feature_test_macros(7) — Linux manual pages

DESCRIPTION

Feature test macros allow the programmer to control the definitions that are exposed by system header files when a program is compiled. This can be useful for creating portable applications, by preventing non-standard definitions from being exposed. Other macros can be used to expose non-standard definitions that are not exposed by default. The precise effects of each of the feature test macros described below can be ascertained by inspecting the <features.h> header file.

In order to be effective, a feature test macro must be defined before including any header files. This can either be done in the compilation command (cc −DMACRO=value) or by defining the macro within the source code before including any headers.

Linux/glibc understands the following feature test macros:

_POSIX_C_SOURCE: Defining this macro with the value 1 causes header files to expose definitions conforming to POSIX.1-1990 and ISO C (1990). Defining with the value 199309 or greater additionally exposes definitions for POSIX.1b (real-time extensions). Defining with the value 199506 or greater additionally exposes definitions for POSIX.1c (threads). Defining with the value 200112 exposes definitions corresponding to the POSIX.1-2001 base specification (excluding the XSI extension).
_POSIX_SOURCE: Defining this obsolete macro with any value is equivalent to defining _POSIX_C_SOURCE with the value 1.
_XOPEN_SOURCE: Defining this macro with any value causes header files to expose definitions conforming to POSIX.1, POSIX.2, and XPG4. Defining with the value 500 or greater additionally exposes definitions for SUSv2 (UNIX 98). Defining with the value 600 or greater additionally exposes definitions for SUSv3 (UNIX 03; i.e., the POSIX.1-2001 base specification plus the XSI extension) and C 99 definitions.
_XOPEN_SOURCE_EXTENDED: If this macro is defined with the value 1, and the _XOPEN_SOURCE is defined, then expose definitions corresponding to the XPG4v2 UNIX extensions.
_ISOC99_SOURCE: Exposes C 99 extensions to ISO C (1990).
_LARGEFILE64_SOURCE: Expose definitions for the alternative API specified by the LFS (Large File Summit) as a "transitional extension" to the Single UNIX Specification. (See http://opengroup.org/platform/lfs.html.)
_FILE_OFFSET_BITS: Defining this macro with the value 64 automatically converts references to 32-bit functions and data types related to file I/O and file system operations into references to their 64-bit counterparts. This is useful for performing I/O on large files (> 2 Gigabytes) on 32-bit systems.
_BSD_SOURCE: Defining this macro with any value cause header files to expose BSD-derived definitions. Defining this macro also causes BSD definitions to be preferred in some situations where standards conflict.
_SVID_SOURCE: Defining this macro with any value cause header files to expose System V-derived definitions. (SVID == System V Interface Definition; see standards(7).)
_GNU_SOURCE: Defining this macro (with any value) is equivalent to defining _BSD_SOURCE, _SVID_SOURCE, _LARGEFILE64_SOURCE, _ISOC99_SOURCE _POSIX_C_SOURCE with the value 1999506, and _XOPEN_SOURCE with the value 600. In addition, various GNU-specific extensions are also exposed.
_REENTRANT: Defining this macro exposes definitions of certain reentrant functions. For multithreaded programs, use cc -pthread instead.
_THREAD_SAFE: Synonym for _REENTRANT, provided for compatibility with some other implementations.
_FORTIFY_SOURCE: Defining this macro causes some lightweight checks to be performed to detect some buffer overflow errors when employing various string and memory manipulation functions. Not all buffer overflows are detected, just some common cases. In the current implementation checks are added for calls to memcpy(3), mempcpy(3), memmove(3), memset(3), stpcpy(3), strcpy(3), strncpy(3), strcat(3), strncat(3), sprintf(3), snprintf(3), vsprintf(3), vsnprintf(3), and gets(3). If _FORTIFY_SOURCE is set to 1, with compiler optimization level 1 (gcc -O1) and above, checks that shouldn't change the behaviour of conforming programs are performed. With _FORTIFY_SOURCE set to 2 some more checking is added, but some conforming programs might fail. Some of the checks can be performed at compile time, and result in compiler warnings; other checks take place at run time, and result in a run-time error if the check fails. Use of this macro requires compiler support, available with gcc(1) since version 4.0.

When gcc(1) is invoked, the following macros are defined by default: _BSD_SOURCE, _SVID_SOURCE, _POSIX_SOURCE, and _POSIX_C_SOURCE=199506. If individual macros are defined, then other macros are disabled unless they are also explicitly defined. (Exception: if _POSIX_C_SOURCE is not otherwise defined, then it is always defined with the value 200112 (199506 in glibc versions before 2.4), unless the compiler is invoked in one of its standard modes, e.g., the −std=c99 flag.) Multiple macros can be defined; the results are additive.

CONFORMING TO

POSIX.1 specifies _POSIX_C_SOURCE, _POSIX_SOURCE, and _XOPEN_SOURCE. _XOPEN_SOURCE_EXTENDED was specified by XPG4v2 (aka SUSv1). _FILE_OFFSET_BITS is not specified by any standard, but is employed on some other implementations. _BSD_SOURCE, _SVID_SOURCE, _GNU_SOURCE, _FORTIFY_SOURCE, _REENTRANT, and _THREAD_SAFE are Linux (glibc) specific.

NOTES

<features.h> is a Linux/glibc specific header file. Other systems have an analogous file, but typically with a different name. This header file is automatically included by other header files as required: it is not necessary to explicitly include it in order to employ feature test macros.

According to which of the above feature test macros are defined, <features.h> internally defines various other macros that are checked by other glibc header files. These macros have names prefixed by two underscores (e.g., __USE_MISC). Programs should never define these macros directly: instead, the appropriate feature test macro(s) from the list above should be employed.

This manpage is Copyright (C) 2006, Michael Kerrisk

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