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@c Copyright (C) 2000-2016 Free Software Foundation, Inc.
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@c This is part of the GCC manual.
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@c For copying conditions, see the file gcc.texi.
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@chapter Language Standards Supported by GCC
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For each language compiled by GCC for which there is a standard, GCC
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attempts to follow one or more versions of that standard, possibly
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with some exceptions, and possibly with some extensions.
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@cindex ANSI C standard
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@cindex ANSI X3.159-1989
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@cindex ISO C standard
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@cindex Technical Corrigenda
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@cindex Technical Corrigendum 1
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@cindex Technical Corrigendum 2
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@cindex Technical Corrigendum 3
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@cindex freestanding implementation
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@cindex freestanding environment
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@cindex hosted implementation
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@cindex hosted environment
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@findex __STDC_HOSTED__
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@opindex pedantic-errors
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The original ANSI C standard (X3.159-1989) was ratified in 1989 and
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published in 1990. This standard was ratified as an ISO standard
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(ISO/IEC 9899:1990) later in 1990. There were no technical
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differences between these publications, although the sections of the
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ANSI standard were renumbered and became clauses in the ISO standard.
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standard, but not the ISO standard, also came with a Rationale
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This standard, in both its forms, is commonly known as @dfn{C89}, or
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occasionally as @dfn{C90}, from the dates of ratification.
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To select this standard in GCC, use one of the options
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@option{-ansi}, @option{-std=c90} or @option{-std=iso9899:1990}; to obtain
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all the diagnostics required by the standard, you should also specify
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@option{-pedantic} (or @option{-pedantic-errors} if you want them to be
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errors rather than warnings). @xref{C Dialect Options,,Options
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Controlling C Dialect}.
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Errors in the 1990 ISO C standard were corrected in two Technical
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Corrigenda published in 1994 and 1996. GCC does not support the
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An amendment to the 1990 standard was published in 1995. This
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amendment added digraphs and @code{__STDC_VERSION__} to the language,
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but otherwise concerned the library. This amendment is commonly known
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as @dfn{AMD1}; the amended standard is sometimes known as @dfn{C94} or
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@dfn{C95}. To select this standard in GCC, use the option
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@option{-std=iso9899:199409} (with, as for other standard versions,
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@option{-pedantic} to receive all required diagnostics).
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A new edition of the ISO C standard was published in 1999 as ISO/IEC
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9899:1999, and is commonly known as @dfn{C99}. (While in
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development, drafts of this standard version were referred to as
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@dfn{C9X}.) GCC has substantially
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complete support for this standard version; see
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@uref{http://gcc.gnu.org/c99status.html} for details. To select this
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standard, use @option{-std=c99} or @option{-std=iso9899:1999}.
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Errors in the 1999 ISO C standard were corrected in three Technical
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Corrigenda published in 2001, 2004 and 2007. GCC does not support the
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A fourth version of the C standard, known as @dfn{C11}, was published
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in 2011 as ISO/IEC 9899:2011. (While in development, drafts of this
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standard version were referred to as @dfn{C1X}.)
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GCC has substantially complete support
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for this standard, enabled with @option{-std=c11} or
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@option{-std=iso9899:2011}.
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By default, GCC provides some extensions to the C language that, on
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rare occasions conflict with the C standard. @xref{C
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Extensions,,Extensions to the C Language Family}.
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Some features that are part of the C99 standard
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are accepted as extensions in C90 mode, and some features that are part
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of the C11 standard are accepted as extensions in C90 and C99 modes.
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@option{-std} options listed above disables these extensions where
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they conflict with the C standard version selected. You may also
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select an extended version of the C language explicitly with
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@option{-std=gnu90} (for C90 with GNU extensions), @option{-std=gnu99}
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(for C99 with GNU extensions) or @option{-std=gnu11} (for C11 with GNU
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The default, if no C language dialect options are given,
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is @option{-std=gnu11}.
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The ISO C standard defines (in clause 4) two classes of conforming
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implementation. A @dfn{conforming hosted implementation} supports the
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whole standard including all the library facilities; a @dfn{conforming
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freestanding implementation} is only required to provide certain
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library facilities: those in @code{<float.h>}, @code{<limits.h>},
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@code{<stdarg.h>}, and @code{<stddef.h>}; since AMD1, also those in
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@code{<iso646.h>}; since C99, also those in @code{<stdbool.h>} and
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@code{<stdint.h>}; and since C11, also those in @code{<stdalign.h>}
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and @code{<stdnoreturn.h>}. In addition, complex types, added in C99, are not
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required for freestanding implementations.
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The standard also defines two environments for programs, a
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@dfn{freestanding environment}, required of all implementations and
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which may not have library facilities beyond those required of
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freestanding implementations, where the handling of program startup
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and termination are implementation-defined; and a @dfn{hosted
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environment}, which is not required, in which all the library
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facilities are provided and startup is through a function @code{int
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main (void)} or @code{int main (int, char *[])}. An OS kernel is an example
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of a program running in a freestanding environment;
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a program using the facilities of an
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operating system is an example of a program running in a hosted environment.
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@opindex ffreestanding
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GCC aims towards being usable as a conforming freestanding
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implementation, or as the compiler for a conforming hosted
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implementation. By default, it acts as the compiler for a hosted
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implementation, defining @code{__STDC_HOSTED__} as @code{1} and
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presuming that when the names of ISO C functions are used, they have
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the semantics defined in the standard. To make it act as a conforming
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freestanding implementation for a freestanding environment, use the
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option @option{-ffreestanding}; it then defines
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@code{__STDC_HOSTED__} to @code{0} and does not make assumptions about the
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meanings of function names from the standard library, with exceptions
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noted below. To build an OS kernel, you may well still need to make
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your own arrangements for linking and startup.
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@xref{C Dialect Options,,Options Controlling C Dialect}.
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GCC does not provide the library facilities required only of hosted
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implementations, nor yet all the facilities required by C99 of
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freestanding implementations on all platforms.
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To use the facilities of a hosted
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environment, you need to find them elsewhere (for example, in the
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GNU C library). @xref{Standard Libraries,,Standard Libraries}.
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Most of the compiler support routines used by GCC are present in
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@file{libgcc}, but there are a few exceptions. GCC requires the
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freestanding environment provide @code{memcpy}, @code{memmove},
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@code{memset} and @code{memcmp}.
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Finally, if @code{__builtin_trap} is used, and the target does
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not implement the @code{trap} pattern, then GCC emits a call
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For references to Technical Corrigenda, Rationale documents and
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information concerning the history of C that is available online, see
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@uref{http://gcc.gnu.org/readings.html}
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@section C++ Language
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GCC supports the original ISO C++ standard published in 1998,
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and the 2011 and 2014 revisions.
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The original ISO C++ standard was published as the ISO standard (ISO/IEC
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14882:1998) and amended by a Technical Corrigenda published in 2003
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(ISO/IEC 14882:2003). These standards are referred to as C++98 and
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C++03, respectively. GCC implements the majority of C++98 (@code{export}
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is a notable exception) and most of the changes in C++03. To select
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this standard in GCC, use one of the options @option{-ansi},
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@option{-std=c++98}, or @option{-std=c++03}; to obtain all the diagnostics
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required by the standard, you should also specify @option{-pedantic} (or
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@option{-pedantic-errors} if you want them to be errors rather than
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A revised ISO C++ standard was published in 2011 as ISO/IEC
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14882:2011, and is referred to as C++11; before its publication it was
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commonly referred to as C++0x. C++11 contains several changes to the
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C++ language, all of which have been implemented in GCC@. For details
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see @uref{https://gcc.gnu.org/projects/@/cxx0x.html}.
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To select this standard in GCC, use the option @option{-std=c++11}.
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Another revised ISO C++ standard was published in 2014 as ISO/IEC
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14882:2014, and is referred to as C++14; before its publication it was
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sometimes referred to as C++1y. C++14 contains several further
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changes to the C++ language, all of which have been implemented in GCC@.
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For details see @uref{https://gcc.gnu.org/projects/@/cxx1y.html}.
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To select this standard in GCC, use the option @option{-std=c++14}.
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GCC also supports the C++ Concepts Technical Specification,
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ISO/IEC TS 19217:2015, which allows constraints to be defined for templates,
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allowing template arguments to be checked and for templates to be
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overloaded or specialized based on the constraints. Support for C++ Concepts
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is included in an experimental C++1z mode that corresponds to the next
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revision of the ISO C++ standard, expected to be published in 2017. To enable
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C++1z support in GCC, use the option @option{-std=c++17} or
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More information about the C++ standards is available on the ISO C++
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committee's web site at @uref{http://www.open-std.org/@/jtc1/@/sc22/@/wg21/}.
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To obtain all the diagnostics required by any of the standard versions
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described above you should specify @option{-pedantic}
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or @option{-pedantic-errors}, otherwise GCC will allow some non-ISO C++
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features as extensions. @xref{Warning Options}.
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By default, GCC also provides some additional extensions to the C++ language
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that on rare occasions conflict with the C++ standard. @xref{C++
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Dialect Options,Options Controlling C++ Dialect}. Use of the
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@option{-std} options listed above disables these extensions where they
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they conflict with the C++ standard version selected. You may also
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select an extended version of the C++ language explicitly with
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@option{-std=gnu++98} (for C++98 with GNU extensions), or
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@option{-std=gnu++11} (for C++11 with GNU extensions), or
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@option{-std=gnu++14} (for C++14 with GNU extensions), or
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@option{-std=gnu++1z} (for C++1z with GNU extensions).
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no C++ language dialect options are given, is @option{-std=gnu++14}.
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@section Objective-C and Objective-C++ Languages
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@cindex Objective-C++
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GCC supports ``traditional'' Objective-C (also known as ``Objective-C
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1.0'') and contains support for the Objective-C exception and
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synchronization syntax. It has also support for a number of
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``Objective-C 2.0'' language extensions, including properties, fast
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enumeration (only for Objective-C), method attributes and the
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@@optional and @@required keywords in protocols. GCC supports
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Objective-C++ and features available in Objective-C are also available
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GCC by default uses the GNU Objective-C runtime library, which is part
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of GCC and is not the same as the Apple/NeXT Objective-C runtime
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library used on Apple systems. There are a number of differences
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documented in this manual. The options @option{-fgnu-runtime} and
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@option{-fnext-runtime} allow you to switch between producing output
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that works with the GNU Objective-C runtime library and output that
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works with the Apple/NeXT Objective-C runtime library.
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There is no formal written standard for Objective-C or Objective-C++@.
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The authoritative manual on traditional Objective-C (1.0) is
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``Object-Oriented Programming and the Objective-C Language'':
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@uref{http://www.gnustep.org/@/resources/@/documentation/@/ObjectivCBook.pdf}
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is the original NeXTstep document;
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@uref{http://objc.toodarkpark.net}
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is the same document in another format.
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The Objective-C exception and synchronization syntax (that is, the
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keywords @code{@@try}, @code{@@throw}, @code{@@catch},
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@code{@@finally} and @code{@@synchronized}) is
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supported by GCC and is enabled with the option
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@option{-fobjc-exceptions}. The syntax is briefly documented in this
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manual and in the Objective-C 2.0 manuals from Apple.
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The Objective-C 2.0 language extensions and features are automatically
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enabled; they include properties (via the @code{@@property},
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@code{@@synthesize} and
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@code{@@dynamic keywords}), fast enumeration (not available in
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Objective-C++), attributes for methods (such as @code{deprecated},
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@code{noreturn}, @code{sentinel}, @code{format}),
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the @code{unused} attribute for method arguments, the
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@code{@@package} keyword for instance variables and the @code{@@optional} and
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@code{@@required} keywords in protocols. You can disable all these
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Objective-C 2.0 language extensions with the option
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@option{-fobjc-std=objc1}, which causes the compiler to recognize the
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same Objective-C language syntax recognized by GCC 4.0, and to produce
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an error if one of the new features is used.
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GCC has currently no support for non-fragile instance variables.
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The authoritative manual on Objective-C 2.0 is available from Apple:
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@uref{https://developer.apple.com/library/mac/documentation/Cocoa/Conceptual/ProgrammingWithObjectiveC/Introduction/Introduction.html}
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For more information concerning the history of Objective-C that is
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available online, see @uref{http://gcc.gnu.org/readings.html}
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As of the GCC 4.7.1 release, GCC supports the Go 1 language standard,
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described at @uref{http://golang.org/doc/go1.html}.
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@section References for Other Languages
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@xref{Top, GNAT Reference Manual, About This Guide, gnat_rm,
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GNAT Reference Manual}, for information on standard
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conformance and compatibility of the Ada compiler.
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@xref{Standards,,Standards, gfortran, The GNU Fortran Compiler}, for details
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of standards supported by GNU Fortran.
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@xref{Compatibility,,Compatibility with the Java Platform, gcj, GNU gcj},
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for details of compatibility between @command{gcj} and the Java Platform.
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