NAME | SYNOPSIS | DESCRIPTION | CONFORMING TO | NOTES | SEE ALSO | COLOPHON
FEATURE_TEST_MACROS(7) Linux Programmer's Manual FEATURE_TEST_MACROS(7)
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.
When a function requires that a feature test macro is defined, the manual page
SYNOPSIS typically includes a note of the following form (this example from
the chmod(2) manual page):
#include <sys/stat.h>
int chmod(const char *path, mode_t mode);
int fchmod(int fd, mode_t mode);
Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
fchmod(): _BSD_SOURCE || _XOPEN_SOURCE >= 500
The || means that in order to obtain the declaration of fchmod(2) from
<sys/stat.h>, either of the following macro definitions must be made before
including any header files:
#define _BSD_SOURCE
#define _XOPEN_SOURCE 500 /* or any value > 500 */
Alternatively, equivalent definitions can be included in the compilation
command:
cc -D_BSD_SOURCE
cc -D_XOPEN_SOURCE=500 # Or any value > 500
Note that, as described below, some feature test macros are defined by
default, so that it may not always be necessary to explicitly specify the
feature test macro(s) shown in the SYNOPSIS.
In a few cases, manual pages use a shorthand for expressing the feature test
macro requirements (this example from readahead(2)):
#define _GNU_SOURCE
#include <fcntl.h>
ssize_t readahead(int fd, off64_t *offset, size_t count);
This format is employed in cases where only a single feature test macro can be
used to expose the function declaration, and that macro is not defined by
default.
The following paragraphs explain how feature test macros are handled in Linux
glibc 2.x, x > 0.
Linux glibc understands the following feature test macros:
__STRICT_ANSI__
ISO Standard C. This macro is implicitly defined by gcc(1) when
invoked with, for example, the -std=c99 or -ansi flag.
_POSIX_C_SOURCE
Defining this macro causes header files to expose definitions as
follows:
o The value 1 exposes definitions conforming to POSIX.1-1990 and ISO C
(1990).
o The value 2 or greater additionally exposes definitions for
POSIX.2-1992.
o The value 199309L or greater additionally exposes definitions for
POSIX.1b (real-time extensions).
o The value 199506L or greater additionally exposes definitions for
POSIX.1c (threads).
o (Since glibc 2.3.3) The value 200112L or greater 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 causes header files to expose definitions as
follows:
o Defining with any value exposes definitions conforming to POSIX.1,
POSIX.2, and XPG4.
o The value 500 or greater additionally exposes definitions for SUSv2
(UNIX 98).
o (Since glibc 2.2) 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 C99 definitions.
_XOPEN_SOURCE_EXTENDED
If this macro is defined, and _XOPEN_SOURCE is defined, then expose
definitions corresponding to the XPG4v2 (SUSv1) UNIX extensions (UNIX
95). This macro is also implicitly defined if _XOPEN_SOURCE is defined
with a value of 500 or more.
_ISOC99_SOURCE
Exposes C99 extensions to ISO C (1990). This macro is recognized since
glibc 2.1.3; earlier glibc 2.1.x versions recognized an equivalent
macro named _ISOC9X_SOURCE (because the C99 standard had not then been
finalized). Although the use of the latter macro is obsolete, glibc
continues to recognize it for backwards compatibility.
_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.) The
alternative API consists of a set of new objects (i.e., functions and
types) whose names are suffixed with "64" (e.g., off64_t versus off_t,
lseek64() versus lseek(), etc.). New programs should not employ this
interface; instead _FILE_OFFSET_BITS=64 should be employed.
_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. (Defining this macro permits correctly written programs to
use large files with only a recompilation being required.) 64-bit
systems naturally permit file sizes greater than 2 Gigabytes, and on
those systems this macro has no effect.
_BSD_SOURCE
Defining this macro with any value causes header files to expose BSD-
derived definitions. Defining this macro also causes BSD definitions
to be preferred in some situations where standards conflict, unless one
or more of _SVID_SOURCE, _POSIX_SOURCE, _POSIX_C_SOURCE, _XOPEN_SOURCE,
_XOPEN_SOURCE_EXTENDED, or _GNU_SOURCE is defined, in which case BSD
definitions are disfavored.
_SVID_SOURCE
Defining this macro with any value causes header files to expose System
V-derived definitions. (SVID == System V Interface Definition; see
standards(7).)
_ATFILE_SOURCE (since glibc 2.4)
Defining this macro with any value causes header files to expose
declarations of a range of functions with the suffix "at"; see
openat(2).
_GNU_SOURCE
Defining this macro (with any value) is equivalent to defining
_BSD_SOURCE, _SVID_SOURCE, _ATFILE_SOURCE, _LARGEFILE64_SOURCE,
_ISOC99_SOURCE, _XOPEN_SOURCE_EXTENDED, _POSIX_SOURCE, _POSIX_C_SOURCE
with the value 200112L (199506L in glibc versions before 2.5), and
_XOPEN_SOURCE with the value 600 (500 in glibc versions before 2.2).
In addition, various GNU-specific extensions are also exposed. Where
standards conflict, BSD definitions are disfavored.
_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 (since glibc 2.3.4)
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 behavior 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.
If no feature test macros are explicitly defined, then the following feature
test macros are defined by default: _BSD_SOURCE, _SVID_SOURCE, _POSIX_SOURCE,
and _POSIX_C_SOURCE=200112L (199506L in glibc versions before 2.4).
If any of __STRICT_ANSI__, _ISOC99_SOURCE, _POSIX_SOURCE, _POSIX_C_SOURCE,
_XOPEN_SOURCE, _XOPEN_SOURCE_EXTENDED, _BSD_SOURCE, or _SVID_SOURCE is
explicitly defined, then _BSD_SOURCE, and _SVID_SOURCE are not defined by
default.
If _POSIX_SOURCE and _POSIX_C_SOURCE are not explicitly defined, and either
__STRICT_ANSI__ is not defined or _XOPEN_SOURCE is defined with a value of 500
or more, then
* _POSIX_SOURCE is defined with the value 1; and
* _POSIX_C_SOURCE is defined with one of the following values:
o 2, if XOPEN_SOURCE is defined with a value less than 500;
o 199506L, if XOPEN_SOURCE is defined with a value greater than or
equal to 500 and less than 600; or
o 200112L (199506L in glibc versions before 2.4), if XOPEN_SOURCE is
undefined, or is defined with a value greater than or equal to
600.
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, _ATFILE_SOURCE, _GNU_SOURCE, _FORTIFY_SOURCE,
_REENTRANT, and _THREAD_SAFE are specific to Linux (glibc).
<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.
standards(7)
The section "Feature Test Macros" under info libc.
/usr/include/features.h
This page is part of release 3.08 of the Linux man-pages project. A
description of the project, and information about reporting bugs, can be found
at http://www.kernel.org/doc/man-pages/.
Linux 2008-01-02 FEATURE_TEST_MACROS(7)