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gnu/glibc/glibc-1.03/NOTES

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+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.
+