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 31.3 Which Version of POSIX is Supported
 ========================================
 
  -- Macro: long int _POSIX_VERSION
      This constant represents the version of the POSIX.1 standard to
      which the implementation conforms.  For an implementation
      conforming to the 1995 POSIX.1 standard, the value is the integer
      `199506L'.
 
      `_POSIX_VERSION' is always defined (in `unistd.h') in any POSIX
      system.
 
      *Usage Note:* Don't try to test whether the system supports POSIX
      by including `unistd.h' and then checking whether `_POSIX_VERSION'
      is defined.  On a non-POSIX system, this will probably fail
      because there is no `unistd.h'.  We do not know of _any_ way you
      can reliably test at compilation time whether your target system
      supports POSIX or whether `unistd.h' exists.
 
      The GNU C compiler predefines the symbol `__POSIX__' if the target
      system is a POSIX system.  Provided you do not use any other
      compilers on POSIX systems, testing `defined (__POSIX__)' will
      reliably detect such systems.
 
  -- Macro: long int _POSIX2_C_VERSION
      This constant represents the version of the POSIX.2 standard which
      the library and system kernel support.  We don't know what value
      this will be for the first version of the POSIX.2 standard,
      because the value is based on the year and month in which the
      standard is officially adopted.
 
      The value of this symbol says nothing about the utilities
      installed on the system.
 
      *Usage Note:* You can use this macro to tell whether a POSIX.1
      system library supports POSIX.2 as well.  Any POSIX.1 system
      contains `unistd.h', so include that file and then test `defined
      (_POSIX2_C_VERSION)'.
 

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