ANSI C

C89

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.

C90

The same standard as C89 was ratified by the International Organization for Standardization as ISO/IEC 9899:1990, with only formatting changes,^[2] 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This standard has been withdrawn by both ANSI/INCITS^[3] and ISO/IEC.^[4]

C95

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

• Improved multi-byte and wide character support in the standard library, introducing <wchar.h> and <wctype.h> as well as multi-byte I/O
• 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__

In addition to the amendment, two technical corrigenda were published by ISO for C90:

• ISO/IEC 9899 TCOR1 in 1995
• ISO/IEC 9899 TCOR2 in 1996

#if defined(__STDC_VERSION__) && __STDC_VERSION__ >= 199409L

/* C95 compatible source code. */
#elif defined(__ANSI__)
/* C89 compatible source code. */
#endif

C99

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:

• New built-in data types: long long, _Bool, _Complex, and _Imaginary
• Several new core language features, including static array indices, designated initializers, compound literals, variable-length arrays, flexible array members, variadic macros, and restrict keyword
• Several new library headers, including stdint.h, <tgmath.h>, fenv.h, <complex.h>
• Improved compatibility with several C++ features, including inline functions, single-line comments with //, mixing declarations and code, and universal character names in identifiers
• Removed several dangerous C89 language features such as implicit function declarations and implicit int

Three technical corrigenda were published by ISO for C99:

• ISO/IEC 9899:1999/Cor.1:2001(E)
• ISO/IEC 9899:1999/Cor.2:2004(E)
• ISO/IEC 9899:1999/Cor.3:2007(E), notable for deprecating the standard library function gets

This standard has been withdrawn by both ANSI/INCITS^[7] and ISO/IEC^[8] in favour of C11.

C11

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 (stdatomic.h).

One technical corrigendum has been published by ISO for C11:

• ISO/IEC 9899:2011/Cor 1:2012^[9]

C18

As of October 2018, "C18" is the current standard for the C programming language.^[10]

Other related ISO publications

As part of the standardization process, ISO also publishes technical reports and specifications related to the C language:

• ISO/IEC TR 19769:2004,^[11] on library extensions to support Unicode transformation formats, integrated into C11
• ISO/IEC TR 24731-1:2007,^[12] on library extensions to support bounds-checked interfaces, integrated into C11
• ISO/IEC TR 18037:2008,^[13] on embedded C extensions
• ISO/IEC TR 24732:2009,^[14] on decimal floating point arithmetic, superseded by ISO/IEC TS 18661-2:2015
• ISO/IEC TR 24747:2009,^[15] on special mathematical functions,
• ISO/IEC TR 24731-2:2010,^[16] on library extensions to support dynamic allocation functions
• ISO/IEC TS 17961:2013,^[17] on secure coding in C
• ISO/IEC TS 18661-1:2014,^[18] on IEC 60559:2011-compatible binary floating-point arithmetic
• ISO/IEC TS 18661-2:2015,^[19] on IEC 60559:2011-compatible decimal floating point arithmetic
• ISO/IEC TS 18661-3:2015,^[20] on IEC 60559:2011-compatible interchange and extended floating-point types
• ISO/IEC TS 18661-4:2015,^[21] on IEC 60559:2011-compatible supplementary functions

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.^[22]

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 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.