5. Preprocessor

  Before the Pike code is sent to the compiler it is fed through the preprocessor. The preprocessor converts the source code from its character encoding into the Pike internal representation, performs some simple normalizations and consistency checks and executes the "preprocessor directives" that the programmer may have put into the file. The preprocessor directives are like a very simple programming language that allows for simple code generation and manipulation. The code preprocessor can be called from within Pike with the cpp call.

5.1. Charset Heuristics

  Pike code is Unicode enabled, so the first thing the preprocessor has to do is to try to determine the character encoding of the file. It will first look at the two first bytes of the file and interpret them according to this chart.

  Byte 0 | Byte 1 | Interpretation
  0 | 0 | 32bit wide string.
  0 | >0 | 16bit Unicode string.
  >0 | 0 | 16bit Unicode string in reverse byte order.
  0xfe | 0xff | 16bit Unicode string.
  0xff | 0xfe | 16bit Unicode string in reverse byte order.
  0x7b | 0x83 | EBCDIC-US ("#c").
  0x7b | 0x40 | EBCDIC-US ("# ").
  0x7b | 0x09 | EBCDIC-US ("#\t").

  - With any other combination of bytes the preprocessor will assume iso-8859-1 encoding until a #charset directive has been found.
  - The file must be an multiple of 4 or 2 bytes in order to be correctly decoded as 32bit or 16bit wide string.
  - It's an error for a program written in EBCDIC not to start with a #charset directive.
  - For obfuscation it is possible to encode the #charset directive in a different charset than the charset stated in the #charset directive.

5.2. Code Normalization

  The preprocessor collapses all consecutive white space characters outside of strings, except for newlines, to single space characters. All // and /**/ comments are removed, as are #! lines. Pike considers ANSI/DEC escape sequences as white space. Supported formats are <ESC>[\040-\077]+[\100-\177] and <CSI>[\040-\077]*[\100-\177]. Note that this means that it is possible to do color markup in the actual source file.

  The preprocessor will treat seven consecutive < characters outside of a string as an CVS conflict error and will return "CVS conflict detected."

5.3. Defines and Macros

  Defining macros or constants is one of the most used preprocessor features. It enables you to make abstractions on a code generation level as well as altering constants cross-application. The simplest use of the #define directive however is to declare a "define" as present.

      #define DO_OVERSAMPLING

  The existence of this definition can now be used by e.g. #ifdef and #ifndef to activate or deactivate blocks of program code.

      #ifdef DO_OVERSAMPLING
        // This code is not always run.
        img->render(size*4)->shrink(4);
      #endif

  Note that defines can be given to pike at execution time. In order to set DO_OVERSAMPLING from a command line, the option -DDO_OVERSAMPLING is added before the name of the pike program. E.g. `pike -DDO_OVERSAMPLING my_program.pike`.

  A define can also be given a specific value, which will be inserted everywhere the define is placed in the source code.

      #define CYCLES 20

      void do_stuff() {
        for(int i; i<CYCLES; i++) do_other_stuff();
      }

  Defines can be given specific values on the command line too, just be sure to quote them as required by your shell.

      ~% pike '-DTEXT="Hello world!"' -e 'write("%s\n", TEXT);'
      Hello world!

  Finally #define can also be used to define macros. Macros are just text expansion with arguments, but it is often very useful to make a cleaner looking code and to write less.

      #define VAR(X) id->misc->variable[X]
      #define ROL(X,Y) (((X)<<(Y))&7+((X)>>(8-(Y))))
      #define PLACEHOLDER(X) void X(mixed ... args) { \
        error("Method " #X " is not implemented yet.\n"); }
      #define ERROR(X,Y ...) werror("MyClass" X "\n", Y)
      #define NEW_CONSTANTS(X) do{ int i=sizeof(all_constants()); \
          X \
          werror("Constant diff is %d\n", sizeof(all_constants())-i); \
        }while(0)
      #define MY_FUNC(X,Y) void my##X##Y()

  - A macro can have up to 254 arguments.
  - It can be wise to put extra parentheses around the arguments expanded since it is a purely textual expansion. E.g. if the macro DOUBLE(X) is defined as X*2, then DOUBLE(2+3) will produce 2+3*2, probably producing a hard to track down bug.
  - Since the preprocessor works with textual expansion, it will not evaluate its arguments. Using one argument several time in the macro will thus cause it to evaluated several times during execution. E.g. #define MSG(X) werror("The value "+(X)+" can differ from "+(X)+"\n") when called with MSG(random(1000));.
  - A backslash (\) at the end of the line can be used to make the definition span several lines.
  - A hash (#) in front of a macro variable "casts" it to a string.
  - It is possible to define macros with a variable list of arguments by using the ... syntax.
  - Macros are often formulated so that a semicolon after it is apropriate, for improved code readability.
  - In Pike code macros and defines are most often written in all caps.
  - If a macro expands into several statements, you are well advised to group them together in containment block, such as do { BODY } while(0). If you do not, your macro could produce other hard to track down bugs, if put as a loop or if body without surrounding curly braces.
  - A double hash (##) in front of a macro variable concatenates it with the text before it.

5.4. Preprocessor Directives

  All of the preprocessor directives except the string-related (`#string` and `#""`) should have the hash character (`#`) as the first character of the line. Even though it is currently allowed to be indented, it is likely that this will generate warnings or errors in the future. Note that it is however allowed to put white-space between the hash character and the rest of the directive to create indentation in code.

  Directive #!

    `#!`

  Description

    All lines beginning with #! will be regarded as comments, to enable shell integration. It is recommended that Pike applications begin with the line `"#! /usr/bin/env pike"` for maximum cross platform compatibility.

  Directive #line
  Directive #<integer>

    `#line`
    `#<integer>`

  Description

    A hash character followed by a number or by the string `"line"` and a number will make the preprocessor line counter set this number as the line number for the next line and adjust the following lines accordingly.

    All error messages from Pike will use these line numbers.

    Optionally the number may be followed by a file name, e.g. `#line 1 "/home/pike/program.pike.in"`. Then this filename will be used instead of the current file for error messages.

  Directive #""

    `#""`

  Description

    If a string literal is opened with `#"` newlines in the string will end up in the string literal, instead of triggering a `"newline in string"` error.

  Note

    Newlines will be converted to `\n` characters in the string even if the newlines in the file are something else.

    This preprocessor directive may appear anywhere a string may appear.

  See also

    #string

  Directive #string

    `#string`

  Description

    The preprocessor directive #string will load the file in the string that follows and insert its contents as a string. This preprocessor directive may appear anywhere a string may appear.

  Example

    do_something(#string "the_file.wks");

See also

  #include

Directive #include

  `#include`

Description

  #include is used to insert the contents of another file into the processed file at the place of the include directive.

  Files can be referenced either by absolute or relative path from the source file, or searched for in the include paths.

  To include a file with absolute or relative path, use double quotes, e.g. `#include "constants.pike"` or `#include "../debug.h"`.

  To include from the include paths, use less than and greater than, e.g. `#include <profiling.h>`.

  It is also possible to include a file whose path is defined in a preprocessor macro, e.g. `#include USER_SETTINGS`.

Directive #if

  `#if`

Description

  The #if directive can evaluate simple expressions and, if the expression is evaluates to true, "activate" the code block that follows. The code block ends when an #endif, #else, #elseif or #elif block is encountered at the same nesting depth.

  The #if expressions may include defines, integer, string and float constants, `?:`, `||` and `&&` operations, `~`, `^`, `!`, `|` and `&` operations, `<`, `>`, `<=`, `>=`, `==` and `!=` operations, `+`, `-`, `*`, `/`, `<<` and `>>` operations and paranthesis.

  Strings may also be indexed with the `[]` index operator. Finally there are three special "functions" available in #if expressions; defined(), efun() and constant().

See also

  #ifdef, #ifndef, #elif, #else, #endif, defined(), constant(), efun()

Directive #ifdef

  `#ifdef`

Description

  Check whether an identifier is a macro.

  The directive

  `#ifdef <identifier>`

  is equvivalent to

  `#if defined(<identifier>)`

See also

  #if, #ifndef, defined

Directive #ifndef

  `#ifndef`

Description

  Check whether an identifier is not a macro.

  This is the inverse of #ifdef.

  The directive

  `#ifndef <identifier>`

  is equvivalent to

  `#if !defined(<identifier>)`

See also

  #if, #ifdef, defined

Directive #endif

  `#endif`

Description

  End a block opened with #if, #ifdef, #ifndef, #else, #elseif or #elif.

Example

  #ifdef DEBUG do_debug_stuff(); #endif // DEBUG

Directive #else

  `#else`

Description

  This directive is used to divide the current code block into another code block with inverse activation.

Example

  #ifdef FAST_ALGORITHM do_fast_algorithm(); #elif defined(EXPERIMENTAL_ALGORITHM) do_experimental_algorithm(); #else do_default_algorithm(); #endif

Directive #elif
Directive #elseif

  `#elif`
  `#elseif`

Description

  These work as a combined #else and #if without adding an extra level of nesting.

Example

      The following two are equvivalent:

  #ifdef A // Code for A. #else #ifdef B // Code for B. #else #ifdef C // Code for C. #else // Code for D. #endif #endif #endif

And

    #ifdef A
         // Code for A.
       #elif defined(B)
         // Code for B.
       #elseif defined(C)
         // Code for C.
       #else
         // Code for D.
       #endif

See also

  #if, #ifdef, #else, defined(), constant()

Directive #define

  `#define`

Description

  This directive is used to define or redefine a cpp macro.

  The simplest way to use define is to write

  #define <identifier> <replacement string>

which will cause all subsequent occurances of `<identifier` to be replaced with the `<replacement string>`.

Define also has the capability to use arguments, thus a line like

    #define <identifier>(arg1, arg2) <replacement string>

would cause `<identifer>` to be a macro. All occurances of '`<identifier>(something1,something2d)`' would be replaced with the `<replacement string>`. And in the `<replacement string>`, `arg1` and `arg2` will be replaced with `something1` and `something2`.

Directive #undef
Directive #undefine

  `#undef`
  `#undefine`

Description

  This removes the effect of a #define, all subsequent occurances of the undefined identifier will not be replaced by anything.

Note

  Note that when undefining a macro, you just give the identifer, not the arguments.

Example

      // Strip debug
         #define werror(X ...) 0
         #include "/home/someone/experimental/stuff.h"
         #undef werror

See also

  #define, defined()

Directive #error

  `#error`

Description

  Throw an error during preprocessing.

  This directive causes a cpp error. It can be used to notify the user that certain functions are missing and similar things.

Note

  Note that this directive will cause cpp() to throw an error at the end of preprocessing, which will cause any compilation to fail.

Example

  #if !constant(Yp) #error Support for NIS not available. #endif

See also

  #warning

Directive #charset

  `#charset`

Description

  Inform the preprocessor about which charset the file is encoded with. The Charset module is called with this string to decode the remainder of the file.

Directive #pike

  `#pike`

Description

  Set the Pike compiler backward compatibility level.

  This tells the compiler which version of Pike it should attempt to emulate from this point on in the current compilation unit.

  This is typically used to "quick-fix" old code to work with more recent versions of Pike.

Example

  // This code was written for Pike 7.2, and depends on // the old behaviour for 7.2::dirname(). #pike 7.2

  // ... Code that uses dirname() ...

This directive is also needed for Pike modules that have been installed globally, and might be used by a Pike that has been started with the `-V` flag.

Example

  // Pike modules that are bundled with Pike are // typically written for the same version of Pike. #pike __REAL_VERSION__

Directive #pragma

  `#pragma`

Description

  This is a generic directive for flags to the compiler.

  These are some of the flags that are available:

  `"all_inline"` | This is the same as adding the modifier `inline` to all functions that follow.
  `"all_final"` | Instructs the compiler to mark all symbols as `final`.
  `"all_nomask"` | Deprecated version of `"all_final"`.
  `"save_parent"` | Cause nested classes to save a reference to their surrounding class even if not strictly needed.
  `"dont_save_parent"` | Inverse of `"save_parent"`. This is needed to override if the global symbol predef::__pragma_save_parent__ has been set.
  `"strict_types"` | Enable warnings for all cases where the compiler isn't certain that the types are correct.

Directive #warning

  `#warning`

Description

  Generate a warning during preprocessing.

  This directive causes a cpp warning, it can be used to notify the user that certain functions are missing and similar things.

Example

  #if !constant(Yp) #warning Support for NIS not available. Some features may not work. #endif

See also

  #error

5.5. Predefined defines

  Constant __VERSION__

    `constant __VERSION__`

  Description

    This define contains the current Pike version as a float. If another Pike version is emulated, this define is updated accordingly.

  See also

    __REAL_VERSION__

  Constant __MAJOR__

    `constant __MAJOR__`

  Description

    This define contains the major part of the current Pike version, represented as an integer. If another Pike version is emulated, this define is updated accordingly.

  See also

    __REAL_MAJOR__

  Constant __MINOR__

    `constant __MINOR__`

  Description

    This define contains the minor part of the current Pike version, represented as an integer. If another Pike version is emulated, this define is updated accordingly.

  See also

    __REAL_MINOR__

  Constant __BUILD__

    `constant __BUILD__`

  Description

    This constant contains the build number of the current Pike version, represented as an integer. If another Pike version is emulated, this constant remains unaltered.

  See also

    __REAL_MINOR__

  Constant __REAL_VERSION__

    `constant __REAL_VERSION__`

  Description

    This define always contains the version of the current Pike, represented as a float.

  See also

    __VERSION__

  Constant __REAL_MAJOR__

    `constant __REAL_MAJOR__`

  Description

    This define always contains the major part of the version of the current Pike, represented as an integer.

  See also

    __MAJOR__

  Constant __REAL_MINOR__

    `constant __REAL_MINOR__`

  Description

    This define always contains the minor part of the version of the current Pike, represented as an integer.

  See also

    __MINOR__

  Constant __REAL_BUILD__

    `constant __REAL_BUILD__`

  Description

    This define always contains the minor part of the version of the current Pike, represented as an integer.

  See also

    __BUILD__

  Constant __DATE__

    `constant __DATE__`

  Description

    This define contains the current date at the time of compilation, e.g. "Jul 28 2001".

  Constant __TIME__

    `constant __TIME__`

  Description

    This define contains the current time at the time of compilation, e.g. "12:20:51".

  Constant __FILE__

    `constant __FILE__`

  Description

    This define contains the file path and name of the source file.

  Constant __DIR__

    `constant __DIR__`

  Description

    This define contains the directory path of the source file.

  Constant __LINE__

    `constant __LINE__`

  Description

    This define contains the current line number, represented as an integer, in the source file.

  Constant __AUTO_BIGNUM__

    `constant __AUTO_BIGNUM__`

  Description

    This define is defined when automatic bignum conversion is enabled. When enabled all integers will automatically be converted to bignums when they get bigger than what can be represented by an integer, hampering performance slightly instead of crashing the program.

  Constant __NT__

    `constant __NT__`

  Description

    This define is defined when the Pike is running on a Microsoft Windows OS, not just Microsoft Windows NT, as the name implies.

  Constant __PIKE__

    `constant __PIKE__`

  Description

    This define is always true.

  Constant __amigaos__

    `constant __amigaos__`

  Description

    This define is defined when the Pike is running on Amiga OS.

  Method _Pragma

    `void _Pragma(string directive)`

  Description

    This macro inserts the corresponding #pragma directive in the source.

    e.g. `_Pragma("strict_types")` is the same as `#pragma strict_types` .

  See also

    #pragma

  Constant static_assert

    `constant static_assert`

  Description

    This define expands to the symbol _Static_assert.

    It is the preferred way to perform static (ie compile-time) assertions.

  Note

    The macro can also be used to check for whether static assertions are supported.

  See also

    predef::_Static_assert()

5.6. Test functions

  These functions are used in #if et al expressions to test for presence of symbols.

  Method constant
  Method efun

    `bool constant(mixed identifier)`
    `__deprecated__ bool efun(mixed identifier)`

  Description

    Check whether the argument resolves to a constant or not.

  See also

    #if, defined()

  Method defined

    `bool defined(mixed identifier)`

  Description

    Check whether an identifier is a cpp macro or not.

  Returns

    defined returns true if the symbol given as argument is defined.

  Note

    `#if defined(MY_DEF)` is equvivalent to `#ifdef MY_DEF`.

  See also

    #if, #ifdef, constant()