Using the GNU Compiler Collection (GCC): Standards

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## 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.

### 2.1 C Language

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 sections of the ANSI standard were renumbered and became clauses in the ISO standard. This standard, in both its forms, is commonly 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=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. GCC has substantially complete support for this standard version; see http://gcc.gnu.org/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 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. GCC has substantially complete support for this standard, enabled with `-std=c11` or `-std=iso9899:2011`. (While in development, drafts of this standard version were referred to as C1X.)

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. 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=gnu90` (for C90 with GNU extensions), `-std=gnu99` (for C99 with GNU extensions) or `-std=gnu11` (for C11 with GNU extensions). The default, if no C language dialect options are given, is `-std=gnu11`. 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.

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 would be a freestanding environment; a program using the facilities of an operating system would normally be in a hosted implementation.

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 `__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 will then define `__STDC_HOSTED__` to `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. 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 will 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`. Finally, if `__builtin_trap` is used, and the target does not implement the `trap` pattern, then GCC will emit a call to `abort`.

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

### 2.2 C++ Language

GCC supports the original ISO C++ standard (1998) and contains experimental support for the second ISO C++ standard (2011).

The original ISO C++ standard was published as the ISO standard (ISO/IEC 14882:1998) and amended by a Technical Corrigenda published in 2003 (ISO/IEC 14882:2003). These standards are referred to as C++98 and C++03, respectively. GCC implements the majority of C++98 (`export` is a notable exception) and most of the changes in C++03. To select this standard in GCC, use one of the options `-ansi`, `-std=c++98`, or `-std=c++03`; 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).

A revised ISO C++ standard was published in 2011 as ISO/IEC 14882:2011, and is referred to as C++11; before its publication it was commonly referred to as C++0x. C++11 contains several changes to the C++ language, most of which have been implemented in an experimental C++11 mode in GCC. For information regarding the C++11 features available in the experimental C++11 mode, see http://gcc.gnu.org/projects/cxx0x.html. To select this standard in GCC, use the option `-std=c++11`; 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).

More information about the C++ standards is available on the ISO C++ committee’s web site at http://www.open-std.org/jtc1/sc22/wg21/.

By default, GCC provides some extensions to the C++ language; See Options Controlling C++ Dialect. Use of the `-std` option listed above will disable these extensions. You may also select an extended version of the C++ language explicitly with `-std=gnu++98` (for C++98 with GNU extensions) or `-std=gnu++11` (for C++11 with GNU extensions). The default, if no C++ language dialect options are given, is `-std=gnu++98`.

### 2.3 Objective-C and Objective-C++ Languages

GCC supports “traditional” Objective-C (also known as “Objective-C 1.0”) and contains support for the Objective-C exception and synchronization syntax. It has also support for a number of “Objective-C 2.0” language extensions, including properties, fast enumeration (only for Objective-C), method attributes and the @optional and @required keywords in protocols. GCC supports Objective-C++ and features available in Objective-C are also available in Objective-C++.

GCC by default uses the GNU Objective-C runtime library, which is part of GCC and is not the same as the Apple/NeXT Objective-C runtime library used on Apple systems. There are a number of differences documented in this manual. The options `-fgnu-runtime` and `-fnext-runtime` allow you to switch between producing output that works with the GNU Objective-C runtime library and output that works with the Apple/NeXT Objective-C runtime library.

There is no formal written standard for Objective-C or Objective-C++. The authoritative manual on traditional Objective-C (1.0) is “Object-Oriented Programming and the Objective-C Language”, available at a number of web sites:

- http://www.gnustep.org/resources/documentation/ObjectivCBook.pdf is the original NeXTstep document;
- http://objc.toodarkpark.net is the same document in another format;
- http://developer.apple.com/mac/library/documentation/Cocoa/Conceptual/ObjectiveC/ has an updated version but make sure you search for “Object Oriented Programming and the Objective-C Programming Language 1.0”, not documentation on the newer “Objective-C 2.0” language

The Objective-C exception and synchronization syntax (that is, the keywords @try, @throw, @catch, @finally and @synchronized) is supported by GCC and is enabled with the option `-fobjc-exceptions`. The syntax is briefly documented in this manual and in the Objective-C 2.0 manuals from Apple.

The Objective-C 2.0 language extensions and features are automatically enabled; they include properties (via the @property, @synthesize and @dynamic keywords), fast enumeration (not available in Objective-C++), attributes for methods (such as deprecated, noreturn, sentinel, format), the unused attribute for method arguments, the @package keyword for instance variables and the @optional and @required keywords in protocols. You can disable all these Objective-C 2.0 language extensions with the option `-fobjc-std=objc1`, which causes the compiler to recognize the same Objective-C language syntax recognized by GCC 4.0, and to produce an error if one of the new features is used.

GCC has currently no support for non-fragile instance variables.

The authoritative manual on Objective-C 2.0 is available from Apple:

- http://developer.apple.com/mac/library/documentation/Cocoa/Conceptual/ObjectiveC/

For more information concerning the history of Objective-C that is available online, see http://gcc.gnu.org/readings.html

### 2.4 Go Language

As of the GCC 4.7.1 release, GCC supports the Go 1 language standard, described at http://golang.org/doc/go1.html.

### 2.5 References for Other Languages

See About This Guide in GNAT Reference Manual, for information on standard conformance and compatibility of the Ada compiler.

See Standards in The GNU Fortran Compiler, for details of standards supported by GNU Fortran.

See Compatibility with the Java Platform in GNU gcj, for details of compatibility between `gcj` and the Java Platform.

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