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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=c89' 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 section 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/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.)
GCC also has some limited support for traditional (pre-ISO) C with the `-traditional' option. This support may be of use for compiling some very old programs that have not been updated to ISO C, but should not be used for new programs. It will not work with some modern C libraries such as the GNU C library.
By default, GCC provides some extensions to the C language that on rare occasions conflict with the C standard. See section 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=gnu89' (for C89 with GNU extensions) or `-std=gnu99' (for C99 with GNU extensions). The default, if no C language dialect options are given, is `-std=gnu89'; 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 C89 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 <float.h>, <limits.h>,
<stdarg.h>, and <stddef.h>; since AMD1, also those in
<iso646.h>; and in C99, also those in <stdbool.h> and
<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.
GNU CC 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
implementation, defining __STDC_HOSTED__ as 1 and
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
__STDC_HOSTED__ to 0 and not make assumptions about the
meanings of function names from the standard library. To build an OS
kernel, you may well still need to make your own arrangements for
linking and startup. See section Options Controlling C Dialect.
GNU CC 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 section Standard Libraries.
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
See section `The GNU Fortran Language' in Using and Porting GNU Fortran, for details of the Fortran language supported by GCC.
See section `Language Definition References' in GNU Chill, for details of the CHILL standard.
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