.\" This manpage is Copyright (C) 2006, Michael Kerrisk
.\"
.\" SPDX-License-Identifier: Linux-man-pages-copyleft
.\"
.TH feature_test_macros 7 2024-05-02 "Linux man-pages 6.8"
.SH NAME
feature_test_macros \- feature test macros
.SH DESCRIPTION
Feature test macros allow the programmer to control the definitions that
are exposed by system header files when a program is compiled.
.P
.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.
The requirement that the macro must be defined before including any
header file exists because header files may freely include one another.
Thus, for example, in the following lines, defining the
.B _GNU_SOURCE
macro may have no effect because the header
.I <abc.h>
itself includes
.I <xyz.h>
(POSIX explicitly allows this):
.P
.in +4n
.EX
#include <abc.h>
#define _GNU_SOURCE
#include <xyz.h>
.EE
.in
.P
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.
.P
The precise effects of each of the feature test macros described below
can be ascertained by inspecting the
.I <features.h>
header file.
.BR Note :
applications do
.I not
need to directly include
.IR <features.h> ;
indeed, doing so is actively discouraged.
See NOTES.
.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):
.P
.RS
.B #include <unistd.h>
.P
.BI "int acct(const char *" filename );
.P
.RS -4
.EX
Feature Test Macro Requirements for glibc (see
.BR feature_test_macros (7)):
.EE
.RE
.P
.BR acct ():
_BSD_SOURCE || (_XOPEN_SOURCE && _XOPEN_SOURCE < 500)
.RE
.P
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:
.P
.in +4n
.EX
#define _BSD_SOURCE
#define _XOPEN_SOURCE        /* or any value < 500 */
.EE
.in
.P
Alternatively, equivalent definitions can be included in the
compilation command:
.P
.in +4n
.EX
cc \-D_BSD_SOURCE
cc \-D_XOPEN_SOURCE           # Or any value < 500
.EE
.in
.P
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.
.P
In a few cases, manual pages use a shorthand for expressing the
feature test macro requirements (this example from
.BR readahead (2)):
.P
.RS +4
.EX
.B #define _GNU_SOURCE
.B #define _FILE_OFFSET_BITS 64
.B #include <fcntl.h>
.P
.BI "ssize_t readahead(int " fd ", off_t *" offset ", size_t " count );
.EE
.RE
.P
This format is employed when the feature test macros ensure
that the proper function declarations are visible,
and the macros are not defined by default.
.SS Feature test macros understood by glibc
The paragraphs below explain how feature test macros are handled
in glibc 2.\fIx\fP,
.I x
> 0.
.P
First, though, a summary of a few details for the impatient:
.IP \[bu] 3
The macros that you most likely need to use in modern source code are
.B _POSIX_C_SOURCE
(for definitions from various versions of POSIX.1),
.B _XOPEN_SOURCE
(for definitions from various versions of SUS),
.B _GNU_SOURCE
(for GNU and/or Linux specific stuff), and
.B _DEFAULT_SOURCE
(to get definitions that would normally be provided by default).
.IP \[bu]
Certain macros are defined with default values.
Thus, although one or more macros may be indicated as being
required in the SYNOPSIS of a man page,
it may not be necessary to define them explicitly.
Full details of the defaults are given later in this man page.
.IP \[bu]
Defining
.B _XOPEN_SOURCE
with a value of 600 or greater produces the same effects as defining
.B _POSIX_C_SOURCE
with a value of 200112L or greater.
Where one sees
.IP
.in +4n
.EX
_POSIX_C_SOURCE >= 200112L
.EE
.in
.IP
in the feature test macro requirements in the SYNOPSIS of a man page,
it is implicit that the following has the same effect:
.IP
.in +4n
.EX
_XOPEN_SOURCE >= 600
.EE
.in
.IP \[bu]
Defining
.B _XOPEN_SOURCE
with a value of 700 or greater produces the same effects as defining
.B _POSIX_C_SOURCE
with a value of 200809L or greater.
Where one sees
.IP
.in +4n
.EX
_POSIX_C_SOURCE >= 200809L
.EE
.in
.IP
in the feature test macro requirements in the SYNOPSIS of a man page,
it is implicit that the following has the same effect:
.IP
.in +4n
.EX
_XOPEN_SOURCE >= 700
.EE
.in
.\" 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.
.P
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 additionally exposes definitions corresponding
to the POSIX.1-2001 base specification (excluding the XSI extension).
This value also causes C95 (since glibc 2.12) and
C99 (since glibc 2.10) features to be exposed
(in other words, the equivalent of defining
.BR _ISOC99_SOURCE ).
.IP \[bu]
(Since glibc 2.10)
The value 200809L or greater additionally 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.
.IP
Since this macro is obsolete,
its usage is generally not documented when discussing
feature test macro requirements in the man pages.
.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
.IP
If
.B __STRICT_ANSI__
is not defined, or
.B _XOPEN_SOURCE
is defined with a value greater than or equal to 500
.I and
neither
.B _POSIX_SOURCE
nor
.B _POSIX_C_SOURCE
is explicitly defined, then
the following macros are implicitly defined:
.RS
.IP \[bu] 3
.B _POSIX_SOURCE
is defined with the value 1.
.IP \[bu]
.B _POSIX_C_SOURCE
is defined, according to the value of
.BR _XOPEN_SOURCE :
.RS
.TP
.BR _XOPEN_SOURCE " < 500"
.B _POSIX_C_SOURCE
is defined with the value 2.
.TP
.RB "500 <= " _XOPEN_SOURCE " < 600"
.B _POSIX_C_SOURCE
is defined with the value 199506L.
.TP
.RB "600 <= " _XOPEN_SOURCE " < 700"
.B _POSIX_C_SOURCE
is defined with the value 200112L.
.TP
.RB "700 <= " _XOPEN_SOURCE " (since glibc 2.10)"
.B _POSIX_C_SOURCE
is defined with the value 200809L.
.RE
.RE
.IP
In addition, defining
.B _XOPEN_SOURCE
with a value of 500 or greater produces the same effects as defining
.BR _XOPEN_SOURCE_EXTENDED .
.TP
.B _XOPEN_SOURCE_EXTENDED
If this macro is defined,
.I and
.B _XOPEN_SOURCE
is defined, then expose definitions corresponding to the XPG4v2
(SUSv1) UNIX extensions (UNIX 95).
Defining
.B _XOPEN_SOURCE
with a value of 500 or more also produces the same effect as defining
.BR _XOPEN_SOURCE_EXTENDED .
Use of
.B _XOPEN_SOURCE_EXTENDED
in new source code should be avoided.
.IP
Since defining
.B _XOPEN_SOURCE
with a value of 500 or more has the same effect as defining
.BR _XOPEN_SOURCE_EXTENDED ,
the latter (obsolete) feature test macro is generally not described in the
SYNOPSIS in man pages.
.TP
.BR _ISOC99_SOURCE " (since glibc 2.1.3)"
Exposes declarations consistent with the ISO C99 standard.
.IP
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 this macro is obsolete, glibc continues
to recognize it for backward compatibility.
.IP
Defining
.B _ISOC99_SOURCE
also exposes ISO C (1990) Amendment 1 ("C95") definitions.
(The primary change in C95 was support for international character sets.)
.IP
Invoking the C compiler with the option
.I \-std=c99
produces the same effects as defining this macro.
.TP
.BR _ISOC11_SOURCE " (since glibc 2.16)"
Exposes declarations consistent with the ISO C11 standard.
Defining this macro also enables C99 and C95 features (like
.BR _ISOC99_SOURCE ).
.IP
Invoking the C compiler with the option
.I \-std=c11
produces the same effects as defining this macro.
.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 macro; instead
.I _FILE_OFFSET_BITS=64
should be employed.
.TP
.B _LARGEFILE_SOURCE
This macro was historically used to expose certain functions (specifically
.BR fseeko (3)
and
.BR ftello (3))
that address limitations of earlier APIs
.RB ( fseek (3)
and
.BR ftell (3))
that use
.I long
for file offsets.
This macro is implicitly defined if
.B _XOPEN_SOURCE
is defined with a value greater than or equal to 500.
New programs should not employ this macro;
defining
.B _XOPEN_SOURCE
as just described or defining
.B _FILE_OFFSET_BITS
with the value 64 is the preferred mechanism to achieve the same result.
.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.
It is also useful when calling functions like
.BR copy_file_range (2)
that were added more recently and that come only in 64-bit flavors.
(Defining this macro permits correctly written programs to use
large files with only a recompilation being required.)
.IP
64-bit systems naturally permit file sizes greater than 2 Gigabytes,
and on those systems this macro has no effect.
.TP
.B _TIME_BITS
Defining this macro with the value 64
changes the width of
.BR time_t (3type)
to 64-bit which allows handling of timestamps beyond
2038.
It is closely related to
.B _FILE_OFFSET_BITS
and depending on implementation, may require it set.
This macro is available as of glibc 2.34.
.TP
.BR _BSD_SOURCE " (deprecated since glibc 2.20)"
Defining this macro with any value causes header files to expose
BSD-derived definitions.
.IP
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.
.IP
Since glibc 2.20, this macro is deprecated.
.\" commit c941736c92fa3a319221f65f6755659b2a5e0a20
.\" commit 498afc54dfee41d33ba519f496e96480badace8e
.\" commit acd7f096d79c181866d56d4aaf3b043e741f1e2c
It now has the same effect as defining
.BR _DEFAULT_SOURCE ,
but generates a compile-time warning (unless
.B _DEFAULT_SOURCE
.\" commit ade40b10ff5fa59a318cf55b9d8414b758e8df78
is also defined).
Use
.B _DEFAULT_SOURCE
instead.
To allow code that requires
.B _BSD_SOURCE
in glibc 2.19 and earlier and
.B _DEFAULT_SOURCE
in glibc 2.20 and later to compile without warnings, define
.I both
.B _BSD_SOURCE
and
.BR _DEFAULT_SOURCE .
.TP
.BR _SVID_SOURCE " (deprecated since glibc 2.20)"
Defining this macro with any value causes header files to expose
System V-derived definitions.
(SVID == System V Interface Definition; see
.BR standards (7).)
.IP
Since glibc 2.20, this macro is deprecated in the same fashion as
.BR _BSD_SOURCE .
.TP
.BR _DEFAULT_SOURCE " (since glibc 2.19)"
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 ).
Defining
.B _DEFAULT_SOURCE
without defining other individual macros
or invoking the compiler in one of its "standard" modes has no effect.
.IP
The "default" definitions comprise those required by POSIX.1-2008 and ISO C99,
as well as various definitions originally derived from BSD and System V.
On glibc 2.19 and earlier, these defaults were approximately equivalent
to explicitly defining the following:
.IP
.in +4n
.EX
cc \-D_BSD_SOURCE \-D_SVID_SOURCE \-D_POSIX_C_SOURCE=200809
.EE
.in
.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
.B _POSIX_C_SOURCE
is defined with a value greater than or equal to 200809L.
.TP
.B _GNU_SOURCE
Defining this macro (with any value) implicitly defines
.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 before glibc 2.10;
199506L before glibc 2.5;
199309L before glibc 2.1)
and
.B _XOPEN_SOURCE
with the value 700
(600 before glibc 2.10;
500 before glibc 2.2).
In addition, various GNU-specific extensions are also exposed.
.IP
Since glibc 2.19, defining
.B _GNU_SOURCE
also has the effect of implicitly defining
.BR _DEFAULT_SOURCE .
Before glibc 2.20, defining
.B _GNU_SOURCE
also had the effect of implicitly defining
.B _BSD_SOURCE
and
.BR _SVID_SOURCE .
.TP
.B _REENTRANT
Historically, on various C libraries
it was necessary to define this macro in all
multithreaded code.
.\" Zack Weinberg
.\"     There did once exist C libraries where it was necessary. The ones
.\"     I remember were proprietary Unix vendor libcs from the mid-1990s
.\"     You would get completely unlocked stdio without _REENTRANT.
(Some C libraries may still require this.)
In glibc,
this macro also exposed definitions of certain reentrant functions.
.IP
However, glibc has been thread-safe by default for many years;
since glibc 2.3, the only effect of defining
.B _REENTRANT
has been to enable one or two of the same declarations that
are also enabled by defining
.B _POSIX_C_SOURCE
with a value of 199606L or greater.
.IP
.B _REENTRANT
is now obsolete.
In glibc 2.25 and later, defining
.B _REENTRANT
is equivalent to defining
.B _POSIX_C_SOURCE
with the value 199606L.
If a higher POSIX conformance level is
selected by any other means (such as
.B _POSIX_C_SOURCE
itself,
.BR _XOPEN_SOURCE ,
.BR _DEFAULT_SOURCE ,
or
.BR _GNU_SOURCE ),
then defining
.B _REENTRANT
has no effect.
.IP
This macro is automatically defined if one compiles with
.IR cc\~\-pthread .
.TP
.B _THREAD_SAFE
Synonym for the (deprecated)
.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 (for example,
.BR memcpy (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),
.BR gets (3),
and wide character variants thereof).
For some functions, argument consistency is checked;
for example, a check is made that
.BR open (2)
has been supplied with a
.I mode
argument when the specified flags include
.BR O_CREAT .
Not all problems are detected, just some common cases.
.\" Look for __USE_FORTIFY_LEVEL in the header files
.IP
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.
.\" For example, given the following code
.\"        int d;
.\"        char buf[1000], buf[1000];
.\"        strcpy(fmt, "Hello world\n%n");
.\"        snprintf(buf, sizeof(buf), fmt, &d);
.\"
.\" Compiling with "gcc -D_FORTIFY_SOURCE=2 -O1" and then running will
.\" cause the following diagnostic at run time at the snprintf() call
.\"
.\"        *** %n in writable segment detected ***
.\"        Aborted (core dumped)
.\"
.IP
Some of the checks can be performed at compile time
(via macros logic implemented in header files),
and result in compiler warnings;
other checks take place at run time,
and result in a run-time error if the check fails.
.IP
With
.B _FORTIFY_SOURCE
set to 3, additional checking is added to intercept
some function calls used with an argument of variable size
where the compiler can deduce an upper bound for its value.
For example, a program where
.BR malloc (3)'s
size argument is variable
can now be fortified.
.IP
Use of this macro requires compiler support, available since
gcc 4.0 and clang 2.6.
Use of
.B _FORTIFY_SOURCE
set to 3 requires gcc 12.0 or later, or clang 9.0 or later,
in conjunction with glibc 2.33 or later.
.\" glibc is not an absolute requirement (gcc has libssp; NetBSD/newlib
.\" and Darwin each have their own implementation), but let's keep it
.\" simple.
.SS 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:
.B _BSD_SOURCE
(in glibc 2.19 and earlier),
.B _SVID_SOURCE
(in glibc 2.19 and earlier),
.B _DEFAULT_SOURCE
(since glibc 2.19),
.BR _POSIX_SOURCE ,
and
.BR _POSIX_C_SOURCE =200809L
(200112L before glibc 2.10;
199506L before glibc 2.4;
199309L before glibc 2.1).
.P
If any of
.BR __STRICT_ANSI__ ,
.BR _ISOC99_SOURCE ,
.B _ISOC11_SOURCE
(since glibc 2.18),
.BR _POSIX_SOURCE ,
.BR _POSIX_C_SOURCE  ,
.BR _XOPEN_SOURCE ,
.B _XOPEN_SOURCE_EXTENDED
(in glibc 2.11 and earlier),
.B _BSD_SOURCE
(in glibc 2.19 and earlier),
or
.B _SVID_SOURCE
(in glibc 2.19 and earlier)
is explicitly defined, then
.BR _BSD_SOURCE ,
.BR _SVID_SOURCE ,
and
.B _DEFAULT_SOURCE
are not defined by default.
.P
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
.IP \[bu] 3
.B _POSIX_SOURCE
is defined with the value 1; and
.IP \[bu]
.B _POSIX_C_SOURCE
is defined with one of the following values:
.RS
.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, before glibc 2.4;
200112L, since glibc 2.4 to glibc 2.9; and
200809L, since glibc 2.10.
.RE
.P
Multiple macros can be defined; the results are additive.
.SH STANDARDS
POSIX.1 specifies
.BR _POSIX_C_SOURCE ,
.BR _POSIX_SOURCE ,
and
.BR _XOPEN_SOURCE .
.P
.B _FILE_OFFSET_BITS
is not specified by any standard,
but is employed on some other implementations.
.P
.BR _BSD_SOURCE ,
.BR _SVID_SOURCE ,
.BR _DEFAULT_SOURCE ,
.BR _ATFILE_SOURCE ,
.BR _GNU_SOURCE ,
.BR _FORTIFY_SOURCE ,
.BR _REENTRANT ,
and
.B _THREAD_SAFE
are specific to glibc.
.SH HISTORY
.B _XOPEN_SOURCE_EXTENDED
was specified by XPG4v2 (aka SUSv1), but is not present in SUSv2 and later.
.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.
.P
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 EXAMPLES
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:
.P
.in +4n
.EX
$ \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
.EE
.in
.SS Program source
\&
.EX
/* ftm.c */
\&
#include <stdint.h>
#include <stdio.h>
#include <unistd.h>
#include <stdlib.h>
\&
int
main(int argc, char *argv[])
{
#ifdef _POSIX_SOURCE
    printf("_POSIX_SOURCE defined\en");
#endif
\&
#ifdef _POSIX_C_SOURCE
    printf("_POSIX_C_SOURCE defined: %jdL\en",
            (intmax_t) _POSIX_C_SOURCE);
#endif
\&
#ifdef _ISOC99_SOURCE
    printf("_ISOC99_SOURCE defined\en");
#endif
\&
#ifdef _ISOC11_SOURCE
    printf("_ISOC11_SOURCE defined\en");
#endif
\&
#ifdef _XOPEN_SOURCE
    printf("_XOPEN_SOURCE defined: %d\en", _XOPEN_SOURCE);
#endif
\&
#ifdef _XOPEN_SOURCE_EXTENDED
    printf("_XOPEN_SOURCE_EXTENDED defined\en");
#endif
\&
#ifdef _LARGEFILE64_SOURCE
    printf("_LARGEFILE64_SOURCE defined\en");
#endif
\&
#ifdef _FILE_OFFSET_BITS
    printf("_FILE_OFFSET_BITS defined: %d\en", _FILE_OFFSET_BITS);
#endif
\&
#ifdef _TIME_BITS
    printf("_TIME_BITS defined: %d\en", _TIME_BITS);
#endif
\&
#ifdef _BSD_SOURCE
    printf("_BSD_SOURCE defined\en");
#endif
\&
#ifdef _SVID_SOURCE
    printf("_SVID_SOURCE defined\en");
#endif
\&
#ifdef _DEFAULT_SOURCE
    printf("_DEFAULT_SOURCE defined\en");
#endif
\&
#ifdef _ATFILE_SOURCE
    printf("_ATFILE_SOURCE defined\en");
#endif
\&
#ifdef _GNU_SOURCE
    printf("_GNU_SOURCE defined\en");
#endif
\&
#ifdef _REENTRANT
    printf("_REENTRANT defined\en");
#endif
\&
#ifdef _THREAD_SAFE
    printf("_THREAD_SAFE defined\en");
#endif
\&
#ifdef _FORTIFY_SOURCE
    printf("_FORTIFY_SOURCE defined\en");
#endif
\&
    exit(EXIT_SUCCESS);
}
.EE
.SH SEE ALSO
.BR libc (7),
.BR standards (7),
.BR system_data_types (7)
.P
The section "Feature Test Macros" under
.IR "info libc" .
.\" But beware: the info libc document is out of date (Jul 07, mtk)
.P
.I /usr/include/features.h