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 ISO C library features, unless you explicitly request additional features by defining one or more of the feature macros. See GNU CC Command Options (The GNU CC Manual), for more information about GCC options.

You should define these macros by using #define preprocessor directives at the top of your source code files.  These directives must come before any #include of a system header file.  It is best to make them the very first thing in the file, preceded only by comments.  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 ISO 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 ISO C facilities.

 -- Macro: _BSD_SOURCE
     If you define this macro, functionality derived from 4.3 BSD Unix is included as well as the ISO 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.

     Due to the nature of some of the conflicts between 4.3 BSD and POSIX.1, you need to use a special BSD compatibility library when linking programs compiled for BSD compatibility.  This is because some functions must be defined in two different ways, one of them in the normal C library, and one of them in the compatibility library.  If your program defines _BSD_SOURCE, you must give the option -lbsd-compat to the compiler or linker when linking the program, to tell it to find functions in this special compatibility library before looking for them in the normal C library.

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

 -- Macro: _XOPEN_SOURCE
     If you define this macro, functionality described in the X/Open Portability Guide is included.  This is a superset of the POSIX.1 and POSIX.2 functionality and in fact _POSIX_SOURCE and _POSIX_C_SOURCE are automatically defined.

     As the unification of all Unices, functionality only available in BSD and SVID is also included.

     If the macro _XOPEN_SOURCE_EXTENDED is also defined, even more functionality is available.  The extra functions will make all functions available which are necessary for the X/Open Unix brand.

 -- Macro: _GNU_SOURCE
     If you define this macro, everything is included: ISO C, POSIX.1, POSIX.2, BSD, SVID, X/Open, 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

     Note that if you do this, you must link your program with the BSD compatibility library by passing the -lbsd-compat option to the compiler or linker.  Note: If you forget to do this, you may get very strange errors at run time.

 -- Macro: _REENTRANT
 -- Macro: _THREAD_SAFE
     If you define one of these macros, reentrant versions of several functions get declared.  Some of the functions are specified in POSIX.1c but many others are only available on a few other systems or are unique to GNU libc. The problem is that the standardization of the thread safe C library interface still is behind.

     Unlike on some other systems no special version of the C library must be used for linking.  There is only one version but while compiling this it must have been specified to compile as thread safe.

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 is the same as defining _POSIX_C_SOURCE to 2 and _POSIX_SOURCE, _SVID_SOURCE, and _BSD_SOURCE to 1.

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.