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FEATURE_TEST_MACROS(7) Linux Programmer's Manual FEATURE_TEST_MACROS(7)
feature_test_macros - feature test macros
#include <features.h>
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.
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 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:
o The value 1 exposes definitions conforming to POSIX.1-1990 and
ISO C (1990).
o The value 2 or greater additionally exposes definitions for
POSIX.2-1992.
o The value 199309L or greater additionally exposes definitions for
POSIX.1b (real-time extensions).
o The value 199506L or greater additionally exposes definitions for
POSIX.1c (threads).
o (Since glibc 2.3.3) The value 200112L or greater exposes
definitions corresponding to the POSIX.1-2001 base specification
(excluding the XSI extension).
o (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:
o Defining with any value exposes definitions conforming to
POSIX.1, POSIX.2, and XPG4.
o The value 500 or greater additionally exposes definitions for
SUSv2 (UNIX 98).
o (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.
o (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.
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:
o 2, if XOPEN_SOURCE is defined with a value less than 500;
o 199506L, if XOPEN_SOURCE is defined with a value greater than
or equal to 500 and less than 600; or
o (since glibc 2.4) 200112L, if XOPEN_SOURCE is defined with a
value greater than or equal to 600 and less than 700.
o (Since glibc 2.10) 200809L, if XOPEN_SOURCE is defined with a
value greater than or equal to 700.
o 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.
o 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).
<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 _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);
}
libc(7), standards(7)
The section "Feature Test Macros" under info libc.
/usr/include/features.h
This page is part of release 3.41 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/.
Linux 2012-01-18 FEATURE_TEST_MACROS(7)
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