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 nonstandard 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 page):
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 spec-
ify 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)):
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 > 0.
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 (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).
o (Since glibc 2.10) The value 200809L or greater exposes definitions corresponding to the POSIX.1-2008 base specification (exclud-
ing 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 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 spec-
ification plus the XSI extension) 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 international character sets. The C95 changes were included in the subsequent C99 standard (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 backward compatibility.
Exposes declarations consistent with the ISO C11 standard. This macro is recognized 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
filesystem operations into references to their 64-bit counterparts. This is useful for performing I/O on large files (> 2 Giga-
bytes) 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
Defining this macro with any value causes header files to expose BSD-derived definitions. Defining this macro also causes BSD defi-
nitions 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 Defini-
tion; 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 ope-
nat(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
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, various GNU-specific extensions are also exposed. Where
standards conflict, BSD definitions are disfavored.
Defining this macro exposes definitions of certain reentrant functions. For multithreaded 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 functions. Not all buffer overflows are detected, just some common cases. In the current implemen-
tation, 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 opti-
mization level 1 (gcc -O1) and above, checks that shouldn't change the behavior of conforming programs are performed. With _FOR-
TIFY_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.
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, 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 (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 version.
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 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.
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 following shell session, on a system with glibc 2.10, shows some examples of what we would see:
$ 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
", (long) _POSIX_C_SOURCE);
printf("_XOPEN_SOURCE defined: %d
printf("_FILE_OFFSET_BITS defined: %d
The section "Feature Test Macros" under info libc.
This page is part of release 3.55 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 2013-09-20 FEATURE_TEST_MACROS(7)