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 <xys.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.
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
#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.
The paragraphs below explain how feature test macros are handled in Linux 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
Linux 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 int 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.
- _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 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.
- Since glibc 2.19, defining _GNU_SOURCE also has the effect of
implicitly defining _DEFAULT_SOURCE. In glibc versions before 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.
- Use of this macro requires compiler support, available with gcc(1)
since version 4.0.
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 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 (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, 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), 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).
<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
/* ftm.c */
#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: %ldL\n", (long) _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 _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)
The section "Feature Test Macros" under
info libc.
/usr/include/features.h