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114 lines
4.7 KiB
Plaintext
114 lines
4.7 KiB
Plaintext
@node Feature Test Macros
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@subsection Feature Test Macros
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@cindex feature test macros
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The exact set of features available when you compile a source file
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is controlled by which @dfn{feature test macros} you define.
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If you compile your programs using @samp{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.
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@xref{Invoking GCC,, GNU CC Command Options, gcc.info, The GNU CC Manual},
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for more information about GCC options.@refill
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You should define these macros by using @samp{#define} preprocessor
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directives at the top of your source code files. These directives
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@emph{must} come before any @code{#include} of a system header file. It
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is best to make them the very first thing in the file, preceded only by
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comments. You could also use the @samp{-D} option to GCC, but it's
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better if you make the source files indicate their own meaning in a
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self-contained way.
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@comment (none)
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@comment POSIX.1
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@defvr 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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@end defvr
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@comment (none)
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@comment POSIX.2
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@defvr Macro _POSIX_C_SOURCE
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If you define this macro with a value of @code{1}, then the
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functionality from the POSIX.1 standard (IEEE Standard 1003.1) is made
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available. If you define this macro with a value of @code{2}, then both
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the functionality from the POSIX.1 standard and the functionality from
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the POSIX.2 standard (IEEE Standard 1003.2) are made available. This is
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in addition to the ANSI C facilities.
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@end defvr
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@comment (none)
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@comment GNU
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@defvr Macro _BSD_SOURCE
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If you define this macro, functionality derived from 4.3 BSD Unix is
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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 POSIX.1,
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you need to use a special @dfn{BSD compatibility library} when linking
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programs compiled for BSD compatibility. This is because some functions
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must be defined in two different ways, one of them in the normal C
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library, and one of them in the compatibility library. If your program
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defines @code{_BSD_SOURCE}, you must give the option @samp{-lbsd-compat}
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to the compiler or linker when linking the program, to tell it to find
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functions in this special compatibility library before looking for them in
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the normal C library.
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@pindex -lbsd-compat
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@pindex bsd-compat
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@cindex BSD compatibility library.
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@end defvr
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@comment (none)
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@comment GNU
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@defvr 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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@end defvr
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@comment (none)
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@comment GNU
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@defvr 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 @code{_GNU_SOURCE} but make the
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BSD definitions take precedence over the POSIX definitions, use this
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sequence of definitions:
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@smallexample
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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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@end smallexample
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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 @samp{-lbsd-compat} option to the
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compiler or linker. @strong{Note:} If you forget to do this, you may
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get very strange errors at run time.
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@end defvr
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We recommend you use @code{_GNU_SOURCE} in new programs. If you don't
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specify the @samp{-ansi} option to GCC and don't define any of these macros
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explicitly, the effect is the same as defining @code{_GNU_SOURCE}.
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When you define a feature test macro to request a larger class of features,
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it is harmless to define in addition a feature test macro for a subset of
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those features. For example, if you define @code{_POSIX_C_SOURCE}, then
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defining @code{_POSIX_SOURCE} as well has no effect. Likewise, if you
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define @code{_GNU_SOURCE}, then defining either @code{_POSIX_SOURCE} or
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@code{_POSIX_C_SOURCE} or @code{_SVID_SOURCE} as well has no effect.
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Note, however, that the features of @code{_BSD_SOURCE} are not a subset of
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any of the other feature test macros supported. This is because it defines
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BSD features that take precedence over the POSIX features that are
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requested by the other macros. For this reason, defining
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@code{_BSD_SOURCE} in addition to the other feature test macros does have
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an effect: it causes the BSD features to take priority over the conflicting
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POSIX features.
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