feature_test_macros — feature test macros
#include <features.h>
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.
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.
<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.
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