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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 suf-
fixed with "64" (e.g., off64_t versus off_t, lseek64() versus
lseek(), etc.). New programs should not employ this inter-
face; 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 natu-
rally 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 Inter-
face 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, _LARGE-
FILE64_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 per-
formed 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 _FOR-
TIFY_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 pro-
grams might fail. Some of the checks can be performed at com-
pile 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
%%%SS%%%
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, _FOR-
TIFY_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 3.51 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 2012-08-05 FEATURE_TEST_MACROS(7)
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