File: autoconf.info,  Node: C and Posix Variants,  Next: Erlang Libraries,  Prev: System Services,  Up: Existing Tests

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.

     The following preprocessor macros are defined unconditionally:

     ‘_ALL_SOURCE’
          Enable extensions on AIX 3 and Interix.
     ‘_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’ (*note
          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
     (http://www.open-std.org/jtc1/sc22/wg14/www/docs/n1967.htm).

     The Autoconf macro ‘AC_USE_SYSTEM_EXTENSIONS’ was introduced in
     Autoconf 2.60.