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.
This can be 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.
In order to be effective, a feature test macro must be defined before
including any header files. This can either be done in the compilation
command (cc -DMACRO=value) or by defining the macro within the source
code before including any headers.
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 chmod(2) manual page):
#include <sys/stat.h>
int chmod(const char *path, mode_t mode);
int fchmod(int fd, mode_t mode);
Feature Test Macro Requirements for glibc (see
feature_test_macros(7)):
fchmod(): _BSD_SOURCE || _XOPEN_SOURCE >= 500
The || means that in order to obtain the declaration of fchmod(2) from
<sys/stat.h>, either of the following macro definitions must be made
before including any header files:
#define _BSD_SOURCE
#define _XOPEN_SOURCE 500 /* or any value > 500 */
Alternatively, equivalent definitions can be included in the
compilation command:
cc -D_BSD_SOURCE
cc -D_XOPEN_SOURCE=500 # 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.
_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 backwards compatibility.
_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.
CONFORMING TO
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
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be found at http://www.kernel.org/doc/man-pages/.