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 section `GNU CC Command Options' in 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
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.
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
_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.
_POSIX_C_SOURCEare 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.
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.
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
_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_SOURCE as well has no effect. Likewise, if you
_GNU_SOURCE, then defining either
_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
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