NAME¶
feature_test_macros - feature test macros
SYNOPSIS¶
#include <features.h>
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.
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):
#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.
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:
- __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 exposes
definitions corresponding to the POSIX.1-2001 base specification
(excluding the XSI extension).
- •
- (Since glibc 2.10) The value 200809L or greater 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.
- _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).
- _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). This macro is also implicitly defined if
_XOPEN_SOURCE is defined with a value of 500 or more.
- _ISOC95_SOURCE
- 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).
- _ISOC99_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.
- _ISOC11_SOURCE
- Exposes declarations consistent with the ISO C11 standard.
This macro is recognized since glibc 2.16.
- _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 interface; instead
_FILE_OFFSET_BITS=64
should be employed.
- _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 file system 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
- Defining this macro with any value causes header files to
expose BSD-derived definitions. 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.
- _SVID_SOURCE
- Defining this macro with any value causes header files to
expose System V-derived definitions. (SVID == System V Interface
Definition; 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 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) 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.
- _REENTRANT
- Defining this macro exposes definitions of certain
reentrant functions. For multithreaded programs, use
cc -pthread instead.
- _THREAD_SAFE
- 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 implementation 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
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, 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:
- •
- 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).
_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).
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.
EXAMPLE¶
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 <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 _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 _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)
The section "Feature Test Macros" under
info libc.
/usr/include/features.h
COLOPHON¶
This page is part of release 3.44 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/.