#### 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, 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.

> 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.

— Macro: _BSD_SOURCE

> If you define this macro, functionality derived from 4.3 BSD Unix is included as well as the ISO C, POSIX.1, and POSIX.2 material.

> Some of the features derived from 4.3 BSD Unix conflict with the corresponding features specified by the POSIX.1 standard. If this macro is defined, the 4.3 BSD definitions take precedence over the POSIX definitions.

> Due to the nature of some of the conflicts between 4.3 BSD and POSIX.1, you need to use a special BSD compatibility library when linking programs compiled for BSD compatibility. This is because some functions must be defined in two different ways, one of them in the normal C library, and one of them in the compatibility library. If your program defines `_BSD_SOURCE`, you must give the option ‘`-lbsd-compat`’ to the compiler or linker when linking the program, to tell it to find functions in this special compatibility library before looking for them in the normal C library.

— Macro: _SVID_SOURCE

> If you define this macro, functionality derived from SVID is included as well as the ISO C, POSIX.1, POSIX.2, and X/Open material.

— 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.

— 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 undefined, or if it is defined to the value `32`, nothing changes. 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`.

> 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: _ISOC99_SOURCE

> Until the revised ISO C standard is widely adopted the new features are not automatically enabled. The GNU C Library nevertheless has a complete implementation of the new standard and to enable the new features the macro `_ISOC99_SOURCE` should be defined.

— 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.

> If you want to get the full effect of `_GNU_SOURCE` but make the BSD definitions take precedence over the POSIX definitions, use this sequence of definitions:

>               #define _GNU_SOURCE
>               #define _BSD_SOURCE
>               #define _SVID_SOURCE

> Note that if you do this, you must link your program with the BSD compatibility library by passing the ‘`-lbsd-compat`’ option to the compiler or linker. NB: If you forget to do this, you may get very strange errors at run time.

— Macro: _REENTRANT
— Macro: _THREAD_SAFE

> If you define one of these macros, reentrant versions of several functions get declared. Some of the functions are specified in POSIX.1c but many others are only available on a few other systems or are unique to the GNU C Library. The problem is the delay in the standardization of the thread safe C library interface.

> Unlike on some other systems, no special version of the C library must be used for linking. There is only one version but while compiling this it must have been specified to compile as thread safe.

We recommend you use `_GNU_SOURCE` in new programs. If you don't specify the ‘`-ansi`’ option to GCC and don't define any of these macros explicitly, the effect is the same as defining `_POSIX_C_SOURCE` to 2 and `_POSIX_SOURCE`, `_SVID_SOURCE`, and `_BSD_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` or `_SVID_SOURCE` as well has no effect.

Note, however, that the features of `_BSD_SOURCE` are not a subset of any of the other feature test macros supported. This is because it defines BSD features that take precedence over the POSIX features that are requested by the other macros. For this reason, defining `_BSD_SOURCE` in addition to the other feature test macros does have an effect: it causes the BSD features to take priority over the conflicting POSIX features.