Chapter 2: Language Standards Supported by GCC 5
2 Language Standards Supported by GCC
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 s ec tions of the ANSI
standard were renumbered and became clauses in the ISO standard. This standard, in
both its forms, is comm only 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 3.4 [Options Controlling C Dialect], page 21.
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-3.4/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 a Technical Corrigendum published
in 2001. GCC does not support the uncorrected version.
By default, GCC provides some extensions to the C language that on rare occasions con-
flict with the C standard. See Chapter 5 [Extensions to the C Language Family], page 179.
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 fa-
cilities; 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 ad-
dition, complex types, added in C99, are not required for freestanding implementations. The
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