What is the difference between C, C99, ANSI C and GNU C?

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  • Everything before standardization is generally called “K&R C”, after the famous book (1st edition and 2nd edition), with Dennis Ritchie, the inventor of the C language, as one of the authors. This was “the C language” from 1972-1989.

  • The first C standard was released 1989 nationally in USA, by their national standard institute ANSI. This release is called C89 or ANSI-C. From 1989-1990 this was “the C language”.

  • The year after, the American standard was accepted internationally and published by ISO (ISO 9899:1990). This release is called C90. Technically, it is the same standard as C89/ANSI-C. Formally, it replaced C89/ANSI-C, making them obsolete. From 1990-1999, C90 was “the C language”.

    Please note that since 1989, ANSI haven’t had anything to do with the C language, other than as one of many instances working on the ISO standard. It is nowadays done in USA through INCITS and the C standard is formally called INCITS/ISO/IEC 9899 in USA. Just as it is for example called EN/ISO/IEC in Europe.

    Programmers still speaking about “ANSI C” generally haven’t got a clue about what it means. ISO “owns” the C language, through the standard ISO 9899.

  • A minor update was released in 1995, sometimes referred to as “C95”. This was not a major revision, but rather a technical amendment formally named ISO/IEC 9899:1990/Amd.1:1995. The main change was introduction of wide character support.

  • In 1999, the C standard went through a major revision (ISO 9899:1999). This version of the standard is called C99. From 1999-2011, this was “the C language”.

  • In 2011, the C standard was changed again (ISO 9899:2011). This version is called C11. Various new features like _Generic, _Static_assert and thread support were added to the language. The update had a lot of focus on multi-core, multi-processing and expression sequencing. From 2011-2017, this was “the C language”.

  • In 2017, C11 was revised and various defect reports were solved. This standard is informally called C17 or C18. It was finished in 2017 (and uses __STDC_VERSION__ = 201710L) but was released by ISO as 9899:2018, hence the ambiguity between C17/C18. It contains no new features, just corrections. It is the current version of the C language.

  • A draft called “C23″/”C2X” is work in progress by the committee, planned to be released in 2023. The current working draft can be found here, at this point called N2731, last changed 2021-10-18.

    This contains a lot of minor defect report fixes like C17/C18 but also some major changes, most notable (so far):

    • the removal of exotic signedness representations in favour of mandatory 2’s complement
    • final removal of “K&R-style” function definitions (flagged obsolescent since C99)
    • some new functions added including memccpy and strdup
    • some new function attributes from C++ deprecated, fallthrough, maybe_unused, and nodiscard
    • binary 0b notation for integer constants (currently not listed as one of the changes to N2731 but present on p.51 of the draft).

“C99 strict” likely refers to a compiler setting forcing a compiler to follow the standard by the letter. There is a term conforming implementation in the C standard. Essentially it means: “this compiler actually implements the C language correctly”. Programs that implement the C language correctly are formally called strictly conforming programs. Such programs may also not contain any form of poorly-defined behavior.

“GNU C” can mean two things. Either the C compiler itself that comes as part of the GNU Compiler Collection (GCC). Or it can mean the non-standard default setup that the GCC C compiler uses. If you compile with gcc program.c then you don’t compile according to the C standard, but rather a non-standard GNU setup, which may be referred to as “GNU C”. For example, the whole Linux kernel is made in non-standard GNU C, and not in standard C.

If you want to compile your programs according to the C standard, you should type gcc -std=c99 -pedantic-errors. Replace c99 with c17 if your GCC version supports it.

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