C89、C99以及__STDC__(轉自維基百科)

History

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 .

C89

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).

C90

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.

C99

Main article: C99

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
 *
)
;

#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 and C90, but their use is discouraged.