feature_test_macros(7) Miscellaneous Information Manual feature_test_macros(7)
NAME
feature_test_macros - feature test macros
DESCRIPTION
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. The requirement that the macro must be defined before including any header file exists because header files may
freely include one another. Thus, for example, in the following lines, defining the _GNU_SOURCE macro may have no effect
because the header <abc.h> itself includes <xyz.h> (POSIX explicitly allows this):
#include <abc.h>
#define _GNU_SOURCE
#include <xyz.h>
Some feature test macros are useful for creating portable applications, by preventing nonstandard definitions from being ex‐
posed. 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. Note: applications do not need to directly include <features.h>; indeed, doing so is actively discouraged.
See NOTES.
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):
#include <unistd.h>
int acct(const char *filename);
Feature Test Macro Requirements for glibc (see
feature_test_macros(7)):
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 _BSD_SOURCE
#define _XOPEN_SOURCE /* or any value < 500 */
Alternatively, equivalent definitions can be included in the compilation command:
cc -D_BSD_SOURCE
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 SYNOPSIS.
In a few cases, manual pages use a shorthand for expressing the feature test macro requirements (this example from reada‐
head(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.
Feature test macros understood by glibc
The paragraphs below explain how feature test macros are handled in glibc 2.x, x > 0.
First, though, a summary of a few details for the impatient:
• The macros that you most likely need to use in modern source code are _POSIX_C_SOURCE (for definitions from various ver‐
sions of POSIX.1), _XOPEN_SOURCE (for definitions from various versions of SUS), _GNU_SOURCE (for GNU and/or Linux spe‐
cific stuff), and _DEFAULT_SOURCE (to get definitions that would normally be provided by default).
• Certain macros are defined with default values. Thus, although one or more macros may be indicated as being required in
the SYNOPSIS of a man page, it may not be necessary to define them explicitly. Full details of the defaults are given
later in this man page.
• Defining _XOPEN_SOURCE with a value of 600 or greater produces the same effects as defining _POSIX_C_SOURCE with a value
of 200112L or greater. Where one sees
_POSIX_C_SOURCE >= 200112L
in the feature test macro requirements in the SYNOPSIS of a man page, it is implicit that the following has the same ef‐
fect:
_XOPEN_SOURCE >= 600
• Defining _XOPEN_SOURCE with a value of 700 or greater produces the same effects as defining _POSIX_C_SOURCE with a value
of 200809L or greater. Where one sees
_POSIX_C_SOURCE >= 200809L
in the feature test macro requirements in the SYNOPSIS of a man page, it is implicit that the following has the same ef‐
fect:
_XOPEN_SOURCE >= 700
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:
• The value 1 exposes definitions conforming to POSIX.1-1990 and ISO C (1990).
• The value 2 or greater additionally exposes definitions for POSIX.2-1992.
• The value 199309L or greater additionally exposes definitions for POSIX.1b (real-time extensions).
• The value 199506L or greater additionally exposes definitions for POSIX.1c (threads).
• (Since glibc 2.3.3) The value 200112L or greater additionally exposes definitions corresponding to the
POSIX.1-2001 base specification (excluding the XSI extension). This value also causes C95 (since glibc 2.12) and
C99 (since glibc 2.10) features to be exposed (in other words, the equivalent of defining _ISOC99_SOURCE).
• (Since glibc 2.10) The value 200809L or greater additionally exposes definitions corresponding to the POSIX.1-2008
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.
Since this macro is obsolete, its usage is generally not documented when discussing feature test macro requirements
in the man pages.
_XOPEN_SOURCE
Defining this macro causes header files to expose definitions as follows:
• Defining with any value exposes definitions conforming to POSIX.1, POSIX.2, and XPG4.
• The value 500 or greater additionally exposes definitions for SUSv2 (UNIX 98).
• (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.
• (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 __STRICT_ANSI__ is not defined, or _XOPEN_SOURCE is defined with a value greater than or equal to 500 and neither
_POSIX_SOURCE nor _POSIX_C_SOURCE is explicitly defined, then the following macros are implicitly defined:
• _POSIX_SOURCE is defined with the value 1.
• _POSIX_C_SOURCE is defined, according to the value of _XOPEN_SOURCE:
_XOPEN_SOURCE < 500
_POSIX_C_SOURCE is defined with the value 2.
500 <= _XOPEN_SOURCE < 600
_POSIX_C_SOURCE is defined with the value 199506L.
600 <= _XOPEN_SOURCE < 700
_POSIX_C_SOURCE is defined with the value 200112L.
700 <= _XOPEN_SOURCE (since glibc 2.10)
_POSIX_C_SOURCE is defined with the value 200809L.
In addition, defining _XOPEN_SOURCE with a value of 500 or greater produces the same effects as defining
_XOPEN_SOURCE_EXTENDED.
_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). Defining _XOPEN_SOURCE with a value of 500 or more also produces the same effect as
defining _XOPEN_SOURCE_EXTENDED. Use of _XOPEN_SOURCE_EXTENDED in new source code should be avoided.
Since defining _XOPEN_SOURCE with a value of 500 or more has the same effect as defining _XOPEN_SOURCE_EXTENDED, the
latter (obsolete) feature test macro is generally not described in the SYNOPSIS in man pages.
_ISOC99_SOURCE (since glibc 2.1.3)
Exposes declarations consistent with the ISO C99 standard.
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 this macro is obsolete, glibc continues to recognize it for backward com‐
patibility.
Defining _ISOC99_SOURCE also exposes ISO C (1990) Amendment 1 ("C95") definitions. (The primary change in C95 was
support for international character sets.)
Invoking the C compiler with the option -std=c99 produces the same effects as defining this macro.
_ISOC11_SOURCE (since glibc 2.16)
Exposes declarations consistent with the ISO C11 standard. Defining this macro also enables C99 and C95 features
(like _ISOC99_SOURCE).
Invoking the C compiler with the option -std=c11 produces the same effects as defining this macro.
_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 macro; instead _FILE_OFFSET_BITS=64 should be
employed.
_LARGEFILE_SOURCE
This macro was historically used to expose certain functions (specifically fseeko(3) and ftello(3)) that address lim‐
itations of earlier APIs (fseek(3) and ftell(3)) that use long for file offsets. This macro is implicitly defined if
_XOPEN_SOURCE is defined with a value greater than or equal to 500. New programs should not employ this macro;
defining _XOPEN_SOURCE as just described or defining _FILE_OFFSET_BITS with the value 64 is the preferred mechanism
to achieve the same result.
_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 filesystem 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.
_TIME_BITS
Defining this macro with the value 64 changes the width of time_t(3type) to 64-bit which allows handling of time‐
stamps beyond 2038. It is closely related to _FILE_OFFSET_BITS and depending on implementation, may require it set.
This macro is available as of glibc 2.34.
_BSD_SOURCE (deprecated since glibc 2.20)
Defining this macro with any value causes header files to expose BSD-derived definitions.
In glibc versions up to and including 2.18, 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.
Since glibc 2.19, _BSD_SOURCE no longer causes BSD definitions to be preferred in case of conflicts.
Since glibc 2.20, this macro is deprecated. It now has the same effect as defining _DEFAULT_SOURCE, but generates a
compile-time warning (unless _DEFAULT_SOURCE is also defined). Use _DEFAULT_SOURCE instead. To allow code that re‐
quires _BSD_SOURCE in glibc 2.19 and earlier and _DEFAULT_SOURCE in glibc 2.20 and later to compile without warnings,
define both _BSD_SOURCE and _DEFAULT_SOURCE.
_SVID_SOURCE (deprecated since glibc 2.20)
Defining this macro with any value causes header files to expose System V-derived definitions. (SVID == System V In‐
terface Definition; see standards(7).)
Since glibc 2.20, this macro is deprecated in the same fashion as _BSD_SOURCE.
_DEFAULT_SOURCE (since glibc 2.19)
This macro can be defined to ensure that the "default" definitions are provided even when the defaults would other‐
wise be disabled, as happens when individual macros are explicitly defined, or the compiler is invoked in one of its
"standard" modes (e.g., cc -std=c99). Defining _DEFAULT_SOURCE without defining other individual macros or invoking
the compiler in one of its "standard" modes has no effect.
The "default" definitions comprise those required by POSIX.1-2008 and ISO C99, as well as various definitions origi‐
nally derived from BSD and System V. On glibc 2.19 and earlier, these defaults were approximately equivalent to ex‐
plicitly defining the following:
cc -D_BSD_SOURCE -D_SVID_SOURCE -D_POSIX_C_SOURCE=200809
_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.
_GNU_SOURCE
Defining this macro (with any value) implicitly defines _ATFILE_SOURCE, _LARGEFILE64_SOURCE, _ISOC99_SOURCE,
_XOPEN_SOURCE_EXTENDED, _POSIX_SOURCE, _POSIX_C_SOURCE with the value 200809L (200112L before glibc 2.10; 199506L be‐
fore glibc 2.5; 199309L before glibc 2.1) and _XOPEN_SOURCE with the value 700 (600 before glibc 2.10; 500 before
glibc 2.2). In addition, various GNU-specific extensions are also exposed.
Since glibc 2.19, defining _GNU_SOURCE also has the effect of implicitly defining _DEFAULT_SOURCE. Before glibc
2.20, defining _GNU_SOURCE also had the effect of implicitly defining _BSD_SOURCE and _SVID_SOURCE.
_REENTRANT
Historically, on various C libraries it was necessary to define this macro in all multithreaded code. (Some C li‐
braries may still require this.) In glibc, this macro also exposed definitions of certain reentrant functions.
However, glibc has been thread-safe by default for many years; since glibc 2.3, the only effect of defining _REEN‐
TRANT has been to enable one or two of the same declarations that are also enabled by defining _POSIX_C_SOURCE with a
value of 199606L or greater.
_REENTRANT is now obsolete. In glibc 2.25 and later, defining _REENTRANT is equivalent to defining _POSIX_C_SOURCE
with the value 199606L. If a higher POSIX conformance level is selected by any other means (such as _POSIX_C_SOURCE
itself, _XOPEN_SOURCE, _DEFAULT_SOURCE, or _GNU_SOURCE), then defining _REENTRANT has no effect.
This macro is automatically defined if one compiles with cc -pthread.
_THREAD_SAFE
Synonym for the (deprecated) _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 employ‐
ing various string and memory manipulation functions (for example, memcpy(3), memset(3), stpcpy(3), strcpy(3),
strncpy(3), strcat(3), strncat(3), sprintf(3), snprintf(3), vsprintf(3), vsnprintf(3), gets(3), and wide character
variants thereof). For some functions, argument consistency is checked; for example, a check is made that open(2)
has been supplied with a mode argument when the specified flags include O_CREAT. Not all problems are detected, just
some common cases.
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 (via macros logic implemented in header files), and result in
compiler warnings; other checks take place at run time, and result in a run-time error if the check fails.
With _FORTIFY_SOURCE set to 3, additional checking is added to intercept some function calls used with an argument of
variable size where the compiler can deduce an upper bound for its value. For example, a program where malloc(3)'s
size argument is variable can now be fortified.
Use of this macro requires compiler support, available with gcc(1) since glibc 4.0.
Use of _FORTIFY_SOURCE set to 3 requires gcc(1) version 12.0 or later.
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
(in glibc 2.19 and earlier), _SVID_SOURCE (in glibc 2.19 and earlier), _DEFAULT_SOURCE (since glibc 2.19), _POSIX_SOURCE,
and _POSIX_C_SOURCE=200809L (200112L before glibc 2.10; 199506L before glibc 2.4; 199309L before glibc 2.1).
If any of __STRICT_ANSI__, _ISOC99_SOURCE, _ISOC11_SOURCE (since glibc 2.18), _POSIX_SOURCE, _POSIX_C_SOURCE, _XOPEN_SOURCE,
_XOPEN_SOURCE_EXTENDED (in glibc 2.11 and earlier), _BSD_SOURCE (in glibc 2.19 and earlier), or _SVID_SOURCE (in glibc 2.19
and earlier) is explicitly defined, then _BSD_SOURCE, _SVID_SOURCE, and _DEFAULT_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:
• 2, if _XOPEN_SOURCE is defined with a value less than 500;
• 199506L, if _XOPEN_SOURCE is defined with a value greater than or equal to 500 and less than 600; or
• (since glibc 2.4) 200112L, if _XOPEN_SOURCE is defined with a value greater than or equal to 600 and less than 700.
• (Since glibc 2.10) 200809L, if _XOPEN_SOURCE is defined with a value greater than or equal to 700.
• 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.
• If _XOPEN_SOURCE is undefined, then the setting of _POSIX_C_SOURCE depends on the glibc version: 199506L, before glibc
2.4; 200112L, since glibc 2.4 to glibc 2.9; and 200809L, since glibc 2.10.
Multiple macros can be defined; the results are additive.
STANDARDS
POSIX.1 specifies _POSIX_C_SOURCE, _POSIX_SOURCE, and _XOPEN_SOURCE.
_XOPEN_SOURCE_EXTENDED was specified by XPG4v2 (aka SUSv1), but is not present in SUSv2 and later. _FILE_OFFSET_BITS is not
specified by any standard, but is employed on some other implementations.
_BSD_SOURCE, _SVID_SOURCE, _DEFAULT_SOURCE, _ATFILE_SOURCE, _GNU_SOURCE, _FORTIFY_SOURCE, _REENTRANT, and _THREAD_SAFE are
specific to Linux (glibc).
NOTES
<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 in‐
clude 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 em‐
ployed.
EXAMPLES
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
$ ./a.out
_POSIX_SOURCE defined
_POSIX_C_SOURCE defined: 200809L
_BSD_SOURCE defined
_SVID_SOURCE defined
_ATFILE_SOURCE defined
$ cc -D_XOPEN_SOURCE=500 ftm.c
$ ./a.out
_POSIX_SOURCE defined
_POSIX_C_SOURCE defined: 199506L
_XOPEN_SOURCE defined: 500
$ cc -D_GNU_SOURCE ftm.c
$ ./a.out
_POSIX_SOURCE defined
_POSIX_C_SOURCE defined: 200809L
_ISOC99_SOURCE defined
_XOPEN_SOURCE defined: 700
_XOPEN_SOURCE_EXTENDED defined
_LARGEFILE64_SOURCE defined
_BSD_SOURCE defined
_SVID_SOURCE defined
_ATFILE_SOURCE defined
_GNU_SOURCE defined
Program source
/* ftm.c */
#include <stdint.h>
#include <stdio.h>
#include <unistd.h>
#include <stdlib.h>
int
main(int argc, char *argv[])
{
#ifdef _POSIX_SOURCE
printf("_POSIX_SOURCE defined\n");
#endif
#ifdef _POSIX_C_SOURCE
printf("_POSIX_C_SOURCE defined: %jdL\n",
(intmax_t) _POSIX_C_SOURCE);
#endif
#ifdef _ISOC99_SOURCE
printf("_ISOC99_SOURCE defined\n");
#endif
#ifdef _ISOC11_SOURCE
printf("_ISOC11_SOURCE defined\n");
#endif
#ifdef _XOPEN_SOURCE
printf("_XOPEN_SOURCE defined: %d\n", _XOPEN_SOURCE);
#endif
#ifdef _XOPEN_SOURCE_EXTENDED
printf("_XOPEN_SOURCE_EXTENDED defined\n");
#endif
#ifdef _LARGEFILE64_SOURCE
printf("_LARGEFILE64_SOURCE defined\n");
#endif
#ifdef _FILE_OFFSET_BITS
printf("_FILE_OFFSET_BITS defined: %d\n", _FILE_OFFSET_BITS);
#endif
#ifdef _TIME_BITS
printf("_TIME_BITS defined: %d\n", _TIME_BITS);
#endif
#ifdef _BSD_SOURCE
printf("_BSD_SOURCE defined\n");
#endif
#ifdef _SVID_SOURCE
printf("_SVID_SOURCE defined\n");
#endif
#ifdef _DEFAULT_SOURCE
printf("_DEFAULT_SOURCE defined\n");
#endif
#ifdef _ATFILE_SOURCE
printf("_ATFILE_SOURCE defined\n");
#endif
#ifdef _GNU_SOURCE
printf("_GNU_SOURCE defined\n");
#endif
#ifdef _REENTRANT
printf("_REENTRANT defined\n");
#endif
#ifdef _THREAD_SAFE
printf("_THREAD_SAFE defined\n");
#endif
#ifdef _FORTIFY_SOURCE
printf("_FORTIFY_SOURCE defined\n");
#endif
exit(EXIT_SUCCESS);
}
SEE ALSO
libc(7), standards(7), system_data_types(7)
The section "Feature Test Macros" under info libc.
/usr/include/features.h
Linux man-pages 6.03 2023-02-05 feature_test_macros(7)