About C language: Standards evolution

Everything before standardization is generally called "K&R C", after thefamous book, 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.

In 1999, the C standard was revised, lots of things changed (ISO 9899:1999). This version of the standard is called C99. From 1999-2011, this was "the C language". Most C compilers still follow this version.

In 2011, the C standard was again changed (ISO 9899:2011). This version is called C11. It is currently the definition 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 termconforming 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.