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

• 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. 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.

"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.

"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 c11 if your GCC version supports it.

I MUST respond regarding ANSI C. Although ANSI has not done anything with it, compilers are still built to it. PIC XC16 compiler for example: "The compiler is a fully validated compiler that conforms to the ANSI C standard as defined by the ANSI specification (ANSI x3.159-1989) and described in Kernighan and Ritchie’s The C Programming Language (second edition). ..."Not all programming is for "big" computers like PCs. Writing a compiler for your device costs, and validating costs time & $. ANSI C is alive & well &living in your embedded / real-time devices.

• ANSI C :The first C language was standardized by the body called ANSI in 1989 that's why it is called c89.

• C99 :
  with the demand from the developers requirements, in 1999-2000 further or additional keywords and features have been included in C99 (ex: inline, boolean.. Added floating point arthematic library functions)

• GNU C: GNU is a unix like operating system (www.gnu.org) & somewhere GNU's project needs C programming language based on ANSI C standard. GNU use GCC (GNU Compiler Collection) compiler to compile the code. It has C library function which defines system calls such as malloc, calloc, exit...etc

ANSI C is a standard which is being used by or refereed the other standards.