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diff --git a/NOTES b/NOTES new file mode 100644 index 0000000000..2af653c5f9 --- /dev/null +++ b/NOTES @@ -0,0 +1,95 @@ +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, 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 + + 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. + + 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 `_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. + |