234 lines
10 KiB
Plaintext
234 lines
10 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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@w{ISO 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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This system exists to allow the library to conform to multiple standards.
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Although the different standards are often described as supersets of each
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other, they are usually incompatible because larger standards require
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functions with names that smaller ones reserve to the user program. This
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is not mere pedantry --- it has been a problem in practice. For instance,
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some non-GNU programs define functions named @code{getline} that have
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nothing to do with this library's @code{getline}. They would not be
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compilable if all features were enabled indiscriminately.
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This should not be used to verify that a program conforms to a limited
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standard. It is insufficient for this purpose, as it will not protect you
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from including header files outside the standard, or relying on semantics
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undefined within the standard.
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@defvr Macro _POSIX_SOURCE
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@standards{POSIX.1, (none)}
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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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@w{ISO C} facilities.
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The state of @code{_POSIX_SOURCE} is irrelevant if you define the
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macro @code{_POSIX_C_SOURCE} to a positive integer.
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@end defvr
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@defvr Macro _POSIX_C_SOURCE
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@standards{POSIX.2, (none)}
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Define this macro to a positive integer to control which POSIX
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functionality is made available. The greater the value of this macro,
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the more functionality is made available.
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If you define this macro to a value greater than or equal to @code{1},
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then the functionality from the 1990 edition of the POSIX.1 standard
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(IEEE Standard 1003.1-1990) is made available.
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If you define this macro to a value greater than or equal to @code{2},
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then the functionality from the 1992 edition of the POSIX.2 standard
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(IEEE Standard 1003.2-1992) is made available.
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If you define this macro to a value greater than or equal to @code{199309L},
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then the functionality from the 1993 edition of the POSIX.1b standard
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(IEEE Standard 1003.1b-1993) is made available.
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Greater values for @code{_POSIX_C_SOURCE} will enable future extensions.
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The POSIX standards process will define these values as necessary, and
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@theglibc{} should support them some time after they become standardized.
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The 1996 edition of POSIX.1 (ISO/IEC 9945-1: 1996) states that
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if you define @code{_POSIX_C_SOURCE} to a value greater than
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or equal to @code{199506L}, then the functionality from the 1996
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edition is made available.
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@end defvr
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@defvr Macro _XOPEN_SOURCE
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@defvrx Macro _XOPEN_SOURCE_EXTENDED
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@standards{X/Open, (none)}
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If you define this macro, functionality described in the X/Open
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Portability Guide is included. This is a superset of the POSIX.1 and
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POSIX.2 functionality and in fact @code{_POSIX_SOURCE} and
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@code{_POSIX_C_SOURCE} are automatically defined.
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As the unification of all Unices, functionality only available in
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BSD and SVID is also included.
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If the macro @code{_XOPEN_SOURCE_EXTENDED} is also defined, even more
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functionality is available. The extra functions will make all functions
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available which are necessary for the X/Open Unix brand.
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If the macro @code{_XOPEN_SOURCE} has the value @math{500} this includes
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all functionality described so far plus some new definitions from the
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Single Unix Specification, @w{version 2}.
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@end defvr
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@defvr Macro _LARGEFILE_SOURCE
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@standards{X/Open, (NONE)}
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If this macro is defined some extra functions are available which
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rectify a few shortcomings in all previous standards. Specifically,
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the functions @code{fseeko} and @code{ftello} are available. Without
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these functions the difference between the @w{ISO C} interface
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(@code{fseek}, @code{ftell}) and the low-level POSIX interface
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(@code{lseek}) would lead to problems.
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This macro was introduced as part of the Large File Support extension (LFS).
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@end defvr
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@defvr Macro _LARGEFILE64_SOURCE
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@standards{X/Open, (NONE)}
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If you define this macro an additional set of functions is made available
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which enables @w{32 bit} systems to use files of sizes beyond
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the usual limit of 2GB. This interface is not available if the system
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does not support files that large. On systems where the natural file
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size limit is greater than 2GB (i.e., on @w{64 bit} systems) the new
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functions are identical to the replaced functions.
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The new functionality is made available by a new set of types and
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functions which replace the existing ones. The names of these new objects
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contain @code{64} to indicate the intention, e.g., @code{off_t}
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vs. @code{off64_t} and @code{fseeko} vs. @code{fseeko64}.
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This macro was introduced as part of the Large File Support extension
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(LFS). It is a transition interface for the period when @w{64 bit}
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offsets are not generally used (see @code{_FILE_OFFSET_BITS}).
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@end defvr
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@defvr Macro _FILE_OFFSET_BITS
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@standards{X/Open, (NONE)}
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This macro determines which file system interface shall be used, one
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replacing the other. Whereas @code{_LARGEFILE64_SOURCE} makes the @w{64
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bit} interface available as an additional interface,
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@code{_FILE_OFFSET_BITS} allows the @w{64 bit} interface to
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replace the old interface.
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If @code{_FILE_OFFSET_BITS} is undefined, or if it is defined to the
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value @code{32}, nothing changes. The @w{32 bit} interface is used and
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types like @code{off_t} have a size of @w{32 bits} on @w{32 bit}
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systems.
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If the macro is defined to the value @code{64}, the large file interface
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replaces the old interface. I.e., the functions are not made available
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under different names (as they are with @code{_LARGEFILE64_SOURCE}).
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Instead the old function names now reference the new functions, e.g., a
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call to @code{fseeko} now indeed calls @code{fseeko64}.
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This macro should only be selected if the system provides mechanisms for
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handling large files. On @w{64 bit} systems this macro has no effect
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since the @code{*64} functions are identical to the normal functions.
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This macro was introduced as part of the Large File Support extension
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(LFS).
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@end defvr
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@defvr Macro _ISOC99_SOURCE
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@standards{GNU, (none)}
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Until the revised @w{ISO C} standard is widely adopted the new features
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are not automatically enabled. @Theglibc{} nevertheless has a complete
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implementation of the new standard and to enable the new features the
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macro @code{_ISOC99_SOURCE} should be defined.
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@end defvr
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@defvr Macro __STDC_WANT_LIB_EXT2__
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@standards{ISO, (none)}
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If you define this macro to the value @code{1}, features from ISO/IEC
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TR 24731-2:2010 (Dynamic Allocation Functions) are enabled. Only some
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of the features from this TR are supported by @theglibc{}.
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@end defvr
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@defvr Macro __STDC_WANT_IEC_60559_BFP_EXT__
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@standards{ISO, (none)}
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If you define this macro, features from ISO/IEC TS 18661-1:2014
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(Floating-point extensions for C: Binary floating-point arithmetic)
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are enabled. Only some of the features from this TS are supported by
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@theglibc{}.
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@end defvr
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@defvr Macro __STDC_WANT_IEC_60559_FUNCS_EXT__
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@standards{ISO, (none)}
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If you define this macro, features from ISO/IEC TS 18661-4:2015
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(Floating-point extensions for C: Supplementary functions) are
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enabled. Only some of the features from this TS are supported by
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@theglibc{}.
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@end defvr
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@defvr Macro __STDC_WANT_IEC_60559_TYPES_EXT__
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@standards{ISO, (none)}
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If you define this macro, features from ISO/IEC TS 18661-3:2015
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(Floating-point extensions for C: Interchange and extended types) are
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enabled. Only some of the features from this TS are supported by
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@theglibc{}.
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@end defvr
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@defvr Macro _GNU_SOURCE
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@standards{GNU, (none)}
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If you define this macro, everything is included: @w{ISO C89}, @w{ISO
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C99}, POSIX.1, POSIX.2, BSD, SVID, X/Open, LFS, and GNU extensions. In
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the cases where POSIX.1 conflicts with BSD, the POSIX definitions take
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precedence.
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@end defvr
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@defvr Macro _DEFAULT_SOURCE
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@standards{GNU, (none)}
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If you define this macro, most features are included apart from
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X/Open, LFS and GNU extensions: the effect is to enable features from
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the 2008 edition of POSIX, as well as certain BSD and SVID features
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without a separate feature test macro to control them. Defining this
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macro, on its own and without using compiler options such as
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@option{-ansi} or @option{-std=c99}, has the same effect as not
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defining any feature test macros; defining it together with other
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feature test macros, or when options such as @option{-ansi} are used,
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enables those features even when the other options would otherwise
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cause them to be disabled.
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@end defvr
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@defvr Macro _REENTRANT
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@defvrx Macro _THREAD_SAFE
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@standards{Obsolete, (none)}
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These macros are obsolete. They have the same effect as defining
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@code{_POSIX_C_SOURCE} with the value @code{199506L}.
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Some very old C libraries required one of these macros to be defined
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for basic functionality (e.g.@: @code{getchar}) to be thread-safe.
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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, or other conformance options
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such as @option{-std=c99}, and don't define any of these macros
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explicitly, the effect is the same as defining @code{_DEFAULT_SOURCE}
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to 1.
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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} as well has no effect.
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