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 the preprocessor directives should be at the beginning of the line. Although indentation is allowed currently, it is possible that it will generate warnings or errors in the future. It is however allowed to put spaces after the hash character to create indentation in code.

5.4.1. #!

  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.

5.4.2. #<integer> and #line

  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. `#1 "/home/pike/program.pike.in"`. Then this filename will be used instead of the current file for error messages.

5.4.3. #""

  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. Newlines will be converted to `\n` characters if the newlines in the file is something else. This preprocessor directive may appear anywhere a string may appear.

5.4.4. #string

  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.

      do_something(#string "the_file.wks");

5.4.5. #include

  `#include` may be 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 by using double quotes, 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 define, e.g. `#include USER_SETTINGS`.

5.4.6. #if

  The `#if` directive can evaluate simple expressions and, if the expression is evaluated to true, "activate" the code block that follows. The code block ends when an `#endif`, `#else`, `#elseif` or `#elif` block is encountered on 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. Define returns true if the symbol given as argument is defined. `#if defined(MY_DEF)` is equal to `#ifdef MY_DEF`. Efun returns true if its argument is an efun and constant returns true if its argument can be resolved into a constant.

5.4.7. #ifdef

  `#ifdef` works as `#if`, but instead of evaluating its arguments it just checks if the first symbol is a defined define or marcro.

5.4.8. #ifndef

  Works as an inverted `#ifndef`; it only "activates" the following block if the symbol is not defined.

5.4.9. #endif

  Ends a block opened by `#if`, `#ifdef`, `#ifndef`, `#else`, `#elseif` or `#elif`.

      #if DEBUG
      do_debug_stuff();
      #endif /* DEBUG */

5.4.10. #else

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

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

5.4.11. #elseif and #elif

  `#elseif` and `#elif` works as `elseif` in the `#if`/`#ifdef`/`#ifndef` context.

5.4.12. #undefine and #undef

  `#undefine` and `#undefine` undefines the symbol given as argument.

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

5.4.13. #error

  Throws an error during preprocessing.

      #ifdef __NT__
      #error "This program can not run on MS Windows."
      #endif

5.4.14. #charset

  Tells the preprocessor which charset the file is encoded with. The Locale.Charset module is called with this string to decode the file.

5.4.15. #pike

  Tells the compiler which version of Pike it should emulate.

      #pike 7.2

5.4.16. #pragma all_inline

5.4.17. #pragma all_final

  Instructs the compiler to mark all symbols as final.

5.4.18. #pragma all_nomask

  Deprecated version of #pragma all_final

5.4.19. #pragma strict_types

5.4.20. #pragma save_parent and #pragma dont_save_parent

5.4.21. #warning

  Generates a warning during compilation.

      #if !constant(Crypto.SHA1.hash)
      #warning SHA1 hash not available.
      #endif

5.5. Predefined defines

  Namespace cpp::

    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 __REAL_VERSION__

      `constant __REAL_VERSION__`

    Description

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

    See also

      __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 __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 __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 __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 __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_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 __LINE__

      `constant __LINE__`

    Description

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

    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 __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 __PIKE__

      `constant __PIKE__`

    Description

      This define is always true.

    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 __amigaos__

      `constant __amigaos__`

    Description

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