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ANSI C is the standard published by the American National Standards Institute (ANSI) for the C programming language. Software developers writing in C are encouraged to conform to the requirements in the document, as it encourages easily portable code.



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, the name ANSI C (rather than ISO C) is still more widely used. 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. After a long and arduous process, 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", or sometimes "C89" (to distinguish it from C99).


In 1990, the ANSI C standard (with a few minor modifications) was adopted by the International Organization for Standardization as ISO/IEC 9899:1990. This version is sometimes called C90. Therefore, the terms "C89" and "C90" refer to essentially the same language.


In March 2000, ANSI adopted the ISO/IEC 9899:1999 standard. This standard is commonly referred to as C99, and it is the current standard for C programming language.

Support from major compilers

ANSI C is now supported by almost all the widely used compilers. Most of the C code being written nowadays is based on ANSI C. 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.

Compliance detectability

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 __STDC__
 extern int getopt(int, char * const *, const char *);
 extern int getopt();

It's better to use "#if __STDC__" as above rather than "#ifdef __STDC__" because some implementation may set __STDC__ to zero to indicate non-ANSI compliance. "#if" will treat any identifiers that couldn't be replaced by a macro as zero (0). Thus even if the macro "__STDC__" is not defined to signify non-ANSI compliance, "#if" will work as shown.

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 and C90, but their use is discouraged.

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