ANSI C

This article is about the programming language standard. For the paper size, seePaper size § ANSI paper sizes.

ANSI C,ISO C,andStandard Care successive standards for theC programming languagepublished by theAmerican National Standards Institute(ANSI) andISO/IEC JTC 1/SC 22/WG 14 of theInternational Organization for Standardization(ISO) and theInternational Electrotechnical Commission(IEC). Historically, the names referred specifically to the original and best-supported version of the standard (known asC89orC90). Software developers writing in C are encouraged to conform to the standards, as doing so helpsportabilitybetween compilers.

History and outlook

The first standard for C was published by ANSI. Although this document was subsequently adopted by ISO/IEC and subsequent revisions published by ISO/IEC have been adopted by ANSI, "ANSI C" is still used to refer to the standard.While some software developers use the term ISO C, others are standards-body neutral and use Standard C.

Informal specification: K&R C (C78)

Main article:K&R C

Informalspecificationin 1978 (Brian KernighanandDennis RitchiebookThe C Programming Language).

Standardizing C

In 1983, the American National Standards Institute formed a committee, X3J11, to establish a standard specification of C. In 1985, the first Standard Draft was released, sometimes referred to asC85.In 1986, another Draft Standard was released, sometimes referred to asC86.The prerelease Standard C was published in 1988, and sometimes referred to asC88.

C89

The ANSI standard was completed in 1989 and ratified as ANSI X3.159-1989 "Programming Language C." This version of the language is often referred to as "ANSI C". Later on sometimes the label "C89" is used to distinguish it from C90 but using the same labeling method.

C90

The same standard as C89 was ratified by ISO/IEC as ISO/IEC 9899:1990, with only formatting changes,which is sometimes referred to as C90. Therefore, the terms "C89" and "C90" refer to a language that is virtually identical.

This standard has been withdrawn by both ANSI/INCITSand ISO/IEC.

C95

In 1995, theISO/IECpublished an extension, called Amendment 1, for the C standard. Its full name finally wasISO/IEC 9899:1990/AMD1:1995or nicknamedC95.Aside from error correction there were further changes to the language capabilities,such as:

• Improvedmulti-byteandwide charactersupport in the standard library, introducing<wchar.h>and<wctype.h>as well asmulti-byteI/O
• Addition ofdigraphsto the language
• Specification of standard macros for the alternative specification of operators, e.g.andfor&&
• Specification of the standard macro__STDC_VERSION__

In addition to the amendment, two technical corrigenda were published by ISO for C90:

• ISO/IEC 9899:1990/Cor 1:1994 TCOR1 in 1994
• ISO/IEC 9899:1990/Cor 2:1996 in 1996

Preprocessor test for C95 compatibility

#if defined(__STDC_VERSION__) && __STDC_VERSION__ >= 199409L

/* C95 compatible source code. */
#elif defined(__STDC__)
/* C89 compatible source code. */
#endif

C99

Main article:C99

In March 2000, ANSI adopted the ISO/IEC 9899:1999 standard.This standard is commonly referred to as C99. Some notable additions to the previous standard include:

• New built-indata types:long long,_Bool,_Complex,and_Imaginary
• Several new core language features, including static array indices, designated initializers, compound literals,variable-length arrays,flexible array members,variadic macros,andrestrictkeyword
• Several new library headers, includingstdint.h,<tgmath.h>,fenv.h,<complex.h>
• Improved compatibility with severalC++features, includinginline functions,single-linecommentswith//,mixingdeclarationsand code, and universal character names inidentifiers
• Removed several dangerous C89 language features such as implicit function declarations and implicitint

Three technical corrigenda were published by ISO for C99:

• ISO/IEC 9899:1999/Cor 1:2001(E)
• ISO/IEC 9899:1999/Cor 2:2004(E)
• ISO/IEC 9899:1999/Cor 3:2007(E), notable for deprecating the standard library functiongets

This standard has been withdrawn by both ANSI/INCITSand ISO/IECin favour of C11.

C11

Main article:C11

C11 was officially ratified and published on December 8, 2011. Notable features include improvedUnicodesupport, type-generic expressions using the new_Generickeyword, a cross-platformmulti-threadingAPI (threads.h), andatomictypes support in both core language and the library (stdatomic.h).

One technical corrigendum has been published by ISO for C11:

• ISO/IEC 9899:2011/Cor 1:2012

C17

Main article:C17

As of October 2018,"C17" is the currentstandardfor theC programming language.

C17 addresses defects in C11 without introducing new language features.

C23

Main article:C23

C23 is the informal name for the next majorC languagestandard revision.

Other related ISO publications

As part of the standardization process, ISO/IEC also publishestechnical reports and specificationsrelated to the C language:

• ISO/IEC TR 19769:2004,on library extensions to support Unicode transformation formats, integrated into C11
• ISO/IEC TR 24731-1:2007,on library extensions to support bounds-checked interfaces, integrated into C11
• ISO/IEC TR 18037:2008,on embedded C extensions
• ISO/IEC TR 24732:2009,ondecimal floating pointarithmetic, superseded by ISO/IEC TS 18661-2:2015
• ISO/IEC TR 24747:2009,on special mathematical functions,
• ISO/IEC TR 24731-2:2010,on library extensions to support dynamic allocation functions
• ISO/IEC TS 17961:2013,on secure coding in C
• ISO/IEC TS 18661-1:2014,onIEC 60559:2011-compatible binary floating-point arithmetic
• ISO/IEC TS 18661-2:2015,on IEC 60559:2011-compatibledecimal floating pointarithmetic
• ISO/IEC TS 18661-3:2015,on IEC 60559:2011-compatible interchange and extended floating-point types
• ISO/IEC TS 18661-4:2015,on IEC 60559:2011-compatible supplementary functions

More technical specifications are in development and pending approval, including the fifth and final part of TS 18661, a software transactional memory specification, and parallel library extensions.

Support from major compilers

ANSI C is now supported by almost all the widely used compilers. GCC and Clang are two major C compilers popular today, both based on the C11 with updates including changes from later specifications such as C17.Any source code writtenonlyin standard C and without any hardware dependent assumptions is virtually guaranteed to compile correctly on anyplatformwith a conforming C implementation. Without such precautions, most programs may compile only on a certain platform or with a particular compiler, due, for example, to the use of non-standard libraries, such asGUIlibraries, or to the reliance on compiler- or platform-specific attributes such as the exact size of certain data types and byteendianness.

Compliance detectability

To mitigate the differences betweenK&R Cand the ANSI C standard, the__STDC__( "standard c" ) macro can be used to split code into ANSI and K&R sections.

#if defined(__STDC__) && __STDC__
externintgetopt(int,char*const*,constchar*);
#else
externintgetopt();
#endif

In the above example, a prototype is used in a function declaration for ANSI compliant implementations, while an obsolescent non-prototype declaration is used otherwise. Those are still ANSI-compliant as of C99. Note how this code checks both definition and evaluation: this is because some implementations may set__STDC__to zero to indicate non-ANSI compliance.

Compiler support

List of compilers supporting ANSI C:

• Acornsoft ANSI C(first version in 1988, revised in 1989)
• Amsterdam Compiler Kit(C K&R and C89/90)
• ARMRealView
• Clang,usingLLVMbackend
• GCC(full C89/90, C99 and C11)
• HP C/ANSI C compiler (C89 and C99)
• IBM XL C/C++(C11, starting with version 12.1)
• Intel's ICC
• LabWindows/CVI
• LCC
• Oracle Developer Studio
• OpenWatcom(C89/90 and some C99)
• Microsoft Visual C++(C89/90 and some C99)
• Pelles C(C99 and C11. Windows only.)
• vbcc(C89/90 and C99)
• Tiny C Compiler(C89/90 and some C99)

See also

• Behavioral Description Language
• Compatibility of C and C++
• C++23,C++20,C++17,C++14,C++11,C++03,C++98,versions of the C++ programming language standard
• C++ Technical Report 1

Further reading

• Schreiner, Axel-Tobias (1993).Object Oriented Programming with ANSI-C.Hanser.ISBN3-446-17426-5.

External links

• ISO C working group
• Draft ANSI C Standard(ANSI X3J11/88-090)(May 13, 1988),Third Public Review
• Draft ANSI C Rationale(ANSI X3J11/88-151)(Nov 18, 1988)
• C Information Bulletin #1(ANSI X3J11/93-007)(May 27, 1992)
• ANSI C Yacc grammar
  • ANSI C grammar, Lex specification
• "ISO/IEC 9899:1999 Programming Languages -- C".American National Standards Institute. Archived fromthe originalon 2011-07-25.Retrieved2009-08-06.
• "ANSI Standards Action Vol. 36, #48"(PDF).American National Standards Institute. 2005-12-02. Archived fromthe original(PDF)on 2016-03-04.Retrieved2009-08-06.