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FEATURE_TEST_MACROS(7) Linux Programmer's Manual FEATURE_TEST_MACROS(7)
NAME
feature_test_macros - feature test macros
SYNOPSIS
#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 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 defini-
tions 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" 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 opera-
tions into references to their 64-bit counterparts. This is useful for per-
forming 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 stan-
dards(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.
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 addi-
tive.
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, and _GNU_SOURCE are Linux (glibc) specific.
NOTES
<features.h> is a Linux/glibc specific header file. Other systems have an analo-
gous 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.
Linux 2006-04-26 FEATURE_TEST_MACROS(7)
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