Ubuntu Manpage: feature_test_macros

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description
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example
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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.

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.

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.

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
#include <fcntl.h>

ssize_t readahead(int fd, off64_t *offset, size_t count);

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.

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

Linux 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 exposes definitions corresponding to the
POSIX.1-2001 base specification (excluding the XSI extension).

• (Since glibc 2.10) The value 200809L or greater 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.

_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).

_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). This macro is also implicitly defined if
_XOPEN_SOURCE is defined with a value of 500 or more.

_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, _ISOC99_SOURCE implies
_ISOC95_SOURCE).

_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 _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.

_ISOC11_SOURCE
Exposes declarations consistent with the ISO C11 standard. This macro is recognized since glibc
2.16.

_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 interface; instead _FILE_OFFSET_BITS=64 should be employed.

_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.

_BSD_SOURCE
Defining this macro with any value causes header files to expose BSD-derived definitions.
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.

_SVID_SOURCE
Defining this macro with any value causes header files to expose System V-derived definitions.
(SVID == System V Interface Definition; see standards(7).)

_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) is equivalent to defining _BSD_SOURCE, _SVID_SOURCE,
_ATFILE_SOURCE, _LARGEFILE64_SOURCE, _ISOC99_SOURCE, _XOPEN_SOURCE_EXTENDED, _POSIX_SOURCE,
_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 _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.

_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 (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. Not all buffer overflows
are detected, just some common cases. In the current implementation, checks are added for calls
to memcpy(3), mempcpy(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.

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, _SVID_SOURCE, _POSIX_SOURCE, and _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).

If any of __STRICT_ANSI__, _ISOC99_SOURCE, _POSIX_SOURCE, _POSIX_C_SOURCE, _XOPEN_SOURCE,
_XOPEN_SOURCE_EXTENDED, _BSD_SOURCE, or _SVID_SOURCE is explicitly defined, then _BSD_SOURCE, and
_SVID_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, in glibc versions before 2.4; 200112L, in glibc 2.4 to 2.9; and 200809L,
since glibc 2.10.

Multiple macros can be defined; the results are additive.

CONFORMING TO

       POSIX.1  specifies  _POSIX_C_SOURCE,  _POSIX_SOURCE,  and  _XOPEN_SOURCE.    _XOPEN_SOURCE_EXTENDED   was
       specified by XPG4v2 (aka SUSv1).

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

       _BSD_SOURCE, _SVID_SOURCE, _ATFILE_SOURCE, _GNU_SOURCE, _FORTIFY_SOURCE, _REENTRANT, and _THREAD_SAFE are
       specific to Linux (glibc).

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.

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:

           $ 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 <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 _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);
       }

COLOPHON

       This  page  is  part  of  release 3.54 of the Linux man-pages project.  A description of the project, and
       information about reporting bugs, can be found at http://www.kernel.org/doc/man-pages/.