@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 undefined, or if it is defined to the

value @code{32}, nothing changes.  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}.

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 _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 __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 _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.

@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.