glibc/manual/creature.texi

@node Feature Test Macros
@subsection Feature Test Macros

@cindex feature test macros
The exact set of features available when you compile a source file
is controlled by which @dfn{feature test macros} you define.

If you compile your programs using @samp{gcc -ansi}, you get only the
@w{ISO C} library features, unless you explicitly request additional
features by defining one or more of the feature macros.
@xref{Invoking GCC,, GNU CC Command Options, gcc, The GNU CC Manual},
for more information about GCC options.@refill

You should define these macros by using @samp{#define} preprocessor
directives at the top of your source code files.  These directives
@emph{must} come before any @code{#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 @samp{-D} option to GCC, but it's
better if you make the source files indicate their own meaning in a
self-contained way.

This system exists to allow the library to conform to multiple standards.
Although the different standards are often described as supersets of each
other, they are usually incompatible because larger standards require
functions with names that smaller ones reserve to the user program.  This
is not mere pedantry --- it has been a problem in practice.  For instance,
some non-GNU programs define functions named @code{getline} that have
nothing to do with this library's @code{getline}.  They would not be
compilable if all features were enabled indiscriminately.

This should not be used to verify that a program conforms to a limited
standard.  It is insufficient for this purpose, as it will not protect you
from including header files outside the standard, or relying on semantics
undefined within the standard.

@defvr Macro _POSIX_SOURCE
@standards{POSIX.1, (none)}
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
@w{ISO C} facilities.

The state of @code{_POSIX_SOURCE} is irrelevant if you define the
macro @code{_POSIX_C_SOURCE} to a positive integer.
@end defvr

@defvr Macro _POSIX_C_SOURCE
@standards{POSIX.2, (none)}
Define this macro to a positive integer to control which POSIX
functionality is made available.  The greater the value of this macro,
the more functionality is made available.

If you define this macro to a value greater than or equal to @code{1},
then the functionality from the 1990 edition of the POSIX.1 standard
(IEEE Standard 1003.1-1990) is made available.

If you define this macro to a value greater than or equal to @code{2},
then the functionality from the 1992 edition of the POSIX.2 standard
(IEEE Standard 1003.2-1992) is made available.

If you define this macro to a value greater than or equal to @code{199309L},
then the functionality from the 1993 edition of the POSIX.1b standard
(IEEE Standard 1003.1b-1993) is made available.

If you define this macro to a value greater than or equal to
@code{199506L}, then the functionality from the 1995 edition of the
POSIX.1c standard (IEEE Standard 1003.1c-1995) is made available.

If you define this macro to a value greater than or equal to
@code{200112L}, then the functionality from the 2001 edition of the
POSIX standard (IEEE Standard 1003.1-2001) is made available.

If you define this macro to a value greater than or equal to
@code{200809L}, then the functionality from the 2008 edition of the
POSIX standard (IEEE Standard 1003.1-2008) is made available.

Greater values for @code{_POSIX_C_SOURCE} will enable future extensions.
The POSIX standards process will define these values as necessary, and
@theglibc{} should support them some time after they become standardized.
The 1996 edition of POSIX.1 (ISO/IEC 9945-1: 1996) states that
if you define @code{_POSIX_C_SOURCE} to a value greater than
or equal to @code{199506L}, then the functionality from the 1996
edition is made available.  In general, in @theglibc{}, bugfixes to
the standards are included when specifying the base version; e.g.,
POSIX.1-2004 will always be included with a value of @code{200112L}.
@end defvr

@defvr Macro _XOPEN_SOURCE
@defvrx Macro _XOPEN_SOURCE_EXTENDED
@standards{X/Open, (none)}
If you define this macro, functionality described in the X/Open
Portability Guide is included.  This is a superset of the POSIX.1 and
POSIX.2 functionality and in fact @code{_POSIX_SOURCE} and
@code{_POSIX_C_SOURCE} are automatically defined.

As the unification of all Unices, functionality only available in
BSD and SVID is also included.

If the macro @code{_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.

If the macro @code{_XOPEN_SOURCE} has the value @math{500} this includes
all functionality described so far plus some new definitions from the
Single Unix Specification, @w{version 2}.  The value @math{600}
(corresponding to the sixth revision) includes definitions from SUSv3,
and using @math{700} (the seventh revision) includes definitions from
SUSv4.
@end defvr

@defvr Macro _LARGEFILE_SOURCE
@standards{X/Open, (NONE)}
If this macro is defined some extra functions are available which
rectify a few shortcomings in all previous standards.  Specifically,
the functions @code{fseeko} and @code{ftello} are available.  Without
these functions the difference between the @w{ISO C} interface
(@code{fseek}, @code{ftell}) and the low-level POSIX interface
(@code{lseek}) would lead to problems.

This macro was introduced as part of the Large File Support extension (LFS).
@end defvr

@defvr Macro _LARGEFILE64_SOURCE
@standards{X/Open, (NONE)}
If you define this macro an additional set of functions is made available
which enables @w{32 bit} systems to use files of sizes beyond
the usual limit of 2GB.  This interface is not available if the system
does not support files that large.  On systems where the natural file
size limit is greater than 2GB (i.e., on @w{64 bit} systems) the new
functions are identical to the replaced functions.

The new functionality is made available by a new set of types and
functions which replace the existing ones.  The names of these new objects
contain @code{64} to indicate the intention, e.g., @code{off_t}
vs. @code{off64_t} and @code{fseeko} vs. @code{fseeko64}.

This macro was introduced as part of the Large File Support extension
(LFS).  It is a transition interface for the period when @w{64 bit}
offsets are not generally used (see @code{_FILE_OFFSET_BITS}).
@end defvr

@defvr Macro _FILE_OFFSET_BITS
@standards{X/Open, (NONE)}
This macro determines which file system interface shall be used, one
replacing the other.  Whereas @code{_LARGEFILE64_SOURCE} makes the @w{64
bit} interface available as an additional interface,
@code{_FILE_OFFSET_BITS} allows the @w{64 bit} interface to
replace the old interface.

If @code{_FILE_OFFSET_BITS} is defined to the
value @code{32}, the @w{32 bit} interface is used and
types like @code{off_t} have a size of @w{32 bits} on @w{32 bit}
systems.

If the macro is defined to the value @code{64}, the large file interface
replaces the old interface.  I.e., the functions are not made available
under different names (as they are with @code{_LARGEFILE64_SOURCE}).
Instead the old function names now reference the new functions, e.g., a
call to @code{fseeko} now indeed calls @code{fseeko64}.

If the macro is not defined it currently defaults to @code{32}, but
this default is planned to change due to a need to update
@code{time_t} for Y2038 safety, and applications should not rely on
the default.

This macro should only be selected if the system provides mechanisms for
handling large files.  On @w{64 bit} systems this macro has no effect
since the @code{*64} functions are identical to the normal functions.

This macro was introduced as part of the Large File Support extension
(LFS).
@end defvr

@defvr Macro _TIME_BITS
This macro determines the bit size of @code{time_t}, and therefore the
bit size of all @code{time_t}-derived types and the prototypes of all
related functions.

@enumerate

@item
If @code{_TIME_BITS} is undefined, the bit size of @code{time_t} is
architecture dependent.  Currently it defaults to 64 bits on most
architectures.  Although it defaults to 32 bits on some traditional
architectures (i686, ARM), this is planned to change and applications
should not rely on this.

@item
If @code{_TIME_BITS} is defined to be 64, @code{time_t} is defined
to be a 64-bit integer.  On platforms where @code{time_t} was
traditionally 32 bits, calls to proper syscalls depend on the
Linux kernel version on which the system is running. For Linux kernel
version above @b{5.1} syscalls supporting 64-bit time are used. Otherwise,
a fallback code is used with legacy (i.e. 32-bit) syscalls.

@item
If @code{_TIME_BITS} is defined to be 32, @code{time_t} is defined to
be a 32-bit integer where that is supported.  This is not recommended,
as 32-bit @code{time_t} stops working in the year 2038.

@item
For any other use case a compile-time error is emitted.
@end enumerate

@code{_TIME_BITS=64} can be defined only when
@code{_FILE_OFFSET_BITS=64} is also defined.

By using this macro certain ports gain support for 64-bit time and as
a result become immune to the Y2038 problem.
@end defvr

@defvr Macro _ISOC99_SOURCE
@standards{GNU, (none)}
If this macro is defined, features from ISO C99 are included.  Since
these features are included by default, this macro is mostly relevant
when the compiler uses an earlier language version.
@end defvr

@defvr Macro _ISOC11_SOURCE
@standards{C11, (none)}
If this macro is defined, ISO C11 extensions to ISO C99 are included.
@end defvr

@defvr Macro _ISOC2X_SOURCE
@standards{C2X, (none)}
If this macro is defined, ISO C2X extensions to ISO C11 are included.
Only some features from this draft standard are supported by
@theglibc{}.
@end defvr

@defvr Macro __STDC_WANT_LIB_EXT2__
@standards{ISO, (none)}
If you define this macro to the value @code{1}, features from ISO/IEC
TR 24731-2:2010 (Dynamic Allocation Functions) are enabled.  Only some
of the features from this TR are supported by @theglibc{}.
@end defvr

@defvr Macro __STDC_WANT_IEC_60559_BFP_EXT__
@standards{ISO, (none)}
If you define this macro, features from ISO/IEC TS 18661-1:2014
(Floating-point extensions for C: Binary floating-point arithmetic)
are enabled.  Only some of the features from this TS are supported by
@theglibc{}.
@end defvr

@defvr Macro __STDC_WANT_IEC_60559_FUNCS_EXT__
@standards{ISO, (none)}
If you define this macro, features from ISO/IEC TS 18661-4:2015
(Floating-point extensions for C: Supplementary functions) are
enabled.  Only some of the features from this TS are supported by
@theglibc{}.
@end defvr

@defvr Macro __STDC_WANT_IEC_60559_TYPES_EXT__
@standards{ISO, (none)}
If you define this macro, features from ISO/IEC TS 18661-3:2015
(Floating-point extensions for C: Interchange and extended types) are
enabled.  Only some of the features from this TS are supported by
@theglibc{}.
@end defvr

@defvr Macro __STDC_WANT_IEC_60559_EXT__
@standards{ISO, (none)}
If you define this macro, ISO C2X features defined in Annex F of that
standard are enabled.  This affects declarations of the
@code{totalorder} functions and functions related to NaN payloads.
@end defvr

@defvr Macro _GNU_SOURCE
@standards{GNU, (none)}
If you define this macro, everything is included: @w{ISO C89}, @w{ISO
C99}, POSIX.1, POSIX.2, BSD, SVID, X/Open, LFS, and GNU extensions.  In
the cases where POSIX.1 conflicts with BSD, the POSIX definitions take
precedence.
@end defvr

@defvr Macro _DEFAULT_SOURCE
@standards{GNU, (none)}
If you define this macro, most features are included apart from
X/Open, LFS and GNU extensions: the effect is to enable features from
the 2008 edition of POSIX, as well as certain BSD and SVID features
without a separate feature test macro to control them.

Be aware that compiler options also affect included features:

@itemize
@item
If you use a strict conformance option, features beyond those from the
compiler's language version will be disabled, though feature test
macros may be used to enable them.

@item
Features enabled by compiler options are not overridden by feature
test macros.
@end itemize
@end defvr

@defvr Macro _ATFILE_SOURCE
@standards{GNU, (none)}
If this macro is defined, additional @code{*at} interfaces are
included.
@end defvr

@defvr Macro _FORTIFY_SOURCE
@standards{GNU, (none)}
If this macro is defined to @math{1}, security hardening is added to
various library functions.  If defined to @math{2}, even stricter
checks are applied. If defined to @math{3}, @theglibc{} may also use
checks that may have an additional performance overhead.
@end defvr

@defvr Macro _SC_SIGSTKSZ_SOURCE
@standards{GNU, (none)}
If this macro is defined, correct (but non compile-time constant)
MINSIGSTKSZ and SIGSTKSZ are defined.
@end defvr

@defvr Macro _REENTRANT
@defvrx Macro _THREAD_SAFE
@standards{Obsolete, (none)}
These macros are obsolete.  They have the same effect as defining
@code{_POSIX_C_SOURCE} with the value @code{199506L}.

Some very old C libraries required one of these macros to be defined
for basic functionality (e.g.@: @code{getchar}) to be thread-safe.
@end defvr

We recommend you use @code{_GNU_SOURCE} in new programs.  If you don't
specify the @samp{-ansi} option to GCC, or other conformance options
such as @option{-std=c99}, and don't define any of these macros
explicitly, the effect is the same as defining @code{_DEFAULT_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 @code{_POSIX_C_SOURCE}, then
defining @code{_POSIX_SOURCE} as well has no effect.  Likewise, if you
define @code{_GNU_SOURCE}, then defining either @code{_POSIX_SOURCE} or
@code{_POSIX_C_SOURCE} as well has no effect.