#### 1.3.4 Feature Test Macros ¶

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.

Macro: _POSIX_SOURCE ¶

  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.

Macro: _POSIX_C_SOURCE ¶

  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.

  If you define this macro to a value greater than or equal to `202405L`, then the functionality from the 2024 edition of the POSIX standard (IEEE Standard 1003.1-2024) 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`.

Macro: _XOPEN_SOURCE ¶
Macro: _XOPEN_SOURCE_EXTENDED ¶

  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. The value 800 includes definitions from POSIX.1-2024.

Macro: _LARGEFILE_SOURCE ¶

  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).

Macro: _LARGEFILE64_SOURCE ¶

  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`).

Macro: _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).

Macro: _TIME_BITS ¶

  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.

  1. If `_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.
  2. If `_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.

     On such platforms, the GNU C Library will also define `__USE_TIME64_REDIRECTS` to indicate whether the declarations are expanded to different ones (either by redefining the symbol name or using a symbol alias). For instance, if the symbol `clock_gettime` expands to `__clock_gettime64`.

  3. If `_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.
  4. For any other use case a compile-time error is emitted.

  `_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.

Macro: _ISOC99_SOURCE ¶

  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.

Macro: _ISOC11_SOURCE ¶

  If this macro is defined, ISO C11 extensions to ISO C99 are included.

Macro: _ISOC23_SOURCE ¶

  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.

Macro: _ISOC2Y_SOURCE ¶

  If this macro is defined, ISO C2Y extensions to ISO C23 are included. Only some features from this draft standard are supported by the GNU C Library.

Macro: __STDC_WANT_LIB_EXT2__ ¶

  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.

Macro: __STDC_WANT_IEC_60559_BFP_EXT__ ¶

  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.

Macro: __STDC_WANT_IEC_60559_FUNCS_EXT__ ¶

  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.

Macro: __STDC_WANT_IEC_60559_TYPES_EXT__ ¶

  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.

Macro: __STDC_WANT_IEC_60559_EXT__ ¶

  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.

Macro: _GNU_SOURCE ¶

  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.

Macro: _DEFAULT_SOURCE ¶

  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 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.
  - Features enabled by compiler options are not overridden by feature test macros.

Macro: _ATFILE_SOURCE ¶

  If this macro is defined, additional `*at` interfaces are included.

Macro: _FORTIFY_SOURCE ¶

  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.

Macro: _DYNAMIC_STACK_SIZE_SOURCE ¶

  If this macro is defined, correct (but non compile-time constant) MINSIGSTKSZ, SIGSTKSZ and PTHREAD_STACK_MIN are defined.

Macro: _REENTRANT ¶
Macro: _THREAD_SAFE ¶

  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.