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 non-standard definitions from being exposed.
Other macros can be used to expose non-standard 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):
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)):
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).
_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.
_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).
_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 200112L (199506L in glibc versions before 2.5),
and
_XOPEN_SOURCE
with the value 600 (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=200112L
(199506L in glibc versions before 2.4).
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
*
200112L (199506L in glibc versions before 2.4),
if
XOPEN_SOURCE
is undefined, or
is defined with a value greater than or equal to 600.
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.
The section "Feature Test Macros" under
info libc.
/usr/include/features.h
COLOPHON
This page is part of release 3.14 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/.
