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 ISO C library features, unless you explicitly request additional features by defining one or more of the feature macros. See GNU CC Command Options in The GNU CC Manual, 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.
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 getline that have
nothing to do with this library’s 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.
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 ISO C facilities.
The state of _POSIX_SOURCE is irrelevant if you define the
macro _POSIX_C_SOURCE to a positive integer.
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 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 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 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
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
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
200809L, then the functionality from the 2008 edition of the
POSIX standard (IEEE Standard 1003.1-2008) is made available.
Greater values for _POSIX_C_SOURCE will enable future extensions.
The POSIX standards process will define these values as necessary, and
the GNU C Library 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 _POSIX_C_SOURCE to a value greater than
or equal to 199506L, then the functionality from the 1996
edition is made available. In general, in the GNU C Library, bugfixes to
the standards are included when specifying the base version; e.g.,
POSIX.1-2004 will always be included with a value of 200112L.
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 _POSIX_SOURCE and
_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 _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 _XOPEN_SOURCE has the value 500 this includes
all functionality described so far plus some new definitions from the
Single Unix Specification, version 2. The value 600
(corresponding to the sixth revision) includes definitions from SUSv3,
and using 700 (the seventh revision) includes definitions from
SUSv4.
If this macro is defined some extra functions are available which
rectify a few shortcomings in all previous standards. Specifically,
the functions fseeko and ftello are available. Without
these functions the difference between the ISO C interface
(fseek, ftell) and the low-level POSIX interface
(lseek) would lead to problems.
This macro was introduced as part of the Large File Support extension (LFS).
If you define this macro an additional set of functions is made available which enables 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 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 64 to indicate the intention, e.g., off_t
vs. off64_t and fseeko vs. fseeko64.
This macro was introduced as part of the Large File Support extension
(LFS). It is a transition interface for the period when 64 bit
offsets are not generally used (see _FILE_OFFSET_BITS).
This macro determines which file system interface shall be used, one
replacing the other. Whereas _LARGEFILE64_SOURCE makes the 64 bit interface available as an additional interface,
_FILE_OFFSET_BITS allows the 64 bit interface to
replace the old interface.
If _FILE_OFFSET_BITS is defined to the
value 32, the 32 bit interface is used and
types like off_t have a size of 32 bits on 32 bit
systems.
If the macro is defined to the value 64, the large file interface
replaces the old interface. I.e., the functions are not made available
under different names (as they are with _LARGEFILE64_SOURCE).
Instead the old function names now reference the new functions, e.g., a
call to fseeko now indeed calls fseeko64.
If the macro is not defined it currently defaults to 32, but
this default is planned to change due to a need to update
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 64 bit systems this macro has no effect
since the *64 functions are identical to the normal functions.
This macro was introduced as part of the Large File Support extension (LFS).
Define this macro to control the bit size of time_t, and therefore
the bit size of all time_t-derived types and the prototypes of all
related functions.
_TIME_BITS is undefined, the bit size of 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.
_TIME_BITS is defined to be 64, time_t is defined
to be a 64-bit integer. On platforms where 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 5.1 syscalls supporting 64-bit time are used. Otherwise,
a fallback code is used with legacy (i.e. 32-bit) syscalls.
_TIME_BITS is defined to be 32, time_t is defined to
be a 32-bit integer where that is supported. This is not recommended,
as 32-bit time_t stops working in the year 2038.
_TIME_BITS=64 can be defined only when
_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.
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.
If this macro is defined, ISO C11 extensions to ISO C99 are included.
If this macro is defined, ISO C23 extensions to ISO C11 are included.
Only some features from this draft standard are supported by
the GNU C Library. The older name _ISOC2X_SOURCE is also supported.
If you define this macro to the value 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 the GNU C Library.
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 the GNU C Library.
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 the GNU C Library.
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 the GNU C Library.
If you define this macro, ISO C23 features defined in Annex F of that
standard are enabled. This affects declarations of the
totalorder functions and functions related to NaN payloads.
If you define this macro, everything is included: ISO C89, 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.
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:
If this macro is defined, additional *at interfaces are
included.
If this macro is defined to 1, security hardening is added to various library functions. If defined to 2, even stricter checks are applied. If defined to 3, the GNU C Library may also use checks that may have an additional performance overhead. See Fortification of function calls.
If this macro is defined, correct (but non compile-time constant) MINSIGSTKSZ, SIGSTKSZ and PTHREAD_STACK_MIN are defined.
These macros are obsolete. They have the same effect as defining
_POSIX_C_SOURCE with the value 199506L.
Some very old C libraries required one of these macros to be defined
for basic functionality (e.g. getchar) to be thread-safe.
We recommend you use _GNU_SOURCE in new programs. If you don’t
specify the ‘-ansi’ option to GCC, or other conformance options
such as -std=c99, and don’t define any of these macros
explicitly, the effect is the same as defining _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 _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 as well has no effect.