Ubuntu Manpage: feature_test_macros

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NAME

       feature_test_macros - feature test macros

DESCRIPTION

Feature test macros allow the programmer to control the definitions that are exposed by system header
files when a program is compiled.

NOTE: In order to be effective, a feature test macro must be defined before including any header files.
This can be done either in the compilation command (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 _GNU_SOURCE macro may have no effect because the header <abc.h>
itself includes <xyz.h> (POSIX explicitly allows this):

#include <abc.h>
#define _GNU_SOURCE
#include <xyz.h>

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 <features.h> header file. Note: applications do not need to directly include <features.h>; indeed,
doing so is actively discouraged. See NOTES.

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 acct(2) manual page):

#include <unistd.h>

int acct(const char *filename);

Feature Test Macro Requirements for glibc (see
feature_test_macros(7)):

acct(): _BSD_SOURCE || (_XOPEN_SOURCE && _XOPEN_SOURCE < 500)

The || means that in order to obtain the declaration of acct(2) from <unistd.h>, either of the following
macro definitions must be made before including any header files:

#define _BSD_SOURCE
#define _XOPEN_SOURCE /* or any value < 500 */

Alternatively, equivalent definitions can be included in the compilation command:

cc -D_BSD_SOURCE
cc -D_XOPEN_SOURCE # 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
#define _FILE_OFFSET_BITS 64
#include <fcntl.h>

ssize_t readahead(int fd, off_t *offset, size_t count);

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.

Feature test macros understood by glibc
The paragraphs below explain how feature test macros are handled in glibc 2.x, x > 0.

First, though, a summary of a few details for the impatient:

• The macros that you most likely need to use in modern source code are _POSIX_C_SOURCE (for definitions
from various versions of POSIX.1), _XOPEN_SOURCE (for definitions from various versions of SUS),
_GNU_SOURCE (for GNU and/or Linux specific stuff), and _DEFAULT_SOURCE (to get definitions that would
normally be provided by default).

• 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.

• Defining _XOPEN_SOURCE with a value of 600 or greater produces the same effects as defining
_POSIX_C_SOURCE with a value of 200112L or greater. Where one sees

_POSIX_C_SOURCE >= 200112L

in the feature test macro requirements in the SYNOPSIS of a man page, it is implicit that the
following has the same effect:

_XOPEN_SOURCE >= 600

• Defining _XOPEN_SOURCE with a value of 700 or greater produces the same effects as defining
_POSIX_C_SOURCE with a value of 200809L or greater. Where one sees

_POSIX_C_SOURCE >= 200809L

in the feature test macro requirements in the SYNOPSIS of a man page, it is implicit that the
following has the same effect:

_XOPEN_SOURCE >= 700

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 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 _ISOC99_SOURCE).

• (Since glibc 2.10) The value 200809L or greater additionally exposes definitions corresponding
to the POSIX.1-2008 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.

Since this macro is obsolete, its usage is generally not documented when discussing feature test
macro requirements in the man pages.

_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.

• (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).

If __STRICT_ANSI__ is not defined, or _XOPEN_SOURCE is defined with a value greater than or equal
to 500 and neither _POSIX_SOURCE nor _POSIX_C_SOURCE is explicitly defined, then the following
macros are implicitly defined:

• _POSIX_SOURCE is defined with the value 1.

• _POSIX_C_SOURCE is defined, according to the value of _XOPEN_SOURCE:

_XOPEN_SOURCE < 500
_POSIX_C_SOURCE is defined with the value 2.

500 <= _XOPEN_SOURCE < 600
_POSIX_C_SOURCE is defined with the value 199506L.

600 <= _XOPEN_SOURCE < 700
_POSIX_C_SOURCE is defined with the value 200112L.

700 <= _XOPEN_SOURCE (since glibc 2.10)
_POSIX_C_SOURCE is defined with the value 200809L.

In addition, defining _XOPEN_SOURCE with a value of 500 or greater produces the same effects as
defining _XOPEN_SOURCE_EXTENDED.

_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). Defining _XOPEN_SOURCE with a value of 500 or more
also produces the same effect as defining _XOPEN_SOURCE_EXTENDED. Use of _XOPEN_SOURCE_EXTENDED
in new source code should be avoided.

Since defining _XOPEN_SOURCE with a value of 500 or more has the same effect as defining
_XOPEN_SOURCE_EXTENDED, the latter (obsolete) feature test macro is generally not described in the
SYNOPSIS in man pages.

_ISOC99_SOURCE (since glibc 2.1.3)
Exposes declarations consistent with the ISO C99 standard.

Earlier glibc 2.1.x versions recognized an equivalent macro named _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.

Defining _ISOC99_SOURCE also exposes ISO C (1990) Amendment 1 ("C95") definitions. (The primary
change in C95 was support for international character sets.)

Invoking the C compiler with the option -std=c99 produces the same effects as defining this macro.

_ISOC11_SOURCE (since glibc 2.16)
Exposes declarations consistent with the ISO C11 standard. Defining this macro also enables C99
and C95 features (like _ISOC99_SOURCE).

Invoking the C compiler with the option -std=c11 produces the same effects as defining this macro.

_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⟩.) 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 macro; instead _FILE_OFFSET_BITS=64 should be employed.

_LARGEFILE_SOURCE
This macro was historically used to expose certain functions (specifically fseeko(3) and
ftello(3)) that address limitations of earlier APIs (fseek(3) and ftell(3)) that use long for file
offsets. This macro is implicitly defined if _XOPEN_SOURCE is defined with a value greater than
or equal to 500. New programs should not employ this macro; defining _XOPEN_SOURCE as just
described or defining _FILE_OFFSET_BITS with the value 64 is the preferred mechanism to achieve
the same result.

_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 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.)

64-bit systems naturally permit file sizes greater than 2 Gigabytes, and on those systems this
macro has no effect.

_TIME_BITS
Defining this macro with the value 64 changes the width of time_t(3type) to 64-bit which allows
handling of timestamps beyond 2038. It is closely related to _FILE_OFFSET_BITS and depending on
implementation, may require it set. This macro is available as of glibc 2.34.

_BSD_SOURCE (deprecated since glibc 2.20)
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 _SVID_SOURCE,
_POSIX_SOURCE, _POSIX_C_SOURCE, _XOPEN_SOURCE, _XOPEN_SOURCE_EXTENDED, or _GNU_SOURCE is defined,
in which case BSD definitions are disfavored. Since glibc 2.19, _BSD_SOURCE no longer causes BSD
definitions to be preferred in case of conflicts.

Since glibc 2.20, this macro is deprecated. It now has the same effect as defining
_DEFAULT_SOURCE, but generates a compile-time warning (unless _DEFAULT_SOURCE is also defined).
Use _DEFAULT_SOURCE instead. To allow code that requires _BSD_SOURCE in glibc 2.19 and earlier
and _DEFAULT_SOURCE in glibc 2.20 and later to compile without warnings, define both _BSD_SOURCE
and _DEFAULT_SOURCE.

_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 standards(7).)

Since glibc 2.20, this macro is deprecated in the same fashion as _BSD_SOURCE.

_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., cc -std=c99). Defining
_DEFAULT_SOURCE without defining other individual macros or invoking the compiler in one of its
"standard" modes has no effect.

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:

cc -D_BSD_SOURCE -D_SVID_SOURCE -D_POSIX_C_SOURCE=200809

_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). Since glibc 2.10, this macro is also implicitly
defined if _POSIX_C_SOURCE is defined with a value greater than or equal to 200809L.

_GNU_SOURCE
Defining this macro (with any value) implicitly defines _ATFILE_SOURCE, _LARGEFILE64_SOURCE,
_ISOC99_SOURCE, _XOPEN_SOURCE_EXTENDED, _POSIX_SOURCE, _POSIX_C_SOURCE with the value 200809L
(200112L before glibc 2.10; 199506L before glibc 2.5; 199309L before glibc 2.1) and _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.

Since glibc 2.19, defining _GNU_SOURCE also has the effect of implicitly defining _DEFAULT_SOURCE.
Before glibc 2.20, defining _GNU_SOURCE also had the effect of implicitly defining _BSD_SOURCE and
_SVID_SOURCE.

_REENTRANT
Historically, on various C libraries it was necessary to define this macro in all multithreaded
code. (Some C libraries may still require this.) In glibc, this macro also exposed definitions
of certain reentrant functions.

However, glibc has been thread-safe by default for many years; since glibc 2.3, the only effect of
defining _REENTRANT has been to enable one or two of the same declarations that are also enabled
by defining _POSIX_C_SOURCE with a value of 199606L or greater.

_REENTRANT is now obsolete. In glibc 2.25 and later, defining _REENTRANT is equivalent to
defining _POSIX_C_SOURCE with the value 199606L. If a higher POSIX conformance level is selected
by any other means (such as _POSIX_C_SOURCE itself, _XOPEN_SOURCE, _DEFAULT_SOURCE, or
_GNU_SOURCE), then defining _REENTRANT has no effect.

This macro is automatically defined if one compiles with cc -pthread.

_THREAD_SAFE
Synonym for the (deprecated) _REENTRANT, provided for compatibility with some other
implementations.

_FORTIFY_SOURCE (since glibc 2.3.4)
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, memcpy(3),
memset(3), stpcpy(3), strcpy(3), strncpy(3), strcat(3), strncat(3), sprintf(3), snprintf(3),
vsprintf(3), vsnprintf(3), gets(3), and wide character variants thereof). For some functions,
argument consistency is checked; for example, a check is made that open(2) has been supplied with
a mode argument when the specified flags include O_CREAT. Not all problems are detected, just
some common cases.

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 (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.

With _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 malloc(3)'s size argument is variable can now be fortified.

Use of this macro requires compiler support, available since gcc 4.0 and clang 2.6. Use of
_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.

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 (in glibc 2.19 and earlier), _SVID_SOURCE (in glibc 2.19 and earlier),
_DEFAULT_SOURCE (since glibc 2.19), _POSIX_SOURCE, and _POSIX_C_SOURCE=200809L (200112L before glibc
2.10; 199506L before glibc 2.4; 199309L before glibc 2.1).

If any of __STRICT_ANSI__, _ISOC99_SOURCE, _ISOC11_SOURCE (since glibc 2.18), _POSIX_SOURCE,
_POSIX_C_SOURCE, _XOPEN_SOURCE, _XOPEN_SOURCE_EXTENDED (in glibc 2.11 and earlier), _BSD_SOURCE (in glibc
2.19 and earlier), or _SVID_SOURCE (in glibc 2.19 and earlier) is explicitly defined, then _BSD_SOURCE,
_SVID_SOURCE, and _DEFAULT_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

• (since glibc 2.4) 200112L, if _XOPEN_SOURCE is defined with a value greater than or equal to 600
and less than 700.

• (Since glibc 2.10) 200809L, if _XOPEN_SOURCE is defined with a value greater than or equal to 700.

• Older versions of glibc do not know about the values 200112L and 200809L for _POSIX_C_SOURCE, and
the setting of this macro will depend on the glibc version.

• If _XOPEN_SOURCE is undefined, then the setting of _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.

Multiple macros can be defined; the results are additive.

STANDARDS

       POSIX.1 specifies _POSIX_C_SOURCE, _POSIX_SOURCE, and _XOPEN_SOURCE.

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

       _BSD_SOURCE, _SVID_SOURCE, _DEFAULT_SOURCE, _ATFILE_SOURCE, _GNU_SOURCE, _FORTIFY_SOURCE, _REENTRANT, and
       _THREAD_SAFE are specific to glibc.

HISTORY

       _XOPEN_SOURCE_EXTENDED was specified by XPG4v2 (aka SUSv1), but is not present in SUSv2 and later.

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, <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  never  define  these  macros directly: instead, the
       appropriate feature test macro(s) from the list above should be employed.

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:

           $ cc ftm.c
           $ ./a.out
           _POSIX_SOURCE defined
           _POSIX_C_SOURCE defined: 200809L
           _BSD_SOURCE defined
           _SVID_SOURCE defined
           _ATFILE_SOURCE defined
           $ cc -D_XOPEN_SOURCE=500 ftm.c
           $ ./a.out
           _POSIX_SOURCE defined
           _POSIX_C_SOURCE defined: 199506L
           _XOPEN_SOURCE defined: 500
           $ cc -D_GNU_SOURCE ftm.c
           $ ./a.out
           _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

   Program source

       /* 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\n");
       #endif

       #ifdef _POSIX_C_SOURCE
           printf("_POSIX_C_SOURCE defined: %jdL\n",
                   (intmax_t) _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 _TIME_BITS
           printf("_TIME_BITS defined: %d\n", _TIME_BITS);
       #endif

       #ifdef _BSD_SOURCE
           printf("_BSD_SOURCE defined\n");
       #endif

       #ifdef _SVID_SOURCE
           printf("_SVID_SOURCE defined\n");
       #endif

       #ifdef _DEFAULT_SOURCE
           printf("_DEFAULT_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);
       }