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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 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.
STANDARDS
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.
HISTORY
_XOPEN_SOURCE_EXTENDED was specified by XPG4v2 (aka SUSv1), but is not present in SUSv2
and later.
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 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.
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