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feature_test_macros - feature test macros
#include <features.h>
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 defini-
tions 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 cor-
responding 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" the Single UNIX Specification. (See http://ftp.sas.com/standards/large.file/.)
_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 use-
ful 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, _LARGE-
FILE64_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.
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 199506, 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 implemen-
tations. _BSD_SOURCE, _SVID_SOURCE, and _GNU_SOURCE 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 neces-
sary to explicitly include it in order to employ feature test macros.
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