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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
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.
Linux/glibc understands the following feature test macros:
_POSIX_C_SOURCE
Defining this macro with the value 1 causes header files to
expose definitions conforming to POSIX.1-1990 and ISO C (1990).
Defining with the value 199309 or greater additionally exposes
definitions for POSIX.1b (real-time extensions). Defining with
the value 199506 or greater additionally exposes definitions for
POSIX.1c (threads). Defining with the value 200112 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 with any value causes header files to expose
definitions conforming to POSIX.1, POSIX.2, and XPG4. Defining
with the value 500 or greater additionally exposes definitions
for SUSv2 (UNIX 98). Defining with 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 C 99
definitions.
_XOPEN_SOURCE_EXTENDED
If this macro is defined with the value 1, and the _XOPEN_SOURCE
is defined, then expose definitions corresponding to the XPG4v2
UNIX extensions.
_ISOC99_SOURCE
Exposes C 99 extensions to ISO C (1990).
_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.)
_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.
_BSD_SOURCE
Defining this macro with any value cause header files to expose
BSD-derived definitions. Defining this macro also causes BSD
definitions to be preferred in some situations where standards
conflict.
_SVID_SOURCE
Defining this macro with any value cause header files to expose
System V-derived definitions. (SVID == System V Interface Defi‐
nition; see standards(7).)
_GNU_SOURCE
Defining this macro (with any value) is equivalent to defining
_BSD_SOURCE, _SVID_SOURCE, _LARGEFILE64_SOURCE, _ISOC99_SOURCE
_POSIX_C_SOURCE with the value 1999506, and _XOPEN_SOURCE with
the value 600. In addition, various GNU-specific extensions are
also exposed.
_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
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 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.
When gcc(1) is invoked, the following macros are defined by default:
_BSD_SOURCE, _SVID_SOURCE, _POSIX_SOURCE, and _POSIX_C_SOURCE=199506.
If individual macros are defined, then other macros are disabled unless
they are also explicitly defined. (Exception: if _POSIX_C_SOURCE is
not otherwise defined, then it is always defined with the value 200112
(199506 in glibc versions before 2.4), unless the compiler is invoked
in one of its standard modes, for example, the -std=c99 flag.) Multi‐
ple 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_OFF
SET_BITS is not specified by any standard, but is employed on some
other implementations. _BSD_SOURCE, _SVID_SOURCE, _GNU_SOURCE, _FOR
TIFY_SOURCE, _REENTRANT, and _THREAD_SAFE are Linux (glibc) specific.
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, <fea‐
tures.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.
standards(7)