FEATURE_TEST_MACROS(7) Linux Programmer's Manual FEATURE_TEST_MACROS(7)
feature_test_macros - feature test macros
Feature test macros allow the programmer to control the definitions that are exposed by
system header files when a program is compiled.
NOTE: In order to be effective, a feature test macro must be defined before including any
header files. This can be done either in the compilation command (cc -DMACRO=value) or by
defining the macro within the source code before including any headers.
Some feature test macros are useful for creating portable applications, by preventing non-
standard definitions from being exposed. Other macros can be used to expose nonstandard
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.
Specification of feature test macro requirements in manual pages
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 acct(2) manual
int acct(const char *filename);
Feature Test Macro Requirements for glibc (see
acct(): _BSD_SOURCE || (_XOPEN_SOURCE && _XOPEN_SOURCE < 500)
The || means that in order to obtain the declaration of acct(2) from <unistd.h>, either of
the following macro definitions must be made before including any header files:
#define _XOPEN_SOURCE /* or any value < 500 */
Alternatively, equivalent definitions can be included in the compilation command:
cc -D_XOPEN_SOURCE # 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
In a few cases, manual pages use a shorthand for expressing the feature test macro
requirements (this example from readahead(2)):
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.
Feature test macros understood by glibc
The following paragraphs explain how feature test macros are handled in Linux glibc 2.x, x
Linux glibc understands the following feature test macros:
ISO Standard C. This macro is implicitly defined by gcc(1) when invoked with, for
example, the -std=c99 or -ansi flag.
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
o The value 199506L or greater additionally exposes definitions for POSIX.1c
o (Since glibc 2.3.3) The value 200112L or greater exposes definitions correspond-
ing to the POSIX.1-2001 base specification (excluding the XSI extension).
o (Since glibc 2.10) The value 200809L or greater exposes definitions correspond-
ing to the POSIX.1-2008 base specification (excluding the XSI extension).
Defining this obsolete macro with any value is equivalent to defining
_POSIX_C_SOURCE with the value 1.
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
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 exten-
sion) and C99 definitions.
o (Since glibc 2.10) The value 700 or greater additionally exposes definitions for
SUSv4 (i.e., the POSIX.1-2008 base specification plus the XSI extension).
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.
Exposes ISO C (1990) Amendment 1 definitions (also known as C95). This macro is
recognized since glibc 2.12. The primary change in C95 was support for interna-
tional character sets. The C95 changes were included in the subsequent C99 stan-
dard (in other words, _ISOC99_SOURCE implies _ISOC95_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 back-
Exposes declarations consistent with the ISO C11 standard. This macro is recog-
nized since glibc 2.16.
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.
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 refer-
ences 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.
Defining this macro with any value causes header files to expose BSD-derived defi-
nitions. 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.
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). Since glibc 2.10, this
macro is also implicitly defined if _POSIX_C_SOURCE is defined with a value greater
than or equal to 200809L.
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 200809L
(200112L in glibc versions before 2.10; 199506L in glibc versions before 2.5;
199309L in glibc versions before 2.1) and _XOPEN_SOURCE with the value 700 (600 in
glibc versions before 2.10; 500 in glibc versions before 2.2). In addition, vari-
ous GNU-specific extensions are also exposed. Where standards conflict, BSD defi-
nitions are disfavored.
Defining this macro exposes definitions of certain reentrant functions. For multi-
threaded programs, use cc -pthread instead.
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 func-
tions. Not all buffer overflows are detected, just some common cases. In the cur-
rent implementation checks are added for calls to memcpy(3), mempcpy(3), mem-
move(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 _FOR-
TIFY_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 pro-
grams 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.
Default definitions, implicit definitions, and combining definitions
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=200809L (200112L in glibc versions before 2.10; 199506L in glibc versions
before 2.4; 199309L in glibc versions before 2.1).
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,
* _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 (since glibc 2.4) 200112L, if XOPEN_SOURCE is defined with a value greater
than or equal to 600 and less than 700.
o (Since glibc 2.10) 200809L, if XOPEN_SOURCE is defined with a value greater
than or equal to 700.
o Older versions of glibc do not know about the values 200112L and 200809L for
_POSIX_C_SOURCE, and the setting of this macro will depend on the glibc ver-
o If _XOPEN_SOURCE is undefined, then the setting of _POSIX_C_SOURCE depends on
the glibc version: 199506L, in glibc versions before 2.4; 200112L, in glibc
2.4 to 2.9; and 200809L, since glibc 2.10.
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 imple-
_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.
The program below can be used to explore how the various feature test macros are set
depending on the glibc version and what feature test macros are explicitly set. The fol-
lowing shell session, on a system with glibc 2.10, shows some examples of what we would
$ cc ftm.c
_POSIX_C_SOURCE defined: 200809L
$ cc -D_XOPEN_SOURCE=500 ftm.c
_POSIX_C_SOURCE defined: 199506L
_XOPEN_SOURCE defined: 500
$ cc -D_GNU_SOURCE ftm.c
_POSIX_C_SOURCE defined: 200809L
_XOPEN_SOURCE defined: 700
/* ftm.c */
main(int argc, char *argv)
printf("_POSIX_C_SOURCE defined: %ldL\n", (long) _POSIX_C_SOURCE);
printf("_XOPEN_SOURCE defined: %d\n", _XOPEN_SOURCE);
printf("_FILE_OFFSET_BITS defined: %d\n", _FILE_OFFSET_BITS);
The section "Feature Test Macros" under info libc.
This page is part of release 3.53 of the Linux man-pages project. A description of the
project, and information about reporting bugs, can be found at
Linux 2012-08-05 FEATURE_TEST_MACROS(7)