ANSI C, also known as C89 and C90 depending on the year of ratification, refers to the family of successive standards published by the American National Standards Institute (ANSI) for the C programming language. Software developers writing in C are encouraged to conform to the standards, as doing so aids 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, 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^{[citation needed]}, 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,^[1] which is sometimes referred to as C90. Therefore, the terms "C89" and "C90" refer to essentially the same language.

This standard has been withdrawn by both INCITS^[2] and ISO/IEC.^[3]

ISO also published an amendment in 1994, referred to as AMD1, (ISO/IEC 9899/Amd.1:1995) introducing minor changes to the language and the library.^[4]

1995 the ISO published an extension, called Amendment 1, for the ANSI-C standard. It full name finally was ISO/IEC 9899/AMD1:1995 or nicknamed C95. Aside to error correction there were further changes to the language capabilities.^{[5] [6]}

• Improvements to the support for multi byte and wide character char sets in the standard library.
• Addition of Digraphs to the language.
• Specification of standard macros for the alternative specification of operators, e.g. and for &&.
• Specification of the standard macro __STDC_VERSION__.

#if defined(__STDC_VERSION__) && __STDC_VERSION__ >= 199409L
/*
 * C95 compatible source code.
 */
#endif

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

This standard has been withdrawn by ISO/IEC in favour of C11,^[7] but is still approved by INCITS.^{[citation needed]}

"C11" is the new standard for the C programming language.

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.

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 *);
 #else
 extern int getopt();
 #endif

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

• Amsterdam Compiler Kit (C K&R and C89/90)
• ARM RealView
• Clang, using LLVM backend
• GCC (Full C89/90, C99 and C11)
• Intel's ICC
• LabWindows/CVI
• LCC
• OpenWatcom (C89/90 and some C99)
• Microsoft Visual C++ (C90. A few features of C99)
• Pelles C (C99 and C11. Windows only.)

• 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
• Schreiner, Axel-Tobias (Hanser). Object oriented programming with ANSI-C. Hanser. ISBN 3-446-17426-5. hdl:1850/8544. ((cite book)): Check date values in: |year= (help)
• "ISO/IEC 9899:1999 Programming Languages -- C". American National Standards Institute.
• "ANSI Standards Action Vol. 36, #48" (PDF). American National Standards Institute. 2005-12-02.