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 Standards Organization (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 aids portability 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 (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". 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/AMD1:1995 or nicknamed C95. Aside to error correction there were further changes to the language capabilities, such as:
- 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.
- Specification of the standard macro
Preprocessor Test for C95 compatibility
#if defined(__STDC_VERSION__) && __STDC_VERSION__ >= 199409L /* C95 compatible source code. */ #elif defined(__ANSI__) /* 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:
- New built-in data types:
- Several new array features, including variable-length arrays and flexible array members
stdint.h, which defines typedefs for fixed-width and minimum-width integral types
restrictkeyword, used to guarantee that two or more pointers do not point to the same address
- Improved compatibility with several C++ features, including inline functions and single-line comments
"C11" is the latest standard for the C programming language. Notable features introduced over the previous standard include improved Unicode support, a type-generic math library
tgmath.h for both floating point and
_Complex types based on the new
_Generic keyword and
threads.h, which defines a cross-platform multi-threading API.
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.
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.
Compilers supporting ANSI C
- Amsterdam Compiler Kit (C K&R and C89/90)
- ARM RealView
- Clang, using LLVM backend
- GCC (Full C89/90, C99 and C11)
- HP C/ANSI C compiler (C89 and C99)
- IBM XL C/C++ (C11, starting with version 12.1)
- Intel's ICC
- OpenWatcom (C89/90 and some C99)
- Microsoft Visual C++ (C89/90 and some C99)
- Pelles C (C99 and C11. Windows only.)
- vbcc (C89/90 and C99)
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- ISO C working group
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- 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
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