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@c Copyright (C) 2000, 2001, 2002, 2004 Free Software Foundation, Inc.

@c This is part of the GCC manual.

@c For copying conditions, see the file gcc.texi.

@node Standards

@chapter Language Standards Supported by GCC

@cindex C standard

@cindex C standards

@cindex ANSI C standard

@cindex ANSI C

@cindex ANSI C89

@cindex C89

@cindex ANSI X3.159-1989

@cindex X3.159-1989

@cindex ISO C standard

@cindex ISO C

@cindex ISO C90

@cindex ISO/IEC 9899

@cindex ISO 9899

@cindex C90

@cindex ISO C94

@cindex C94

@cindex ISO C95

@cindex C95

@cindex ISO C99

@cindex C99

@cindex ISO C9X

@cindex C9X

@cindex Technical Corrigenda

@cindex TC1

@cindex Technical Corrigendum 1

@cindex TC2

@cindex Technical Corrigendum 2

@cindex AMD1

@cindex freestanding implementation

@cindex freestanding environment

@cindex hosted implementation

@cindex hosted environment

@findex __STDC_HOSTED__

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.

@opindex std

@opindex ansi

@opindex pedantic

@opindex pedantic-errors

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 @dfn{C89}, or

occasionally as @dfn{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

@option{-ansi}, @option{-std=c89} or @option{-std=iso9899:1990}; to obtain

all the diagnostics required by the standard, you should also specify

@option{-pedantic} (or @option{-pedantic-errors} if you want them to be

errors rather than warnings).  @xref{C Dialect Options,,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 @code{__STDC_VERSION__} to the language,

but otherwise concerned the library.  This amendment is commonly known

as @dfn{AMD1}; the amended standard is sometimes known as @dfn{C94} or

@dfn{C95}.  To select this standard in GCC, use the option

@option{-std=iso9899:199409} (with, as for other standard versions,

@option{-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 @dfn{C99}.  GCC has incomplete

support for this standard version; see

@uref{http://gcc.gnu.org/gcc-4.2/c99status.html} for details.  To select this

standard, use @option{-std=c99} or @option{-std=iso9899:1999}.  (While in

development, drafts of this standard version were referred to as

@dfn{C9X}.)

Errors in the 1999 ISO C standard were corrected in two Technical

Corrigenda published in 2001 and 2004.  GCC does not support the uncorrected

version.

By default, GCC provides some extensions to the C language that on

rare occasions conflict with the C standard.  @xref{C

Extensions,,Extensions to the C Language Family}.  Use of the

@option{-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

@option{-std=gnu89} (for C89 with GNU extensions) or @option{-std=gnu99}

(for C99 with GNU extensions).  The default, if no C language dialect

options are given, is @option{-std=gnu89}; this will change to

@option{-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 @dfn{conforming hosted implementation} supports the

whole standard including all the library facilities; a @dfn{conforming

freestanding implementation} is only required to provide certain

library facilities: those in @code{<float.h>}, @code{<limits.h>},

@code{<stdarg.h>}, and @code{<stddef.h>}; since AMD1, also those in

@code{<iso646.h>}; and in C99, also those in @code{<stdbool.h>} and

@code{<stdint.h>}.  In addition, complex types, added in C99, are not

required for freestanding implementations.  The standard also defines

two environments for programs, a @dfn{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 @dfn{hosted environment}, which is not

required, in which all the library facilities are provided and startup

is through a function @code{int main (void)} or @code{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.

@opindex ffreestanding

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

implementation, defining @code{__STDC_HOSTED__} as @code{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 @option{-ffreestanding}; it will then define

@code{__STDC_HOSTED__} to @code{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.

@xref{C Dialect Options,,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).  @xref{Standard Libraries,,Standard Libraries}.

Most of the compiler support routines used by GCC are present in

@file{libgcc}, but there are a few exceptions.  GCC requires the

freestanding environment provide @code{memcpy}, @code{memmove},

@code{memset} and @code{memcmp}.

Finally, if @code{__builtin_trap} is used, and the target does

not implement the @code{trap} pattern, then GCC will emit a call

to @code{abort}.

For references to Technical Corrigenda, Rationale documents and

information concerning the history of C that is available online, see

@uref{http://gcc.gnu.org/readings.html}

@c FIXME: details of C++ standard.

@cindex Objective-C

@cindex Objective-C++

There is no formal written standard for Objective-C or Objective-C++@.  The most

authoritative manual is ``Object-Oriented Programming and the

Objective-C Language'', available at a number of web sites:

@itemize

@item

@uref{http://developer.apple.com/documentation/Cocoa/Conceptual/ObjectiveC/}

is a recent (and periodically updated) version;

@item

@uref{http://www.toodarkpark.org/computers/objc/}

is an older example;

@item

@uref{http://www.gnustep.org}

and

@uref{http://gcc.gnu.org/readings.html}

have additional useful information.

@end itemize

@cindex treelang

There is no standard for treelang, which is a sample language front end

for GCC@.  Its only purpose is as a sample for people wishing to write a

new language for GCC@.  The language is documented in

@file{gcc/treelang/treelang.texi} which can be turned into info or

HTML format.

@xref{Top, GNAT Reference Manual, About This Guide, gnat_rm,

GNAT Reference Manual}, for information on standard

conformance and compatibility of the Ada compiler.

@xref{Standards,,Standards, gfortran, The GNU Fortran Compiler}, for details

of standards supported by GNU Fortran.

@xref{Compatibility,,Compatibility with the Java Platform, gcj, GNU gcj},

for details of compatibility between @command{gcj} and the Java Platform.