For each language compiled by GCC for which there is a standard, GCC attempts to follow one or more versions of that standard, possibly with some exceptions, and possibly with some extensions.
GCC supports three versions of the C standard, although support for the most recent version is not yet complete.
The original ANSI C standard (X3.159-1989) was ratified in 1989 and published in 1990. This standard was ratified as an ISO standard (ISO/IEC 9899:1990) later in 1990. There were no technical differences between these publications, although the sections of the ANSI standard were renumbered and became clauses in the ISO standard. This standard, in both its forms, is commonly known as C89, or occasionally as C90, from the dates of ratification. The ANSI standard, but not the ISO standard, also came with a Rationale document. To select this standard in GCC, use one of the options -ansi, -std=c90 or -std=iso9899:1990; to obtain all the diagnostics required by the standard, you should also specify -pedantic (or -pedantic-errors if you want them to be errors rather than warnings). See Options Controlling C Dialect.
Errors in the 1990 ISO C standard were corrected in two Technical Corrigenda published in 1994 and 1996. GCC does not support the uncorrected version.
An amendment to the 1990 standard was published in 1995. This
amendment added digraphs and
__STDC_VERSION__ to the language,
but otherwise concerned the library. This amendment is commonly known
as AMD1; the amended standard is sometimes known as C94 or
C95. To select this standard in GCC, use the option
-std=iso9899:199409 (with, as for other standard versions,
-pedantic to receive all required diagnostics).
A new edition of the ISO C standard was published in 1999 as ISO/IEC 9899:1999, and is commonly known as C99. GCC has incomplete support for this standard version; see http://gcc.gnu.org/gcc-4.6/c99status.html for details. To select this standard, use -std=c99 or -std=iso9899:1999. (While in development, drafts of this standard version were referred to as C9X.)
Errors in the 1999 ISO C standard were corrected in three Technical Corrigenda published in 2001, 2004 and 2007. GCC does not support the uncorrected version.
A fourth version of the C standard, known as C1X, is under development; GCC has limited preliminary support for parts of this standard, enabled with -std=c1x.
By default, GCC provides some extensions to the C language that on rare occasions conflict with the C standard. See Extensions to the C Language Family. Use of the -std options listed above will disable these extensions where they conflict with the C standard version selected. You may also select an extended version of the C language explicitly with -std=gnu90 (for C90 with GNU extensions), -std=gnu99 (for C99 with GNU extensions) or -std=gnu1x (for C1X with GNU extensions). The default, if no C language dialect options are given, is -std=gnu90; this will change to -std=gnu99 in some future release when the C99 support is complete. Some features that are part of the C99 standard are accepted as extensions in C90 mode.
The ISO C standard defines (in clause 4) two classes of conforming
implementation. A conforming hosted implementation supports the
whole standard including all the library facilities; a conforming
freestanding implementation is only required to provide certain
library facilities: those in
<stddef.h>; since AMD1, also those in
<iso646.h>; and in C99, also those in
<stdint.h>. In addition, complex types, added in C99, are not
required for freestanding implementations. The standard also defines
two environments for programs, a freestanding environment,
required of all implementations and which may not have library
facilities beyond those required of freestanding implementations,
where the handling of program startup and termination are
implementation-defined, and a hosted environment, which is not
required, in which all the library facilities are provided and startup
is through a function
int main (void) or
int main (int,
char *). An OS kernel would be a freestanding environment; a
program using the facilities of an operating system would normally be
in a hosted implementation.
GCC aims towards being usable as a conforming freestanding
implementation, or as the compiler for a conforming hosted
implementation. By default, it will act as the compiler for a hosted
presuming that when the names of ISO C functions are used, they have
the semantics defined in the standard. To make it act as a conforming
freestanding implementation for a freestanding environment, use the
option -ffreestanding; it will then define
0 and not make assumptions about the
meanings of function names from the standard library, with exceptions
noted below. To build an OS kernel, you may well still need to make
your own arrangements for linking and startup.
See Options Controlling C Dialect.
GCC does not provide the library facilities required only of hosted implementations, nor yet all the facilities required by C99 of freestanding implementations; to use the facilities of a hosted environment, you will need to find them elsewhere (for example, in the GNU C library). See Standard Libraries.
Most of the compiler support routines used by GCC are present in
libgcc, but there are a few exceptions. GCC requires the
freestanding environment provide
__builtin_trap is used, and the target does
not implement the
trap pattern, then GCC will emit a call
For references to Technical Corrigenda, Rationale documents and information concerning the history of C that is available online, see http://gcc.gnu.org/readings.html
GCC supports the ISO C++ standard (1998) and contains experimental support for the upcoming ISO C++ standard (200x).
The original ISO C++ standard was published as the ISO standard (ISO/IEC
14882:1998) and amended by a Technical Corrigenda published in 2003
(ISO/IEC 14882:2003). These standards are referred to as C++98 and
C++03, respectively. GCC implements the majority of C++98 (
is a notable exception) and most of the changes in C++03. To select
this standard in GCC, use one of the options -ansi or
-std=c++98; to obtain all the diagnostics required by the
standard, you should also specify -pedantic (or
-pedantic-errors if you want them to be errors rather than
The ISO C++ committee is working on a new ISO C++ standard, dubbed C++0x, that is intended to be published by 2009. C++0x contains several changes to the C++ language, some of which have been implemented in an experimental C++0x mode in GCC. The C++0x mode in GCC tracks the draft working paper for the C++0x standard; the latest working paper is available on the ISO C++ committee's web site at http://www.open-std.org/jtc1/sc22/wg21/. For information regarding the C++0x features available in the experimental C++0x mode, see http://gcc.gnu.org/projects/cxx0x.html. To select this standard in GCC, use the option -std=c++0x; to obtain all the diagnostics required by the standard, you should also specify -pedantic (or -pedantic-errors if you want them to be errors rather than warnings).
By default, GCC provides some extensions to the C++ language; See Options Controlling C++ Dialect. Use of the -std option listed above will disable these extensions. You may also select an extended version of the C++ language explicitly with -std=gnu++98 (for C++98 with GNU extensions) or -std=gnu++0x (for C++0x with GNU extensions). The default, if no C++ language dialect options are given, is -std=gnu++98.
GCC supports “traditional” Objective-C (also known as “Objective-C 1.0”) and contains support for the Objective-C exception and synchronization syntax. It has also support for a number of “Objective-C 2.0” language extensions, including properties, fast enumeration (only for Objective-C), method attributes and the @optional and @required keywords in protocols. GCC supports Objective-C++ and features available in Objective-C are also available in Objective-C++.
GCC by default uses the GNU Objective-C runtime library, which is part of GCC and is not the same as the Apple/NeXT Objective-C runtime library used on Apple systems. There are a number of differences documented in this manual. The options -fgnu-runtime and -fnext-runtime allow you to switch between producing output that works with the GNU Objective-C runtime library and output that works with the Apple/NeXT Objective-C runtime library.
There is no formal written standard for Objective-C or Objective-C++. The authoritative manual on traditional Objective-C (1.0) is “Object-Oriented Programming and the Objective-C Language”, available at a number of web sites:
The Objective-C exception and synchronization syntax (that is, the keywords @try, @throw, @catch, @finally and @synchronized) is supported by GCC and is enabled with the option -fobjc-exceptions. The syntax is briefly documented in this manual and in the Objective-C 2.0 manuals from Apple.
The Objective-C 2.0 language extensions and features are automatically enabled; they include properties (via the @property, @synthesize and @dynamic keywords), fast enumeration (not available in Objective-C++), attributes for methods (such as deprecated, noreturn, sentinel, format), the unused attribute for method arguments, the @package keyword for instance variables and the @optional and @required keywords in protocols. You can disable all these Objective-C 2.0 language extensions with the option -fobjc-std=objc1, which causes the compiler to recognize the same Objective-C language syntax recognized by GCC 4.0, and to produce an error if one of the new features is used.
GCC has currently no support for non-fragile instance variables.
The authoritative manual on Objective-C 2.0 is available from Apple:
For more information concerning the history of Objective-C that is available online, see http://gcc.gnu.org/readings.html
The Go language continues to evolve as of this writing; see the current language specifications. At present there are no specific versions of Go, and there is no way to describe the language supported by GCC in terms of a specific version. In general GCC tracks the evolving specification closely, and any given release will support the language as of the date that the release was frozen.
See GNAT Reference Manual, for information on standard conformance and compatibility of the Ada compiler.
See Standards, for details of standards supported by GNU Fortran.
See Compatibility with the Java Platform, for details of compatibility between gcj and the Java Platform.