man-pages/man7/feature_test_macros.7

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.TH FEATURE_TEST_MACROS 7 2006-04-26 "Linux" "Linux Programmer's Manual"
.SH NAME
feature_test_macros \- feature test macros
.SH SYNOPSIS
.nf
.B #include <features.h>
.SH 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
.I <features.h>
header file.

In order to be effective, a feature test macro
.IR "must be defined before including any header files" .
This can either be done in the compilation command
.RI ( "cc \-DMACRO=value" )
or by defining the macro within the source code before
including any headers.

Linux/glibc understands the following feature test macros:
.TP
.B _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 definitions corresponding
to the POSIX.1-2001 base specification (excluding the XSI extension).
.TP
.B _POSIX_SOURCE
Defining this obsolete macro with any value is equivalent to defining
.B _POSIX_C_SOURCE
with the value 1.
.TP
.B _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.
.TP
.B _XOPEN_SOURCE_EXTENDED
If this macro is defined with the value 1, and the
.BR _XOPEN_SOURCE
is defined, then expose definitions corresponding to the XPG4v2
UNIX extensions.
.TP
.B _ISOC99_SOURCE
Exposes C 99 extensions to ISO C (1990).
.TP
.B _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.)
.TP
.B _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.
.TP
.B _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.
.TP
.B _SVID_SOURCE
Defining this macro with any value cause header files to expose
System V-derived definitions.
(SVID == System V Interface Definition; see
.BR standards (7).)
.TP
.B _GNU_SOURCE
Defining this macro (with any value) is equivalent to defining
.BR _BSD_SOURCE ,
.BR _SVID_SOURCE ,
.BR _LARGEFILE64_SOURCE ,
.BR _ISOC99_SOURCE
.BR _POSIX_C_SOURCE
with the value 1999506,
and
.BR _XOPEN_SOURCE
with the value 600.
In addition, various GNU-specific extensions are also exposed.
.TP
.B _REENTRANT
Defining this macro exposes definitions of certain reentrant functions.
For multithreaded programs, use
.I "cc -pthread"
instead.
.TP
.B _THREAD_SAFE
Synonym for
.BR _REENTRANT ,
provided for compatibility with some other implementations.
.TP
.B _FORTIFY_SOURCE
.\" For more detail, see:
.\" http://gcc.gnu.org/ml/gcc-patches/2004-09/msg02055.html
.\" [PATCH] Object size checking to prevent (some) buffer overflows
.\" * From: Jakub Jelinek <jakub at redhat dot com>
.\" * To: gcc-patches at gcc dot gnu dot org
.\" * Date: Tue, 21 Sep 2004 04:16:40 -0400
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
.BR memcpy (3),
.BR mempcpy (3),
.BR memmove (3),
.BR memset (3),
.BR stpcpy (3),
.BR strcpy (3),
.BR strncpy (3),
.BR strcat (3),
.BR strncat (3),
.BR sprintf (3),
.BR snprintf (3),
.BR vsprintf (3),
.BR vsnprintf (3),
and
.BR gets (3).
If
.B _FORTIFY_SOURCE
is set to 1, with compiler optimization level 1
.RI ( "gcc -O1" )
and above, checks that shouldn't change the behavior of
conforming programs are performed.
With
.B _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
.BR gcc (1)
since version 4.0.
.PP
When
.BR gcc (1)
is invoked, the following macros are defined by default:
.BR _BSD_SOURCE ,
.BR _SVID_SOURCE ,
.BR _POSIX_SOURCE ,
and
.BR _POSIX_C_SOURCE =199506.
If individual macros are defined, then other macros are disabled
unless they are also explicitly defined.
(Exception: if
.BR _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
.I -std=c99
flag.)
Multiple macros can be defined; the results are additive.
.SH CONFORMING TO
POSIX.1 specifies
.BR _POSIX_C_SOURCE ,
.BR _POSIX_SOURCE ,
and
.BR _XOPEN_SOURCE .
.BR _XOPEN_SOURCE_EXTENDED
was specified by XPG4v2 (aka SUSv1).
.BR _FILE_OFFSET_BITS
is not specified by any standard,
but is employed on some other implementations.
.BR _BSD_SOURCE ,
.BR _SVID_SOURCE ,
.BR _GNU_SOURCE ,
.BR _FORTIFY_SOURCE ,
.BR _REENTRANT ,
and
.BR _THREAD_SAFE
are Linux (glibc) specific.
.SH NOTES
.I <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,
.I <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 \fInever\fP define these macros directly:
instead, the appropriate feature test macro(s) from the
list above should be employed.
.SH
.BR standards (7)