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NAME | DESCRIPTION | STANDARDS | HISTORY | NOTES | EXAMPLES | SEE ALSO | COLOPHON |
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featur..._macros(7) Miscellaneous Information Manual featur..._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. 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
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. 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 readahead(2)):
#define _GNU_SOURCE
#define _FILE_OFFSET_BITS 64
#include <fcntl.h>
ssize_t readahead(int fd, off_t *offset, size_t count);
This format is employed when the feature test macros ensure that
the proper function declarations are visible, and the macros are
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 versions
of POSIX.1), _XOPEN_SOURCE (for definitions from various
versions of SUS), _GNU_SOURCE (for GNU and/or Linux specific
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 effect:
_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 effect:
_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
compatibility.
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 limitations 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. It is also useful when calling functions like
copy_file_range(2) that were added more recently and that
come only in 64-bit flavors. (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 timestamps
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
requires _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
Interface 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
otherwise 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
originally derived from BSD and System V. On glibc 2.19
and earlier, these defaults were approximately equivalent
to explicitly 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
before 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
libraries 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
_REENTRANT 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
employing 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
since gcc 4.0 and clang 2.6. Use of _FORTIFY_SOURCE set to
3 requires gcc 12.0 or later, or clang 9.0 or later, in
conjunction with glibc 2.33 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.
POSIX.1 specifies _POSIX_C_SOURCE, _POSIX_SOURCE, and
_XOPEN_SOURCE.
_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 glibc.
_XOPEN_SOURCE_EXTENDED was specified by XPG4v2 (aka SUSv1), but is
not present in SUSv2 and later.
<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
$ ./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);
}
libc(7), standards(7), system_data_types(7)
The section "Feature Test Macros" under info libc.
/usr/include/features.h
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Linux man-pages 6.15 2025-05-17 featur..._macros(7)
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