96 lines
4.4 KiB
Plaintext
96 lines
4.4 KiB
Plaintext
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Feature Test Macros
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-------------------
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The exact set of features available when you compile a source file
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is controlled by which "feature test macros" you define.
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If you compile your programs using `gcc -ansi', you get only the
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ANSI C library features, unless you explicitly request additional
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features by defining one or more of the feature macros. *Note GNU CC
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Command Options: (gcc.info)Invoking GCC, for more information about GCC
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options.
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You should define these macros by using `#define' preprocessor
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directives at the top of your source code files. These directives
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*must* come before any `#include' of a system header file. It is best
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to make them the very first thing in the file, preceded only by
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comments. You could also use the `-D' option to GCC, but it's better
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if you make the source files indicate their own meaning in a
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self-contained way.
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- Macro: _POSIX_SOURCE
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If you define this macro, then the functionality from the POSIX.1
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standard (IEEE Standard 1003.1) is available, as well as all of the
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ANSI C facilities.
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- Macro: _POSIX_C_SOURCE
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If you define this macro with a value of `1', then the
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functionality from the POSIX.1 standard (IEEE Standard 1003.1) is
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made available. If you define this macro with a value of `2',
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then both the functionality from the POSIX.1 standard and the
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functionality from the POSIX.2 standard (IEEE Standard 1003.2) are
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made available. This is in addition to the ANSI C facilities.
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- Macro: _BSD_SOURCE
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If you define this macro, functionality derived from 4.3 BSD Unix
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is included as well as the ANSI C, POSIX.1, and POSIX.2 material.
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Some of the features derived from 4.3 BSD Unix conflict with the
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corresponding features specified by the POSIX.1 standard. If this
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macro is defined, the 4.3 BSD definitions take precedence over the
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POSIX definitions.
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Due to the nature of some of the conflicts between 4.3 BSD and
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POSIX.1, you need to use a special "BSD compatibility library"
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when linking programs compiled for BSD compatibility. This is
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because some functions must be defined in two different ways, one
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of them in the normal C library, and one of them in the
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compatibility library. If your program defines `_BSD_SOURCE', you
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must give the option `-lbsd-compat' to the compiler or linker when
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linking the program, to tell it to find functions in this special
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compatibility library before looking for them in the normal C
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library.
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- Macro: _SVID_SOURCE
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If you define this macro, functionality derived from SVID is
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included as well as the ANSI C, POSIX.1, and POSIX.2 material.
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- Macro: _GNU_SOURCE
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If you define this macro, everything is included: ANSI C, POSIX.1,
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POSIX.2, BSD, SVID, and GNU extensions. In the cases where POSIX.1
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conflicts with BSD, the POSIX definitions take precedence.
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If you want to get the full effect of `_GNU_SOURCE' but make the
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BSD definitions take precedence over the POSIX definitions, use
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this sequence of definitions:
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#define _GNU_SOURCE
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#define _BSD_SOURCE
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#define _SVID_SOURCE
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Note that if you do this, you must link your program with the BSD
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compatibility library by passing the `-lbsd-compat' option to the
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compiler or linker. *Note:* If you forget to do this, you may get
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very strange errors at run time.
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We recommend you use `_GNU_SOURCE' in new programs. If you don't
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specify the `-ansi' option to GCC and don't define any of these macros
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explicitly, the effect is the same as defining `_GNU_SOURCE'.
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When you define a feature test macro to request a larger class of
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features, it is harmless to define in addition a feature test macro for
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a subset of those features. For example, if you define
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`_POSIX_C_SOURCE', then defining `_POSIX_SOURCE' as well has no effect.
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Likewise, if you define `_GNU_SOURCE', then defining either
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`_POSIX_SOURCE' or `_POSIX_C_SOURCE' or `_SVID_SOURCE' as well has no
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effect.
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Note, however, that the features of `_BSD_SOURCE' are not a subset of
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any of the other feature test macros supported. This is because it
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defines BSD features that take precedence over the POSIX features that
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are requested by the other macros. For this reason, defining
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`_BSD_SOURCE' in addition to the other feature test macros does have an
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effect: it causes the BSD features to take priority over the conflicting
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POSIX features.
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