| 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 GNU CC |
| Command Options: (gcc.info)Invoking GCC, 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 |
| 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. |
| |
| 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 ANSI C, POSIX.1, POSIX.2, and X/Open |
| material. |
| |
| - Macro: _XOPEN_SOURCE |
| If you define these macro, functionality described in the X/Open |
| Portability Guide is included. This is an superset of the POSIX.1 |
| and POSIX.2 functionality and in fact `_POSIX_SOURCE' and |
| `_POSIX_C_SOURCE' get automatically be defined. |
| |
| But as the great unifaction of all Unices there is also |
| functionality only available in BSD and SVID is 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: ANSI 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,_THREAD_SAFE |
| If you define one this macro, 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. |
| |