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ANSI C refers to the family of successive standards published by the American National Standards Institute
American National Standards Institute
The American National Standards Institute is a private non-profit organization that oversees the development of voluntary consensus standards for products, services, processes, systems, and personnel in the United States. The organization also coordinates U.S. standards with international...

 (ANSI) for the C programming language
C (programming language)
C is a general-purpose computer programming language developed between 1969 and 1973 by Dennis Ritchie at the Bell Telephone Laboratories for use with the Unix operating system....

. Software developers writing in C are encouraged to conform to the standards, as doing so aids portability
In computer science, porting is the process of adapting software so that an executable program can be created for a computing environment that is different from the one for which it was originally designed...

 between compilers.

History and outlook

The first standard for C was published by ANSI. Although this document was subsequently adopted by International Organization for Standardization
International Organization for Standardization
The International Organization for Standardization , widely known as ISO, is an international standard-setting body composed of representatives from various national standards organizations. Founded on February 23, 1947, the organization promulgates worldwide proprietary, industrial and commercial...

 (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 most recent standard for the C programming language.


"C1X" is the unofficial name of the planned new standard
Open standard
An open standard is a standard that is publicly available and has various rights to use associated with it, and may also have various properties of how it was designed . There is no single definition and interpretations vary with usage....

 for the C programming language. It is intended to supersede the existing C standard, informally known as C99
C99 is a modern dialect of the C programming language. It extends the previous version with new linguistic and library features, and helps implementations make better use of available computer hardware and compiler technology.-History:...


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
Graphical user interface
In computing, a graphical user interface is a type of user interface that allows users to interact with electronic devices with images rather than text commands. GUIs can be used in computers, hand-held devices such as MP3 players, portable media players or gaming devices, household appliances and...

 libraries, or to the reliance on compiler- or platform-specific attributes such as the exact size of certain data types and byte endianness
In computing, the term endian or endianness refers to the ordering of individually addressable sub-components within the representation of a larger data item as stored in external memory . Each sub-component in the representation has a unique degree of significance, like the place value of digits...


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.

Compilers supporting ANSI C

  • GCC
    GNU Compiler Collection
    The GNU Compiler Collection is a compiler system produced by the GNU Project supporting various programming languages. GCC is a key component of the GNU toolchain...

  • Microsoft Visual C++
    Visual C++
    Microsoft Visual C++ is a commercial , integrated development environment product from Microsoft for the C, C++, and C++/CLI programming languages...

     (C90. A few features of C99)
  • ARM
    ARM Holdings
    ARM Holdings plc is a British multinational semiconductor and software company headquartered in Cambridge. Its largest business is in processors, although it also designs, licenses and sells software development tools under the RealView and KEIL brands, systems and platforms, system-on-a-chip...

  • LCC
  • OpenWatcom (C89/90 and some C99)

External links