSIZE`. The former name, `HAVE_ST_BLKSIZE` is to be avoided, as its support will cease in the future. This macro is obsoleted, and should be replaced by

      AC_CHECK_MEMBERS([struct stat.st_blksize])

  See AC_CHECK_MEMBERS.

Macro: AC_STRUCT_ST_RDEV ¶

  If `struct stat` contains an `st_rdev` member, define `HAVE_STRUCT_STAT_ST_RDEV`. The former name for this macro, `HAVE_ST_RDEV`, is to be avoided as it will cease to be supported in the future. Actually, even the new macro is obsolete and should be replaced by:

      AC_CHECK_MEMBERS([struct stat.st_rdev])

  See AC_CHECK_MEMBERS.

Macro: AC_ST_BLKSIZE ¶

  Replaced by `AC_CHECK_MEMBERS` (see AC_CHECK_MEMBERS).

Macro: AC_ST_BLOCKS ¶

  Replaced by `AC_STRUCT_ST_BLOCKS` (see AC_STRUCT_ST_BLOCKS).

Macro: AC_ST_RDEV ¶

  Replaced by `AC_CHECK_MEMBERS` (see AC_CHECK_MEMBERS).

Macro: AC_SYS_RESTARTABLE_SYSCALLS ¶

  If the system automatically restarts a system call that is interrupted by a signal, define `HAVE_RESTARTABLE_SYSCALLS`. This macro does not check whether system calls are restarted in general—it checks whether a signal handler installed with `signal` (but not `sigaction`) causes system calls to be restarted. It does not check whether system calls can be restarted when interrupted by signals that have no handler.

  These days portable programs should use `sigaction` with `SA_RESTART` if they want restartable system calls. They should not rely on `HAVE_RESTARTABLE_SYSCALLS`, since nowadays whether a system call is restartable is a dynamic issue, not a configuration-time issue.

Macro: AC_SYS_SIGLIST_DECLARED ¶

  This macro was renamed `AC_DECL_SYS_SIGLIST`. However, even that name is obsolete, as the same functionality is now achieved via `AC_CHECK_DECLS` (see AC_CHECK_DECLS).

Macro: AC_TEST_CPP ¶

  This macro was renamed `AC_TRY_CPP`, which in turn was replaced by `AC_PREPROC_IFELSE` (see AC_PREPROC_IFELSE).

Macro: AC_TEST_PROGRAM ¶

  This macro was renamed `AC_TRY_RUN`, which in turn was replaced by `AC_RUN_IFELSE` (see AC_RUN_IFELSE).

Macro: AC_TIMEZONE ¶

  Replaced by `AC_STRUCT_TIMEZONE` (see AC_STRUCT_TIMEZONE).

Macro: AC_TIME_WITH_SYS_TIME ¶

  Replaced by `AC_HEADER_TIME` (see AC_HEADER_TIME), which is itself obsolete; nowadays one need only do ‘`AC_CHECK_HEADERS([sys/time.h])`’.

Macro: AC_TRY_COMPILE(includes, function-body, [action-if-true], [action-if-false]) ¶

  Same as:

      AC_COMPILE_IFELSE(
        [AC_LANG_PROGRAM([[includes]],
           [[function-body]])],
        [action-if-true],
        [action-if-false])

  See Running the Compiler.

  This macro double quotes both includes and function-body.

  For C and C++, includes is any `#include` statements needed by the code in function-body (includes is ignored if the currently selected language is Fortran or Fortran 77). The compiler and compilation flags are determined by the current language (see Language Choice).

Macro: AC_TRY_CPP(input, [action-if-true], [action-if-false]) ¶

  Same as:

      AC_PREPROC_IFELSE(
        [AC_LANG_SOURCE([[input]])],
        [action-if-true],
        [action-if-false])

  See Running the Preprocessor.

  This macro double quotes the input.

Macro: AC_TRY_LINK(includes, function-body, [action-if-true], [action-if-false]) ¶

  Same as:

      AC_LINK_IFELSE(
        [AC_LANG_PROGRAM([[includes]],
           [[function-body]])],
        [action-if-true],
        [action-if-false])

  See Running the Linker.

  This macro double quotes both includes and function-body.

  Depending on the current language (see Language Choice), create a test program to see whether a function whose body consists of function-body can be compiled and linked. If the file compiles and links successfully, run shell commands action-if-found, otherwise run action-if-not-found.

  This macro double quotes both includes and function-body.

  For C and C++, includes is any `#include` statements needed by the code in function-body (includes is ignored if the currently selected language is Fortran or Fortran 77). The compiler and compilation flags are determined by the current language (see Language Choice), and in addition `LDFLAGS` and `LIBS` are used for linking.

Macro: AC_TRY_LINK_FUNC(function, [action-if-found], [action-if-not-found]) ¶

  This macro is equivalent to

      AC_LINK_IFELSE([AC_LANG_CALL([], [function])],
        [action-if-found], [action-if-not-found])

  See Running the Linker.

Macro: AC_TRY_RUN(program, [action-if-true], [action-if-false], [action-if-cross-compiling = ‘`AC_MSG_FAILURE`’]) ¶

  Same as:

      AC_RUN_IFELSE(
        [AC_LANG_SOURCE([[program]])],
        [action-if-true],
        [action-if-false],
        [action-if-cross-compiling])

  See Checking Runtime Behavior.

Macro: AC_TYPE_SIGNAL ¶

  If `signal.h` declares `signal` as returning a pointer to a function returning `void`, define `RETSIGTYPE` to be `void`; otherwise, define it to be `int`. These days, it is portable to assume C89, and that signal handlers return `void`, without needing to use this macro or `RETSIGTYPE`.

Macro: AC_UID_T ¶

  Replaced by `AC_TYPE_UID_T` (see AC_TYPE_UID_T).

Macro: AC_UNISTD_H ¶

  Same as ‘`AC_CHECK_HEADERS([unistd.h])`’ (see AC_CHECK_HEADERS), which is one of the tests done as a side effect by `AC_INCLUDES_DEFAULT` (see Default Includes), so usually unnecessary to write explicitly.

Macro: AC_USG ¶

  Define `USG` if the BSD string functions (`bcopy`, `bzero`, `index`, `rindex`, etc) are not defined in `strings.h`. Modern code should assume `string.h` exists and should use the standard C string functions (`memmove`, `memset`, `strchr`, `strrchr`, etc) unconditionally.

  `strings.h` may be the only header that declares `strcasecmp`, `strncasecmp`, and `ffs`. `AC_INCLUDES_DEFAULT` checks for it (see Default Includes); test `HAVE_STRINGS_H`.

Macro: AC_UTIME_NULL ¶

  Replaced by `AC_FUNC_UTIME_NULL` (see AC_FUNC_UTIME_NULL).

Macro: AC_VALIDATE_CACHED_SYSTEM_TUPLE([cmd]) ¶

  If the cache file is inconsistent with the current host, target and build system types, it used to execute cmd or print a default error message. This is now handled by default.

Macro: AC_VERBOSE(result-description) ¶

  Replaced by `AC_MSG_RESULT` (see AC_MSG_RESULT).

Macro: AC_VFORK ¶

  Replaced by `AC_FUNC_FORK` (see AC_FUNC_FORK).

Macro: AC_VPRINTF ¶

  Replaced by `AC_FUNC_VPRINTF` (see AC_FUNC_VPRINTF).

Macro: AC_WAIT3 ¶

  This macro was renamed `AC_FUNC_WAIT3`. However, these days portable programs should use `waitpid`, not `wait3`, as `wait3` has been removed from POSIX.

Macro: AC_WARN ¶

  Replaced by `AC_MSG_WARN` (see AC_MSG_WARN).

Macro: AC_WARNING(message) ¶

  Replaced by `m4_warn` (see m4_warn).

Macro: AC_WITH(package, action-if-given, [action-if-not-given]) ¶

  This is an obsolete version of `AC_ARG_WITH` that does not support providing a help string (see AC_ARG_WITH).

Macro: AC_WORDS_BIGENDIAN ¶

  Replaced by `AC_C_BIGENDIAN` (see AC_C_BIGENDIAN).

Macro: AC_XENIX_DIR ¶

  This macro is equivalent to the obsolescent `AC_HEADER_DIRENT` macro, plus it also sets the shell variable `XENIX`. Don’t use this macro, the dignified means to check the nature of the host is using `AC_CANONICAL_HOST` (see Getting the Canonical System Type).

Macro: AC_YYTEXT_POINTER ¶

  This macro was renamed `AC_DECL_YYTEXT`, which in turn was integrated into `AC_PROG_LEX` (see AC_PROG_LEX).

### 18.5 Upgrading From Version 1 ¶

Autoconf version 2 is mostly backward compatible with version 1. However, it introduces better ways to do some things, and doesn’t support some of the ugly things in version 1. So, depending on how sophisticated your `configure.ac` files are, you might have to do some manual work in order to upgrade to version 2. This chapter points out some problems to watch for when upgrading. Also, perhaps your `configure` scripts could benefit from some of the new features in version 2; the changes are summarized in the file `NEWS` in the Autoconf distribution.

#### 18.5.1 Changed File Names ¶

If you have an `aclocal.m4` installed with Autoconf (as opposed to in a particular package’s source directory), you must rename it to `acsite.m4`. See Using `autoconf` to Create `configure`.

If you distribute `install.sh` with your package, rename it to `install-sh` so `make` builtin rules don’t inadvertently create a file called `install` from it. `AC_PROG_INSTALL` looks for the script under both names, but it is best to use the new name.

If you were using `config.h.top`, `config.h.bot`, or `acconfig.h`, you still can, but you have less clutter if you use the `AH_` macros. See Autoheader Macros.

#### 18.5.2 Changed Makefiles ¶

Add ‘`@CFLAGS@`’, ‘`@CPPFLAGS@`’, and ‘`@LDFLAGS@`’ in your `Makefile.in` files, so they can take advantage of the values of those variables in the environment when `configure` is run. Doing this isn’t necessary, but it’s a convenience for users.

Also add ‘`@configure_input@`’ in a comment to each input file for `AC_OUTPUT`, so that the output files contain a comment saying they were produced by `configure`. Automatically selecting the right comment syntax for all the kinds of files that people call `AC_OUTPUT` on became too much work.

Add `config.log` and `config.cache` to the list of files you remove in `distclean` targets.

If you have the following in `Makefile.in`:

    prefix = /usr/local
    exec_prefix = $(prefix)

you must change it to:

    prefix = @prefix@
    exec_prefix = @exec_prefix@

The old behavior of replacing those variables without ‘`@`’ characters around them has been removed.

#### 18.5.3 Changed Macros ¶

Many of the macros were renamed in Autoconf version 2. You can still use the old names, but the new ones are clearer, and it’s easier to find the documentation for them. See Obsolete Macros, for a table showing the new names for the old macros. Use the `autoupdate` program to convert your `configure.ac` to using the new macro names. See Using `autoupdate` to Modernize `configure.ac`.

Some macros have been superseded by similar ones that do the job better, but are not call-compatible. If you get warnings about calling obsolete macros while running `autoconf`, you may safely ignore them, but your `configure` script generally works better if you follow the advice that is printed about what to replace the obsolete macros with. In particular, the mechanism for reporting the results of tests has changed. If you were using `echo` or `AC_VERBOSE` (perhaps via `AC_COMPILE_CHECK`), your `configure` script’s output looks better if you switch to `AC_MSG_CHECKING` and `AC_MSG_RESULT`. See Printing Messages. Those macros work best in conjunction with cache variables. See Caching Results.

#### 18.5.4 Changed Results ¶

If you were checking the results of previous tests by examining the shell variable `DEFS`, you need to switch to checking the values of the cache variables for those tests. `DEFS` no longer exists while `configure` is running; it is only created when generating output files. This difference from version 1 is because properly quoting the contents of that variable turned out to be too cumbersome and inefficient to do every time `AC_DEFINE` is called. See Cache Variable Names.

For example, here is a `configure.ac` fragment written for Autoconf version 1:

    AC_HAVE_FUNCS(syslog)
    case "$DEFS" in
    *-DHAVE_SYSLOG*) ;;
    *) # syslog is not in the default libraries.  See if it's in some other.
      saved_LIBS="$LIBS"
      for lib in bsd socket inet; do
        AC_CHECKING(for syslog in -l$lib)
        LIBS="-l$lib $saved_LIBS"
        AC_HAVE_FUNCS(syslog)
        case "$DEFS" in
        *-DHAVE_SYSLOG*) break ;;
        *) ;;
        esac
        LIBS="$saved_LIBS"
      done ;;
    esac

Here is a way to write it for version 2:

    AC_CHECK_FUNCS([syslog])
    AS_IF([test "x$ac_cv_func_syslog" = xno],
      [# syslog is not in the default libraries.  See if it's in some other.
       for lib in bsd socket inet; do
         AC_CHECK_LIB([$lib], [syslog],
           [AC_DEFINE([HAVE_SYSLOG])
            LIBS="-l$lib $LIBS"; break])
       done])

If you were working around bugs in `AC_DEFINE_UNQUOTED` by adding backslashes before quotes, you need to remove them. It now works predictably, and does not treat quotes (except back quotes) specially. See Setting Output Variables.

All of the Boolean shell variables set by Autoconf macros now use ‘`yes`’ for the true value. Most of them use ‘`no`’ for false, though for backward compatibility some use the empty string instead. If you were relying on a shell variable being set to something like 1 or ‘`t`’ for true, you need to change your tests.

#### 18.5.5 Changed Macro Writing ¶

When defining your own macros, you should now use `AC_DEFUN` instead of `define`. `AC_DEFUN` automatically calls `AC_PROVIDE` and ensures that macros called via `AC_REQUIRE` do not interrupt other macros, to prevent nested ‘`checking…`’ messages on the screen. There’s no actual harm in continuing to use the older way, but it’s less convenient and attractive. See Macro Definitions.

You probably looked at the macros that came with Autoconf as a guide for how to do things. It would be a good idea to take a look at the new versions of them, as the style is somewhat improved and they take advantage of some new features.

If you were doing tricky things with undocumented Autoconf internals (macros, variables, diversions), check whether you need to change anything to account for changes that have been made. Perhaps you can even use an officially supported technique in version 2 instead of kludging. Or perhaps not.

To speed up your locally written feature tests, add caching to them. See whether any of your tests are of general enough usefulness to encapsulate them into macros that you can share.

### 18.6 Upgrading From Version 2.13 ¶

The introduction of the previous section (see Upgrading From Version 1) perfectly suits this section...

> Autoconf version 2.50 is mostly backward compatible with version 2.13. However, it introduces better ways to do some things, and doesn’t support some of the ugly things in version 2.13. So, depending on how sophisticated your `configure.ac` files are, you might have to do some manual work in order to upgrade to version 2.50. This chapter points out some problems to watch for when upgrading. Also, perhaps your `configure` scripts could benefit from some of the new features in version 2.50; the changes are summarized in the file `NEWS` in the Autoconf distribution.

#### 18.6.1 Changed Quotation ¶

The most important changes are invisible to you: the implementation of most macros have completely changed. This allowed more factorization of the code, better error messages, a higher uniformity of the user’s interface etc. Unfortunately, as a side effect, some construct which used to (miraculously) work might break starting with Autoconf 2.50. The most common culprit is bad quotation.

For instance, in the following example, the message is not properly quoted:

    AC_INIT
    AC_CHECK_HEADERS(foo.h, ,
      AC_MSG_ERROR(cannot find foo.h, bailing out))
    AC_OUTPUT

Autoconf 2.13 simply ignores it:

    $ autoconf-2.13; ./configure --silent
    creating cache ./config.cache
    configure: error: cannot find foo.h
    $

while Autoconf 2.50 produces a broken `configure`:

    $ autoconf-2.50; ./configure --silent
    configure: error: cannot find foo.h
    ./configure: exit: bad non-numeric arg `bailing'
    ./configure: exit: bad non-numeric arg `bailing'
    $

The message needs to be quoted, and the `AC_MSG_ERROR` invocation too!

    AC_INIT([Example], [1.0], [bug-example@example.org])
    AC_CHECK_HEADERS([foo.h], [],
      [AC_MSG_ERROR([cannot find foo.h, bailing out])])
    AC_OUTPUT

Many many (and many more) Autoconf macros were lacking proper quotation, including no less than… `AC_DEFUN` itself!

    $ cat configure.in
    AC_DEFUN([AC_PROG_INSTALL],
    [# My own much better version
    ])
    AC_INIT
    AC_PROG_INSTALL
    AC_OUTPUT
    $ autoconf-2.13
    autoconf: Undefined macros:
    ***BUG in Autoconf--please report*** AC_FD_MSG
    ***BUG in Autoconf--please report*** AC_EPI
    configure.in:1:AC_DEFUN([AC_PROG_INSTALL],
    configure.in:5:AC_PROG_INSTALL
    $ autoconf-2.50
    $

#### 18.6.2 New Macros ¶

While Autoconf was relatively dormant in the late 1990s, Automake provided Autoconf-like macros for a while. Starting with Autoconf 2.50 in 2001, Autoconf provided versions of these macros, integrated in the `AC_` namespace, instead of `AM_`. But in order to ease the upgrading via `autoupdate`, bindings to such `AM_` macros are provided.

Unfortunately older versions of Automake (e.g., Automake 1.4) did not quote the names of these macros. Therefore, when `m4` finds something like ‘`AC_DEFUN(AM_TYPE_PTRDIFF_T, …)`’ in `aclocal.m4`, `AM_TYPE_PTRDIFF_T` is expanded, replaced with its Autoconf definition.

Fortunately Autoconf catches pre-`AC_INIT` expansions, and complains, in its own words:

    $ cat configure.ac
    AC_INIT([Example], [1.0], [bug-example@example.org])
    AM_TYPE_PTRDIFF_T
    $ aclocal-1.4
    $ autoconf
    aclocal.m4:17: error: m4_defn: undefined macro: _m4_divert_diversion
    aclocal.m4:17: the top level
    autom4te: m4 failed with exit status: 1
    $

Modern versions of Automake no longer define most of these macros, and properly quote the names of the remaining macros. If you must use an old Automake, do not depend upon macros from Automake as it is simply not its job to provide macros (but the one it requires itself):

    $ cat configure.ac
    AC_INIT([Example], [1.0], [bug-example@example.org])
    AM_TYPE_PTRDIFF_T
    $ rm aclocal.m4
    $ autoupdate
    autoupdate: 'configure.ac' is updated
    $ cat configure.ac
    AC_INIT([Example], [1.0], [bug-example@example.org])
    AC_CHECK_TYPES([ptrdiff_t])
    $ aclocal-1.4
    $ autoconf
    $

#### 18.6.3 Hosts and Cross-Compilation ¶

Based on the experience of compiler writers, and after long public debates, many aspects of the cross-compilation chain have changed:

- the relationship between the build, host, and target architecture types,
- the command line interface for specifying them to `configure`,
- the variables defined in `configure`,
- the enabling of cross-compilation mode.

The relationship between build, host, and target have been cleaned up: the chain of default is now simply: target defaults to host, host to build, and build to the result of `config.guess`. Nevertheless, in order to ease the transition from 2.13 to 2.50, the following transition scheme is implemented. Do not rely on it, as it will be completely disabled in a couple of releases (we cannot keep it, as it proves to cause more problems than it cures).

They all default to the result of running `config.guess`, unless you specify either `--build` or `--host`. In this case, the default becomes the system type you specified. If you specify both, and they’re different, `configure` enters cross compilation mode, so it doesn’t run any tests that require execution.

Hint: if you mean to override the result of `config.guess`, prefer `--build` over `--host`.

For backward compatibility, `configure` accepts a system type as an option by itself. Such an option overrides the defaults for build, host, and target system types. The following configure statement configures a cross toolchain that runs on NetBSD/aarch64 but generates code for GNU Hurd/riscv64, which is also the build platform.

    ./configure --host=aarch64-netbsd riscv64-gnu

In Autoconf 2.13 and before, the variables `build`, `host`, and `target` had a different semantics before and after the invocation of `AC_CANONICAL_BUILD` etc. Now, the argument of `--build` is strictly copied into `build_alias`, and is left empty otherwise. After the `AC_CANONICAL_BUILD`, `build` is set to the canonicalized build type. To ease the transition, before, its contents is the same as that of `build_alias`. Do not rely on this broken feature.

For consistency with the backward compatibility scheme exposed above, when `--host` is specified but `--build` isn’t, the build system is assumed to be the same as `--host`, and ‘`build_alias`’ is set to that value. Eventually, this historically incorrect behavior will go away.

The former scheme to enable cross-compilation proved to cause more harm than good, in particular, it used to be triggered too easily, leaving regular end users puzzled in front of cryptic error messages. `configure` could even enter cross-compilation mode only because the compiler was not functional. This is mainly because `configure` used to try to detect cross-compilation, instead of waiting for an explicit flag from the user.

Now, `configure` enters cross-compilation mode if and only if `--host` is passed.

That’s the short documentation. To ease the transition between 2.13 and its successors, a more complicated scheme is implemented. Do not rely on the following, as it will be removed in the near future.

If you specify `--host`, but not `--build`, when `configure` performs the first compiler test it tries to run an executable produced by the compiler. If the execution fails, it enters cross-compilation mode. This is fragile. Moreover, by the time the compiler test is performed, it may be too late to modify the build-system type: other tests may have already been performed. Therefore, whenever you specify `--host`, be sure to specify `--build` too.

    ./configure --build=x86_64-pc-linux-gnu --host=x86_64-w64-mingw64

enters cross-compilation mode. The former interface, which consisted in setting the compiler to a cross-compiler without informing `configure` is obsolete. For instance, `configure` fails if it can’t run the code generated by the specified compiler if you configure as follows:

    ./configure CC=x86_64-w64-mingw64-gcc

#### 18.6.4 `AC_LIBOBJ` vs. `LIBOBJS` ¶

Up to Autoconf 2.13, the replacement of functions was triggered via the variable `LIBOBJS`. Since Autoconf 2.50, the macro `AC_LIBOBJ` should be used instead (see Generic Function Checks). Starting at Autoconf 2.53, the use of `LIBOBJS` is an error.

This change is mandated by the unification of the GNU Build System components. In particular, the various fragile techniques used to parse a `configure.ac` are all replaced with the use of traces. As a consequence, any action must be traceable, which obsoletes critical variable assignments. Fortunately, `LIBOBJS` was the only problem, and it can even be handled gracefully (read, “without your having to change something”).

There were two typical uses of `LIBOBJS`: asking for a replacement function, and adjusting `LIBOBJS` for Automake and/or Libtool.

As for function replacement, the fix is immediate: use `AC_LIBOBJ`. For instance:

    LIBOBJS="$LIBOBJS fnmatch.o"
    LIBOBJS="$LIBOBJS malloc.$ac_objext"

should be replaced with:

    AC_LIBOBJ([fnmatch])
    AC_LIBOBJ([malloc])

When used with Automake 1.10 or newer, a suitable value for `LIBOBJDIR` is set so that the `LIBOBJS` and `LTLIBOBJS` can be referenced from any `Makefile.am`. Even without Automake, arranging for `LIBOBJDIR` to be set correctly enables referencing `LIBOBJS` and `LTLIBOBJS` in another directory. The `LIBOBJDIR` feature is experimental.

#### 18.6.5 `AC_ACT_IFELSE` vs. `AC_TRY_ACT` ¶

Since Autoconf 2.50, internal codes uses `AC_PREPROC_IFELSE`, `AC_COMPILE_IFELSE`, `AC_LINK_IFELSE`, and `AC_RUN_IFELSE` on one hand and `AC_LANG_SOURCE`, and `AC_LANG_PROGRAM` on the other hand instead of the deprecated `AC_TRY_CPP`, `AC_TRY_COMPILE`, `AC_TRY_LINK`, and `AC_TRY_RUN`. The motivations where:

- a more consistent interface: `AC_TRY_COMPILE` etc. were double quoting their arguments;
- the combinatorial explosion is solved by decomposing on the one hand the generation of sources, and on the other hand executing the program;
- this scheme helps supporting more languages than plain C and C++.

In addition to the change of syntax, the philosophy has changed too: while emphasis was put on speed at the expense of accuracy, today’s Autoconf promotes accuracy of the testing framework at, ahem…, the expense of speed.

As a perfect example of what is not to be done, here is how to find out whether a header file contains a particular declaration, such as a typedef, a structure, a structure member, or a function. Use `AC_EGREP_HEADER` instead of running `grep` directly on the header file; on some systems the symbol might be defined in another header file that the file you are checking includes.

As a (bad) example, here is how you should not check for C preprocessor symbols, either defined by header files or predefined by the C preprocessor: using `AC_EGREP_CPP`:

    AC_EGREP_CPP(yes,
    [#ifdef _AIX
      yes
    #endif
    ], is_aix=yes, is_aix=no)

The above example, properly written would (i) use `AC_LANG_PROGRAM`, and (ii) run the compiler:

    AC_COMPILE_IFELSE([AC_LANG_PROGRAM(
    [[#ifndef _AIX
     error: This isn't AIX!
    #endif
    ]])],
                       [is_aix=yes],
                       [is_aix=no])

## 19 Generating Test Suites with Autotest ¶

    N.B.: This section describes a feature which is still
    stabilizing.  Although we believe that Autotest is useful as-is, this
    documentation describes an interface which might change in the future:
    do not depend upon Autotest without subscribing to the Autoconf mailing
    lists.

It is paradoxical that portable projects depend on nonportable tools to run their test suite. Autoconf by itself is the paragon of this problem: although it aims at perfectly portability, up to 2.13 its test suite was using DejaGNU, a rich and complex testing framework, but which is far from being standard on POSIX systems. Worse yet, it was likely to be missing on the most fragile platforms, the very platforms that are most likely to torture Autoconf and exhibit deficiencies.

To circumvent this problem, many package maintainers have developed their own testing framework, based on simple shell scripts whose sole outputs are exit status values describing whether the test succeeded. Most of these tests share common patterns, and this can result in lots of duplicated code and tedious maintenance.

Following exactly the same reasoning that yielded to the inception of Autoconf, Autotest provides a test suite generation framework, based on M4 macros building a portable shell script. The suite itself is equipped with automatic logging and tracing facilities which greatly diminish the interaction with bug reporters, and simple timing reports.

Autoconf itself has been using Autotest for years, and we do attest that it has considerably improved the strength of the test suite and the quality of bug reports. Other projects are known to use some generation of Autotest, such as Bison, GNU Wdiff, GNU Tar, each of them with different needs, and this usage has validated Autotest as a general testing framework.

Nonetheless, compared to DejaGNU, Autotest is inadequate for interactive tool testing, which is probably its main limitation.

### 19.1 Using an Autotest Test Suite ¶

#### 19.1.1 `testsuite` Scripts ¶

Generating testing or validation suites using Autotest is rather easy. The whole validation suite is held in a file to be processed through `autom4te`, itself using GNU M4 under the hood, to produce a stand-alone Bourne shell script which then gets distributed. Neither `autom4te` nor GNU M4 are needed at the installer’s end.

Each test of the validation suite should be part of some test group. A test group is a sequence of interwoven tests that ought to be executed together, usually because one test in the group creates data files that a later test in the same group needs to read. Complex test groups make later debugging more tedious. It is much better to keep only a few tests per test group. Ideally there is only one test per test group.

For all but the simplest packages, some file such as `testsuite.at` does not fully hold all test sources, as these are often easier to maintain in separate files. Each of these separate files holds a single test group, or a sequence of test groups all addressing some common functionality in the package. In such cases, `testsuite.at` merely initializes the validation suite, and sometimes does elementary health checking, before listing include statements for all other test files. The special file `package.m4`, containing the identification of the package, is automatically included if found.

A convenient alternative consists in moving all the global issues (local Autotest macros, elementary health checking, and `AT_INIT` invocation) into the file `local.at`, and making `testsuite.at` be a simple list of `m4_include`s of sub test suites. In such case, generating the whole test suite or pieces of it is only a matter of choosing the `autom4te` command line arguments.

The validation scripts that Autotest produces are by convention called `testsuite`. When run, `testsuite` executes each test group in turn, producing only one summary line per test to say if that particular test succeeded or failed. At end of all tests, summarizing counters get printed. One debugging directory is left for each test group which failed, if any: such directories are named `testsuite.dir/nn`, where nn is the sequence number of the test group, and they include:

- a debugging script named `run` which reruns the test in debug mode (see Running `testsuite` Scripts). The automatic generation of debugging scripts has the purpose of easing the chase for bugs.
- all the files created with `AT_DATA`
- all the Erlang source code files created with `AT_CHECK_EUNIT`
- a log of the run, named `testsuite.log`

In the ideal situation, none of the tests fail, and consequently no debugging directory is left behind for validation.

It often happens in practice that individual tests in the validation suite need to get information coming out of the configuration process. Some of this information, common for all validation suites, is provided through the file `atconfig`, automatically created by `AC_CONFIG_TESTDIR`. For configuration information which your testing environment specifically needs, you might prepare an optional file named `atlocal.in`, instantiated by `AC_CONFIG_FILES`. The configuration process produces `atconfig` and `atlocal` out of these two input files, and these two produced files are automatically read by the `testsuite` script.

Here is a diagram showing the relationship between files.

Files used in preparing a software package for distribution:

                    [package.m4] -->.
                                     \
    subfile-1.at ->.  [local.at] ---->+
        ...         \                  \
    subfile-i.at ---->-- testsuite.at -->-- autom4te* -->testsuite
        ...         /
    subfile-n.at ->'

Files used in configuring a software package:

                                         .--> atconfig
                                        /
    [atlocal.in] -->  config.status* --<
                                        \
                                         `--> [atlocal]

Files created during test suite execution:

    atconfig -->.                    .--> testsuite.log
                 \                  /
                  >-- testsuite* --<
                 /                  \
    [atlocal] ->'                    `--> [testsuite.dir]

#### 19.1.2 Autotest Logs ¶

When run, the test suite creates a log file named after itself, e.g., a test suite named `testsuite` creates `testsuite.log`. It contains a lot of information, usually more than maintainers actually need, but therefore most of the time it contains all that is needed:

command line arguments

  A bad but unfortunately widespread habit consists of setting environment variables before the command, such as in ‘`CC=my-home-grown-cc ./testsuite`’. The test suite does not know this change, hence (i) it cannot report it to you, and (ii) it cannot preserve the value of `CC` for subsequent runs. Autoconf faced exactly the same problem, and solved it by asking users to pass the variable definitions as command line arguments. Autotest requires this rule, too, but has no means to enforce it; the log then contains a trace of the variables that were changed by the user.

`ChangeLog` excerpts

  The topmost lines of all the `ChangeLog` files found in the source hierarchy. This is especially useful when bugs are reported against development versions of the package, since the version string does not provide sufficient information to know the exact state of the sources the user compiled. Of course, this relies on the use of a `ChangeLog`.

build machine

  Running a test suite in a cross-compile environment is not an easy task, since it would mean having the test suite run on a machine build, while running programs on a machine host. It is much simpler to run both the test suite and the programs on host, but then, from the point of view of the test suite, there remains a single environment, host = build. The log contains relevant information on the state of the build machine, including some important environment variables.

tested programs

  The absolute file name and answers to `--version` of the tested programs (see Writing `testsuite.at`, `AT_TESTED`).

configuration log

  The contents of `config.log`, as created by `configure`, are appended. It contains the configuration flags and a detailed report on the configuration itself.

### 19.2 Writing `testsuite.at` ¶

The `testsuite.at` is a Bourne shell script making use of special Autotest M4 macros. It often contains a call to `AT_INIT` near its beginning followed by one call to `m4_include` per source file for tests. Each such included file, or the remainder of `testsuite.at` if include files are not used, contain a sequence of test groups. Each test group begins with a call to `AT_SETUP`, then an arbitrary number of shell commands or calls to `AT_CHECK`, and then completes with a call to `AT_CLEANUP`. Multiple test groups can be categorized by a call to `AT_BANNER`.

All of the public Autotest macros have all-uppercase names in the namespace ‘`^AT_`’ to prevent them from accidentally conflicting with other text; Autoconf also reserves the namespace ‘`^_AT_`’ for internal macros. All shell variables used in the testsuite for internal purposes have mostly-lowercase names starting with ‘`at_`’. Autotest also uses here-document delimiters in the namespace ‘`^_AT[A-Z]`’, and makes use of the file system namespace ‘`^at-`’.

Since Autoconf is built on top of M4sugar (see Programming in M4sugar) and M4sh (see Programming in M4sh), you must also be aware of those namespaces (‘`^_?\(m4\|AS\)_`’). In general, you should not use the namespace of a package that does not own the macro or shell code you are writing.

Macro: AT_INIT([name]) ¶

  Initialize Autotest. Giving a name to the test suite is encouraged if your package includes several test suites. Before this macro is called, `AT_PACKAGE_STRING` and `AT_PACKAGE_BUGREPORT` must be defined, which are used to display information about the testsuite to the user. Typically, these macros are provided by a file `package.m4` built by `make` (see Making `testsuite` Scripts), in order to inherit the package name, version, and bug reporting address from `configure.ac`.

Macro: AT_COPYRIGHT(copyright-notice) ¶

  State that, in addition to the Free Software Foundation’s copyright on the Autotest macros, parts of your test suite are covered by copyright-notice.

  The copyright-notice shows up in both the head of `testsuite` and in ‘`testsuite --version`’.

Macro: AT_ARG_OPTION(options, help-text, [action-if-given], [action-if-not-given]) ¶

  Accept options from the space-separated list options, a list that has leading dashes removed from the options. Long options will be prefixed with ‘`--`’, single-character options with ‘`-`’. The first word in this list is the primary option, any others are assumed to be short-hand aliases. The variable associated with it is `at_arg_option`, with any dashes in option replaced with underscores.

  If the user passes `--option` to the `testsuite`, the variable will be set to ‘`:`’. If the user does not pass the option, or passes `--no-option`, then the variable will be set to ‘`false`’.

  action-if-given is run each time the option is encountered; here, the variable `at_optarg` will be set to ‘`:`’ or ‘`false`’ as appropriate. `at_optarg` is actually just a copy of `at_arg_option`.

  action-if-not-given will be run once after option parsing is complete and if no option from options was used.

  help-text is added to the end of the list of options shown in `testsuite --help` (see AS_HELP_STRING).

  It is recommended that you use a package-specific prefix to options names in order to avoid clashes with future Autotest built-in options.

Macro: AT_ARG_OPTION_ARG(options, help-text, [action-if-given], [action-if-not-given]) ¶

  Accept options with arguments from the space-separated list options, a list that has leading dashes removed from the options. Long options will be prefixed with ‘`--`’, single-character options with ‘`-`’. The first word in this list is the primary option, any others are assumed to be short-hand aliases. The variable associated with it is `at_arg_option`, with any dashes in option replaced with underscores.

  If the user passes `--option=arg` or `--option arg` to the `testsuite`, the variable will be set to ‘`arg`’.

  action-if-given is run each time the option is encountered; here, the variable `at_optarg` will be set to ‘`arg`’. `at_optarg` is actually just a copy of `at_arg_option`.

  action-if-not-given will be run once after option parsing is complete and if no option from options was used.

  help-text is added to the end of the list of options shown in `testsuite --help` (see AS_HELP_STRING).

  It is recommended that you use a package-specific prefix to options names in order to avoid clashes with future Autotest built-in options.

Macro: AT_COLOR_TESTS ¶

  Enable colored test results by default when the output is connected to a terminal.

Macro: AT_TESTED(executables) ¶

  Log the file name and answer to `--version` of each program in space-separated list executables. Several invocations register new executables, in other words, don’t fear registering one program several times.

  Autotest test suites rely on `PATH` to find the tested program. This avoids the need to generate absolute names of the various tools, and makes it possible to test installed programs. Therefore, knowing which programs are being exercised is crucial to understanding problems in the test suite itself, or its occasional misuses. It is a good idea to also subscribe foreign programs you depend upon, to avoid incompatible diagnostics.

  executables is implicitly wrapped in shell double quotes, but it will still use shell variable expansion (‘`$`’), command substitution (‘```’), and backslash escaping (‘`\`’). In particular, the `EXEEXT` variable is available if it is passed to the testsuite via `atlocal` or `atconfig`.

Macro: AT_PREPARE_TESTS(shell-code) ¶

  Execute shell-code in the main testsuite process, after initializing the test suite and processing command-line options, but before running any tests. If this macro is used several times, all of the shell-codes will be executed, in the order they appeared in `testsuite.at`.

  One reason to use `AT_PREPARE_TESTS` is when the programs under test are sensitive to environment variables: you can unset all these variables or reset them to safe values in shell-code.

  shell-code is only executed if at least one test is going to be run. In particular, it will not be executed if any of the `--help`, `--version`, `--list`, or `--clean` options are given to `testsuite` (see Running `testsuite` Scripts).

Macro: AT_PREPARE_EACH_TEST(shell-code) ¶

  Execute shell-code in each test group’s subshell, at the point of the `AT_SETUP` that starts the test group.

Macro: AT_TEST_HELPER_FN(name, args, description, code) ¶

  Define a shell function that will be available to the code for each test group. Its name will be `ath_fn_name`, and its body will be code. (The prefix prevents name conflicts with shell functions defined by M4sh and Autotest.)

  args should describe the function’s arguments and description what it does; these are used only for documentation comments in the generated testsuite script.

Macro: AT_BANNER(test-category-name) ¶

  This macro identifies the start of a category of related test groups. When the resulting `testsuite` is invoked with more than one test group to run, its output will include a banner containing test-category-name prior to any tests run from that category. The banner should be no more than about 40 or 50 characters. A blank banner indicates uncategorized tests; an empty line will be inserted after tests from an earlier category, effectively ending that category.

Macro: AT_SETUP(test-group-name) ¶

  This macro starts a group of related tests, all to be executed in the same subshell. It accepts a single argument, which holds a few words (no more than about 30 or 40 characters) quickly describing the purpose of the test group being started. test-group-name must not expand to unbalanced quotes, although quadrigraphs can be used.

Macro: AT_KEYWORDS(keywords) ¶

  Associate the space-separated list of keywords to the enclosing test group. This makes it possible to run “slices” of the test suite. For instance, if some of your test groups exercise some ‘`foo`’ feature, then using ‘`AT_KEYWORDS(foo)`’ lets you run ‘`./testsuite -k foo`’ to run exclusively these test groups. The test-group-name of the test group is automatically recorded to `AT_KEYWORDS`.

  Several invocations within a test group accumulate new keywords. In other words, don’t fear registering the same keyword several times in a test group.

Macro: AT_CAPTURE_FILE(file) ¶

  If the current test group fails, log the contents of file. Several identical calls within one test group have no additional effect.

Macro: AT_FAIL_IF(shell-condition) ¶

  Make the test group fail and skip the rest of its execution, if shell-condition is true. shell-condition is a shell expression such as a `test` command. Tests before `AT_FAIL_IF` will be executed and may still cause the test group to be skipped. You can instantiate this macro many times from within the same test group.

  You should use this macro only for very simple failure conditions. If the shell-condition could emit any kind of output you should instead use `AT_CHECK` like

      AT_CHECK([if shell-condition; then exit 99; fi])

  so that such output is properly recorded in the `testsuite.log` file.

Macro: AT_SKIP_IF(shell-condition) ¶

  Determine whether the test should be skipped because it requires features that are unsupported on the machine under test. shell-condition is a shell expression such as a `test` command. Tests before `AT_SKIP_IF` will be executed and may still cause the test group to fail. You can instantiate this macro many times from within the same test group.

  You should use this macro only for very simple skip conditions. If the shell-condition could emit any kind of output you should instead use `AT_CHECK` like

      AT_CHECK([if shell-condition; then exit 77; fi])

  so that such output is properly recorded in the `testsuite.log` file.

Macro: AT_XFAIL_IF(shell-condition) ¶

  Determine whether the test is expected to fail because it is a known bug (for unsupported features, you should skip the test). shell-condition is a shell expression such as a `test` command; you can instantiate this macro many times from within the same test group, and one of the conditions is enough to turn the test into an expected failure.

Macro: AT_CLEANUP ¶

  End the current test group.

Macro: AT_DATA(file, contents) ¶
Macro: AT_DATA_UNQUOTED(file, contents) ¶

  Initialize an input data file with given contents. Of course, the contents have to be properly quoted between square brackets to protect against included commas or spurious M4 expansion. contents must be empty or end with a newline. file must be a single shell word that expands into a single file name.

  The difference between `AT_DATA` and `AT_DATA_UNQUOTED` is that only the latter performs shell variable expansion (‘`$`’), command substitution (‘```’), and backslash escaping (‘`\`’) on contents.

Macro: AT_CHECK(commands, [status = ‘`0`’], [stdout], [stderr], [run-if-fail], [run-if-pass]) ¶
Macro: AT_CHECK_UNQUOTED(commands, [status = ‘`0`’], [stdout], [stderr], [run-if-fail], [run-if-pass]) ¶

  Perform a test, by running the shell commands in a subshell. commands is output as-is, so shell expansions are honored. These commands are expected to have a final exit status of status, and to produce output as described by stdout and stderr (see below).

  This macro must be invoked in between `AT_SETUP` and `AT_CLEANUP`.

  If commands exit with unexpected status 77, then the rest of the test group is skipped. If commands exit with unexpected status 99, then the test group is immediately failed; this is called a hard failure. Otherwise, the test is considered to have succeeded if all of the status, stdout, and stderr expectations were met.

  If run-if-fail is nonempty, it provides extra shell commands to run when the test fails; if run-if-pass is nonempty, it provides extra shell commands to run when the test succeeds. These commands are not run in a subshell, and they are not run when the test group is skipped (exit code 77) or hard-failed (exit code 99). They may change whether the test group is considered to have succeeded, by modifying the shell variable `at_failed`; set it to `:` to indicate that the test group has failed, or `false` to indicate that it has succeeded.

  The exit status of commands is available to run-if-fail and run-if-pass commands in the `at_status` shell variable. The output from commands is also available, in the files named by the `at_stdout` and `at_stderr` variables.

  If status is the literal ‘`ignore`’, then the exit status of commands is not checked, except for the special cases of 77 (skip) and 99 (hard failure). The existence of hard failures allows one to mark a test as an expected failure with `AT_XFAIL_IF` because a feature has not yet been implemented, but to still distinguish between gracefully handling the missing feature and dumping core.

  If the value of the stdout or stderr parameter is one of the literals in the following table, then the test treats the output according to the rules of that literal.

  ‘`ignore`’

    The content of the output is ignored, but still captured in the test group log (if the testsuite is run with the `-v` option, the test group log is displayed as the test is run; if the test group later fails, the test group log is also copied into the overall testsuite log). This action is valid for both stdout and stderr.

  ‘`ignore-nolog`’

    The content of the output is ignored, and nothing is captured in the log files. If commands are likely to produce binary output (including long lines) or large amounts of output, then logging the output can make it harder to locate details related to subsequent tests within the group, and could potentially corrupt terminal display of a user running `testsuite -v`. This action is valid for both stdout and stderr.

  ‘`stdout`’

    Only valid as the stdout parameter. Capture the content of standard output in both a file named `stdout` and the test group log. Subsequent commands in the test group can then post-process the file. This action is often used when it is desired to use `grep` to look for a substring in the output, or when the output must be post-processed to normalize error messages into a common form.

  ‘`stderr`’

    Only valid as the stderr parameter. Capture the content of standard error in both a file named `stderr` and the test group log.

  ‘`stdout-nolog`’
  ‘`stderr-nolog`’

    Like ‘`stdout`’ or ‘`stderr`’, except that the captured output is not duplicated into the test group log. This action is particularly useful for an intermediate check that produces large amounts of data, which will be followed by another check that filters down to the relevant data, as it makes it easier to locate details in the log.

  ‘`expout`’

    Only valid as the stdout parameter. Compare standard output with the previously created file `expout`, and list any differences in the testsuite log.

  ‘`experr`’

    Only valid as the stderr parameter. Compare standard error with the previously created file `experr`, and list any differences in the testsuite log.

  Otherwise, the values of the stdout and stderr parameters are treated as text that must exactly match the output given by commands on standard output and standard error (including an empty parameter for no output); any differences are captured in the testsuite log and the test is failed (unless an unexpected exit status of 77 skipped the test instead).

  `AT_CHECK_UNQUOTED` performs shell variable expansion (‘`$`’), command substitution (‘```’), and backslash escaping (‘`\`’) on comparison text given in the stdout and stderr parameters; `AT_CHECK` does not. There is no difference in the interpretation of commands.

Macro: AT_CHECK_EUNIT(module, test-spec, [erlflags], [run-if-fail], [run-if-pass]) ¶

  Initialize and execute an Erlang module named module that performs tests following the test-spec EUnit test specification. test-spec must be a valid EUnit test specification, as defined in the EUnit Reference Manual. erlflags are optional command-line options passed to the Erlang interpreter to execute the test Erlang module. Typically, erlflags defines at least the paths to directories containing the compiled Erlang modules under test, as ‘`-pa path1 path2 ...`’.

  For example, the unit tests associated with Erlang module ‘`testme`’, which compiled code is in subdirectory `src`, can be performed with:

      AT_CHECK_EUNIT([testme_testsuite], [{module, testme}],
                     [-pa "${abs_top_builddir}/src"])

  This macro must be invoked in between `AT_SETUP` and `AT_CLEANUP`.

  Variables `ERL`, `ERLC`, and (optionally) `ERLCFLAGS` must be defined as the path of the Erlang interpreter, the path of the Erlang compiler, and the command-line flags to pass to the compiler, respectively. Those variables should be configured in `configure.ac` using the `AC_ERLANG_PATH_ERL` and `AC_ERLANG_PATH_ERLC` macros, and the configured values of those variables are automatically defined in the testsuite. If `ERL` or `ERLC` is not defined, the test group is skipped.

  If the EUnit library cannot be found, i.e. if module `eunit` cannot be loaded, the test group is skipped. Otherwise, if test-spec is an invalid EUnit test specification, the test group fails. Otherwise, if the EUnit test passes, shell commands run-if-pass are executed or, if the EUnit test fails, shell commands run-if-fail are executed and the test group fails.

  Only the generated test Erlang module is automatically compiled and executed. If test-spec involves testing other Erlang modules, e.g. module ‘`testme`’ in the example above, those modules must be already compiled.

  If the testsuite is run in verbose mode and with the `--verbose` option, EUnit is also run in verbose mode to output more details about individual unit tests.

### 19.3 Running `testsuite` Scripts ¶

Autotest test suites support the following options:

`--help`
`-h`

  Display the list of options and exit successfully.

`--version`
`-V`

  Display the version of the test suite and exit successfully.

`--directory=dir`
`-C dir`

  Change the current directory to dir before creating any files. Useful for running the testsuite in a subdirectory from a top-level Makefile.

`--jobs[=n]`
`-j[n]`

  Run n tests in parallel, if possible. If n is not given, run all given tests in parallel. Note that there should be no space before the argument to `-j`, as `-j number` denotes the separate arguments `-j` and `number`, see below.

  In parallel mode, the standard input device of the testsuite script is not available to commands inside a test group. Furthermore, banner lines are not printed, and the summary line for each test group is output after the test group completes. Summary lines may appear unordered. If verbose and trace output are enabled (see below), they may appear intermixed from concurrently running tests.

  Parallel mode requires the `mkfifo` command to work, and will be silently disabled otherwise.

`--clean`
`-c`

  Remove all the files the test suite might have created and exit. Meant for `clean` Make targets.

`--list`
`-l`

  List all the tests (or only the selection), including their possible keywords.

By default all tests are performed (or described with `--list`) silently in the default environment, but the environment, set of tests, and verbosity level can be tuned:

‘`variable=value`’

  Set the environment variable to value. Use this rather than ‘`FOO=foo ./testsuite`’ as debugging scripts would then run in a different environment.

  The variable `AUTOTEST_PATH` specifies the testing path to prepend to `PATH`. Relative directory names (not starting with ‘`/`’) are considered to be relative to the top level of the package being built. All directories are made absolute, first starting from the top level build tree, then from the source tree. For instance ‘`./testsuite AUTOTEST_PATH=tests:bin`’ for a `/src/foo-1.0` source package built in `/tmp/foo` results in ‘`/tmp/foo/tests:/tmp/foo/bin`’ and then ‘`/src/foo-1.0/tests:/src/foo-1.0/bin`’ being prepended to `PATH`.

‘`number`’
‘`number-number`’
‘`number-`’
‘`-number`’

  Add the corresponding test groups, with obvious semantics, to the selection.

‘`--keywords=keywords`’
‘`-k keywords`’

  Add to the selection the test groups with title or keywords (arguments to `AT_SETUP` or `AT_KEYWORDS`) that match all keywords of the comma separated list keywords, case-insensitively. Use ‘`!`’ immediately before the keyword to invert the selection for this keyword. By default, the keywords match whole words; enclose them in ‘`.*`’ to also match parts of words.

  For example, running

      ./testsuite -k 'autoupdate,.*FUNC.*'

  selects all tests tagged ‘`autoupdate`’ and with tags containing ‘`FUNC`’ (as in ‘`AC_CHECK_FUNC`’, ‘`AC_FUNC_ALLOCA`’, etc.), while

      ./testsuite -k '!autoupdate' -k '.*FUNC.*'

  selects all tests not tagged ‘`autoupdate`’ or with tags containing ‘`FUNC`’.

‘`--errexit`’
‘`-e`’

  If any test fails, immediately abort testing. This implies `--debug`: post test group clean up, and top-level logging are inhibited. This option is meant for the full test suite, it is not really useful for generated debugging scripts. If the testsuite is run in parallel mode using `--jobs`, then concurrently running tests will finish before exiting.

‘`--verbose`’
‘`-v`’

  Force more verbosity in the detailed output of what is being done. This is the default for debugging scripts.

‘`--color`’
‘`--color[=never|auto|always]`’

  Enable colored test results. Without an argument, or with ‘`always`’, test results will be colored. With ‘`never`’, color mode is turned off. Otherwise, if either the macro `AT_COLOR_TESTS` is used by the testsuite author, or the argument ‘`auto`’ is given, then test results are colored if standard output is connected to a terminal.

‘`--debug`’
‘`-d`’

  Do not remove the files after a test group was performed—but they are still removed before, therefore using this option is sane when running several test groups. Create debugging scripts. Do not overwrite the top-level log (in order to preserve a supposedly existing full log file). This is the default for debugging scripts, but it can also be useful to debug the testsuite itself.

‘`--recheck`’

  Add to the selection all test groups that failed or passed unexpectedly during the last non-debugging test run.

‘`--trace`’
‘`-x`’

  Trigger shell tracing of the test groups.

Besides these options accepted by every Autotest testsuite, the testsuite author might have added package-specific options via the `AT_ARG_OPTION` and `AT_ARG_OPTION_ARG` macros (see Writing `testsuite.at`); refer to `testsuite --help` and the package documentation for details.

### 19.4 Making `testsuite` Scripts ¶

For putting Autotest into movement, you need some configuration and makefile machinery. We recommend, at least if your package uses deep or shallow hierarchies, that you use `tests/` as the name of the directory holding all your tests and their makefile. Here is a check list of things to do, followed by an example, taking into consideration whether you are also using Automake.

- Make sure to create the file `package.m4`, which defines the identity of the package. It must define `AT_PACKAGE_STRING`, the full signature of the package, and `AT_PACKAGE_BUGREPORT`, the address to which bug reports should be sent. For sake of completeness, we suggest that you also define `AT_PACKAGE_NAME`, `AT_PACKAGE_TARNAME`, `AT_PACKAGE_VERSION`, and `AT_PACKAGE_URL`. See Initializing `configure`, for a description of these variables. Be sure to distribute `package.m4` and to put it into the source hierarchy: the test suite ought to be shipped! See below for an example.
- Invoke `AC_CONFIG_TESTDIR` in your `configure.ac`.
  Macro: AC_CONFIG_TESTDIR(directory, [test-path = directory]) ¶

    An Autotest test suite is to be configured in directory. This macro causes `directory/atconfig` to be created by `config.status` and sets the default `AUTOTEST_PATH` to test-path (see Running `testsuite` Scripts).

- Still within `configure.ac`, as appropriate, ensure that some `AC_CONFIG_FILES` command includes substitution for `tests/atlocal`.
- Also within your `configure.ac`, arrange for the `AUTOM4TE` variable to be set.
- The appropriate `Makefile` should be modified so the validation in your package is triggered by ‘`make check`’.

The following example demonstrates the above checklist, first by assuming that you are using Automake (see below for tweaks to make to get the same results without Automake). Begin by adding the following lines to your `configure.ac`:

    # Initialize the test suite.
    AC_CONFIG_TESTDIR([tests])
    AC_CONFIG_FILES([tests/Makefile tests/atlocal])
    AM_MISSING_PROG([AUTOM4TE], [autom4te])

Next, add the following lines to your `tests/Makefile.am`, in order to link ‘`make check`’ with a validation suite.

    $(srcdir)/package.m4: $(top_srcdir)/configure.ac
            printf >'$@' '%s\n' \
              '# Signature of the current package.' \
              'm4_define([AT_PACKAGE_NAME], [$(PACKAGE_NAME)])' \
              'm4_define([AT_PACKAGE_TARNAME], [$(PACKAGE_TARNAME)])' \
              'm4_define([AT_PACKAGE_VERSION], [$(PACKAGE_VERSION)])' \
              'm4_define([AT_PACKAGE_STRING], [$(PACKAGE_STRING)])' \
              'm4_define([AT_PACKAGE_URL], [$(PACKAGE_URL)])' \
              'm4_define([AT_PACKAGE_BUGREPORT], [$(PACKAGE_BUGREPORT)])'

    EXTRA_DIST = testsuite.at $(srcdir)/package.m4 $(TESTSUITE) atlocal.in
    TESTSUITE = $(srcdir)/testsuite

    check-local: atconfig atlocal $(TESTSUITE)
            $(SHELL) '$(TESTSUITE)' $(TESTSUITEFLAGS)

    installcheck-local: atconfig atlocal $(TESTSUITE)
            $(SHELL) '$(TESTSUITE)' AUTOTEST_PATH='$(bindir)' \
              $(TESTSUITEFLAGS)

    clean-local:
            test ! -f '$(TESTSUITE)' || \
             $(SHELL) '$(TESTSUITE)' --clean

    AUTOTEST = $(AUTOM4TE) --language=autotest
    $(TESTSUITE): $(srcdir)/testsuite.at $(srcdir)/package.m4
            $(AUTOTEST) -I '$(srcdir)' -o $@.tmp $@.at
            mv $@.tmp $@

Note that the built testsuite is distributed; this is necessary because users might not have Autoconf installed, and thus would not be able to rebuild it. Likewise, the use of Automake’s `AM_MISSING_PROG` will arrange for the definition of `$AUTOM4TE` within the Makefile to provide the user with a nicer error message if they modify a source file to the testsuite, and accidentally trigger the rebuild rules.

You might want to list explicitly the dependencies, i.e., the list of the files `testsuite.at` includes.

If you don’t use Automake, you should make the following tweaks. In your `configure.ac`, replace the `AM_MISSING_PROG` line above with `AC_PATH_PROG([AUTOM4TE], [autom4te], [false])`. You are welcome to also try using the `missing` script from the Automake project instead of `false`, to try to get a nicer error message when the user modifies prerequisites but did not have Autoconf installed, but at that point you may be better off using Automake. Then, take the code suggested above for `tests/Makefile.am` and place it in your `tests/Makefile.in` instead. Add code to your `tests/Makefile.in` to ensure that `$(EXTRA_DIST)` files are distributed, as well as adding the following additional lines to prepare the set of needed Makefile variables:

    subdir = tests
    PACKAGE_NAME = @PACKAGE_NAME@
    PACKAGE_TARNAME = @PACKAGE_TARNAME@
    PACKAGE_VERSION = @PACKAGE_VERSION@
    PACKAGE_STRING = @PACKAGE_STRING@
    PACKAGE_BUGREPORT = @PACKAGE_BUGREPORT@
    PACKAGE_URL = @PACKAGE_URL@
    AUTOM4TE = @AUTOM4TE@

    atconfig: $(top_builddir)/config.status
            cd $(top_builddir) && \
               $(SHELL) ./config.status $(subdir)/$@

    atlocal: $(srcdir)/atlocal.in $(top_builddir)/config.status
            cd $(top_builddir) && \
               $(SHELL) ./config.status $(subdir)/$@

Using the above example (with or without Automake), and assuming you were careful to not initialize ‘`TESTSUITEFLAGS`’ within your makefile, you can now fine-tune test suite execution at runtime by altering this variable, for example:

    make check TESTSUITEFLAGS='-v -d -x 75 -k AC_PROG_CC CFLAGS=-g'

## 20 Frequent Autoconf Questions, with answers ¶

Several questions about Autoconf come up occasionally. Here some of them are addressed.

### 20.1 Distributing `configure` Scripts ¶

    What are the restrictions on distributing configure
    scripts that Autoconf generates?  How does that affect my
    programs that use them?

There are no restrictions on how the configuration scripts that Autoconf produces may be distributed or used. In Autoconf version 1, they were covered by the GNU General Public License. We still encourage software authors to distribute their work under terms like those of the GPL, but doing so is not required to use Autoconf.

Of the other files that might be used with `configure`, `config.h.in` is under whatever copyright you use for your `configure.ac`. `config.sub` and `config.guess` have an exception to the GPL when they are used with an Autoconf-generated `configure` script, which permits you to distribute them under the same terms as the rest of your package. `install-sh` is from the X Consortium and is not copyrighted.

### 20.2 Why Require GNU M4? ¶

    Why does Autoconf require GNU M4?

Many M4 implementations have hard-coded limitations on the size and number of macros that Autoconf exceeds. They also lack several builtin macros that it would be difficult to get along without in a sophisticated application like Autoconf, including:

    m4_builtin
    m4_indir
    m4_bpatsubst
    __file__
    __line__

Autoconf requires version 1.4.8 or later of GNU M4. It works better with version 1.4.16 or later.

Since only software maintainers need to use Autoconf, and since GNU M4 is simple to configure and install, it seems reasonable to require GNU M4 to be installed also. Many maintainers of GNU and other free software already have most of the GNU utilities installed, since they prefer them.

### 20.3 How Can I Bootstrap? ¶

    If Autoconf requires GNU M4 and GNU M4 has an Autoconf
    configure script, how do I bootstrap?  It seems like a chicken
    and egg problem!

This is a misunderstanding. Although GNU M4 does come with a `configure` script produced by Autoconf, Autoconf is not required in order to run the script and install GNU M4. Autoconf is only required if you want to change the M4 `configure` script, which few people have to do (mainly its maintainer).

### 20.4 Why Not Imake? ¶

    Why not use Imake instead of configure scripts?

Several people have written addressing this question, so adaptations of their explanations are included here.

The following answer is based on one written by Richard Pixley:

> Autoconf generated scripts frequently work on machines that it has never been set up to handle before. That is, it does a good job of inferring a configuration for a new system. Imake cannot do this.

> Imake uses a common database of host specific data. For X11, this makes sense because the distribution is made as a collection of tools, by one central authority who has control over the database.

> GNU tools are not released this way. Each GNU tool has a maintainer; these maintainers are scattered across the world. Using a common database would be a maintenance nightmare. Autoconf may appear to be this kind of database, but in fact it is not. Instead of listing host dependencies, it lists program requirements.

> If you view the GNU suite as a collection of native tools, then the problems are similar. But the GNU development tools can be configured as cross tools in almost any host+target permutation. All of these configurations can be installed concurrently. They can even be configured to share host independent files across hosts. Imake doesn’t address these issues.

> Imake templates are a form of standardization. The GNU coding standards address the same issues without necessarily imposing the same restrictions.

Here is some further explanation, written by Per Bothner:

> One of the advantages of Imake is that it is easy to generate large makefiles using the ‘`#include`’ and macro mechanisms of `cpp`. However, `cpp` is not programmable: it has limited conditional facilities, and no looping. And `cpp` cannot inspect its environment.

> All of these problems are solved by using `sh` instead of `cpp`. The shell is fully programmable, has macro substitution, can execute (or source) other shell scripts, and can inspect its environment.

Paul Eggert elaborates more:

> With Autoconf, installers need not assume that Imake itself is already installed and working well. This may not seem like much of an advantage to people who are accustomed to Imake. But on many hosts Imake is not installed or the default installation is not working well, and requiring Imake to install a package hinders the acceptance of that package on those hosts. For example, the Imake template and configuration files might not be installed properly on a host, or the Imake build procedure might wrongly assume that all source files are in one big directory tree, or the Imake configuration might assume one compiler whereas the package or the installer needs to use another, or there might be a version mismatch between the Imake expected by the package and the Imake supported by the host. These problems are much rarer with Autoconf, where each package comes with its own independent configuration processor.

> Also, Imake often suffers from unexpected interactions between `make` and the installer’s C preprocessor. The fundamental problem here is that the C preprocessor was designed to preprocess C programs, not makefiles. This is much less of a problem with Autoconf, which uses the general-purpose preprocessor M4, and where the package’s author (rather than the installer) does the preprocessing in a standard way.

Finally, Mark Eichin notes:

> Imake isn’t all that extensible, either. In order to add new features to Imake, you need to provide your own project template, and duplicate most of the features of the existing one. This means that for a sophisticated project, using the vendor-provided Imake templates fails to provide any leverage—since they don’t cover anything that your own project needs (unless it is an X11 program).

> On the other side, though:

> The one advantage that Imake has over `configure`: `Imakefile` files tend to be much shorter (likewise, less redundant) than `Makefile.in` files. There is a fix to this, however—at least for the Kerberos V5 tree, we’ve modified things to call in common `post.in` and `pre.in` makefile fragments for the entire tree. This means that a lot of common things don’t have to be duplicated, even though they normally are in `configure` setups.

### 20.5 How Do I `#define` Installation Directories? ¶

    My program needs library files, installed in datadir and
    similar.  If I use

    AC_DEFINE_UNQUOTED([DATADIR], [$datadir],
      [Define to the read-only architecture-independent
       data directory.])

    I get

    #define DATADIR "${prefix}/share"

As already explained, this behavior is on purpose, mandated by the GNU Coding Standards, see Installation Directory Variables. There are several means to achieve a similar goal:

- Do not use `AC_DEFINE` but use your makefile to pass the actual value of `datadir` via compilation flags. See Installation Directory Variables, for the details.
- This solution can be simplified when compiling a program: you may either extend the `CPPFLAGS`:

      CPPFLAGS = -DDATADIR='"$(datadir)"' @CPPFLAGS@

  If you are using Automake, you should use `AM_CPPFLAGS` instead:

      AM_CPPFLAGS = -DDATADIR='"$(datadir)"'

  Alternatively, create a dedicated header file:

      DISTCLEANFILES = myprog-paths.h
      myprog-paths.h: Makefile
              printf '%s\n' '#define DATADIR "$(datadir)"' >$@

  The Gnulib module ‘`configmake`’ provides such a header with all the standard directory variables defined, see configmake in GNU Gnulib.

- Use `AC_DEFINE` but have `configure` compute the literal value of `datadir` and others. Many people have wrapped macros to automate this task; for an example, see the macro `AC_DEFINE_DIR` from the Autoconf Macro Archive.

  This solution does not conform to the GNU Coding Standards.

- Note that all the previous solutions hard wire the absolute name of these directories in the executables, which is not a good property. You may try to compute the names relative to `prefix`, and try to find `prefix` at runtime, this way your package is relocatable.

### 20.6 What is `autom4te.cache`? ¶

    What is this directory autom4te.cache?  Can I safely remove it?

In the GNU Build System, `configure.ac` plays a central role and is read by many tools: `autoconf` to create `configure`, `autoheader` to create `config.h.in`, `automake` to create `Makefile.in`, `autoscan` to check the completeness of `configure.ac`, `autoreconf` to check the GNU Build System components that are used. To “read `configure.ac`” actually means to compile it with M4, which can be a long process for complex `configure.ac`.

This is why all these tools, instead of running directly M4, invoke `autom4te` (see Invoking `autom4te`) which, while answering to a specific demand, stores additional information in `autom4te.cache` for future runs. For instance, if you run `autoconf`, behind the scenes, `autom4te` also stores information for the other tools, so that when you invoke `autoheader` or `automake` etc., reprocessing `configure.ac` is not needed. The speed up is frequently 30%, and is increasing with the size of `configure.ac`.

But it is and remains being simply a cache: you can safely remove it.

    Can I permanently get rid of it?

The creation of this cache can be disabled from `~/.autom4te.cfg`, see Customizing `autom4te`, for more details. You should be aware that disabling the cache slows down the Autoconf test suite by 40%. The more GNU Build System components are used, the more the cache is useful; for instance running ‘`autoreconf -f`’ on the Core Utilities is twice slower without the cache although `--force` implies that the cache is not fully exploited, and eight times slower than without `--force`.

### 20.7 Header Present But Cannot Be Compiled ¶

The most important guideline to bear in mind when checking for features is to mimic as much as possible the intended use. Unfortunately, old versions of `AC_CHECK_HEADER` and `AC_CHECK_HEADERS` failed to follow this idea, and called the preprocessor, instead of the compiler, to check for headers. As a result, incompatibilities between headers went unnoticed during configuration, and maintainers finally had to deal with this issue elsewhere.

The transition began with Autoconf 2.56. As of Autoconf 2.64 both checks are performed, and `configure` complains loudly if the compiler and the preprocessor do not agree. However, only the compiler result is considered. As of Autoconf 2.70, only the compiler check is performed.

Consider the following example:

    $ cat number.h
    typedef int number;
    $ cat pi.h
    const number pi = 3;
    $ cat configure.ac
    AC_INIT([Example], [1.0], [bug-example@example.org])
    AC_CHECK_HEADERS([pi.h])
    $ autoconf -Wall
    $ ./configure CPPFLAGS='-I.'
    checking for gcc... gcc
    checking whether the C compiler works... yes
    checking for C compiler default output file name... a.out
    checking for suffix of executables...
    checking whether we are cross compiling... no
    checking for suffix of object files... o
    checking whether the compiler supports GNU C... yes
    checking whether gcc accepts -g... yes
    checking for gcc option to enable C23 features... -std=gnu23
    checking for sys/types.h... yes
    checking for sys/stat.h... yes
    checking for strings.h... yes
    checking for inttypes.h... yes
    checking for stdint.h... yes
    checking for unistd.h... yes
    checking for pi.h... no

The proper way to handle this case is using the fourth argument (see Generic Header Checks):

    $ cat configure.ac
    AC_INIT([Example], [1.0], [bug-example@example.org])
    AC_CHECK_HEADERS([number.h pi.h], [], [],
    [[#ifdef HAVE_NUMBER_H
    # include <number.h>
    #endif
    ]])
    $ autoconf -Wall
    $ ./configure CPPFLAGS='-I.'
    checking for gcc... gcc
    checking whether the C compiler works... yes
    checking for C compiler default output file name... a.out
    checking for suffix of executables...
    checking whether we are cross compiling... no
    checking for suffix of object files... o
    checking whether the compiler supports GNU C... yes
    checking whether gcc accepts -g... yes
    checking for gcc option to enable C23 features... -std=gnu23
    checking for number.h... yes
    checking for pi.h... yes

See Particular Header Checks, for a list of headers with their prerequisites.

### 20.8 Expanded Before Required ¶

Older versions of Autoconf silently built files with incorrect ordering between dependent macros if an outer macro first expanded, then later indirectly required, an inner macro. Starting with Autoconf 2.64, this situation no longer generates out-of-order code, but results in duplicate output and a syntax warning:

    $ cat configure.ac
    ⇒AC_DEFUN([TESTA], [[echo in A
    ⇒if test -n "$SEEN_A" ; then echo duplicate ; fi
    ⇒SEEN_A=:]])
    ⇒AC_DEFUN([TESTB], [AC_REQUIRE([TESTA])[echo in B
    ⇒if test -z "$SEEN_A" ; then echo bug ; fi]])
    ⇒AC_DEFUN([TESTC], [AC_REQUIRE([TESTB])[echo in C]])
    ⇒AC_DEFUN([OUTER], [[echo in OUTER]
    ⇒TESTA
    ⇒TESTC])
    ⇒AC_INIT
    ⇒OUTER
    ⇒AC_OUTPUT
    $ autoconf
    ⇒configure.ac:11: warning: AC_REQUIRE:
    ⇒ 'TESTA' was expanded before it was required
    ⇒configure.ac:4: TESTB is expanded from...
    ⇒configure.ac:6: TESTC is expanded from...
    ⇒configure.ac:7: OUTER is expanded from...
    ⇒configure.ac:11: the top level

To avoid this warning, decide what purpose the macro in question serves. If it only needs to be expanded once (for example, if it provides initialization text used by later macros), then the simplest fix is to change the macro to be declared with `AC_DEFUN_ONCE` (see One-Shot Macros), although this only works in Autoconf 2.64 and newer. A more portable fix is to change all instances of direct calls to instead go through `AC_REQUIRE` (see Prerequisite Macros). If, instead, the macro is parameterized by arguments or by the current definition of other macros in the m4 environment, then the macro should always be directly expanded instead of required.

For another case study, consider this example trimmed down from an actual package. Originally, the package contained shell code and multiple macro invocations at the top level of `configure.ac`:

    AC_DEFUN([FOO], [AC_COMPILE_IFELSE([...])])
    foobar=
    AC_PROG_CC
    FOO

but that was getting complex, so the author wanted to offload some of the text into a new macro in another file included via `aclocal.m4`. The naïve approach merely wraps the text in a new macro:

    AC_DEFUN([FOO], [AC_COMPILE_IFELSE([...])])
    AC_DEFUN([BAR], [
    foobar=
    AC_PROG_CC
    FOO
    ])
    BAR

With older versions of Autoconf, the setting of ‘`foobar=`’ occurs before the single compiler check, as the author intended. But with Autoconf 2.64, this issues the “expanded before it was required” warning for `AC_PROG_CC`, and outputs two copies of the compiler check, one before ‘`foobar=`’, and one after. To understand why this is happening, remember that the use of `AC_COMPILE_IFELSE` includes a call to `AC_REQUIRE([AC_PROG_CC])` under the hood. According to the documented semantics of `AC_REQUIRE`, this means that `AC_PROG_CC` must occur before the body of the outermost `AC_DEFUN`, which in this case is `BAR`, thus preceding the use of ‘`foobar=`’. The older versions of Autoconf were broken with regards to the rules of `AC_REQUIRE`, which explains why the code changed from one over to two copies of `AC_PROG_CC` when upgrading autoconf. In other words, the author was unknowingly relying on a bug exploit to get the desired results, and that exploit broke once the bug was fixed.

So, what recourse does the author have, to restore their intended semantics of setting ‘`foobar=`’ prior to a single compiler check, regardless of whether Autoconf 2.63 or 2.64 is used? One idea is to remember that only `AC_DEFUN` is impacted by `AC_REQUIRE`; there is always the possibility of using the lower-level `m4_define`:

    AC_DEFUN([FOO], [AC_COMPILE_IFELSE([...])])
    m4_define([BAR], [
    foobar=
    AC_PROG_CC
    FOO
    ])
    BAR

This works great if everything is in the same file. However, it does not help in the case where the author wants to have `aclocal` find the definition of `BAR` from its own file, since `aclocal` requires the use of `AC_DEFUN`. In this case, a better fix is to recognize that if `BAR` also uses `AC_REQUIRE`, then there will no longer be direct expansion prior to a subsequent require. Then, by creating yet another helper macro, the author can once again guarantee a single invocation of `AC_PROG_CC`, which will still occur after `foobar=`. The author can also use `AC_BEFORE` to make sure no other macro appearing before `BAR` has triggered an unwanted expansion of `AC_PROG_CC`.

    AC_DEFUN([FOO], [AC_COMPILE_IFELSE([...])])
    AC_DEFUN([BEFORE_CC], [
    foobar=
    ])
    AC_DEFUN([BAR], [
    AC_BEFORE([$0], [AC_PROG_CC])dnl
    AC_REQUIRE([BEFORE_CC])dnl
    AC_REQUIRE([AC_PROG_CC])dnl
    FOO
    ])
    BAR

### 20.9 Debugging `configure` scripts ¶

While in general, `configure` scripts generated by Autoconf strive to be fairly portable to various systems, compilers, shells, and other tools, it may still be necessary to debug a failing test, broken script or makefile, or fix or override an incomplete, faulty, or erroneous test, especially during macro development. Failures can occur at all levels, in M4 syntax or semantics, shell script issues, or due to bugs in the test or the tools invoked by `configure`. Together with the rather arcane error message that `m4` and `make` may produce when their input contains syntax errors, this can make debugging rather painful.

Nevertheless, here is a list of hints and strategies that may help:

- When `autoconf` fails, common causes for error include:
  - mismatched or unbalanced parentheses or braces (see Dealing with unbalanced parentheses),
  - under- or over-quoted macro arguments (see The Autoconf Language, see Quoting and Parameters, see Quotation and Nested Macros),
  - spaces between macro name and opening parenthesis (see The Autoconf Language).

  Typically, it helps to go back to the last working version of the input and compare the differences for each of these errors. Another possibility is to sprinkle pairs of `m4_traceon` and `m4_traceoff` judiciously in the code, either without a parameter or listing some macro names and watch `m4` expand its input verbosely (see Debugging via autom4te).

- Sometimes `autoconf` succeeds but the generated `configure` script has invalid shell syntax. You can detect this case by running ‘`bash -n configure`’ or ‘`sh -n configure`’. If this command fails, the same tips apply, as if `autoconf` had failed.
- Debugging `configure` script execution may be done by sprinkling pairs of `set -x` and `set +x` into the shell script before and after the region that contains a bug. Running the whole script with ‘`shell -vx ./configure 2>&1 | tee log-file`’ with a decent shell may work, but produces lots of output. Here, it can help to search for markers like ‘`checking for`’ a particular test in the log-file.
- Alternatively, you might use a shell with debugging capabilities like bashdb.
- When `configure` tests produce invalid results for your system, it may be necessary to override them:
  - For programs, tools or libraries variables, preprocessor, compiler, or linker flags, it is often sufficient to override them at `make` run time with some care (see `make macro=value` and Submakes). Since this normally won’t cause `configure` to be run again with these changed settings, it may fail if the changed variable would have caused different test results from `configure`, so this may work only for simple differences.
  - Most tests which produce their result in a substituted variable allow to override the test by setting the variable on the `configure` command line (see Compilers and Options, see Defining Variables).
  - Many tests store their result in a cache variable (see Caching Results). This lets you override them either on the `configure` command line as above, or through a primed cache or site file (see Cache Files, see Setting Site Defaults). The name of a cache variable is documented with a test macro or may be inferred from Cache Variable Names; the precise semantics of undocumented variables are often internal details, subject to change.
- Alternatively, `configure` may produce invalid results because of uncaught programming errors, in your package or in an upstream library package. For example, when `AC_CHECK_LIB` fails to find a library with a specified function, always check `config.log`. This will reveal the exact error that produced the failing result: the library linked by `AC_CHECK_LIB` probably has a fatal bug.

Conversely, as macro author, you can make it easier for users of your macro:

- by minimizing dependencies between tests and between test results as far as possible,
- by using `make` variables to factorize and allow override of settings at `make` run time,
- by honoring the GNU Coding Standards and not overriding flags reserved for the user except temporarily during `configure` tests,
- by not requiring users of your macro to use the cache variables. Instead, expose the result of the test via run-if-true and run-if-false parameters. If the result is not a boolean, then provide it through documented shell variables.

## 21 History of Autoconf ¶

This chapter was written by the original author, David MacKenzie.

You may be wondering, Why was Autoconf originally written? How did it get into its present form? (Why does it look like gorilla spit?) If you’re not wondering, then this chapter contains no information useful to you, and you might as well skip it. If you are wondering, then let there be light...

### 21.1 Genesis ¶

In June 1991 I was maintaining many of the GNU utilities for the Free Software Foundation. As they were ported to more platforms and more programs were added, the number of `-D` options that users had to select in the makefile (around 20) became burdensome. Especially for me—I had to test each new release on a bunch of different systems. So I wrote a little shell script to guess some of the correct settings for the fileutils package, and released it as part of fileutils 2.0. That `configure` script worked well enough that the next month I adapted it (by hand) to create similar `configure` scripts for several other GNU utilities packages. Brian Berliner also adapted one of my scripts for his CVS revision control system.

Later that summer, I learned that Richard Stallman and Richard Pixley were developing similar scripts to use in the GNU compiler tools; so I adapted my `configure` scripts to support their evolving interface: using the file name `Makefile.in` as the templates; adding ‘`+srcdir`’, the first option (of many); and creating `config.status` files.

### 21.2 Exodus ¶

As I got feedback from users, I incorporated many improvements, using Emacs to search and replace, cut and paste, similar changes in each of the scripts. As I adapted more GNU utilities packages to use `configure` scripts, updating them all by hand became impractical. Rich Murphey, the maintainer of the GNU graphics utilities, sent me mail saying that the `configure` scripts were great, and asking if I had a tool for generating them that I could send him. No, I thought, but I should! So I started to work out how to generate them. And the journey from the slavery of hand-written `configure` scripts to the abundance and ease of Autoconf began.

Cygnus `configure`, which was being developed at around that time, is table driven; it is meant to deal mainly with a discrete number of system types with a small number of mainly unguessable features (such as details of the object file format). The automatic configuration system that Brian Fox had developed for Bash takes a similar approach. For general use, it seems to me a hopeless cause to try to maintain an up-to-date database of which features each variant of each operating system has. It’s easier and more reliable to check for most features on the fly—especially on hybrid systems that people have hacked on locally or that have patches from vendors installed.

I considered using an architecture similar to that of Cygnus `configure`, where there is a single `configure` script that reads pieces of `configure.in` when run. But I didn’t want to have to distribute all of the feature tests with every package, so I settled on having a different `configure` made from each `configure.in` by a preprocessor. That approach also offered more control and flexibility.

I looked briefly into using the Metaconfig package, by Larry Wall, Harlan Stenn, and Raphael Manfredi, but I decided not to for several reasons. The `Configure` scripts it produces are interactive, which I find quite inconvenient; I didn’t like the ways it checked for some features (such as library functions); I didn’t know that it was still being maintained, and the `Configure` scripts I had seen didn’t work on many modern systems (such as System V R4 and NeXT); it wasn’t flexible in what it could do in response to a feature’s presence or absence; I found it confusing to learn; and it was too big and complex for my needs (I didn’t realize then how much Autoconf would eventually have to grow).

I considered using Perl to generate my style of `configure` scripts, but decided that M4 was better suited to the job of simple textual substitutions: it gets in the way less, because output is implicit. Plus, everyone already has it. (Initially I didn’t rely on the GNU extensions to M4.) Also, some of my friends at the University of Maryland had recently been putting M4 front ends on several programs, including `tvtwm`, and I was interested in trying out a new language.

### 21.3 Leviticus ¶

Since my `configure` scripts determine the system’s capabilities automatically, with no interactive user intervention, I decided to call the program that generates them Autoconfig. But with a version number tacked on, that name would be too long for old Unix file systems, so I shortened it to Autoconf.

In the fall of 1991 I called together a group of fellow questers after the Holy Grail of portability (er, that is, alpha testers) to give me feedback as I encapsulated pieces of my handwritten scripts in M4 macros and continued to add features and improve the techniques used in the checks. Prominent among the testers were François Pinard, who came up with the idea of making an Autoconf shell script to run M4 and check for unresolved macro calls; Richard Pixley, who suggested running the compiler instead of searching the file system to find include files and symbols, for more accurate results; Karl Berry, who got Autoconf to configure TeX and added the macro index to the documentation; and Ian Lance Taylor, who added support for creating a C header file as an alternative to putting `-D` options in a makefile, so he could use Autoconf for his UUCP package. The alpha testers cheerfully adjusted their files again and again as the names and calling conventions of the Autoconf macros changed from release to release. They all contributed many specific checks, great ideas, and bug fixes.

### 21.4 Numbers ¶

In July 1992, after months of alpha testing, I released Autoconf 1.0, and converted many GNU packages to use it. I was surprised by how positive the reaction to it was. More people started using it than I could keep track of, including people working on software that wasn’t part of the GNU Project (such as TCL, FSP, and Kerberos V5). Autoconf continued to improve rapidly, as many people using the `configure` scripts reported problems they encountered.

Autoconf turned out to be a good torture test for M4 implementations. Unix M4 started to dump core because of the length of the macros that Autoconf defined, and several bugs showed up in GNU M4 as well. Eventually, we realized that we needed to use some features that only GNU M4 has. 4.3BSD M4, in particular, has an impoverished set of builtin macros; the System V version is better, but still doesn’t provide everything we need.

More development occurred as people put Autoconf under more stresses (and to uses I hadn’t anticipated). Karl Berry added checks for X11. david zuhn contributed C++ support. François Pinard made it diagnose invalid arguments. Jim Blandy bravely coerced it into configuring GNU Emacs, laying the groundwork for several later improvements. Roland McGrath got it to configure the GNU C Library, wrote the `autoheader` script to automate the creation of C header file templates, and added a `--verbose` option to `configure`. Noah Friedman added the `--autoconf-dir` option and `AC_MACRODIR` environment variable. (He also coined the term autoconfiscate to mean “adapt a software package to use Autoconf”.) Roland and Noah improved the quoting protection in `AC_DEFINE` and fixed many bugs, especially when I got sick of dealing with portability problems from February through June, 1993.

### 21.5 Deuteronomy ¶

A long wish list for major features had accumulated, and the effect of several years of patching by various people had left some residual cruft. In April 1994, while working for Cygnus Support, I began a major revision of Autoconf. I added most of the features of the Cygnus `configure` that Autoconf had lacked, largely by adapting the relevant parts of Cygnus `configure` with the help of david zuhn and Ken Raeburn. These features include support for using `config.sub`, `config.guess`, `--host`, and `--target`; making links to files; and running `configure` scripts in subdirectories. Adding these features enabled Ken to convert GNU `as`, and Rob Savoye to convert DejaGNU, to using Autoconf.

I added more features in response to other peoples’ requests. Many people had asked for `configure` scripts to share the results of the checks between runs, because (particularly when configuring a large source tree, like Cygnus does) they were frustratingly slow. Mike Haertel suggested adding site-specific initialization scripts. People distributing software that had to unpack on MS-DOS asked for a way to override the `.in` extension on the file names, which produced file names like `config.h.in` containing two dots. Jim Avera did an extensive examination of the problems with quoting in `AC_DEFINE` and `AC_SUBST`; his insights led to significant improvements. Richard Stallman asked that compiler output be sent to `config.log` instead of `/dev/null`, to help people debug the Emacs `configure` script.

I made some other changes because of my dissatisfaction with the quality of the program. I made the messages showing results of the checks less ambiguous, always printing a result. I regularized the names of the macros and cleaned up coding style inconsistencies. I added some auxiliary utilities that I had developed to help convert source code packages to use Autoconf. With the help of François Pinard, I made the macros not interrupt each others’ messages. (That feature revealed some performance bottlenecks in GNU M4, which he hastily corrected!) I reorganized the documentation around problems people want to solve. And I began a test suite, because experience had shown that Autoconf has a pronounced tendency to regress when we change it.

Again, several alpha testers gave invaluable feedback, especially François Pinard, Jim Meyering, Karl Berry, Rob Savoye, Ken Raeburn, and Mark Eichin.

Finally, version 2.0 was ready. And there was much rejoicing. (And I have free time again. I think. Yeah, right.)

## Appendix A GNU Free Documentation License ¶

Version 1.3, 3 November 2008

    Copyright © 2000, 2001, 2002, 2007, 2008 Free Software Foundation, Inc.
    https://fsf.org/

    Everyone is permitted to copy and distribute verbatim copies
    of this license document, but changing it is not allowed.

0. PREAMBLE

   The purpose of this License is to make a manual, textbook, or other functional and useful document free in the sense of freedom: to assure everyone the effective freedom to copy and redistribute it, with or without modifying it, either commercially or noncommercially. Secondarily, this License preserves for the author and publisher a way to get credit for their work, while not being considered responsible for modifications made by others.

   This License is a kind of “copyleft”, which means that derivative works of the document must themselves be free in the same sense. It complements the GNU General Public License, which is a copyleft license designed for free software.

   We have designed this License in order to use it for manuals for free software, because free software needs free documentation: a free program should come with manuals providing the same freedoms that the software does. But this License is not limited to software manuals; it can be used for any textual work, regardless of subject matter or whether it is published as a printed book. We recommend this License principally for works whose purpose is instruction or reference.

1. APPLICABILITY AND DEFINITIONS

   This License applies to any manual or other work, in any medium, that contains a notice placed by the copyright holder saying it can be distributed under the terms of this License. Such a notice grants a world-wide, royalty-free license, unlimited in duration, to use that work under the conditions stated herein. The “Document”, below, refers to any such manual or work. Any member of the public is a licensee, and is addressed as “you”. You accept the license if you copy, modify or distribute the work in a way requiring permission under copyright law.

   A “Modified Version” of the Document means any work containing the Document or a portion of it, either copied verbatim, or with modifications and/or translated into another language.

   A “Secondary Section” is a named appendix or a front-matter section of the Document that deals exclusively with the relationship of the publishers or authors of the Document to the Document’s overall subject (or to related matters) and contains nothing that could fall directly within that overall subject. (Thus, if the Document is in part a textbook of mathematics, a Secondary Section may not explain any mathematics.) The relationship could be a matter of historical connection with the subject or with related matters, or of legal, commercial, philosophical, ethical or political position regarding them.

   The “Invariant Sections” are certain Secondary Sections whose titles are designated, as being those of Invariant Sections, in the notice that says that the Document is released under this License. If a section does not fit the above definition of Secondary then it is not allowed to be designated as Invariant. The Document may contain zero Invariant Sections. If the Document does not identify any Invariant Sections then there are none.

   The “Cover Texts” are certain short passages of text that are listed, as Front-Cover Texts or Back-Cover Texts, in the notice that says that the Document is released under this License. A Front-Cover Text may be at most 5 words, and a Back-Cover Text may be at most 25 words.

   A “Transparent” copy of the Document means a machine-readable copy, represented in a format whose specification is available to the general public, that is suitable for revising the document straightforwardly with generic text editors or (for images composed of pixels) generic paint programs or (for drawings) some widely available drawing editor, and that is suitable for input to text formatters or for automatic translation to a variety of formats suitable for input to text formatters. A copy made in an otherwise Transparent file format whose markup, or absence of markup, has been arranged to thwart or discourage subsequent modification by readers is not Transparent. An image format is not Transparent if used for any substantial amount of text. A copy that is not “Transparent” is called “Opaque”.

   Examples of suitable formats for Transparent copies include plain ASCII without markup, Texinfo input format, LaTeX input format, SGML or XML using a publicly available DTD, and standard-conforming simple HTML, PostScript or PDF designed for human modification. Examples of transparent image formats include PNG, XCF and JPG. Opaque formats include proprietary formats that can be read and edited only by proprietary word processors, SGML or XML for which the DTD and/or processing tools are not generally available, and the machine-generated HTML, PostScript or PDF produced by some word processors for output purposes only.

   The “Title Page” means, for a printed book, the title page itself, plus such following pages as are needed to hold, legibly, the material this License requires to appear in the title page. For works in formats which do not have any title page as such, “Title Page” means the text near the most prominent appearance of the work’s title, preceding the beginning of the body of the text.

   The “publisher” means any person or entity that distributes copies of the Document to the public.

   A section “Entitled XYZ” means a named subunit of the Document whose title either is precisely XYZ or contains XYZ in parentheses following text that translates XYZ in another language. (Here XYZ stands for a specific section name mentioned below, such as “Acknowledgements”, “Dedications”, “Endorsements”, or “History”.) To “Preserve the Title” of such a section when you modify the Document means that it remains a section “Entitled XYZ” according to this definition.

   The Document may include Warranty Disclaimers next to the notice which states that this License applies to the Document. These Warranty Disclaimers are considered to be included by reference in this License, but only as regards disclaiming warranties: any other implication that these Warranty Disclaimers may have is void and has no effect on the meaning of this License.

2. VERBATIM COPYING

   You may copy and distribute the Document in any medium, either commercially or noncommercially, provided that this License, the copyright notices, and the license notice saying this License applies to the Document are reproduced in all copies, and that you add no other conditions whatsoever to those of this License. You may not use technical measures to obstruct or control the reading or further copying of the copies you make or distribute. However, you may accept compensation in exchange for copies. If you distribute a large enough number of copies you must also follow the conditions in section 3.

   You may also lend copies, under the same conditions stated above, and you may publicly display copies.

3. COPYING IN QUANTITY

   If you publish printed copies (or copies in media that commonly have printed covers) of the Document, numbering more than 100, and the Document’s license notice requires Cover Texts, you must enclose the copies in covers that carry, clearly and legibly, all these Cover Texts: Front-Cover Texts on the front cover, and Back-Cover Texts on the back cover. Both covers must also clearly and legibly identify you as the publisher of these copies. The front cover must present the full title with all words of the title equally prominent and visible. You may add other material on the covers in addition. Copying with changes limited to the covers, as long as they preserve the title of the Document and satisfy these conditions, can be treated as verbatim copying in other respects.

   If the required texts for either cover are too voluminous to fit legibly, you should put the first ones listed (as many as fit reasonably) on the actual cover, and continue the rest onto adjacent pages.

   If you publish or distribute Opaque copies of the Document numbering more than 100, you must either include a machine-readable Transparent copy along with each Opaque copy, or state in or with each Opaque copy a computer-network location from which the general network-using public has access to download using public-standard network protocols a complete Transparent copy of the Document, free of added material. If you use the latter option, you must take reasonably prudent steps, when you begin distribution of Opaque copies in quantity, to ensure that this Transparent copy will remain thus accessible at the stated location until at least one year after the last time you distribute an Opaque copy (directly or through your agents or retailers) of that edition to the public.

   It is requested, but not required, that you contact the authors of the Document well before redistributing any large number of copies, to give them a chance to provide you with an updated version of the Document.

4. MODIFICATIONS

   You may copy and distribute a Modified Version of the Document under the conditions of sections 2 and 3 above, provided that you release the Modified Version under precisely this License, with the Modified Version filling the role of the Document, thus licensing distribution and modification of the Modified Version to whoever possesses a copy of it. In addition, you must do these things in the Modified Version:

   1. Use in the Title Page (and on the covers, if any) a title distinct from that of the Document, and from those of previous versions (which should, if there were any, be listed in the History section of the Document). You may use the same title as a previous version if the original publisher of that version gives permission.
   2. List on the Title Page, as authors, one or more persons or entities responsible for authorship of the modifications in the Modified Version, together with at least five of the principal authors of the Document (all of its principal authors, if it has fewer than five), unless they release you from this requirement.
   3. State on the Title page the name of the publisher of the Modified Version, as the publisher.
   4. Preserve all the copyright notices of the Document.
   5. Add an appropriate copyright notice for your modifications adjacent to the other copyright notices.
   6. Include, immediately after the copyright notices, a license notice giving the public permission to use the Modified Version under the terms of this License, in the form shown in the Addendum below.
   7. Preserve in that license notice the full lists of Invariant Sections and required Cover Texts given in the Document’s license notice.
   8. Include an unaltered copy of this License.
   9. Preserve the section Entitled “History”, Preserve its Title, and add to it an item stating at least the title, year, new authors, and publisher of the Modified Version as given on the Title Page. If there is no section Entitled “History” in the Document, create one stating the title, year, authors, and publisher of the Document as given on its Title Page, then add an item describing the Modified Version as stated in the previous sentence.
   10. Preserve the network location, if any, given in the Document for public access to a Transparent copy of the Document, and likewise the network locations given in the Document for previous versions it was based on. These may be placed in the “History” section. You may omit a network location for a work that was published at least four years before the Document itself, or if the original publisher of the version it refers to gives permission.
   11. For any section Entitled “Acknowledgements” or “Dedications”, Preserve the Title of the section, and preserve in the section all the substance and tone of each of the contributor acknowledgements and/or dedications given therein.
   12. Preserve all the Invariant Sections of the Document, unaltered in their text and in their titles. Section numbers or the equivalent are not considered part of the section titles.
   13. Delete any section Entitled “Endorsements”. Such a section may not be included in the Modified Version.
   14. Do not retitle any existing section to be Entitled “Endorsements” or to conflict in title with any Invariant Section.
   15. Preserve any Warranty Disclaimers.

   If the Modified Version includes new front-matter sections or appendices that qualify as Secondary Sections and contain no material copied from the Document, you may at your option designate some or all of these sections as invariant. To do this, add their titles to the list of Invariant Sections in the Modified Version’s license notice. These titles must be distinct from any other section titles.

   You may add a section Entitled “Endorsements”, provided it contains nothing but endorsements of your Modified Version by various parties—for example, statements of peer review or that the text has been approved by an organization as the authoritative definition of a standard.

   You may add a passage of up to five words as a Front-Cover Text, and a passage of up to 25 words as a Back-Cover Text, to the end of the list of Cover Texts in the Modified Version. Only one passage of Front-Cover Text and one of Back-Cover Text may be added by (or through arrangements made by) any one entity. If the Document already includes a cover text for the same cover, previously added by you or by arrangement made by the same entity you are acting on behalf of, you may not add another; but you may replace the old one, on explicit permission from the previous publisher that added the old one.

   The author(s) and publisher(s) of the Document do not by this License give permission to use their names for publicity for or to assert or imply endorsement of any Modified Version.

5. COMBINING DOCUMENTS

   You may combine the Document with other documents released under this License, under the terms defined in section 4 above for modified versions, provided that you include in the combination all of the Invariant Sections of all of the original documents, unmodified, and list them all as Invariant Sections of your combined work in its license notice, and that you preserve all their Warranty Disclaimers.

   The combined work need only contain one copy of this License, and multiple identical Invariant Sections may be replaced with a single copy. If there are multiple Invariant Sections with the same name but different contents, make the title of each such section unique by adding at the end of it, in parentheses, the name of the original author or publisher of that section if known, or else a unique number. Make the same adjustment to the section titles in the list of Invariant Sections in the license notice of the combined work.

   In the combination, you must combine any sections Entitled “History” in the various original documents, forming one section Entitled “History”; likewise combine any sections Entitled “Acknowledgements”, and any sections Entitled “Dedications”. You must delete all sections Entitled “Endorsements.”

6. COLLECTIONS OF DOCUMENTS

   You may make a collection consisting of the Document and other documents released under this License, and replace the individual copies of this License in the various documents with a single copy that is included in the collection, provided that you follow the rules of this License for verbatim copying of each of the documents in all other respects.

   You may extract a single document from such a collection, and distribute it individually under this License, provided you insert a copy of this License into the extracted document, and follow this License in all other respects regarding verbatim copying of that document.

7. AGGREGATION WITH INDEPENDENT WORKS

   A compilation of the Document or its derivatives with other separate and independent documents or works, in or on a volume of a storage or distribution medium, is called an “aggregate” if the copyright resulting from the compilation is not used to limit the legal rights of the compilation’s users beyond what the individual works permit. When the Document is included in an aggregate, this License does not apply to the other works in the aggregate which are not themselves derivative works of the Document.

   If the Cover Text requirement of section 3 is applicable to these copies of the Document, then if the Document is less than one half of the entire aggregate, the Document’s Cover Texts may be placed on covers that bracket the Document within the aggregate, or the electronic equivalent of covers if the Document is in electronic form. Otherwise they must appear on printed covers that bracket the whole aggregate.

8. TRANSLATION

   Translation is considered a kind of modification, so you may distribute translations of the Document under the terms of section 4. Replacing Invariant Sections with translations requires special permission from their copyright holders, but you may include translations of some or all Invariant Sections in addition to the original versions of these Invariant Sections. You may include a translation of this License, and all the license notices in the Document, and any Warranty Disclaimers, provided that you also include the original English version of this License and the original versions of those notices and disclaimers. In case of a disagreement between the translation and the original version of this License or a notice or disclaimer, the original version will prevail.

   If a section in the Document is Entitled “Acknowledgements”, “Dedications”, or “History”, the requirement (section 4) to Preserve its Title (section 1) will typically require changing the actual title.

9. TERMINATION

   You may not copy, modify, sublicense, or distribute the Document except as expressly provided under this License. Any attempt otherwise to copy, modify, sublicense, or distribute it is void, and will automatically terminate your rights under this License.

   However, if you cease all violation of this License, then your license from a particular copyright holder is reinstated (a) provisionally, unless and until the copyright holder explicitly and finally terminates your license, and (b) permanently, if the copyright holder fails to notify you of the violation by some reasonable means prior to 60 days after the cessation.

   Moreover, your license from a particular copyright holder is reinstated permanently if the copyright holder notifies you of the violation by some reasonable means, this is the first time you have received notice of violation of this License (for any work) from that copyright holder, and you cure the violation prior to 30 days after your receipt of the notice.

   Termination of your rights under this section does not terminate the licenses of parties who have received copies or rights from you under this License. If your rights have been terminated and not permanently reinstated, receipt of a copy of some or all of the same material does not give you any rights to use it.

10. FUTURE REVISIONS OF THIS LICENSE

    The Free Software Foundation may publish new, revised versions of the GNU Free Documentation License from time to time. Such new versions will be similar in spirit to the present version, but may differ in detail to address new problems or concerns. See https://www.gnu.org/licenses/.

    Each version of the License is given a distinguishing version number. If the Document specifies that a particular numbered version of this License “or any later version” applies to it, you have the option of following the terms and conditions either of that specified version or of any later version that has been published (not as a draft) by the Free Software Foundation. If the Document does not specify a version number of this License, you may choose any version ever published (not as a draft) by the Free Software Foundation. If the Document specifies that a proxy can decide which future versions of this License can be used, that proxy’s public statement of acceptance of a version permanently authorizes you to choose that version for the Document.

11. RELICENSING

    “Massive Multiauthor Collaboration Site” (or “MMC Site”) means any World Wide Web server that publishes copyrightable works and also provides prominent facilities for anybody to edit those works. A public wiki that anybody can edit is an example of such a server. A “Massive Multiauthor Collaboration” (or “MMC”) contained in the site means any set of copyrightable works thus published on the MMC site.

    “CC-BY-SA” means the Creative Commons Attribution-Share Alike 3.0 license published by Creative Commons Corporation, a not-for-profit corporation with a principal place of business in San Francisco, California, as well as future copyleft versions of that license published by that same organization.

    “Incorporate” means to publish or republish a Document, in whole or in part, as part of another Document.

    An MMC is “eligible for relicensing” if it is licensed under this License, and if all works that were first published under this License somewhere other than this MMC, and subsequently incorporated in whole or in part into the MMC, (1) had no cover texts or invariant sections, and (2) were thus incorporated prior to November 1, 2008.

    The operator of an MMC Site may republish an MMC contained in the site under CC-BY-SA on the same site at any time before August 1, 2009, provided the MMC is eligible for relicensing.

### ADDENDUM: How to use this License for your documents ¶

To use this License in a document you have written, include a copy of the License in the document and put the following copyright and license notices just after the title page:

      Copyright (C)  year  your name.
      Permission is granted to copy, distribute and/or modify this document
      under the terms of the GNU Free Documentation License, Version 1.3
      or any later version published by the Free Software Foundation;
      with no Invariant Sections, no Front-Cover Texts, and no Back-Cover
      Texts.  A copy of the license is included in the section entitled ``GNU
      Free Documentation License''.

If you have Invariant Sections, Front-Cover Texts and Back-Cover Texts, replace the “with…Texts.” line with this:

        with the Invariant Sections being list their titles, with
        the Front-Cover Texts being list, and with the Back-Cover Texts
        being list.

If you have Invariant Sections without Cover Texts, or some other combination of the three, merge those two alternatives to suit the situation.

If your document contains nontrivial examples of program code, we recommend releasing these examples in parallel under your choice of free software license, such as the GNU General Public License, to permit their use in free software.

## Appendix B Indices ¶

### B.1 Environment Variable Index ¶

This is an alphabetical list of the environment variables that might influence Autoconf checks.

Jump to: | _ A C D E F G H I L M N O P R S T W X Y

| Index Entry | Section

_
| `_` | Special Shell Variables

A
| `A68FLAGS` | Preset Output Variables

C
| `CC` | C Compiler
| `CDPATH` | Special Shell Variables
| `CFLAGS` | Preset Output Variables
| `CFLAGS` | C Compiler
| `CLICOLOR_FORCE` | Special Shell Variables
| `CONFIG_COMMANDS` | Obsolete config.status Use
| `CONFIG_FILES` | Obsolete config.status Use
| `CONFIG_HEADERS` | Obsolete config.status Use
| `CONFIG_LINKS` | Obsolete config.status Use
| `CONFIG_SHELL` | config.status Invocation
| `CONFIG_SITE` | Site Defaults
| `CONFIG_STATUS` | config.status Invocation
| `CPP` | C Compiler
| `CPPFLAGS` | Preset Output Variables
| `CXX` | C++ Compiler
| `CXXCPP` | C++ Compiler
| `CXXFLAGS` | Preset Output Variables
| `CXXFLAGS` | C++ Compiler
| `CYGWIN` | Obsolete Macros

D
| `DUALCASE` | Special Shell Variables

E
| `ENV` | Special Shell Variables
| `ERL` | Erlang Compiler and Interpreter
| `ERLC` | Erlang Compiler and Interpreter
| `ERLCFLAGS` | Preset Output Variables
| `ERLCFLAGS` | Erlang Compiler and Interpreter

F
| `F77` | Fortran Compiler
| `FC` | Fortran Compiler
| `FCFLAGS` | Preset Output Variables
| `FCFLAGS` | Fortran Compiler
| `FFLAGS` | Preset Output Variables
| `FFLAGS` | Fortran Compiler
| `FPATH` | Special Shell Variables

G
| `GOFLAGS` | Preset Output Variables
| `GREP_OPTIONS` | Special Shell Variables

H
| `HOME` | Special Shell Variables
| `HOME` | Limitations of Builtins

I
| `IFS` | Special Shell Variables

L
| `LANG` | Special Shell Variables
| `LANGUAGE` | Special Shell Variables
| `LC_ADDRESS` | Special Shell Variables
| `LC_ALL` | Initialization Macros
| `LC_ALL` | Special Shell Variables
| `LC_COLLATE` | Special Shell Variables
| `LC_CTYPE` | Special Shell Variables
| `LC_IDENTIFICATION` | Special Shell Variables
| `LC_MEASUREMENT` | Special Shell Variables
| `LC_MESSAGES` | Special Shell Variables
| `LC_MONETARY` | Special Shell Variables
| `LC_NAME` | Special Shell Variables
| `LC_NUMERIC` | Special Shell Variables
| `LC_PAPER` | Special Shell Variables
| `LC_TELEPHONE` | Special Shell Variables
| `LC_TIME` | Special Shell Variables
| `LDFLAGS` | Preset Output Variables
| `LIBS` | Preset Output Variables
| `LINENO` | Initialization Macros
| `LINENO` | Special Shell Variables

M
| `M4` | autom4te Invocation
| `MAIL` | Special Shell Variables
| `MAILPATH` | Special Shell Variables

N
| `NULLCMD` | Special Shell Variables

O
| `OBJC` | Objective C Compiler
| `OBJCFLAGS` | Preset Output Variables
| `OBJCFLAGS` | Objective C Compiler
| `OBJCPP` | Objective C Compiler
| `OBJCXX` | Objective C++ Compiler
| `OBJCXXCPP` | Objective C++ Compiler
| `OBJCXXFLAGS` | Preset Output Variables
| `OBJCXXFLAGS` | Objective C++ Compiler
| `OLDPWD` | Special Shell Variables
| `OLDPWD` | Limitations of Builtins
| `options` | Special Shell Variables

P
| `PATH_SEPARATOR` | Special Shell Variables
| `POSIXLY_CORRECT` | Special Shell Variables
| `PS1` | Special Shell Variables
| `PS2` | Special Shell Variables
| `PS4` | Special Shell Variables
| `PWD` | Special Shell Variables
| `PWD` | Limitations of Builtins

R
| `RANDOM` | Special Shell Variables

S
| `SHELL` | Initialization Macros
| `SIMPLE_BACKUP_SUFFIX` | autoupdate Invocation
| `status` | Special Shell Variables

T
| `TMPDIR` | Initialization Macros

W
| `WARNINGS` | autoconf Invocation
| `WARNINGS` | autoreconf Invocation
| `WARNINGS` | autoheader Invocation
| `WARNINGS` | autom4te Invocation

X
| `XMKMF` | System Services

Y
| `YACC` | Particular Programs
| `YFLAGS` | Particular Programs

### B.2 Output Variable Index ¶

This is an alphabetical list of the variables that Autoconf can substitute into files that it creates, typically one or more makefiles. See Setting Output Variables, for more information on how this is done.

Jump to: | A B C D E F G H I K L M N O P R S T X Y

| Index Entry | Section

A
| `A68FLAGS` | Preset Output Variables
| `abs_builddir` | Preset Output Variables
| `abs_srcdir` | Preset Output Variables
| `abs_top_builddir` | Preset Output Variables
| `abs_top_srcdir` | Preset Output Variables
| `ac_empty` | Fortran Compiler
| `ALLOCA` | Particular Functions
| `AR` | Particular Programs
| `AWK` | Particular Programs

B
| `bindir` | Installation Directory Variables
| `build` | Canonicalizing
| `build_alias` | Canonicalizing
| `build_cpu` | Canonicalizing
| `build_os` | Canonicalizing
| `build_vendor` | Canonicalizing
| `builddir` | Preset Output Variables

C
| `CC` | C Compiler
| `CC` | System Services
| `CFLAGS` | Preset Output Variables
| `CFLAGS` | C Compiler
| `configure_input` | Preset Output Variables
| `CPP` | C Compiler
| `CPP` | C Compiler
| `CPPFLAGS` | Preset Output Variables
| `cross_compiling` | Runtime
| `CXX` | C++ Compiler
| `CXXCPP` | C++ Compiler
| `CXXFLAGS` | Preset Output Variables
| `CXXFLAGS` | C++ Compiler

D
| `datadir` | Installation Directory Variables
| `datarootdir` | Installation Directory Variables
| `DEFS` | Preset Output Variables
| `docdir` | Installation Directory Variables
| `dvidir` | Installation Directory Variables

E
| `ECHO_C` | Preset Output Variables
| `ECHO_N` | Preset Output Variables
| `ECHO_T` | Preset Output Variables
| `EGREP` | Particular Programs
| `ERL` | Erlang Compiler and Interpreter
| `ERL` | Language Choice
| `ERL` | Running the Compiler
| `ERLANG_ERTS_VER` | Erlang Libraries
| `ERLANG_INSTALL_LIB_DIR` | Installation Directory Variables
| `ERLANG_INSTALL_LIB_DIR` | Erlang Libraries
| `ERLANG_INSTALL_LIB_DIR_library` | Installation Directory Variables
| `ERLANG_INSTALL_LIB_DIR_library` | Erlang Libraries
| `ERLANG_LIB_DIR` | Erlang Libraries
| `ERLANG_LIB_DIR_library` | Erlang Libraries
| `ERLANG_LIB_VER_library` | Erlang Libraries
| `ERLANG_ROOT_DIR` | Erlang Libraries
| `ERLC` | Erlang Compiler and Interpreter
| `ERLC` | Language Choice
| `ERLCFLAGS` | Preset Output Variables
| `ERLCFLAGS` | Erlang Compiler and Interpreter
| `ERLCFLAGS` | Language Choice
| `exec_prefix` | Installation Directory Variables
| `EXEEXT` | Compilers and Preprocessors
| `EXEEXT` | Obsolete Macros

F
| `F77` | Fortran Compiler
| `FC` | Fortran Compiler
| `FC_MODEXT` | Fortran Compiler
| `FC_MODINC` | Fortran Compiler
| `FC_MODOUT` | Fortran Compiler
| `FCFLAGS` | Preset Output Variables
| `FCFLAGS` | Fortran Compiler
| `FCLIBS` | Fortran Compiler
| `FFLAGS` | Preset Output Variables
| `FFLAGS` | Fortran Compiler
| `FGREP` | Particular Programs
| `FLIBS` | Fortran Compiler

G
| `GETGROUPS_LIB` | Particular Functions
| `GETLOADAVG_LIBS` | Particular Functions
| `GOFLAGS` | Preset Output Variables
| `GREP` | Particular Programs

H
| `host` | Canonicalizing
| `host_alias` | Canonicalizing
| `host_cpu` | Canonicalizing
| `host_os` | Canonicalizing
| `host_vendor` | Canonicalizing
| `htmldir` | Installation Directory Variables

I
| `includedir` | Installation Directory Variables
| `infodir` | Installation Directory Variables
| `INSTALL` | Particular Programs
| `INSTALL_DATA` | Particular Programs
| `INSTALL_PROGRAM` | Particular Programs
| `INSTALL_SCRIPT` | Particular Programs

K
| `KMEM_GROUP` | Particular Functions

L
| `LDFLAGS` | Preset Output Variables
| `LEX` | Particular Programs
| `LEX_OUTPUT_ROOT` | Particular Programs
| `LEXLIB` | Particular Programs
| `libdir` | Installation Directory Variables
| `libexecdir` | Installation Directory Variables
| `LIBOBJDIR` | AC_LIBOBJ vs LIBOBJS
| `LIBOBJS` | Particular Functions
| `LIBOBJS` | Particular Functions
| `LIBOBJS` | Particular Functions
| `LIBOBJS` | Generic Functions
| `LIBOBJS` | Generic Functions
| `LIBOBJS` | Particular Structures
| `LIBS` | Preset Output Variables
| `LIBS` | Obsolete Macros
| `LIBS` | Obsolete Macros
| `LIBS` | Obsolete Macros
| `LN_S` | Particular Programs
| `localedir` | Installation Directory Variables
| `localstatedir` | Installation Directory Variables

M
| `mandir` | Installation Directory Variables
| `MKDIR_P` | Particular Programs

N
| `NEED_SETGID` | Particular Functions

O
| `OBJC` | Objective C Compiler
| `OBJCFLAGS` | Preset Output Variables
| `OBJCFLAGS` | Objective C Compiler
| `OBJCPP` | Objective C Compiler
| `OBJCXX` | Objective C++ Compiler
| `OBJCXXCPP` | Objective C++ Compiler
| `OBJCXXFLAGS` | Preset Output Variables
| `OBJCXXFLAGS` | Objective C++ Compiler
| `OBJEXT` | Compilers and Preprocessors
| `OBJEXT` | Obsolete Macros
| `oldincludedir` | Installation Directory Variables
| `OPENMP_CFLAGS` | Generic Compiler Characteristics
| `OPENMP_CXXFLAGS` | Generic Compiler Characteristics
| `OPENMP_FCFLAGS` | Generic Compiler Characteristics
| `OPENMP_FFLAGS` | Generic Compiler Characteristics

P
| `PACKAGE_BUGREPORT` | Initializing configure
| `PACKAGE_NAME` | Initializing configure
| `PACKAGE_STRING` | Initializing configure
| `PACKAGE_TARNAME` | Initializing configure
| `PACKAGE_URL` | Initializing configure
| `PACKAGE_VERSION` | Initializing configure
| `pdfdir` | Installation Directory Variables
| `POW_LIB` | Particular Functions
| `prefix` | Installation Directory Variables
| `program_transform_name` | Transforming Names
| `psdir` | Installation Directory Variables

R
| `RANLIB` | Particular Programs
| `runstatedir` | Installation Directory Variables

S
| `sbindir` | Installation Directory Variables
| `SED` | Particular Programs
| `SET_MAKE` | Output
| `sharedstatedir` | Installation Directory Variables
| `srcdir` | Preset Output Variables
| `subdirs` | Subdirectories
| `sysconfdir` | Installation Directory Variables

T
| `target` | Canonicalizing
| `target_alias` | Canonicalizing
| `target_cpu` | Canonicalizing
| `target_os` | Canonicalizing
| `target_vendor` | Canonicalizing
| `tmp` | Initialization Macros
| `top_build_prefix` | Preset Output Variables
| `top_builddir` | Preset Output Variables
| `top_srcdir` | Preset Output Variables

X
| `X_CFLAGS` | System Services
| `X_EXTRA_LIBS` | System Services
| `X_LIBS` | System Services
| `X_PRE_LIBS` | System Services

Y
| `YACC` | Particular Programs

### B.3 Preprocessor Symbol Index ¶

This is an alphabetical list of the C preprocessor symbols that the Autoconf macros define. To work with Autoconf, C source code needs to use these names in `#if` or `#ifdef` directives.

Jump to: | _ A C D F G H I L M N O P R S T U V W X Y

| Index Entry | Section

_
| `__CHAR_UNSIGNED__` | C Compiler
| `__EXTENSIONS__` | C and POSIX Variants
| `__PROTOTYPES` | C Compiler
| `__STDC_NO_VLA__` | C Compiler
| `__STDC_WANT_DEC_FP__` | C and POSIX Variants
| `__STDC_WANT_IEC_60559_ATTRIBS_EXT__` | C and POSIX Variants
| `__STDC_WANT_IEC_60559_BFP_EXT__` | C and POSIX Variants
| `__STDC_WANT_IEC_60559_DFP_EXT__` | C and POSIX Variants
| `__STDC_WANT_IEC_60559_EXT__` | C and POSIX Variants
| `__STDC_WANT_IEC_60559_FUNCS_EXT__` | C and POSIX Variants
| `__STDC_WANT_IEC_60559_TYPES_EXT__` | C and POSIX Variants
| `__STDC_WANT_LIB_EXT1__` | C and POSIX Variants
| `__STDC_WANT_LIB_EXT2__` | C and POSIX Variants
| `__STDC_WANT_MATH_SPEC_FUNCS__` | C and POSIX Variants
| `_ALL_SOURCE` | C and POSIX Variants
| `_ALL_SOURCE` | Obsolete Macros
| `_COSMO_SOURCE` | C and POSIX Variants
| `_DARWIN_C_SOURCE` | C and POSIX Variants
| `_FILE_OFFSET_BITS` | System Services
| `_Generic` | C Compiler
| `_GNU_SOURCE` | C and POSIX Variants
| `_GNU_SOURCE` | Obsolete Macros
| `_LARGEFILE_SOURCE` | Particular Functions
| `_MINIX` | C and POSIX Variants
| `_MINIX` | Obsolete Macros
| `_NETBSD_SOURCE` | C and POSIX Variants
| `_OPENBSD_SOURCE` | C and POSIX Variants
| `_OPENMP` | Generic Compiler Characteristics
| `_POSIX_1_SOURCE` | C and POSIX Variants
| `_POSIX_1_SOURCE` | Obsolete Macros
| `_POSIX_PTHREAD_SEMANTICS` | C and POSIX Variants
| `_POSIX_SOURCE` | C and POSIX Variants
| `_POSIX_SOURCE` | Obsolete Macros
| `_POSIX_VERSION` | Particular Headers
| `_TANDEM_SOURCE` | C and POSIX Variants
| `_TIME_BITS` | System Services
| `_XOPEN_SOURCE` | C and POSIX Variants

A
| `ALIGNOF_type` | Generic Compiler Characteristics

C
| `C_ALLOCA` | Particular Functions
| `C_GETLOADAVG` | Particular Functions
| `CLOSEDIR_VOID` | Particular Functions
| `const` | C Compiler
| `CXX_NO_MINUS_C_MINUS_O` | C++ Compiler

D
| `DGUX` | Particular Functions
| `DIRENT` | Obsolete Macros

F
| `F77_DUMMY_MAIN` | Fortran Compiler
| `F77_FUNC` | Fortran Compiler
| `F77_FUNC_` | Fortran Compiler
| `F77_MAIN` | Fortran Compiler
| `F77_NO_MINUS_C_MINUS_O` | Fortran Compiler
| `FC_DUMMY_MAIN` | Fortran Compiler
| `FC_FUNC` | Fortran Compiler
| `FC_FUNC_` | Fortran Compiler
| `FC_MAIN` | Fortran Compiler
| `FC_NO_MINUS_C_MINUS_O` | Fortran Compiler
| `FLEXIBLE_ARRAY_MEMBER` | C Compiler

G
| `GETGROUPS_T` | Particular Types
| `GETLOADAVG_PRIVILEGED` | Particular Functions
| `GETPGRP_VOID` | Particular Functions
| `gid_t` | Particular Types
| `GWINSZ_IN_SYS_IOCTL` | Particular Headers

H
| `HAVE__BOOL` | Particular Headers
| `HAVE__BOOL` | Particular Headers
| `HAVE_aggregate_member` | Generic Structures
| `HAVE_ALLOCA_H` | Particular Functions
| `HAVE_C_BACKSLASH_A` | C Compiler
| `HAVE_C_VARARRAYS` | C Compiler
| `HAVE_CHOWN` | Particular Functions
| `HAVE_CONFIG_H` | Configuration Headers
| `HAVE_DECL_STRERROR_R` | Particular Functions
| `HAVE_DECL_symbol` | Generic Declarations
| `HAVE_DECL_symbol` | Generic Declarations
| `HAVE_DECL_TZNAME` | Particular Structures
| `HAVE_DIRENT_H` | Particular Headers
| `HAVE_DOPRNT` | Particular Functions
| `HAVE_FSEEKO` | Particular Functions
| `HAVE_function` | Generic Functions
| `HAVE_function` | Generic Functions
| `HAVE_function` | Generic Functions
| `HAVE_GETGROUPS` | Particular Functions
| `HAVE_GETMNTENT` | Particular Functions
| `HAVE_header` | Generic Headers
| `HAVE_header` | Generic Headers
| `HAVE_INT16_T` | Particular Types
| `HAVE_INT32_T` | Particular Types
| `HAVE_INT64_T` | Particular Types
| `HAVE_INT8_T` | Particular Types
| `HAVE_INTMAX_T` | Particular Types
| `HAVE_INTPTR_T` | Particular Types
| `HAVE_LONG_DOUBLE` | Particular Types
| `HAVE_LONG_DOUBLE` | Obsolete Macros
| `HAVE_LONG_DOUBLE_WIDER` | Particular Types
| `HAVE_LONG_FILE_NAMES` | System Services
| `HAVE_LONG_LONG_INT` | Particular Types
| `HAVE_LSTAT_EMPTY_STRING_BUG` | Particular Functions
| `HAVE_MALLOC` | Particular Functions
| `HAVE_MBRTOWC` | Particular Functions
| `HAVE_MMAP` | Particular Functions
| `HAVE_NDIR_H` | Particular Headers
| `HAVE_NLIST_H` | Particular Functions
| `HAVE_OBSTACK` | Particular Functions
| `HAVE_REALLOC` | Particular Functions
| `HAVE_RESOLV_H` | Particular Headers
| `HAVE_RESTARTABLE_SYSCALLS` | Obsolete Macros
| `HAVE_ST_BLKSIZE` | Obsolete Macros
| `HAVE_ST_BLOCKS` | Particular Structures
| `HAVE_ST_RDEV` | Obsolete Macros
| `HAVE_STAT_EMPTY_STRING_BUG` | Particular Functions
| `HAVE_STDBOOL_H` | Particular Headers
| `HAVE_STRCOLL` | Particular Functions
| `HAVE_STRERROR_R` | Particular Functions
| `HAVE_STRFTIME` | Particular Functions
| `HAVE_STRINGIZE` | C Compiler
| `HAVE_STRNLEN` | Particular Functions
| `HAVE_STRTOLD` | Particular Functions
| `HAVE_STRUCT_DIRENT_D_INO` | Particular Structures
| `HAVE_STRUCT_DIRENT_D_TYPE` | Particular Structures
| `HAVE_STRUCT_STAT_ST_BLKSIZE` | Obsolete Macros
| `HAVE_STRUCT_STAT_ST_BLOCKS` | Particular Structures
| `HAVE_STRUCT_STAT_ST_RDEV` | Obsolete Macros
| `HAVE_STRUCT_TM_TM_ZONE` | Particular Structures
| `HAVE_SYS_DIR_H` | Particular Headers
| `HAVE_SYS_NDIR_H` | Particular Headers
| `HAVE_SYS_WAIT_H` | Particular Headers
| `HAVE_TM_ZONE` | Particular Structures
| `HAVE_type` | Generic Types
| `HAVE_TYPEOF` | C Compiler
| `HAVE_TZNAME` | Particular Structures
| `HAVE_UINT16_T` | Particular Types
| `HAVE_UINT32_T` | Particular Types
| `HAVE_UINT64_T` | Particular Types
| `HAVE_UINT8_T` | Particular Types
| `HAVE_UINTMAX_T` | Particular Types
| `HAVE_UINTPTR_T` | Particular Types
| `HAVE_UNSIGNED_LONG_LONG_INT` | Particular Types
| `HAVE_UTIME_NULL` | Particular Functions
| `HAVE_VFORK_H` | Particular Functions
| `HAVE_VPRINTF` | Particular Functions
| `HAVE_WAIT3` | Obsolete Macros
| `HAVE_WORKING_FORK` | Particular Functions
| `HAVE_WORKING_VFORK` | Particular Functions

I
| `inline` | C Compiler
| `INT_16_BITS` | Obsolete Macros
| `int16_t` | Particular Types
| `int32_t` | Particular Types
| `int64_t` | Particular Types
| `int8_t` | Particular Types
| `intmax_t` | Particular Types
| `intptr_t` | Particular Types

L
| `LONG_64_BITS` | Obsolete Macros
| `LSTAT_FOLLOWS_SLASHED_SYMLINK` | Particular Functions

M
| `MAJOR_IN_MKDEV` | Particular Headers
| `MAJOR_IN_SYSMACROS` | Particular Headers
| `malloc` | Particular Functions
| `mbstate_t` | Particular Types
| `mode_t` | Particular Types

N
| `NDEBUG` | Particular Headers
| `NDIR` | Obsolete Macros
| `NEED_MEMORY_H` | Obsolete Macros
| `NEED_SETGID` | Particular Functions
| `NLIST_NAME_UNION` | Particular Functions
| `NO_MINUS_C_MINUS_O` | C Compiler

O
| `off_t` | Particular Types

P
| `PACKAGE_BUGREPORT` | Initializing configure
| `PACKAGE_NAME` | Initializing configure
| `PACKAGE_STRING` | Initializing configure
| `PACKAGE_TARNAME` | Initializing configure
| `PACKAGE_URL` | Initializing configure
| `PACKAGE_VERSION` | Initializing configure
| `PARAMS` | C Compiler
| `pid_t` | Particular Types
| `PROTOTYPES` | C Compiler

R
| `realloc` | Particular Functions
| `restrict` | C Compiler
| `RETSIGTYPE` | Obsolete Macros

S
| `SELECT_TYPE_ARG1` | Particular Functions
| `SELECT_TYPE_ARG234` | Particular Functions
| `SELECT_TYPE_ARG5` | Particular Functions
| `SETPGRP_VOID` | Particular Functions
| `SETVBUF_REVERSED` | Obsolete Macros
| `size_t` | Particular Types
| `SIZEOF_type-or-expr` | Generic Compiler Characteristics
| `ssize_t` | Particular Types
| `STAT_MACROS_BROKEN` | Particular Headers
| `STDC_HEADERS` | Particular Headers
| `STRERROR_R_CHAR_P` | Particular Functions
| `SVR4` | Particular Functions
| `SYS_SIGLIST_DECLARED` | Obsolete Macros
| `SYSDIR` | Obsolete Macros
| `SYSNDIR` | Obsolete Macros

T
| `TIME_WITH_SYS_TIME` | Obsolete Macros
| `TM_IN_SYS_TIME` | Particular Structures
| `typeof` | C Compiler

U
| `uid_t` | Particular Types
| `uint16_t` | Particular Types
| `uint32_t` | Particular Types
| `uint64_t` | Particular Types
| `uint8_t` | Particular Types
| `uintmax_t` | Particular Types
| `uintptr_t` | Particular Types
| `UMAX` | Particular Functions
| `UMAX4_3` | Particular Functions
| `USG` | Obsolete Macros

V
| `variable` | Defining Symbols
| `variable` | Defining Symbols
| `vfork` | Particular Functions
| `volatile` | C Compiler

W
| `WORDS_BIGENDIAN` | C Compiler

X
| `X_DISPLAY_MISSING` | System Services

Y
| `YYTEXT_POINTER` | Particular Programs

### B.4 Cache Variable Index ¶

This is an alphabetical list of documented cache variables used by macros defined in Autoconf. Autoconf macros may use additional cache variables internally.

| Index Entry | Section

A
| `ac_cv_alignof_type-or-expr` | Generic Compiler Characteristics
| `ac_cv_c_const` | C Compiler
| `ac_cv_c_int16_t` | Particular Types
| `ac_cv_c_int32_t` | Particular Types
| `ac_cv_c_int64_t` | Particular Types
| `ac_cv_c_int8_t` | Particular Types
| `ac_cv_c_restrict` | C Compiler
| `ac_cv_c_uint16_t` | Particular Types
| `ac_cv_c_uint32_t` | Particular Types
| `ac_cv_c_uint64_t` | Particular Types
| `ac_cv_c_uint8_t` | Particular Types
| `ac_cv_f77_compiler_gnu` | Fortran Compiler
| `ac_cv_f77_dummy_main` | Fortran Compiler
| `ac_cv_f77_implicit_none` | Fortran Compiler
| `ac_cv_f77_libs` | Fortran Compiler
| `ac_cv_f77_main` | Fortran Compiler
| `ac_cv_f77_mangling` | Fortran Compiler
| `ac_cv_fc_check_bounds` | Fortran Compiler
| `ac_cv_fc_compiler_gnu` | Fortran Compiler
| `ac_cv_fc_cray_pointer` | Fortran Compiler
| `ac_cv_fc_dummy_main` | Fortran Compiler
| `ac_cv_fc_fixedform` | Fortran Compiler
| `ac_cv_fc_freeform` | Fortran Compiler
| `ac_cv_fc_implicit_none` | Fortran Compiler
| `ac_cv_fc_libs` | Fortran Compiler
| `ac_cv_fc_line_length` | Fortran Compiler
| `ac_cv_fc_main` | Fortran Compiler
| `ac_cv_fc_mangling` | Fortran Compiler
| `ac_cv_fc_module_ext` | Fortran Compiler
| `ac_cv_fc_module_flag` | Fortran Compiler
| `ac_cv_fc_module_output_flag` | Fortran Compiler
| `ac_cv_fc_pp_define` | Fortran Compiler
| `ac_cv_fc_pp_srcext_ext` | Fortran Compiler
| `ac_cv_fc_srcext_ext` | Fortran Compiler
| `ac_cv_file_file` | Files
| `ac_cv_file_file` | Files
| `ac_cv_func_chown_works` | Particular Functions
| `ac_cv_func_closedir_void` | Particular Functions
| `ac_cv_func_fnmatch_gnu` | Particular Functions
| `ac_cv_func_fnmatch_works` | Particular Functions
| `ac_cv_func_fnmatch_works` | Particular Functions
| `ac_cv_func_function` | Generic Functions
| `ac_cv_func_getgroups_works` | Particular Functions
| `ac_cv_func_getpgrp_void` | Particular Functions
| `ac_cv_func_lstat_dereferences_slashed_symlink` | Particular Functions
| `ac_cv_func_lstat_empty_string_bug` | Particular Functions
| `ac_cv_func_malloc_0_nonnull` | Particular Functions
| `ac_cv_func_mbrtowc` | Particular Functions
| `ac_cv_func_memcmp_working` | Particular Functions
| `ac_cv_func_mmap_fixed_mapped` | Particular Functions
| `ac_cv_func_obstack` | Particular Functions
| `ac_cv_func_pow` | Particular Functions
| `ac_cv_func_realloc_0_nonnull` | Particular Functions
| `ac_cv_func_setpgrp_void` | Particular Functions
| `ac_cv_func_stat_empty_string_bug` | Particular Functions
| `ac_cv_func_strcoll_works` | Particular Functions
| `ac_cv_func_strerror_r_char_p` | Particular Functions
| `ac_cv_func_strnlen_working` | Particular Functions
| `ac_cv_func_strtod` | Particular Functions
| `ac_cv_func_strtold` | Particular Functions
| `ac_cv_func_utime_null` | Particular Functions
| `ac_cv_func_working_mktime` | Particular Functions
| `ac_cv_have_decl_symbol` | Generic Declarations
| `ac_cv_have_decl_symbol` | Generic Declarations
| `ac_cv_header_header-file` | Generic Headers
| `ac_cv_header_header-file` | Generic Headers
| `ac_cv_header_stdbool_h` | Particular Headers
| `ac_cv_header_stdbool_h` | Particular Headers
| `ac_cv_header_stdc` | Particular Headers
| `ac_cv_header_sys_wait_h` | Particular Headers
| `ac_cv_header_time` | Obsolete Macros
| `ac_cv_lib_error_at_line` | Particular Functions
| `ac_cv_lib_library_function` | Libraries
| `ac_cv_member_aggregate_member` | Generic Structures
| `ac_cv_member_struct_stat_st_blocks` | Particular Structures
| `ac_cv_path_install` | Particular Programs
| `ac_cv_path_mkdir` | Particular Programs
| `ac_cv_path_SED` | Particular Programs
| `ac_cv_path_variable` | Generic Programs
| `ac_cv_path_variable` | Generic Programs
| `ac_cv_path_variable` | Generic Programs
| `ac_cv_prog_AWK` | Particular Programs
| `ac_cv_prog_c_openmp` | Generic Compiler Characteristics
| `ac_cv_prog_cc_compiler_c_o` | C Compiler
| `ac_cv_prog_cxx_openmp` | Generic Compiler Characteristics
| `ac_cv_prog_EGREP` | Particular Programs
| `ac_cv_prog_f77_c_o` | Fortran Compiler
| `ac_cv_prog_f77_g` | Fortran Compiler
| `ac_cv_prog_f77_openmp` | Generic Compiler Characteristics
| `ac_cv_prog_f77_v` | Fortran Compiler
| `ac_cv_prog_fc_c_o` | Fortran Compiler
| `ac_cv_prog_fc_g` | Fortran Compiler
| `ac_cv_prog_fc_openmp` | Generic Compiler Characteristics
| `ac_cv_prog_fc_v` | Fortran Compiler
| `ac_cv_prog_FGREP` | Particular Programs
| `ac_cv_prog_GREP` | Particular Programs
| `ac_cv_prog_LEX` | Particular Programs
| `ac_cv_prog_variable` | Generic Programs
| `ac_cv_prog_variable` | Generic Programs
| `ac_cv_prog_YACC` | Particular Programs
| `ac_cv_search_function` | Libraries
| `ac_cv_search_getmntent` | Particular Functions
| `ac_cv_sizeof_type-or-expr` | Generic Compiler Characteristics
| `ac_cv_sys_posix_termios` | System Services
| `ac_cv_type_getgroups` | Particular Types
| `ac_cv_type_long_double` | Particular Types
| `ac_cv_type_long_double_wider` | Particular Types
| `ac_cv_type_long_long_int` | Particular Types
| `ac_cv_type_mbstate_t` | Particular Types
| `ac_cv_type_mode_t` | Particular Types
| `ac_cv_type_off_t` | Particular Types
| `ac_cv_type_pid_t` | Particular Types
| `ac_cv_type_size_t` | Particular Types
| `ac_cv_type_ssize_t` | Particular Types
| `ac_cv_type_type` | Generic Types
| `ac_cv_type_uid_t` | Particular Types
| `ac_cv_type_unsigned_long_long_int` | Particular Types

### B.5 Autoconf Macro Index ¶

This is an alphabetical list of the Autoconf macros.

| Index Entry | Section

A
| `AC_ACT_IFELSE` | AC_ACT_IFELSE vs AC_TRY_ACT
| `AC_AIX` | Obsolete Macros
| `AC_ALLOCA` | Obsolete Macros
| `AC_ARG_ARRAY` | Obsolete Macros
| `AC_ARG_ENABLE` | Package Options
| `AC_ARG_PROGRAM` | Transforming Names
| `AC_ARG_VAR` | Setting Output Variables
| `AC_ARG_WITH` | External Software
| `AC_AUTOCONF_VERSION` | Versioning
| `AC_BEFORE` | Suggested Ordering
| `AC_C__GENERIC` | C Compiler
| `AC_C_BACKSLASH_A` | C Compiler
| `AC_C_BIGENDIAN` | C Compiler
| `AC_C_CHAR_UNSIGNED` | C Compiler
| `AC_C_CONST` | C Compiler
| `AC_C_CROSS` | Obsolete Macros
| `AC_C_FLEXIBLE_ARRAY_MEMBER` | C Compiler
| `AC_C_INLINE` | C Compiler
| `AC_C_LONG_DOUBLE` | Obsolete Macros
| `AC_C_PROTOTYPES` | C Compiler
| `AC_C_RESTRICT` | C Compiler
| `AC_C_STRINGIZE` | C Compiler
| `AC_C_TYPEOF` | C Compiler
| `AC_C_VARARRAYS` | C Compiler
| `AC_C_VOLATILE` | C Compiler
| `AC_CACHE_CHECK` | Caching Results
| `AC_CACHE_LOAD` | Cache Checkpointing
| `AC_CACHE_SAVE` | Cache Checkpointing
| `AC_CACHE_VAL` | Caching Results
| `AC_CANONICAL_BUILD` | Canonicalizing
| `AC_CANONICAL_HOST` | Canonicalizing
| `AC_CANONICAL_SYSTEM` | Obsolete Macros
| `AC_CANONICAL_TARGET` | Canonicalizing
| `AC_CHAR_UNSIGNED` | Obsolete Macros
| `AC_CHECK_ALIGNOF` | Generic Compiler Characteristics
| `AC_CHECK_DECL` | Generic Declarations
| `AC_CHECK_DECLS` | Generic Declarations
| `AC_CHECK_DECLS_ONCE` | Generic Declarations
| `AC_CHECK_FILE` | Files
| `AC_CHECK_FILES` | Files
| `AC_CHECK_FUNC` | Generic Functions
| `AC_CHECK_FUNCS` | Generic Functions
| `AC_CHECK_FUNCS_ONCE` | Generic Functions
| `AC_CHECK_HEADER` | Generic Headers
| `AC_CHECK_HEADER_STDBOOL` | Particular Headers
| `AC_CHECK_HEADERS` | Generic Headers
| `AC_CHECK_HEADERS_ONCE` | Generic Headers
| `AC_CHECK_INCLUDES_DEFAULT` | Default Includes
| `AC_CHECK_LIB` | Libraries
| `AC_CHECK_MEMBER` | Generic Structures
| `AC_CHECK_MEMBERS` | Generic Structures
| `AC_CHECK_PROG` | Generic Programs
| `AC_CHECK_PROGS` | Generic Programs
| `AC_CHECK_SIZEOF` | Generic Compiler Characteristics
| `AC_CHECK_TARGET_TOOL` | Generic Programs
| `AC_CHECK_TARGET_TOOLS` | Generic Programs
| `AC_CHECK_TOOL` | Generic Programs
| `AC_CHECK_TOOLS` | Generic Programs
| `AC_CHECK_TYPE` | Generic Types
| `AC_CHECK_TYPE` | Obsolete Macros
| `AC_CHECK_TYPES` | Generic Types
| `AC_CHECKING` | Obsolete Macros
| `AC_COMPILE_CHECK` | Obsolete Macros
| `AC_COMPILE_IFELSE` | Running the Compiler
| `AC_COMPUTE_INT` | Generic Compiler Characteristics
| `AC_CONFIG_AUX_DIR` | Input
| `AC_CONFIG_COMMANDS` | Configuration Commands
| `AC_CONFIG_COMMANDS_POST` | Configuration Commands
| `AC_CONFIG_COMMANDS_PRE` | Configuration Commands
| `AC_CONFIG_FILES` | Configuration Files
| `AC_CONFIG_HEADERS` | Configuration Headers
| `AC_CONFIG_ITEMS` | Configuration Actions
| `AC_CONFIG_LIBOBJ_DIR` | Generic Functions
| `AC_CONFIG_LINKS` | Configuration Links
| `AC_CONFIG_MACRO_DIR` | Input
| `AC_CONFIG_MACRO_DIR_TRACE` | Input
| `AC_CONFIG_MACRO_DIRS` | Input
| `AC_CONFIG_SRCDIR` | Input
| `AC_CONFIG_SUBDIRS` | Subdirectories
| `AC_CONFIG_TESTDIR` | Making testsuite Scripts
| `AC_CONST` | Obsolete Macros
| `AC_COPYRIGHT` | Notices
| `AC_CROSS_CHECK` | Obsolete Macros
| `AC_CYGWIN` | Obsolete Macros
| `AC_DATAROOTDIR_CHECKED` | Changed Directory Variables
| `AC_DECL_SYS_SIGLIST` | Obsolete Macros
| `AC_DECL_YYTEXT` | Obsolete Macros
| `AC_DEFINE` | Defining Symbols
| `AC_DEFINE_UNQUOTED` | Defining Symbols
| `AC_DEFUN` | Macro Definitions
| `AC_DEFUN_ONCE` | One-Shot Macros
| `AC_DIAGNOSE` | Obsolete Macros
| `AC_DIR_HEADER` | Obsolete Macros
| `AC_DISABLE_OPTION_CHECKING` | Option Checking
| `AC_DYNIX_SEQ` | Obsolete Macros
| `AC_EGREP_CPP` | Running the Preprocessor
| `AC_EGREP_HEADER` | Running the Preprocessor
| `AC_EMXOS2` | Obsolete Macros
| `AC_ENABLE` | Obsolete Macros
| `AC_ERLANG_CHECK_LIB` | Erlang Libraries
| `AC_ERLANG_NEED_ERL` | Erlang Compiler and Interpreter
| `AC_ERLANG_NEED_ERLC` | Erlang Compiler and Interpreter
| `AC_ERLANG_PATH_ERL` | Erlang Compiler and Interpreter
| `AC_ERLANG_PATH_ERLC` | Erlang Compiler and Interpreter
| `AC_ERLANG_SUBST_ERTS_VER` | Erlang Libraries
| `AC_ERLANG_SUBST_INSTALL_LIB_DIR` | Installation Directory Variables
| `AC_ERLANG_SUBST_INSTALL_LIB_DIR` | Erlang Libraries
| `AC_ERLANG_SUBST_INSTALL_LIB_SUBDIR` | Installation Directory Variables
| `AC_ERLANG_SUBST_INSTALL_LIB_SUBDIR` | Erlang Libraries
| `AC_ERLANG_SUBST_LIB_DIR` | Erlang Libraries
| `AC_ERLANG_SUBST_ROOT_DIR` | Erlang Libraries
| `AC_ERROR` | Obsolete Macros
| `AC_EXEEXT` | Obsolete Macros
| `AC_F77_CRAY_POINTERS` | Fortran Compiler
| `AC_F77_DUMMY_MAIN` | Fortran Compiler
| `AC_F77_FUNC` | Fortran Compiler
| `AC_F77_IMPLICIT_NONE` | Fortran Compiler
| `AC_F77_LIBRARY_LDFLAGS` | Fortran Compiler
| `AC_F77_MAIN` | Fortran Compiler
| `AC_F77_WRAPPERS` | Fortran Compiler
| `AC_FATAL` | Obsolete Macros
| `AC_FC_CHECK_BOUNDS` | Fortran Compiler
| `AC_FC_CRAY_POINTERS` | Fortran Compiler
| `AC_FC_DUMMY_MAIN` | Fortran Compiler
| `AC_FC_FIXEDFORM` | Fortran Compiler
| `AC_FC_FREEFORM` | Fortran Compiler
| `AC_FC_FUNC` | Fortran Compiler
| `AC_FC_IMPLICIT_NONE` | Fortran Compiler
| `AC_FC_LIBRARY_LDFLAGS` | Fortran Compiler
| `AC_FC_LINE_LENGTH` | Fortran Compiler
| `AC_FC_MAIN` | Fortran Compiler
| `AC_FC_MODULE_EXTENSION` | Fortran Compiler
| `AC_FC_MODULE_FLAG` | Fortran Compiler
| `AC_FC_MODULE_OUTPUT_FLAG` | Fortran Compiler
| `AC_FC_PP_DEFINE` | Fortran Compiler
| `AC_FC_PP_SRCEXT` | Fortran Compiler
| `AC_FC_SRCEXT` | Fortran Compiler
| `AC_FC_WRAPPERS` | Fortran Compiler
| `AC_FIND_X` | Obsolete Macros
| `AC_FIND_XTRA` | Obsolete Macros
| `AC_FOREACH` | Obsolete Macros
| `AC_FUNC_ALLOCA` | Particular Functions
| `AC_FUNC_CHECK` | Obsolete Macros
| `AC_FUNC_CHOWN` | Particular Functions
| `AC_FUNC_CLOSEDIR_VOID` | Particular Functions
| `AC_FUNC_ERROR_AT_LINE` | Particular Functions
| `AC_FUNC_FNMATCH` | Particular Functions
| `AC_FUNC_FNMATCH_GNU` | Particular Functions
| `AC_FUNC_FORK` | Particular Functions
| `AC_FUNC_FSEEKO` | Particular Functions
| `AC_FUNC_GETGROUPS` | Particular Functions
| `AC_FUNC_GETLOADAVG` | Particular Functions
| `AC_FUNC_GETMNTENT` | Particular Functions
| `AC_FUNC_GETPGRP` | Particular Functions
| `AC_FUNC_LSTAT` | Particular Functions
| `AC_FUNC_LSTAT_FOLLOWS_SLASHED_SYMLINK` | Particular Functions
| `AC_FUNC_MALLOC` | Particular Functions
| `AC_FUNC_MBRTOWC` | Particular Functions
| `AC_FUNC_MEMCMP` | Particular Functions
| `AC_FUNC_MKTIME` | Particular Functions
| `AC_FUNC_MMAP` | Particular Functions
| `AC_FUNC_OBSTACK` | Particular Functions
| `AC_FUNC_REALLOC` | Particular Functions
| `AC_FUNC_SELECT_ARGTYPES` | Particular Functions
| `AC_FUNC_SETPGRP` | Particular Functions
| `AC_FUNC_SETVBUF_REVERSED` | Obsolete Macros
| `AC_FUNC_STAT` | Particular Functions
| `AC_FUNC_STRCOLL` | Particular Functions
| `AC_FUNC_STRERROR_R` | Particular Functions
| `AC_FUNC_STRFTIME` | Particular Functions
| `AC_FUNC_STRNLEN` | Particular Functions
| `AC_FUNC_STRTOD` | Particular Functions
| `AC_FUNC_STRTOLD` | Particular Functions
| `AC_FUNC_UTIME_NULL` | Particular Functions
| `AC_FUNC_VPRINTF` | Particular Functions
| `AC_FUNC_WAIT3` | Obsolete Macros
| `AC_GCC_TRADITIONAL` | Obsolete Macros
| `AC_GETGROUPS_T` | Obsolete Macros
| `AC_GETLOADAVG` | Obsolete Macros
| `AC_GNU_SOURCE` | Obsolete Macros
| `AC_HAVE_FUNCS` | Obsolete Macros
| `AC_HAVE_HEADERS` | Obsolete Macros
| `AC_HAVE_LIBRARY` | Obsolete Macros
| `AC_HAVE_POUNDBANG` | Obsolete Macros
| `AC_HEADER_ASSERT` | Particular Headers
| `AC_HEADER_CHECK` | Obsolete Macros
| `AC_HEADER_DIRENT` | Particular Headers
| `AC_HEADER_EGREP` | Obsolete Macros
| `AC_HEADER_MAJOR` | Particular Headers
| `AC_HEADER_RESOLV` | Particular Headers
| `AC_HEADER_STAT` | Particular Headers
| `AC_HEADER_STDBOOL` | Particular Headers
| `AC_HEADER_STDC` | Particular Headers
| `AC_HEADER_SYS_WAIT` | Particular Headers
| `AC_HEADER_TIME` | Obsolete Macros
| `AC_HEADER_TIOCGWINSZ` | Particular Headers
| `AC_HELP_STRING` | Obsolete Macros
| `AC_INCLUDES_DEFAULT` | Default Includes
| `AC_INIT` | Initializing configure
| `AC_INIT` | Obsolete Macros
| `AC_INLINE` | Obsolete Macros
| `AC_INT_16_BITS` | Obsolete Macros
| `AC_IRIX_SUN` | Obsolete Macros
| `AC_ISC_POSIX` | Obsolete Macros
| `AC_LANG` | Language Choice
| `AC_LANG_ASSERT` | Language Choice
| `AC_LANG_C` | Obsolete Macros
| `AC_LANG_CALL` | Generating Sources
| `AC_LANG_CONFTEST` | Generating Sources
| `AC_LANG_CPLUSPLUS` | Obsolete Macros
| `AC_LANG_DEFINES_PROVIDED` | Generating Sources
| `AC_LANG_FORTRAN77` | Obsolete Macros
| `AC_LANG_FUNC_LINK_TRY` | Generating Sources
| `AC_LANG_POP` | Language Choice
| `AC_LANG_PROGRAM` | Generating Sources
| `AC_LANG_PUSH` | Language Choice
| `AC_LANG_RESTORE` | Obsolete Macros
| `AC_LANG_SAVE` | Obsolete Macros
| `AC_LANG_SOURCE` | Generating Sources
| `AC_LANG_WERROR` | Generic Compiler Characteristics
| `AC_LIBOBJ` | Generic Functions
| `AC_LIBSOURCE` | Generic Functions
| `AC_LIBSOURCES` | Generic Functions
| `AC_LINK_FILES` | Obsolete Macros
| `AC_LINK_IFELSE` | Running the Linker
| `AC_LN_S` | Obsolete Macros
| `AC_LONG_64_BITS` | Obsolete Macros
| `AC_LONG_DOUBLE` | Obsolete Macros
| `AC_LONG_FILE_NAMES` | Obsolete Macros
| `AC_MAJOR_HEADER` | Obsolete Macros
| `AC_MEMORY_H` | Obsolete Macros
| `AC_MINGW32` | Obsolete Macros
| `AC_MINIX` | Obsolete Macros
| `AC_MINUS_C_MINUS_O` | Obsolete Macros
| `AC_MMAP` | Obsolete Macros
| `AC_MODE_T` | Obsolete Macros
| `AC_MSG_CHECKING` | Printing Messages
| `AC_MSG_ERROR` | Printing Messages
| `AC_MSG_FAILURE` | Printing Messages
| `AC_MSG_NOTICE` | Printing Messages
| `AC_MSG_RESULT` | Printing Messages
| `AC_MSG_WARN` | Printing Messages
| `AC_OBJEXT` | Obsolete Macros
| `AC_OBSOLETE` | Obsolete Macros
| `AC_OFF_T` | Obsolete Macros
| `AC_OPENMP` | Generic Compiler Characteristics
| `AC_OUTPUT` | Output
| `AC_OUTPUT` | Obsolete Macros
| `AC_OUTPUT_COMMANDS` | Obsolete Macros
| `AC_PACKAGE_BUGREPORT` | Initializing configure
| `AC_PACKAGE_NAME` | Initializing configure
| `AC_PACKAGE_STRING` | Initializing configure
| `AC_PACKAGE_TARNAME` | Initializing configure
| `AC_PACKAGE_URL` | Initializing configure
| `AC_PACKAGE_VERSION` | Initializing configure
| `AC_PATH_PROG` | Generic Programs
| `AC_PATH_PROGS` | Generic Programs
| `AC_PATH_PROGS_FEATURE_CHECK` | Generic Programs
| `AC_PATH_TARGET_TOOL` | Generic Programs
| `AC_PATH_TOOL` | Generic Programs
| `AC_PATH_X` | System Services
| `AC_PATH_XTRA` | System Services
| `AC_PID_T` | Obsolete Macros
| `AC_PREFIX` | Obsolete Macros
| `AC_PREFIX_DEFAULT` | Default Prefix
| `AC_PREFIX_PROGRAM` | Default Prefix
| `AC_PREPROC_IFELSE` | Running the Preprocessor
| `AC_PREREQ` | Versioning
| `AC_PRESERVE_HELP_ORDER` | Help Formatting
| `AC_PROG_AR` | Particular Programs
| `AC_PROG_AWK` | Particular Programs
| `AC_PROG_CC` | C Compiler
| `AC_PROG_CC_C_O` | C Compiler
| `AC_PROG_CC_C89` | Obsolete Macros
| `AC_PROG_CC_C99` | Obsolete Macros
| `AC_PROG_CC_STDC` | Obsolete Macros
| `AC_PROG_CPP` | C Compiler
| `AC_PROG_CPP_WERROR` | C Compiler
| `AC_PROG_CXX` | C++ Compiler
| `AC_PROG_CXX_C_O` | C++ Compiler
| `AC_PROG_CXXCPP` | C++ Compiler
| `AC_PROG_EGREP` | Particular Programs
| `AC_PROG_F77` | Fortran Compiler
| `AC_PROG_F77_C_O` | Fortran Compiler
| `AC_PROG_FC` | Fortran Compiler
| `AC_PROG_FC_C_O` | Fortran Compiler
| `AC_PROG_FGREP` | Particular Programs
| `AC_PROG_GCC_TRADITIONAL` | Obsolete Macros
| `AC_PROG_GREP` | Particular Programs
| `AC_PROG_INSTALL` | Particular Programs
| `AC_PROG_LEX` | Particular Programs
| `AC_PROG_LN_S` | Particular Programs
| `AC_PROG_MAKE_SET` | Output
| `AC_PROG_MKDIR_P` | Particular Programs
| `AC_PROG_OBJC` | Objective C Compiler
| `AC_PROG_OBJCPP` | Objective C Compiler
| `AC_PROG_OBJCXX` | Objective C++ Compiler
| `AC_PROG_OBJCXXCPP` | Objective C++ Compiler
| `AC_PROG_RANLIB` | Particular Programs
| `AC_PROG_SED` | Particular Programs
| `AC_PROG_YACC` | Particular Programs
| `AC_PROGRAM_CHECK` | Obsolete Macros
| `AC_PROGRAM_EGREP` | Obsolete Macros
| `AC_PROGRAM_PATH` | Obsolete Macros
| `AC_PROGRAMS_CHECK` | Obsolete Macros
| `AC_PROGRAMS_PATH` | Obsolete Macros
| `AC_REMOTE_TAPE` | Obsolete Macros
| `AC_REPLACE_FNMATCH` | Particular Functions
| `AC_REPLACE_FUNCS` | Generic Functions
| `AC_REQUIRE` | Prerequisite Macros
| `AC_REQUIRE_AUX_FILE` | Input
| `AC_REQUIRE_CPP` | Language Choice
| `AC_RESTARTABLE_SYSCALLS` | Obsolete Macros
| `AC_RETSIGTYPE` | Obsolete Macros
| `AC_REVISION` | Notices
| `AC_RSH` | Obsolete Macros
| `AC_RUN_IFELSE` | Runtime
| `AC_SCO_INTL` | Obsolete Macros
| `AC_SEARCH_LIBS` | Libraries
| `AC_SET_MAKE` | Obsolete Macros
| `AC_SETVBUF_REVERSED` | Obsolete Macros
| `AC_SIZE_T` | Obsolete Macros
| `AC_SIZEOF_TYPE` | Obsolete Macros
| `AC_ST_BLKSIZE` | Obsolete Macros
| `AC_ST_BLOCKS` | Obsolete Macros
| `AC_ST_RDEV` | Obsolete Macros
| `AC_STAT_MACROS_BROKEN` | Obsolete Macros
| `AC_STDC_HEADERS` | Obsolete Macros
| `AC_STRCOLL` | Obsolete Macros
| `AC_STRUCT_DIRENT_D_INO` | Particular Structures
| `AC_STRUCT_DIRENT_D_TYPE` | Particular Structures
| `AC_STRUCT_ST_BLKSIZE` | Obsolete Macros
| `AC_STRUCT_ST_BLOCKS` | Particular Structures
| `AC_STRUCT_ST_RDEV` | Obsolete Macros
| `AC_STRUCT_TIMEZONE` | Particular Structures
| `AC_STRUCT_TM` | Particular Structures
| `AC_SUBST` | Setting Output Variables
| `AC_SUBST_FILE` | Setting Output Variables
| `AC_SYS_INTERPRETER` | System Services
| `AC_SYS_LARGEFILE` | System Services
| `AC_SYS_LONG_FILE_NAMES` | System Services
| `AC_SYS_POSIX_TERMIOS` | System Services
| `AC_SYS_RESTARTABLE_SYSCALLS` | Obsolete Macros
| `AC_SYS_SIGLIST_DECLARED` | Obsolete Macros
| `AC_SYS_YEAR2038` | System Services
| `AC_SYS_YEAR2038_RECOMMENDED` | System Services
| `AC_TEST_CPP` | Obsolete Macros
| `AC_TEST_PROGRAM` | Obsolete Macros
| `AC_TIME_WITH_SYS_TIME` | Obsolete Macros
| `AC_TIMEZONE` | Obsolete Macros
| `AC_TRY_ACT` | AC_ACT_IFELSE vs AC_TRY_ACT
| `AC_TRY_COMPILE` | Obsolete Macros
| `AC_TRY_CPP` | Obsolete Macros
| `AC_TRY_LINK` | Obsolete Macros
| `AC_TRY_LINK_FUNC` | Obsolete Macros
| `AC_TRY_RUN` | Obsolete Macros
| `AC_TYPE_GETGROUPS` | Particular Types
| `AC_TYPE_INT16_T` | Particular Types
| `AC_TYPE_INT32_T` | Particular Types
| `AC_TYPE_INT64_T` | Particular Types
| `AC_TYPE_INT8_T` | Particular Types
| `AC_TYPE_INTMAX_T` | Particular Types
| `AC_TYPE_INTPTR_T` | Particular Types
| `AC_TYPE_LONG_DOUBLE` | Particular Types
| `AC_TYPE_LONG_DOUBLE_WIDER` | Particular Types
| `AC_TYPE_LONG_LONG_INT` | Particular Types
| `AC_TYPE_MBSTATE_T` | Particular Types
| `AC_TYPE_MODE_T` | Particular Types
| `AC_TYPE_OFF_T` | Particular Types
| `AC_TYPE_PID_T` | Particular Types
| `AC_TYPE_SIGNAL` | Obsolete Macros
| `AC_TYPE_SIZE_T` | Particular Types
| `AC_TYPE_SSIZE_T` | Particular Types
| `AC_TYPE_UID_T` | Particular Types
| `AC_TYPE_UINT16_T` | Particular Types
| `AC_TYPE_UINT32_T` | Particular Types
| `AC_TYPE_UINT64_T` | Particular Types
| `AC_TYPE_UINT8_T` | Particular Types
| `AC_TYPE_UINTMAX_T` | Particular Types
| `AC_TYPE_UINTPTR_T` | Particular Types
| `AC_TYPE_UNSIGNED_LONG_LONG_INT` | Particular Types
| `AC_UID_T` | Obsolete Macros
| `AC_UNISTD_H` | Obsolete Macros
| `AC_USE_SYSTEM_EXTENSIONS` | C and POSIX Variants
| `AC_USG` | Obsolete Macros
| `AC_UTIME_NULL` | Obsolete Macros
| `AC_VALIDATE_CACHED_SYSTEM_TUPLE` | Obsolete Macros
| `AC_VERBOSE` | Obsolete Macros
| `AC_VFORK` | Obsolete Macros
| `AC_VPRINTF` | Obsolete Macros
| `AC_WAIT3` | Obsolete Macros
| `AC_WARN` | Obsolete Macros
| `AC_WARNING` | Obsolete Macros
| `AC_WITH` | Obsolete Macros
| `AC_WORDS_BIGENDIAN` | Obsolete Macros
| `AC_XENIX_DIR` | Obsolete Macros
| `AC_YYTEXT_POINTER` | Obsolete Macros
| `AH_BOTTOM` | Autoheader Macros
| `AH_HEADER` | Configuration Headers
| `AH_TEMPLATE` | Autoheader Macros
| `AH_TOP` | Autoheader Macros
| `AH_VERBATIM` | Autoheader Macros
| `AU_ALIAS` | Obsoleting Macros
| `AU_DEFUN` | Obsoleting Macros

### B.6 M4 Macro Index ¶

This is an alphabetical list of the M4, M4sugar, and M4sh macros.

Jump to: | _ A D M

| Index Entry | Section

_
| `__file__` | Redefined M4 Macros
| `__line__` | Redefined M4 Macros
| `__oline__` | Redefined M4 Macros

A
| `AS_BOURNE_COMPATIBLE` | Initialization Macros
| `AS_BOX` | Common Shell Constructs
| `AS_CASE` | Common Shell Constructs
| `AS_DIRNAME` | Common Shell Constructs
| `AS_ECHO` | Common Shell Constructs
| `AS_ECHO_N` | Common Shell Constructs
| `AS_ESCAPE` | Common Shell Constructs
| `AS_EXECUTABLE_P` | Common Shell Constructs
| `AS_EXIT` | Common Shell Constructs
| `AS_HELP_STRING` | Pretty Help Strings
| `AS_IF` | Common Shell Constructs
| `AS_INIT` | Initialization Macros
| `AS_INIT_GENERATED` | Initialization Macros
| `AS_LINENO_PREPARE` | Initialization Macros
| `AS_LITERAL_IF` | Polymorphic Variables
| `AS_LITERAL_WORD_IF` | Polymorphic Variables
| `AS_ME_PREPARE` | Initialization Macros
| `AS_MESSAGE_FD` | File Descriptor Macros
| `AS_MESSAGE_LOG_FD` | File Descriptor Macros
| `AS_MKDIR_P` | Common Shell Constructs
| `AS_ORIGINAL_STDIN_FD` | File Descriptor Macros
| `AS_SET_CATFILE` | Common Shell Constructs
| `AS_SET_STATUS` | Common Shell Constructs
| `AS_SHELL_SANITIZE` | Initialization Macros
| `AS_TMPDIR` | Initialization Macros
| `AS_TR_CPP` | Common Shell Constructs
| `AS_TR_SH` | Common Shell Constructs
| `AS_UNSET` | Common Shell Constructs
| `AS_VAR_APPEND` | Polymorphic Variables
| `AS_VAR_ARITH` | Polymorphic Variables
| `AS_VAR_COPY` | Polymorphic Variables
| `AS_VAR_IF` | Polymorphic Variables
| `AS_VAR_POPDEF` | Polymorphic Variables
| `AS_VAR_PUSHDEF` | Polymorphic Variables
| `AS_VAR_SET` | Polymorphic Variables
| `AS_VAR_SET_IF` | Polymorphic Variables
| `AS_VAR_TEST_SET` | Polymorphic Variables
| `AS_VERSION_COMPARE` | Common Shell Constructs

D
| `dnl` | Redefined M4 Macros

M
| `m4_append` | Text processing Macros
| `m4_append_uniq` | Text processing Macros
| `m4_append_uniq_w` | Text processing Macros
| `m4_apply` | Evaluation Macros
| `m4_argn` | Looping constructs
| `m4_assert` | Diagnostic Macros
| `m4_bmatch` | Conditional constructs
| `m4_bpatsubst` | Redefined M4 Macros
| `m4_bpatsubsts` | Conditional constructs
| `m4_bregexp` | Redefined M4 Macros
| `m4_builtin` | Redefined M4 Macros
| `m4_car` | Looping constructs
| `m4_case` | Conditional constructs
| `m4_cdr` | Looping constructs
| `m4_changecom` | Redefined M4 Macros
| `m4_changequote` | Redefined M4 Macros
| `m4_chomp` | Text processing Macros
| `m4_chomp_all` | Text processing Macros
| `m4_cleardivert` | Diversion support
| `m4_cmp` | Number processing Macros
| `m4_combine` | Text processing Macros
| `m4_cond` | Conditional constructs
| `m4_copy` | Redefined M4 Macros
| `m4_copy_force` | Redefined M4 Macros
| `m4_count` | Evaluation Macros
| `m4_curry` | Evaluation Macros
| `m4_debugfile` | Redefined M4 Macros
| `m4_debugmode` | Redefined M4 Macros
| `m4_decr` | Redefined M4 Macros
| `m4_default` | Conditional constructs
| `m4_default_nblank` | Conditional constructs
| `m4_default_nblank_quoted` | Conditional constructs
| `m4_default_quoted` | Conditional constructs
| `m4_define` | Redefined M4 Macros
| `m4_define_default` | Conditional constructs
| `m4_defn` | Redefined M4 Macros
| `m4_divert` | Redefined M4 Macros
| `m4_divert_once` | Diversion support
| `m4_divert_pop` | Diversion support
| `m4_divert_push` | Diversion support
| `m4_divert_text` | Diversion support
| `m4_divnum` | Redefined M4 Macros
| `m4_do` | Evaluation Macros
| `m4_dquote` | Evaluation Macros
| `m4_dquote_elt` | Evaluation Macros
| `m4_dumpdef` | Redefined M4 Macros
| `m4_dumpdefs` | Redefined M4 Macros
| `m4_echo` | Evaluation Macros
| `m4_errprint` | Redefined M4 Macros
| `m4_errprintn` | Diagnostic Macros
| `m4_escape` | Text processing Macros
| `m4_esyscmd` | Redefined M4 Macros
| `m4_esyscmd_s` | Redefined M4 Macros
| `m4_eval` | Redefined M4 Macros
| `m4_exit` | Redefined M4 Macros
| `m4_expand` | Evaluation Macros
| `m4_fatal` | Diagnostic Macros
| `m4_flatten` | Text processing Macros
| `m4_for` | Looping constructs
| `m4_foreach` | Looping constructs
| `m4_foreach_w` | Looping constructs
| `m4_format` | Redefined M4 Macros
| `m4_if` | Redefined M4 Macros
| `m4_ifblank` | Conditional constructs
| `m4_ifdef` | Redefined M4 Macros
| `m4_ifnblank` | Conditional constructs
| `m4_ifndef` | Conditional constructs
| `m4_ifset` | Conditional constructs
| `m4_ifval` | Conditional constructs
| `m4_ifvaln` | Conditional constructs
| `m4_ignore` | Evaluation Macros
| `m4_include` | Redefined M4 Macros
| `m4_incr` | Redefined M4 Macros
| `m4_index` | Redefined M4 Macros
| `m4_indir` | Redefined M4 Macros
| `m4_init` | Diversion support
| `m4_join` | Text processing Macros
| `m4_joinall` | Text processing Macros
| `m4_len` | Redefined M4 Macros
| `m4_list_cmp` | Number processing Macros
| `m4_location` | Diagnostic Macros
| `m4_make_list` | Evaluation Macros
| `m4_maketemp` | Redefined M4 Macros
| `m4_map` | Looping constructs
| `m4_map_args` | Looping constructs
| `m4_map_args_pair` | Looping constructs
| `m4_map_args_sep` | Looping constructs
| `m4_map_args_w` | Looping constructs
| `m4_map_sep` | Looping constructs
| `m4_mapall` | Looping constructs
| `m4_mapall_sep` | Looping constructs
| `m4_max` | Number processing Macros
| `m4_min` | Number processing Macros
| `m4_mkstemp` | Redefined M4 Macros
| `m4_n` | Conditional constructs
| `m4_newline` | Text processing Macros
| `m4_normalize` | Text processing Macros
| `m4_pattern_allow` | Forbidden Patterns
| `m4_pattern_forbid` | Forbidden Patterns
| `m4_popdef` | Redefined M4 Macros
| `m4_pushdef` | Redefined M4 Macros
| `m4_quote` | Evaluation Macros
| `m4_re_escape` | Text processing Macros
| `m4_rename` | Redefined M4 Macros
| `m4_rename_force` | Redefined M4 Macros
| `m4_reverse` | Evaluation Macros
| `m4_set_add` | Set manipulation Macros
| `m4_set_add_all` | Set manipulation Macros
| `m4_set_contains` | Set manipulation Macros
| `m4_set_contents` | Set manipulation Macros
| `m4_set_delete` | Set manipulation Macros
| `m4_set_difference` | Set manipulation Macros
| `m4_set_dump` | Set manipulation Macros
| `m4_set_empty` | Set manipulation Macros
| `m4_set_foreach` | Set manipulation Macros
| `m4_set_intersection` | Set manipulation Macros
| `m4_set_list` | Set manipulation Macros
| `m4_set_listc` | Set manipulation Macros
| `m4_set_map` | Set manipulation Macros
| `m4_set_map_sep` | Set manipulation Macros
| `m4_set_remove` | Set manipulation Macros
| `m4_set_size` | Set manipulation Macros
| `m4_set_union` | Set manipulation Macros
| `m4_shift` | Redefined M4 Macros
| `m4_shift2` | Looping constructs
| `m4_shift3` | Looping constructs
| `m4_shiftn` | Looping constructs
| `m4_sign` | Number processing Macros
| `m4_sinclude` | Redefined M4 Macros
| `m4_split` | Text processing Macros
| `m4_stack_foreach` | Looping constructs
| `m4_stack_foreach_lifo` | Looping constructs
| `m4_stack_foreach_sep` | Looping constructs
| `m4_stack_foreach_sep_lifo` | Looping constructs
| `m4_strip` | Text processing Macros
| `m4_substr` | Redefined M4 Macros
| `m4_syscmd` | Redefined M4 Macros
| `m4_sysval` | Redefined M4 Macros
| `m4_text_box` | Text processing Macros
| `m4_text_wrap` | Text processing Macros
| `m4_tolower` | Text processing Macros
| `m4_toupper` | Text processing Macros
| `m4_traceoff` | Redefined M4 Macros
| `m4_traceon` | Redefined M4 Macros
| `m4_translit` | Redefined M4 Macros
| `m4_undefine` | Redefined M4 Macros
| `m4_undivert` | Redefined M4 Macros
| `m4_unquote` | Evaluation Macros
| `m4_version_compare` | Number processing Macros
| `m4_version_prereq` | Number processing Macros
| `m4_warn` | Diagnostic Macros
| `m4_wrap` | Redefined M4 Macros
| `m4_wrap_lifo` | Redefined M4 Macros

### B.7 Autotest Macro Index ¶

This is an alphabetical list of the Autotest macros.

| Index Entry | Section

A
| `AT_ARG_OPTION` | Writing Testsuites
| `AT_ARG_OPTION_ARG` | Writing Testsuites
| `AT_AT_PREPARE_EACH_TEST` | Writing Testsuites
| `AT_BANNER` | Writing Testsuites
| `AT_CAPTURE_FILE` | Writing Testsuites
| `AT_CHECK` | Writing Testsuites
| `AT_CHECK_EUNIT` | Writing Testsuites
| `AT_CHECK_UNQUOTED` | Writing Testsuites
| `AT_CLEANUP` | Writing Testsuites
| `AT_COLOR_TESTS` | Writing Testsuites
| `AT_COPYRIGHT` | Writing Testsuites
| `AT_DATA` | Writing Testsuites
| `AT_FAIL_IF` | Writing Testsuites
| `AT_INIT` | Writing Testsuites
| `AT_KEYWORDS` | Writing Testsuites
| `AT_PACKAGE_BUGREPORT` | Making testsuite Scripts
| `AT_PACKAGE_NAME` | Making testsuite Scripts
| `AT_PACKAGE_STRING` | Making testsuite Scripts
| `AT_PACKAGE_TARNAME` | Making testsuite Scripts
| `AT_PACKAGE_URL` | Making testsuite Scripts
| `AT_PACKAGE_VERSION` | Making testsuite Scripts
| `AT_PREPARE_TESTS` | Writing Testsuites
| `AT_SETUP` | Writing Testsuites
| `AT_SKIP_IF` | Writing Testsuites
| `AT_TESTED` | Writing Testsuites
| `AT_XFAIL_IF` | Writing Testsuites

### B.8 Program and Function Index ¶

This is an alphabetical list of the programs and functions whose portability is discussed in this document.

Jump to: | ! . { / & | A B C D E F G I J K L M N O P R S T U V W X

| Index Entry | Section

!
| `!` | Limitations of Builtins

.
| `.` | Limitations of Builtins

{
| `{...}` | Limitations of Builtins

/
| `/usr/bin/ksh` on Solaris | Shellology
| `/usr/dt/bin/dtksh` on Solaris | Shellology
| `/usr/xpg4/bin/sh` on Solaris | Shellology

&
| `&&` | Limitations of Builtins

| `||` | Limitations of Builtins

A
| `alloca` | Particular Functions
| `alloca.h` | Particular Functions
| `assert.h` | Default Includes
| `assert.h` | Particular Headers
| `awk` | Limitations of Usual Tools

B
| `basename` | Limitations of Usual Tools
| `break` | Limitations of Builtins

C
| `case` | Limitations of Builtins
| `cat` | Limitations of Usual Tools
| `cc` | Limitations of Usual Tools
| `cd` | Limitations of Builtins
| `chgrp` | Limitations of Usual Tools
| `chmod` | Limitations of Usual Tools
| `chown` | Particular Functions
| `chown` | Limitations of Usual Tools
| `closedir` | Particular Functions
| `cmp` | Limitations of Usual Tools
| `config.guess` | Input
| `config.guess` | Manual Configuration
| `config.sub` | Input
| `config.sub` | Manual Configuration
| `cp` | Limitations of Usual Tools
| `ctype.h` | Default Includes

D
| `date` | Limitations of Usual Tools
| `dirent.h` | Particular Headers
| `dirname` | Limitations of Usual Tools

E
| `echo` | Limitations of Builtins
| `egrep` | Limitations of Usual Tools
| `errno.h` | Default Includes
| `error_at_line` | Particular Functions
| `eval` | Limitations of Builtins
| `exec` | Limitations of Builtins
| `exit` | Function Portability
| `exit` | Limitations of Builtins
| `export` | Limitations of Builtins
| `expr` | Limitations of Usual Tools
| `expr` | Limitations of Usual Tools
| `expr` (‘`|`’) | Limitations of Usual Tools

F
| `false` | Limitations of Builtins
| `fgrep` | Limitations of Usual Tools
| `find` | Limitations of Usual Tools
| `float.h` | Default Includes
| `fnmatch` | Particular Functions
| `fnmatch` | Particular Functions
| `fnmatch` | Particular Functions
| `fnmatch.h` | Particular Functions
| `for` | Limitations of Builtins
| `fork` | Particular Functions
| `fseeko` | Particular Functions
| `ftello` | Particular Functions

G
| `getgroups` | Particular Functions
| `getloadavg` | Particular Functions
| `getmntent` | Particular Functions
| `getpgid` | Particular Functions
| `getpgrp` | Particular Functions
| `grep` | Limitations of Usual Tools

I
| `if` | Limitations of Builtins
| `install-sh` | Input
| `install-sh` | Particular Programs
| `install-sh` | Particular Programs
| `inttypes.h` | Header Portability
| `inttypes.h` | Particular Types
| `iso646.h` | Default Includes

J
| `join` | Limitations of Usual Tools

K
| ‘`ksh`’ | Shellology
| ‘`ksh88`’ | Shellology
| ‘`ksh93`’ | Shellology

L
| `limits.h` | Default Includes
| `linux/irda.h` | Header Portability
| `linux/random.h` | Header Portability
| `ln` | Limitations of Usual Tools
| `locale.h` | Default Includes
| `ls` | Limitations of Usual Tools
| `lstat` | Particular Functions
| `lstat` | Particular Functions

M
| `make` | Portable Make
| `malloc` | Function Portability
| `malloc` | Particular Functions
| `math.h` | Default Includes
| `mbrtowc` | Particular Functions
| `memcmp` | Particular Functions
| `memory.h` | Header Portability
| `mkdir` | Limitations of Usual Tools
| `mkfifo` | Limitations of Usual Tools
| `mknod` | Limitations of Usual Tools
| `mktemp` | Limitations of Usual Tools
| `mktime` | Particular Functions
| `mmap` | Particular Functions
| `mv` | Limitations of Usual Tools

N
| `ndir.h` | Particular Headers
| `net/if.h` | Header Portability
| `netinet/if_ether.h` | Header Portability
| `nlist.h` | Particular Functions

O
| `od` | Limitations of Usual Tools

P
| ‘`pdksh`’ | Shellology
| `printf` | Limitations of Builtins
| `putenv` | Function Portability
| `pwd` | Limitations of Builtins

R
| `read` | Limitations of Builtins
| `realloc` | Function Portability
| `realloc` | Particular Functions
| `resolv.h` | Particular Headers
| `rm` | Limitations of Usual Tools
| `rmdir` | Limitations of Usual Tools

S
| `sed` | Limitations of Usual Tools
| `sed` (‘`t`’) | Limitations of Usual Tools
| `sed` (‘`w`’) | Limitations of Usual Tools
| `select` | Particular Functions
| `set` | Limitations of Builtins
| `setjmp.h` | Default Includes
| `setpgrp` | Particular Functions
| `setvbuf` | Obsolete Macros
| `shift` | Limitations of Builtins
| `sigaction` | Function Portability
| `signal` | Function Portability
| `signal.h` | Default Includes
| `signal.h` | Obsolete Macros
| `sleep` | Limitations of Usual Tools
| `snprintf` | Function Portability
| `sort` | Limitations of Usual Tools
| `source` | Limitations of Builtins
| `stat` | Particular Functions
| `stdarg.h` | Default Includes
| `stdbool.h` | Particular Headers
| `stdbool.h` | Particular Headers
| `stddef.h` | Default Includes
| `stdint.h` | Header Portability
| `stdint.h` | Particular Types
| `stdio.h` | Default Includes
| `stdlib.h` | Default Includes
| `stdlib.h` | Particular Types
| `strcoll` | Particular Functions
| `strerror_r` | Function Portability
| `strerror_r` | Particular Functions
| `strftime` | Particular Functions
| `string.h` | Default Includes
| `strings.h` | Header Portability
| `strnlen` | Function Portability
| `strnlen` | Particular Functions
| `strtod` | Particular Functions
| `strtold` | Particular Functions
| `sys/dir.h` | Particular Headers
| `sys/ioctl.h` | Particular Headers
| `sys/mkdev.h` | Particular Headers
| `sys/mount.h` | Header Portability
| `sys/ndir.h` | Particular Headers
| `sys/ptem.h` | Header Portability
| `sys/socket.h` | Header Portability
| `sys/stat.h` | Particular Headers
| `sys/sysmacros.h` | Particular Headers
| `sys/time.h` | Particular Structures
| `sys/time.h` | Obsolete Macros
| `sys/types.h` | Particular Types
| `sys/ucred.h` | Header Portability
| `sys/wait.h` | Particular Headers
| `sysconf` | Function Portability

T
| `tar` | Limitations of Usual Tools
| `termios.h` | Particular Headers
| `test` | Limitations of Builtins
| `time.h` | Default Includes
| `time.h` | Particular Structures
| `time.h` | Obsolete Macros
| `touch` | Limitations of Usual Tools
| `tr` | Limitations of Usual Tools
| `trap` | Limitations of Builtins
| `true` | Limitations of Builtins

U
| `unistd.h` | Particular Headers
| `unlink` | Function Portability
| `unset` | Limitations of Builtins
| `unsetenv` | Function Portability
| `utime` | Particular Functions

V
| `va_copy` | Function Portability
| `va_list` | Function Portability
| `vfork` | Particular Functions
| `vfork.h` | Particular Functions
| `vprintf` | Particular Functions
| `vsnprintf` | Function Portability
| `vsprintf` | Particular Functions

W
| `wait` | Limitations of Builtins
| `wait3` | Obsolete Macros
| `wchar.h` | Default Includes
| `wchar.h` | Particular Types
| `wctype.h` | Default Includes

X
| `X11/extensions/scrnsaver.h` | Header Portability

### B.9 Concept Index ¶

This is an alphabetical list of the files, tools, and concepts introduced in this document.

Jump to: | _ " @ ` ^ $ 6 A B C D E F G H I K L M N O P Q R S T U V W X Y Z

| Index Entry | Section

_
| `_m4_divert_diversion` | New Macros

"
| `"$@"` | Shell Substitutions

@
| ‘`@:}@`’ | Quadrigraphs
| ‘`@:>@`’ | Quadrigraphs
| ‘`@{:@`’ | Quadrigraphs
| ‘`@&t@`’ | Quadrigraphs
| ‘`@%:@`’ | Quadrigraphs
| ‘`@<:@`’ | Quadrigraphs
| ‘`@S|@`’ | Quadrigraphs

`
| ``commands`` | Shell Substitutions

^
| `^` quoting | Shell Substitutions

$
| `$((expression))` | Shell Substitutions
| `$(commands)` | Shell Substitutions
| `${#var}` | Shell Substitutions
| `${var-value}` | Shell Substitutions
| `${var:-value}` | Shell Substitutions
| `${var:?value}` | Shell Substitutions
| `${var:+value}` | Shell Substitutions
| `${var:=value}` | Shell Substitutions
| `${var?value}` | Shell Substitutions
| `${var##word}` | Shell Substitutions
| `${var#word}` | Shell Substitutions
| `${var%%word}` | Shell Substitutions
| `${var%word}` | Shell Substitutions
| `${var+value}` | Shell Substitutions
| `${var=expanded-value}` | Shell Substitutions
| `${var=literal}` | Shell Substitutions
| `${var=value}` | Shell Substitutions
| `${var=value}` | Shell Substitutions
| `$<`, explicit rules, and `VPATH` | $< in Explicit Rules

6
| 64-bit libraries | Site Defaults

A
| `A68FLAGS` | Preset Output Variables
| `abs_builddir` | Preset Output Variables
| `abs_srcdir` | Preset Output Variables
| `abs_top_builddir` | Preset Output Variables
| `abs_top_srcdir` | Preset Output Variables
| absolute file names, detect | File System Conventions
| `ac_aux_dir` | Input
| `ac_objext` | Generic Functions
| `ac_path_variable` | Generic Programs
| `ac_path_variable_found` | Generic Programs
| `ac_srcdir` | Configuration Actions
| `ac_top_build_prefix` | Configuration Actions
| `ac_top_srcdir` | Configuration Actions
| `acconfig.h` | acconfig Header
| `aclocal.m4` | Making configure Scripts
| Algol 68 | Algol 68 Compiler
| Ash | Shellology
| `at_arg_option` | Writing Testsuites
| `at_arg_option` | Writing Testsuites
| `at_optarg` | Writing Testsuites
| `at_optarg` | Writing Testsuites
| `at_optarg_option` | Writing Testsuites
| `at_status` | Writing Testsuites
| `autoconf` | autoconf Invocation
| Autoconf upgrading | Autoconf 1
| Autoconf upgrading | Autoconf 2.13
| Autoconf version | Versioning
| `autoheader` | autoheader Invocation
| Autoheader macros | Autoheader Macros
| autom4te debugging tips | Debugging via autom4te
| Autom4te Library | autom4te Invocation
| `autom4te.cache` | autom4te Invocation
| `autom4te.cfg` | autom4te Invocation
| Automake | Automake
| Automatic remaking | Automatic Remaking
| automatic rule rewriting and `VPATH` | Automatic Rule Rewriting
| `autopoint` | autoreconf Invocation
| `autoreconf` | autoreconf Invocation
| `autoscan` | autoscan Invocation
| Autotest | Using Autotest
| `AUTOTEST_PATH` | testsuite Invocation
| `autoupdate` | autoupdate Invocation

B
| balancing parentheses | Balancing Parentheses
| Bash | Shellology
| Bash 2.05 and later | Shellology
| `bindir` | Installation Directory Variables
| Bootstrap | Bootstrapping
| BSD `make` and `obj/` | obj/ and Make
| buffer overruns | Buffer Overruns
| Build directories | Build Directories
| `builddir` | Preset Output Variables

C
| C function portability | Function Portability
| C types | Types
| C89, C99, C11, C17, and C23 | Portable C and C++
| Cache | Caching Results
| Cache variable | Cache Variable Names
| Cache, enabling | configure Invocation
| Canonical system type | Canonicalizing
| carriage return, deleting | Limitations of Usual Tools
| `CFLAGS` | Preset Output Variables
| `changequote` | Changequote is Evil
| Coding style | Coding Style
| Command Substitution | Shell Substitutions
| command-line, macros set on | Command-line Macros and whitespace
| Commands for configuration | Configuration Commands
| Comments in `Makefile` macros | Comments in Make Macros
| Comments in `Makefile` rules | Comments in Make Rules
| Common autoconf behavior | Common Behavior
| Compilers | Compilers and Preprocessors
| composing variable names | Polymorphic Variables
| `CONFIG_COMMANDS` | Obsolete config.status Use
| `CONFIG_FILES` | Obsolete config.status Use
| `CONFIG_HEADERS` | Obsolete config.status Use
| `CONFIG_LINKS` | Obsolete config.status Use
| `CONFIG_SHELL` | config.status Invocation
| `CONFIG_STATUS` | config.status Invocation
| `config.h` | Configuration Headers
| `config.h.bot` | acconfig Header
| `config.h.in` | Header Templates
| `config.h.top` | acconfig Header
| config.site | Site Defaults
| `config.status` | config.status Invocation
| `config.sub` | Specifying Target Triplets
| Configuration actions | Configuration Actions
| Configuration commands | Configuration Commands
| Configuration file creation | Configuration Files
| Configuration Header | Configuration Headers
| Configuration Header Template | Header Templates
| Configuration links | Configuration Links
| `configure` | Making configure Scripts
| `configure` | Running configure Scripts
| Configure subdirectories | Subdirectories
| `configure_input` | Preset Output Variables
| `configure.ac` | Making configure Scripts
| `configure.in` | Writing Autoconf Input
| Copyright Notice | Notices
| Copyright Notice | Writing Testsuites
| `CPPFLAGS` | Preset Output Variables
| Creating configuration files | Configuration Files
| Creating temporary files | Limitations of Usual Tools
| Cross compilation | Hosts and Cross-Compilation
| `CXXFLAGS` | Preset Output Variables

D
| Darwin | Systemology
| Data structure, set | Set manipulation Macros
| `datadir` | Installation Directory Variables
| `datarootdir` | Changed Directory Variables
| `datarootdir` | Installation Directory Variables
| debugging tips | Debugging via autom4te
| Declaration, checking | Declarations
| Default includes | Default Includes
| `DEFS` | Preset Output Variables
| deleting carriage return | Limitations of Usual Tools
| Dependencies between macros | Dependencies Between Macros
| descriptors | File Descriptor Macros
| Descriptors | File Descriptors
| Directories, build | Build Directories
| Directories, installation | Installation Directory Variables
| division, integer | Signed Integer Division
| `dnl` | Macro Definitions
| `dnl` | Coding Style
| `docdir` | Installation Directory Variables
| double-colon rules and `VPATH` | VPATH and Double-colon
| `dvidir` | Installation Directory Variables

E
| `ECHO_C` | Preset Output Variables
| `ECHO_N` | Preset Output Variables
| `ECHO_T` | Preset Output Variables
| Endianness | C Compiler
| environment, macros set from | Command-line Macros and whitespace
| Erlang | Erlang Compiler and Interpreter
| `ERLANG_INSTALL_LIB_DIR` | Installation Directory Variables
| `ERLANG_INSTALL_LIB_DIR_library` | Installation Directory Variables
| Erlang, Library, checking | Erlang Libraries
| `ERLCFLAGS` | Preset Output Variables
| `exec_prefix` | Installation Directory Variables
| exiting portably | Exiting Portably
| expanded before required | Expanded Before Required
| explicit rules, `$<`, and `VPATH` | $< in Explicit Rules
| External software | External Software

F
| F77 | Fortran Compiler
| `FCFLAGS` | Preset Output Variables
| `FFLAGS` | Preset Output Variables
| FHS | Site Defaults
| file descriptors | File Descriptor Macros
| File descriptors | File Descriptors
| File system conventions | File System Conventions
| File, checking | Files
| Filesystem Hierarchy Standard | Site Defaults
| floating point | Floating Point Portability
| Forbidden patterns | Forbidden Patterns
| Fortran | Fortran Compiler
| Function, checking | Particular Functions

G
| Gettext | autoreconf Invocation
| GNU build system | The GNU Build System
| Gnulib | Gnulib
| Go | Go Compiler
| `GOFLAGS` | Preset Output Variables

H
| Header portability | Header Portability
| Header templates | Header Templates
| Header, checking | Header Files
| Help strings | Pretty Help Strings
| Here-documents | Here-Documents
| History of autoconf | History
| `htmldir` | Installation Directory Variables

I
| `ifnames` | ifnames Invocation
| Imake | Why Not Imake
| `includedir` | Installation Directory Variables
| Includes, default | Default Includes
| indirection, variable name | Polymorphic Variables
| `infodir` | Installation Directory Variables
| input | File Descriptor Macros
| Install prefix | Default Prefix
| Installation directories | Installation Directory Variables
| Instantiation | Output
| integer overflow | Integer Overflow
| integer overflow | Integer Overflow Basics
| integer overflow | Signed Overflow Examples
| integer overflow | Signed Overflow Advice
| Introduction | Introduction
| invoking the shell | Invoking the Shell

K
| K&R C | Portable C and C++
| Korn shell | Shellology
| Ksh | Shellology

L
| Language | Language Choice
| Large file support | System Services
| `LDFLAGS` | Preset Output Variables
| LFS | System Services
| `lib64` | Site Defaults
| `libdir` | Installation Directory Variables
| `libexecdir` | Installation Directory Variables
| Library, checking | Libraries
| `LIBS` | Preset Output Variables
| Libtool | Libtool
| License | Distributing
| Limitations of `make` | Portable Make
| Limitations of shell builtins | Limitations of Builtins
| Limitations of usual tools | Limitations of Usual Tools
| Links | Configuration Links
| Links for configuration | Configuration Links
| Listing directories | Limitations of Usual Tools
| `localedir` | Installation Directory Variables
| `localstatedir` | Installation Directory Variables
| loop induction | Optimization and Wraparound
| low-level output | File Descriptor Macros

M
| M4 | Programming in M4
| M4 quotation | M4 Quotation
| M4sugar | Programming in M4sugar
| m4sugar debugging tips | Debugging via autom4te
| Mac OS X | Systemology
| macOS | Systemology
| Macros, called once | One-Shot Macros
| Macros, obsoleting | Obsoleting Macros
| Macros, ordering | Suggested Ordering
| Macros, prerequisites | Prerequisite Macros
| `make -k` | make -k Status
| `make` and `MAKEFLAGS` | The Make Macro MAKEFLAGS
| `make` and `SHELL` | The Make Macro SHELL
| `Makefile` macros and comments | Comments in Make Macros
| `Makefile` macros and whitespace | Trailing whitespace in Make Macros
| `Makefile` rules and comments | Comments in Make Rules
| `Makefile` rules and newlines | Newlines in Make Rules
| Makefile substitutions | Makefile Substitutions
| `MAKEFLAGS` and `make` | The Make Macro MAKEFLAGS
| Making directories | Limitations of Usual Tools
| `mandir` | Installation Directory Variables
| Messages, from `configure` | Printing Messages
| Messages, from `M4sugar` | Diagnostic Macros
| Moving open files | Limitations of Usual Tools

N
| newline, deleting | Limitations of Usual Tools
| Newlines in `Makefile` rules | Newlines in Make Rules
| Notices in `configure` | Notices
| null pointers | Null Pointers

O
| `obj/`, subdirectory | obj/ and Make
| `OBJCFLAGS` | Preset Output Variables
| `OBJCXXFLAGS` | Preset Output Variables
| Obsolete constructs | Obsolete Constructs
| Obsoleting macros | Obsoleting Macros
| obstack | Particular Functions
| `oldincludedir` | Installation Directory Variables
| One-shot macros | One-Shot Macros
| Options, package | Package Options
| Options, Package | Option Checking
| Ordering macros | Suggested Ordering
| Output variables | Preset Output Variables
| Output variables | Setting Output Variables
| Output variables, special characters in | Special Chars in Variables
| output, low-level | File Descriptor Macros
| Outputting files | Output
| overflow, signed integer | Integer Overflow
| overflow, signed integer | Integer Overflow Basics
| overflow, signed integer | Signed Overflow Examples
| overflow, signed integer | Signed Overflow Advice

P
| Package options | Package Options
| `package.m4` | Making testsuite Scripts
| Parallel `make` | Parallel Make
| parentheses, balancing | Balancing Parentheses
| Patterns, forbidden | Forbidden Patterns
| `pdfdir` | Installation Directory Variables
| polymorphic variable name | Polymorphic Variables
| portability | Varieties of Unportability
| Portability of C functions | Function Portability
| Portability of headers | Header Portability
| Portable C and C++ programming | Portable C and C++
| Portable shell programming | Portable Shell
| positional parameters | Shell Substitutions
| POSIX termios headers | System Services
| Precious Variable | Setting Output Variables
| `prefix` | Installation Directory Variables
| Prefix for install | Default Prefix
| preprocessor arithmetic | Preprocessor Arithmetic
| Preprocessors | Compilers and Preprocessors
| Prerequisite macros | Prerequisite Macros
| Program names, transforming | Transforming Names
| Programs, checking | Alternative Programs
| `psdir` | Installation Directory Variables

Q
| QNX | Systemology
| quadrigraphs | Quadrigraphs
| quotation | Autoconf Language
| quotation | M4 Quotation

R
| Remaking automatically | Automatic Remaking
| Revision | Notices
| Rule, Single Suffix Inference | Single Suffix Rules
| `runstatedir` | Installation Directory Variables

S
| `sbindir` | Installation Directory Variables
| Separated Dependencies | Single Suffix Rules
| `set -b` | Limitations of Builtins
| `set -e` | Limitations of Builtins
| `set -m` | Limitations of Builtins
| `set -n` | Limitations of Builtins
| Set manipulation | Set manipulation Macros
| `sharedstatedir` | Installation Directory Variables
| `SHELL` and `make` | The Make Macro SHELL
| Shell assignments | Assignments
| Shell builtins | Limitations of Builtins
| Shell file descriptors | File Descriptors
| Shell Functions | Shell Functions
| Shell here-documents | Here-Documents
| shell invocation | Invoking the Shell
| Shell parentheses | Parentheses
| Shell pattern matching | Shell Pattern Matching
| Shell substitutions | Shell Substitutions
| Shell variables | Special Shell Variables
| Shellology | Shellology
| Signal handling in the shell | Signal Handling
| Signals, shells and | Signal Handling
| signed integer overflow | Integer Overflow
| signed integer overflow | Integer Overflow Basics
| signed integer overflow | Signed Overflow Examples
| signed integer overflow | Signed Overflow Advice
| Single Suffix Inference Rule | Single Suffix Rules
| Site defaults | Site Defaults
| Site details | Site Details
| Special shell variables | Special Shell Variables
| `srcdir` | Configuration Actions
| `srcdir` | Preset Output Variables
| standard input | File Descriptor Macros
| Standard symbols | Standard Symbols
| Structure, checking | Structures
| Subdirectory configure | Subdirectories
| Substitutions in makefiles | Makefile Substitutions
| Symbolic links | Limitations of Usual Tools
| `sysconfdir` | Installation Directory Variables
| System type | Specifying Target Triplets
| System type | Canonicalizing
| Systemology | Systemology

T
| Target triplet | Specifying Target Triplets
| termios POSIX headers | System Services
| test group | testsuite Scripts
| `testsuite` | testsuite Scripts
| `testsuite` | testsuite Invocation
| timestamp resolution | Limitations of Usual Tools
| timestamp resolution | Limitations of Usual Tools
| timestamp resolution | Timestamps and Make
| `tmp` | Configuration Actions
| `top_build_prefix` | Preset Output Variables
| `top_builddir` | Preset Output Variables
| `top_srcdir` | Preset Output Variables
| Transforming program names | Transforming Names
| Types | Types

U
| unbalanced parentheses, managing | Balancing Parentheses
| undefined macro | New Macros
| Unix version 7 | Systemology
| Unordered set manipulation | Set manipulation Macros
| Upgrading autoconf | Autoconf 1
| Upgrading autoconf | Autoconf 2.13

V
| V7 | Systemology
| variable name indirection | Polymorphic Variables
| variable names, composing | Polymorphic Variables
| Variable, Precious | Setting Output Variables
| variables and `VPATH` | Variables listed in VPATH
| Version | Versioning
| version, Autoconf | Versioning
| volatile objects | Volatile Objects
| `VPATH` | VPATH and Make
| `VPATH` and automatic rule rewriting | Automatic Rule Rewriting
| `VPATH` and double-colon rules | VPATH and Double-colon
| `VPATH` and variables | Variables listed in VPATH
| `VPATH`, explicit rules, and `$<` | $< in Explicit Rules
| `VPATH`, resolving target pathnames | Make Target Lookup

W
| whitespace in command-line macros | Command-line Macros and whitespace
| whitespace in `Makefile` macros | Trailing whitespace in Make Macros
| wraparound arithmetic | Integer Overflow
| wraparound arithmetic | Integer Overflow Basics
| wraparound arithmetic | Signed Overflow Examples
| wraparound arithmetic | Signed Overflow Advice

X
| X Window System | System Services

Y
| Year 2038 | System Services

Z
| Zsh | Shellology

#### Footnotes

##### (1)

GNU Autoconf, Automake and Libtool, by G. V. Vaughan, B. Elliston, T. Tromey, and I. L. Taylor. SAMS (originally New Riders), 2000, ISBN 1578701902.

##### (2)

Because M4 is not aware of Sh code, especially conditionals, some optimizations that look nice statically may produce incorrect results at runtime.

##### (3)

By itself, M4 uses ‘```’ and ‘`'`’; it is the M4sugar layer that sets up the preferred quotes of ‘`[`’ and ‘`]`’.

##### (4)

Using `defn`.

##### (5)

Yet another great name from Lars J. Aas.