gcc/doc/gcc/language-standards-supported-by-gcc/c-language.rst

   1 ..
   2   Copyright 1988-2022 Free Software Foundation, Inc.
   3   This is part of the GCC manual.
   4   For copying conditions, see the copyright.rst file.
   5
   6 .. index:: C standard, C standards, ANSI C standard, ANSI C, ANSI C89, C89, ANSI X3.159-1989, X3.159-1989, ISO C standard, ISO C, ISO C90, ISO/IEC 9899, ISO 9899, C90, ISO C94, C94, ISO C95, C95, ISO C99, C99, ISO C9X, C9X, ISO C11, C11, ISO C1X, C1X, ISO C17, C17, ISO C2X, C2X, Technical Corrigenda, TC1, Technical Corrigendum 1, TC2, Technical Corrigendum 2, TC3, Technical Corrigendum 3, AMD1, freestanding implementation, freestanding environment, hosted implementation, hosted environment, __STDC_HOSTED__, std, ansi, pedantic, pedantic-errors
   7
   8 C Language
   9 **********
  10
  11 The original ANSI C standard (X3.159-1989) was ratified in 1989 and
  12 published in 1990.  This standard was ratified as an ISO standard
  13 (ISO/IEC 9899:1990) later in 1990.  There were no technical
  14 differences between these publications, although the sections of the
  15 ANSI standard were renumbered and became clauses in the ISO standard.
  16 The ANSI
  17 standard, but not the ISO standard, also came with a Rationale
  18 document.
  19 This standard, in both its forms, is commonly known as :dfn:`C89`, or
  20 occasionally as :dfn:`C90`, from the dates of ratification.
  21 To select this standard in GCC, use one of the options
  22 :option:`-ansi`, :option:`-std=c90` or :option:`-std=iso9899:1990` ; to obtain
  23 all the diagnostics required by the standard, you should also specify
  24 :option:`-pedantic` (or :option:`-pedantic-errors` if you want them to be
  25 errors rather than warnings).  See :ref:`c-dialect-options`.
  26
  27 Errors in the 1990 ISO C standard were corrected in two Technical
  28 Corrigenda published in 1994 and 1996.  GCC does not support the
  29 uncorrected version.
  30
  31 An amendment to the 1990 standard was published in 1995.  This
  32 amendment added digraphs and ``__STDC_VERSION__`` to the language,
  33 but otherwise concerned the library.  This amendment is commonly known
  34 as :dfn:`AMD1`; the amended standard is sometimes known as :dfn:`C94` or
  35 :dfn:`C95`.  To select this standard in GCC, use the option
  36 :option:`-std=iso9899:199409` (with, as for other standard versions,
  37 :option:`-pedantic` to receive all required diagnostics).
  38
  39 A new edition of the ISO C standard was published in 1999 as ISO/IEC
  40 9899:1999, and is commonly known as :dfn:`C99`.  (While in
  41 development, drafts of this standard version were referred to as
  42 :dfn:`C9X`.)  GCC has substantially
  43 complete support for this standard version; see
  44 https://gcc.gnu.org/c99status.html for details.  To select this
  45 standard, use :option:`-std=c99` or :option:`-std=iso9899:1999`.
  46
  47 Errors in the 1999 ISO C standard were corrected in three Technical
  48 Corrigenda published in 2001, 2004 and 2007.  GCC does not support the
  49 uncorrected version.
  50
  51 A fourth version of the C standard, known as :dfn:`C11`, was published
  52 in 2011 as ISO/IEC 9899:2011.  (While in development, drafts of this
  53 standard version were referred to as :dfn:`C1X`.)
  54 GCC has substantially complete support
  55 for this standard, enabled with :option:`-std=c11` or
  56 :option:`-std=iso9899:2011`.  A version with corrections integrated was
  57 prepared in 2017 and published in 2018 as ISO/IEC 9899:2018; it is
  58 known as :dfn:`C17` and is supported with :option:`-std=c17` or
  59 :option:`-std=iso9899:2017` ; the corrections are also applied with
  60 :option:`-std=c11`, and the only difference between the options is the
  61 value of ``__STDC_VERSION__``.
  62
  63 A further version of the C standard, known as :dfn:`C2X`, is under
  64 development; experimental and incomplete support for this is enabled
  65 with :option:`-std=c2x`.
  66
  67 By default, GCC provides some extensions to the C language that, on
  68 rare occasions conflict with the C standard.  See :ref:`c-extensions`.
  69 Some features that are part of the C99 standard
  70 are accepted as extensions in C90 mode, and some features that are part
  71 of the C11 standard are accepted as extensions in C90 and C99 modes.
  72 Use of the
  73 :option:`-std` options listed above disables these extensions where
  74 they conflict with the C standard version selected.  You may also
  75 select an extended version of the C language explicitly with
  76 :option:`-std=gnu90` (for C90 with GNU extensions), :option:`-std=gnu99`
  77 (for C99 with GNU extensions) or :option:`-std=gnu11` (for C11 with GNU
  78 extensions).
  79
  80 The default, if no C language dialect options are given,
  81 is :option:`-std=gnu17`.
  82
  83 The ISO C standard defines (in clause 4) two classes of conforming
  84 implementation.  A :dfn:`conforming hosted implementation` supports the
  85 whole standard including all the library facilities; a :dfn:`conforming
  86 freestanding implementation` is only required to provide certain
  87 library facilities: those in ``<float.h>``, ``<limits.h>``,
  88 ``<stdarg.h>``, and ``<stddef.h>`` ; since AMD1, also those in
  89 ``<iso646.h>`` ; since C99, also those in ``<stdbool.h>`` and
  90 ``<stdint.h>`` ; and since C11, also those in ``<stdalign.h>``
  91 and ``<stdnoreturn.h>``.  In addition, complex types, added in C99, are not
  92 required for freestanding implementations.
  93
  94 The standard also defines two environments for programs, a
  95 :dfn:`freestanding environment`, required of all implementations and
  96 which may not have library facilities beyond those required of
  97 freestanding implementations, where the handling of program startup
  98 and termination are implementation-defined; and a :dfn:`hosted
  99 environment`, which is not required, in which all the library
 100 facilities are provided and startup is through a function ``int
 101 main (void)`` or ``int main (int, char *[])``.  An OS kernel is an example
 102 of a program running in a freestanding environment;
 103 a program using the facilities of an
 104 operating system is an example of a program running in a hosted environment.
 105
 106 .. index:: ffreestanding
 107
 108 GCC aims towards being usable as a conforming freestanding
 109 implementation, or as the compiler for a conforming hosted
 110 implementation.  By default, it acts as the compiler for a hosted
 111 implementation, defining ``__STDC_HOSTED__`` as ``1`` and
 112 presuming that when the names of ISO C functions are used, they have
 113 the semantics defined in the standard.  To make it act as a conforming
 114 freestanding implementation for a freestanding environment, use the
 115 option :option:`-ffreestanding` ; it then defines
 116 ``__STDC_HOSTED__`` to ``0`` and does not make assumptions about the
 117 meanings of function names from the standard library, with exceptions
 118 noted below.  To build an OS kernel, you may well still need to make
 119 your own arrangements for linking and startup.
 120 See :ref:`c-dialect-options`.
 121
 122 GCC does not provide the library facilities required only of hosted
 123 implementations, nor yet all the facilities required by C99 of
 124 freestanding implementations on all platforms.
 125 To use the facilities of a hosted
 126 environment, you need to find them elsewhere (for example, in the
 127 GNU C library).  See :ref:`standard-libraries`.
 128
 129 Most of the compiler support routines used by GCC are present in
 130 :samp:`libgcc`, but there are a few exceptions.  GCC requires the
 131 freestanding environment provide ``memcpy``, ``memmove``,
 132 ``memset`` and ``memcmp``.
 133 Finally, if ``__builtin_trap`` is used, and the target does
 134 not implement the ``trap`` pattern, then GCC emits a call
 135 to ``abort``.
 136
 137 For references to Technical Corrigenda, Rationale documents and
 138 information concerning the history of C that is available online, see
 139 https://gcc.gnu.org/readings.html