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.TH FEATURE_TEST_MACROS 7 2014-01-06 "Linux" "Linux Programmer's Manual"
.SH NAME
feature_test_macros \- feature test macros
.SH SYNOPSIS
.nf
.B #include <features.h>
.fi
.SH DESCRIPTION
Feature test macros allow the programmer to control the definitions that
are exposed by system header files when a program is compiled.

.B NOTE:
In order to be effective, a feature test macro
.IR "must be defined before including any header files" .
This can be done either in the compilation command
.RI ( "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
.I <features.h>
header file.
.SS 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
.BR acct (2)
manual page):
.RS 8
.sp
.B #include <unistd.h>
.sp
.BI "int acct(const char *" filename );
.sp
.nf
.in -4n
Feature Test Macro Requirements for glibc (see
.BR feature_test_macros (7)):
.fi
.in
.sp
.BR acct ():
_BSD_SOURCE || (_XOPEN_SOURCE && _XOPEN_SOURCE\ <\ 500)
.RE
.PP
The
.B ||
means that in order to obtain the declaration of
.BR acct (2)
from
.IR <unistd.h> ,
.I either
of the following macro
definitions must be made before including any header files:
.RS
.nf

#define _BSD_SOURCE
#define _XOPEN_SOURCE        /* or any value < 500 */
.fi
.RE
.PP
Alternatively, equivalent definitions can be included in the
compilation command:
.RS
.nf

cc \-D_BSD_SOURCE
cc \-D_XOPEN_SOURCE           # Or any value < 500
.fi
.RE
.PP
Note that, as described below,
.BR "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
.BR readahead (2)):
.RS
.nf

.B #define _GNU_SOURCE
.B #include <fcntl.h>
.sp
.BI "ssize_t readahead(int " fd ", off64_t *" offset ", size_t " count );
.fi
.RE
.PP
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.
.SS Feature test macros understood by glibc
The following paragraphs explain how feature test macros are handled
in Linux glibc 2.\fIx\fP,
.I x
> 0.
.\" The details in glibc 2.0 are simpler, but combining a
.\" a description of them with the details in later glibc versions
.\" would make for a complicated description.

Linux glibc understands the following feature test macros:
.TP
.B __STRICT_ANSI__
ISO Standard C.
This macro is implicitly defined by
.BR gcc (1)
when invoked with, for example, the
.I -std=c99
or
.I -ansi
flag.
.TP
.B _POSIX_C_SOURCE
Defining this macro causes header files to expose definitions as follows:
.RS
.IP \(bu 3
The value 1 exposes definitions conforming to POSIX.1-1990 and
ISO C (1990).
.IP \(bu
The value 2 or greater additionally exposes
definitions for POSIX.2-1992.
.IP \(bu
The value 199309L or greater additionally exposes
definitions for POSIX.1b (real-time extensions).
.\" 199506L functionality is available only since glibc 2.1
.IP \(bu
The value 199506L or greater additionally exposes
definitions for POSIX.1c (threads).
.IP \(bu
(Since glibc 2.3.3)
The value 200112L or greater exposes definitions corresponding
to the POSIX.1-2001 base specification (excluding the XSI extension).
.IP \(bu
(Since glibc 2.10)
The value 200809L or greater exposes definitions corresponding
to the POSIX.1-2008 base specification (excluding the XSI extension).
.RE
.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 causes header files to expose definitions as follows:
.RS
.IP \(bu 3
Defining with any value exposes
definitions conforming to POSIX.1, POSIX.2, and XPG4.
.IP \(bu
The value 500 or greater additionally exposes
definitions for SUSv2 (UNIX 98).
.IP \(bu
(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.
.IP \(bu
(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).
.RE
.TP
.B _XOPEN_SOURCE_EXTENDED
If this macro is defined, and
.B _XOPEN_SOURCE
is defined, then expose definitions corresponding to the XPG4v2
(SUSv1) UNIX extensions (UNIX 95).
This macro is also implicitly defined if
.B _XOPEN_SOURCE
is defined with a value of 500 or more.
.TP
.B _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,
.B _ISOC99_SOURCE
implies
.BR _ISOC95_SOURCE ).
.TP
.B _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
.B _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.
.TP
.B _ISOC11_SOURCE
Exposes declarations consistent with the ISO C11 standard.
This macro is recognized since glibc 2.16.
.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
.UR http:\:/\:/opengroup.org\:/platform\:/lfs.html
.UE )
The alternative API consists of a set of new objects
(i.e., functions and types) whose names are suffixed with "64"
(e.g.,
.I off64_t
versus
.IR off_t ,
.BR lseek64 ()
versus
.BR lseek (),
etc.).
New programs should not employ this interface; instead
.I _FILE_OFFSET_BITS=64
should be employed.
.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 filesystem 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.
.TP
.B _BSD_SOURCE
Defining this macro with any value causes header files to expose
BSD-derived definitions.
In glibc versions up to and including 2.18,
defining this macro also causes BSD definitions to be preferred in
some situations where standards conflict, unless one or more of
.BR _SVID_SOURCE ,
.BR _POSIX_SOURCE ,
.BR _POSIX_C_SOURCE ,
.BR _XOPEN_SOURCE ,
.BR _XOPEN_SOURCE_EXTENDED ,
or
.B _GNU_SOURCE
is defined, in which case BSD definitions are disfavored.
Since glibc 2.19,
.B _BSD_SOURCE
no longer causes BSD definitions to be preferred in case of conflicts.
.TP
.B _SVID_SOURCE
Defining this macro with any value causes header files to expose
System V-derived definitions.
(SVID == System V Interface Definition; see
.BR standards (7).)
.TP
.BR _DEFAULT_SOURCE " (since glibc 2.19)"
Defining this macro provides an effect similar to
the feature test macros that are defined by default; that is:

    cc \-D_BSD_SOURCE \-D_SVID_SOURCE \-D_POSIX_C_SOURCE=200809

This macro can be defined to ensure that the "default"
definitions are provided even when the defaults would otherwise
be disabled,
as happens when individual macros are explicitly defined,
or the compiler is invoked in one of its "standard" modes (e.g.,
.IR "cc\ \-std=c99" ).
.TP
.BR _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
.BR openat (2).
Since glibc 2.10, this macro is also implicitly defined if
.BR _POSIX_C_SOURCE
is defined with a value greater than or equal to 200809L.
.TP
.B _GNU_SOURCE
Defining this macro (with any value) is equivalent to defining
.BR _BSD_SOURCE ,
.BR _SVID_SOURCE ,
.BR _ATFILE_SOURCE ,
.BR _LARGEFILE64_SOURCE ,
.BR _ISOC99_SOURCE ,
.BR _XOPEN_SOURCE_EXTENDED ,
.BR _POSIX_SOURCE ,
.B _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
.B _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.
.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
.BR _FORTIFY_SOURCE " (since glibc 2.3.4)"
.\" 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.
.SS Default definitions, implicit definitions, and combining definitions
.PP
If no feature test macros are explicitly defined,
then the following feature test macros are defined by default:
.BR _BSD_SOURCE ,
.BR _SVID_SOURCE ,
.BR _POSIX_SOURCE ,
and
.BR _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).
.PP
If any of
.BR __STRICT_ANSI__ ,
.BR _ISOC99_SOURCE ,
.BR _POSIX_SOURCE ,
.BR _POSIX_C_SOURCE  ,
.BR _XOPEN_SOURCE ,
.BR _XOPEN_SOURCE_EXTENDED ,
.BR _BSD_SOURCE ,
or
.B _SVID_SOURCE
is explicitly defined, then
.BR _BSD_SOURCE
and
.B _SVID_SOURCE
are not defined by default.

If
.B _POSIX_SOURCE
and
.B _POSIX_C_SOURCE
are not explicitly defined,
and either
.B __STRICT_ANSI__
is not defined or
.B _XOPEN_SOURCE
is defined with a value of 500 or more, then
.RS 3
.IP * 3
.B _POSIX_SOURCE
is defined with the value 1; and
.IP *
.B _POSIX_C_SOURCE
is defined with one of the following values:
.RS 6
.IP \(bu 3
2,
if
.B XOPEN_SOURCE
is defined with a value less than 500;
.IP \(bu
199506L,
if
.B XOPEN_SOURCE
is defined with a value greater than or equal to 500 and less than 600;
or
.IP \(bu
(since glibc 2.4) 200112L,
if
.B XOPEN_SOURCE
is defined with a value greater than or equal to 600 and less than 700.
.IP \(bu
(Since glibc 2.10)
200809L,
if
.B XOPEN_SOURCE
is defined with a value greater than or equal to 700.
.IP \(bu
Older versions of glibc do not know about the values
200112L and 200809L for
.BR _POSIX_C_SOURCE ,
and the setting of this macro will depend on the glibc version.
.IP \(bu
If
.B _XOPEN_SOURCE
is undefined, then the setting of
.B _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.
.RE
.RE
.PP
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 .
.B _XOPEN_SOURCE_EXTENDED
was specified by XPG4v2 (aka SUSv1).

.B _FILE_OFFSET_BITS
is not specified by any standard,
but is employed on some other implementations.

.BR _BSD_SOURCE ,
.BR _SVID_SOURCE ,
.BR _ATFILE_SOURCE ,
.BR _GNU_SOURCE ,
.BR _FORTIFY_SOURCE ,
.BR _REENTRANT ,
and
.B _THREAD_SAFE
are specific to Linux (glibc).
.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.,
.BR __USE_MISC ).
Programs should
.I never
define these macros directly:
instead, the appropriate feature test macro(s) from the
list above should be employed.
.SH EXAMPLE
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:
.in +4n
.nf

$ \fBcc ftm.c\fP
$ \fB./a.out\fP
_POSIX_SOURCE defined
_POSIX_C_SOURCE defined: 200809L
_BSD_SOURCE defined
_SVID_SOURCE defined
_ATFILE_SOURCE defined
$ \fBcc \-D_XOPEN_SOURCE=500 ftm.c\fP
$ \fB./a.out\fP
_POSIX_SOURCE defined
_POSIX_C_SOURCE defined: 199506L
_XOPEN_SOURCE defined: 500
$ \fBcc \-D_GNU_SOURCE ftm.c\fP
$ \fB./a.out\fP
_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
.fi
.in
.SS Program source
\&
.nf
/* 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 _ISOC11_SOURCE
    printf("_ISOC11_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);
}
.fi
.SH SEE ALSO
.BR libc (7),
.BR standards (7)

The section "Feature Test Macros" under
.IR "info libc" .
.\" But beware: the info libc document is out of date (Jul 07, mtk)

.I /usr/include/features.h