Feature Test Macros
--------------------

   The exact set of features available when you compile a source file
is controlled by which "feature test macros" you define.

   If you compile your programs using `gcc -ansi', you get only the
ANSI C library features, unless you explicitly request additional
features by defining one or more of the feature macros.  *Note
Options: (gcc.info)Options, for more information about GCC options.

   You should define these macros by using `#define' preprocessor
directives at the top of your source code files.  You could also use
the `-D' option to GCC, but it's better if you make the source files
indicate their own meaning in a self-contained way.

 -- Macro: _POSIX_SOURCE
     If you define this macro, then the functionality from the POSIX.1
     standard (IEEE Standard 1003.1) is available, as well as all of
     the ANSI C facilities.

 -- Macro: _POSIX_C_SOURCE
     If you define this macro with a value of `1', then the
     functionality from the POSIX.1 standard (IEEE Standard 1003.1) is
     made available.  If you define this macro with a value of `2',
     then both the functionality from the POSIX.1 standard and the
     functionality from the POSIX.2 standard (IEEE Standard 1003.2)
     are made available.  This is in addition to the ANSI C facilities.

 -- Macro: _BSD_SOURCE
     If you define this macro, functionality derived from 4.3 BSD Unix
     is included as well as the ANSI C, POSIX.1, and POSIX.2 material.

     Some of the features derived from 4.3 BSD Unix conflict with the
     corresponding features specified by the POSIX.1 standard.  If this
     macro is defined, the 4.3 BSD definitions take precedence over the
     POSIX definitions.

 -- Macro: _SVID_SOURCE
     If you define this macro, functionality derived from SVID is
     included as well as the ANSI C, POSIX.1, and POSIX.2 material.

 -- Macro: _GNU_SOURCE
     If you define this macro, everything is included: ANSI C, POSIX.1,
     POSIX.2, BSD, SVID, and GNU extensions.  In the cases where
     POSIX.1 conflicts with BSD, the POSIX definitions take precedence.

     If you want to get the full effect of `_GNU_SOURCE' but make the
     BSD definitions take precedence over the POSIX definitions, use
     this sequence of definitions:

          #define _GNU_SOURCE
          #define _BSD_SOURCE
          #define _SVID_SOURCE

   We recommend you use `_GNU_SOURCE' in new programs.  If you don't
specify the `-ansi' option to GCC and don't define any of these macros
explicitly, the effect as the same as defining `_GNU_SOURCE'.

   When you define a feature test macro to request a larger class of
features, it is harmless to define in addition a feature test macro for
a subset of those features.  For example, if you define
`_POSIX_C_SOURCE', then defining `_POSIX_SOURCE' as well has no
effect.  Likewise, if you define `_GNU_SOURCE', then defining either
`_POSIX_SOURCE' or `_POSIX_C_SOURCE' or `_SVID_SOURCE' as well has no
effect.

   Note, however, that the features of `_BSD_SOURCE' are not a subset
of any of the other feature test macros supported.  This is because it
defines BSD features that take precedence over the POSIX features that
are requested by the other macros.  For this reason, defining
`_BSD_SOURCE' in addition to the other feature test macros does have
an effect: it causes the BSD features to take priority over the
conflicting POSIX features.