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
<features.h>
header file.
In order to be effective, a feature test macro
must be defined before including any header files.
This can either be done in the compilation command
(cc -DMACRO=value)
or by defining the macro within the source code before
including any headers.
Linux/glibc understands the following feature test macros:
_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).
_POSIX_C_SOURCE
Defining this obsolete macro with any value is equivalent to defining
_POSIX_C_SOURCE
with the value 1.
_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 base specification
plus the XSI extension) and C 99 definitions.
_XOPEN_SOURCE_EXTENDED
If this macro is defined with the value 1, and the
_XOPEN_SOURCE
is defined, then expose definitions corresponding to the XPG4v2
UNIX extensions.
_ISOC99_SOURCE
Exposes C 99 extensions to ISO C (1990).
_LARGEFILE64_SOURCE
Expose definitions for the alternative API specified by the
LFS (Large File Summit) as a "transitional extension" the
Single UNIX Specification.
_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.
_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.
_SVID_SOURCE
Defining this macro with any value cause header files to expose
System V-derived definitions.
(SVID == System V Interface Definition.)
_GNU_SOURCE
Defining this macro (with any value) is equivalent to defining
_BSD_SOURCE,
_SVID_SOURCE,
_LARGEFILE64_SOURCE,
_ISOC99_SOURCE_POSIX_C_SOURCE
with the value 1999506,
and
_XOPEN_SOURCE
with the value 600.
In addition, various GNU-specific extensions are also exposed.
When
gcc(1)
is invoked, then the following macros are defined by default:
_BSD_SOURCE,
_SVID_SOURCE,
_POSIX_SOURCE,
and
POSIX_C_SOURCE=199506.
If individual macros are defined, then other macros are disabled
unless they are also explicitly defined. (Exception:
if
POSIX_C_SOURCE
is not otherwise defined,
then it is always defined with the value 199506,
unless the compiler is invoked in one of is standard modes, e.g., the
-std=c99
flag.)
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,
and
_GNU_SOURCE
are Linux (glibc) specific.
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.
