FreeBSD/Linux Kernel Cross Reference
sys/include/ansi.h

     /* The <ansi.h> header attempts to decide whether the compiler has enough
      * conformance to Standard C for Minix to take advantage of.  If so, the
      * symbol _ANSI is defined (as 31459).  Otherwise _ANSI is not defined
      * here, but it may be defined by applications that want to bend the rules.
      * The magic number in the definition is to inhibit unnecessary bending
      * of the rules.  (For consistency with the new '#ifdef _ANSI" tests in
      * the headers, _ANSI should really be defined as nothing, but that would
      * break many library routines that use "#if _ANSI".)
     
     * If _ANSI ends up being defined, a macro
     *
     *      _PROTOTYPE(function, params)
     *
     * is defined.  This macro expands in different ways, generating either
     * ANSI Standard C prototypes or old-style K&R (Kernighan & Ritchie)
     * prototypes, as needed.  Finally, some programs use _CONST, _VOIDSTAR etc
     * in such a way that they are portable over both ANSI and K&R compilers.
     * The appropriate macros are defined here.
     */
    
    #ifndef _ANSI_H
    #define _ANSI_H
    
    #if __STDC__ == 1
    #define _ANSI           31459   /* compiler claims full ANSI conformance */
    #endif
    
    #ifdef __GNUC__
    #define _ANSI           31459   /* gcc conforms enough even in non-ANSI mode */
    #endif
    
    #ifdef _ANSI
    
    /* Keep everything for ANSI prototypes. */
    #define _PROTOTYPE(function, params)    function params
    #define _ARGS(params)                   params
    
    #define _VOIDSTAR       void *
    #define _VOID           void
    #define _CONST          const
    #define _VOLATILE       volatile
    #define _SIZET          size_t
    
    #else
    
    /* Throw away the parameters for K&R prototypes. */
    #define _PROTOTYPE(function, params)    function()
    #define _ARGS(params)                   ()
    
    #define _VOIDSTAR       void *
    #define _VOID           void
    #define _CONST
    #define _VOLATILE
    #define _SIZET          int
    
    #endif /* _ANSI */
    
    /* This should be defined as restrict when a C99 compiler is used. */
    #define _RESTRICT
    
    /* Setting any of _MINIX, _POSIX_C_SOURCE or _POSIX2_SOURCE implies
     * _POSIX_SOURCE.  (Seems wrong to put this here in ANSI space.)
     */
    #if defined(_MINIX) || _POSIX_C_SOURCE > 0 || defined(_POSIX2_SOURCE)
    #undef _POSIX_SOURCE
    #define _POSIX_SOURCE   1
    #endif
    
    #endif /* ANSI_H */

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