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ANSI C,ISO C, andStandard C are successive standards for theC programming language published 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 asC89 orC90). Software developers writing in C are encouraged to conform to the standards, as doing so helpsportability between compilers.
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.[1] While some software developers use the term ISO C, others are standards-body neutral and use Standard C.
Informalspecification in 1978 (Brian Kernighan andDennis Ritchie bookThe C Programming Language).
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.[2]
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.
The same standard as C89 was ratified by ISO/IEC as ISO/IEC 9899:1990, with only formatting changes,[3] 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This standard has been withdrawn by both ANSI/INCITS[4] and ISO/IEC.[5]
In 1995, theISO/IEC published an extension, called Amendment 1, for the C standard. Its full name finally wasISO/IEC 9899:1990/AMD1:1995, nicknamedC94 orC95. Aside from error correction there were further changes to the language capabilities,[6][7] such as:
<wchar.h> and<wctype.h> as well asmulti-byteI/Oand for&&__STDC_VERSION__.This was both the first standard with a __STDC_VERSION__ value (199409L) and the first version in which the year in that value did not match the year of publication (1995), leading to common names of bothC94 andC95. This would happen again in C17 (2018) and C23 (2024), but they are more commonly known by their earlier years, while this standard is often referred to by its later year.[8]
In addition to the amendment, two technical corrigenda were published by ISO for C90:
#if defined(__STDC_VERSION__) && __STDC_VERSION__ >= 199409L/* C95 compatible source code. */#elif defined(__STDC__)/* C89 compatible source code. */#endif
In March 2000, ANSI adopted the ISO/IEC 9899:1999 standard.[11] This standard is commonly referred to as C99. Some notable additions to the previous standard include:
long long,_Bool,_Complex, and_Imaginaryrestrict keywordstdint.h,<tgmath.h>,fenv.h,<complex.h>//, mixingdeclarations and code, and universal character names inidentifiersintThree technical corrigenda were published by ISO for C99:
This standard has been withdrawn by both ANSI/INCITS[12] and ISO/IEC[13] in favour of C11.
C11 was officially ratified and published on December 8, 2011. Notable features include improvedUnicode support, type-generic expressions using the new_Generic keyword, a cross-platformmulti-threading API (threads.h), andatomic types support in both core language and the library (stdatomic.h).
One technical corrigendum has been published by ISO for C11:
C17 was published in June 2018.[15] Rather than introducing new language features, it only addresses defects in C11.[16]
C23 was published in October 2024, and is the currentstandard for theC programming language.[17]
C2Y is an informal name for the next revision of the C programming language that is hoped to be released in the later 2020s.
As part of the standardization process, ISO/IEC also publishestechnical reports and specifications related to the C language:
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.[29]
ANSI C is 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.[30][31] Any source code writtenonly in standard C and without any hardware dependent assumptions is virtually guaranteed to compile correctly on anyplatform with 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 asGUI libraries, or to the reliance on compiler- or platform-specific attributes such as the exact size of certain data types and byteendianness.
To mitigate the differences betweenK&R C and 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*);#elseexternintgetopt();#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.[32]
List of compilers supporting ANSI C:
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