ANSI C, ISO C and Standard C refer to the successive standards for the C programming language published by the American National Standards Institute (ANSI) and the International Organization for Standardization (ISO). Historically, the names referred specifically to the original and best-supported version of the standard (known as C89 or C90). Software developers writing in C are encouraged to conform to the standards, as doing so helps portability between compilers.
The first standard for C was published by ANSI. Although this document was subsequently adopted by International Organization for Standardization (ISO) and subsequent revisions published by ISO 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.
In 1983, the American National Standards Institute formed a committee, X3J11, to establish a standard specification of C. The 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 C99 but using the same labelling method.
The same standard as C89 was ratified by the International Organization for Standardization as ISO/IEC 9899:1990, with only formatting changes, which is sometimes referred to as C90. Therefore, the terms "C89" and "C90" refer to essentially the same language.
In 1995, the ISO published an extension, called Amendment 1, for the ANSI-C standard. Its full name finally was ISO/IEC 9899:1990/AMD1:1995 or nicknamed C95. Aside from error correction there were further changes to the language capabilities, such as:
<wctype.h>as well as multi-byte I/O
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. This standard is commonly referred to as C99. Some notable additions to the previous standard include:
//, mixing declarations and code, and universal character names in identifiers
Three technical corrigenda were published by ISO for C99:
As of 2018, "C11" is the previous standard for the C programming language. Notable features introduced over the previous revision include improved Unicode support, type-generic expressions using the new
_Generic keyword, a cross-platform multi-threading API (
threads.h) and atomic types support in both core language and the library (
One technical corrigendum has been published by ISO for C11:
As part of the standardization process, ISO also publishes technical 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.
ANSI C is now supported by almost all the widely used compilers. GCC and Clang are two major C compilers popular today, both are based on the C11 with updates including changes from later specifications such as C17 and C18. Any program written only in standard C and without any hardware dependent assumptions is virtually guaranteed to compile correctly on any platform 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 as GUI libraries, or to the reliance on compiler- or platform-specific attributes such as the exact size of certain data types and byte endianness.
To mitigate the differences between K&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__ extern int getopt(int, char * const *, const char *); #else extern int getopt(); #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.
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