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FEATURE_TEST_MACROS(7) Linux Programmers Manual FEATURE_TEST_MACROS(7)

NAME
feature_test_macros - feature test macros

SYNOPSIS
#include

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
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 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 man
ual page SYNOPSIS typically includes a note of the following form (this
example from the chmod(2) manual page):

#include

int chmod(const char *path, mode_t mode);
int fchmod(int fd, mode_t mode);

Feature Test Macro Requirements for glibc (see fea
ture_test_macros(7)):

fchmod(): _BSD_SOURCE || _XOPEN_SOURCE >= 500

The || means that in order to obtain the declaration of fchmod(2) from
, 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 compila
tion 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

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 def
initions 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 recog
nized since glibc 2.1.3; earlier glibc 2.1.x versions recognized
an equivalent macro named _ISOC9X_SOURCE (because the C99 stan
dard had not then been finalized). Although the use of the lat
ter macro is obsolete, glibc continues to recognize it for back
wards 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/plat
form/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 recom
pilation 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 disfa
vored.

_SVID_SOURCE
Defining this macro with any value causes header files to expose
System V-derived definitions. (SVID == System V Interface Defi
nition; 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 ver
sions 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 def
initions 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 var
ious 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), mem
pcpy(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 shouldnt 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, _FOR
TIFY_SOURCE, _REENTRANT, and _THREAD_SAFE are specific to Linux
(glibc).

NOTES
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, 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.