Standards (Using The GNU Compiler Collection (GCC))

2.1 C Language ¶

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. The ANSI standard, but not the ISO standard, also came with a Rationale document. This standard, in both its forms, is commonly known as C89, or occasionally as C90, from the dates of ratification. To select this standard in GCC, use one of the options -ansi, -std=c90 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 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. (While in development, drafts of this standard version were referred to as C9X.) GCC has substantially complete support for this standard version; see https://gcc.gnu.org/c99status.html for details. To select this standard, use -std=c99 or -std=iso9899:1999.

Errors in the 1999 ISO C standard were corrected in three Technical Corrigenda published in 2001, 2004 and 2007. GCC does not support the uncorrected version.

A fourth version of the C standard, known as C11, was published in 2011 as ISO/IEC 9899:2011. (While in development, drafts of this standard version were referred to as C1X.) GCC has substantially complete support for this standard, enabled with -std=c11 or -std=iso9899:2011. A version with corrections integrated was prepared in 2017 and published in 2018 as ISO/IEC 9899:2018; it is known as C17 and is supported with -std=c17 or -std=iso9899:2017; the corrections are also applied with -std=c11, and the only difference between the options is the value of __STDC_VERSION__.

A fifth version of the C standard, known as C23, was published in 2024 as ISO/IEC 9899:2024. (While in development, drafts of this standard version were referred to as C2X.) Support for this is enabled with -std=c23 or -std=iso9899:2024.

A further version of the C standard, known as C2Y, is under development; experimental and incomplete support for this is enabled with -std=c2y.

By default, GCC provides some extensions to the C language that, on rare occasions conflict with the C standard. See Extensions to the C Language Family. Some features that are part of the C99 standard are accepted as extensions in C90 mode, and some features that are part of the C11 standard are accepted as extensions in C90 and C99 modes. Use of the -std options listed above disables 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=gnu90 (for C90 with GNU extensions), -std=gnu99 (for C99 with GNU extensions), -std=gnu11 (for C11 with GNU extensions), -std=gnu17 (for C17 with GNU extensions) or -std=gnu23 (for C23 with GNU extensions).

The default, if no C language dialect options are given, is -std=gnu23.

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>; since C99, also those in <stdbool.h> and <stdint.h>; and since C11, also those in <stdalign.h> and <stdnoreturn.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 is an example of a program running in a freestanding environment; a program using the facilities of an operating system is an example of a program running in a hosted environment.

GCC aims towards being usable as a conforming freestanding implementation, or as the compiler for a conforming hosted implementation. By default, it acts 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 then defines __STDC_HOSTED__ to 0 and does 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. See 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 on all platforms. To use the facilities of a hosted environment, you need to find them elsewhere (for example, in the GNU C library). See Standard Libraries.

Most of the compiler support routines used by GCC are present in libgcc, but there are a few exceptions. GCC requires the freestanding environment provide memcpy, memmove, memset and memcmp. Contrary to the standards covering memcpy GCC expects the case of an exact overlap of source and destination to work and not invoke undefined behavior. Finally, if __builtin_trap is used, and the target does not implement the trap pattern, then GCC emits a call to abort.

For references to Technical Corrigenda, Rationale documents and information concerning the history of C that is available online, see https://gcc.gnu.org/readings.html