Are prototypes required for all functions in C89, C90 or C99?

Question

To be truly standards-compliant, must all functions in C (except for main) have a prototype, even if they are only used after their definition in the same translation unit?<

Answer 1

A prototype is a function declaration that specifies the types of the function's parameters.

Pre-ANSI C (the language described by the 1978 first edition of Kernighan & Ritchie's "The C Programming Language") did not have prototypes; it was not possible for a function declaration to describe the number or types of the parameters. It was up to the caller to pass the correct number and type of arguments.

ANSI C introduced "prototypes", declarations that specify the types of the parameters (a feature borrowed from early C++).

As of C89/C90 (the ANSI and ISO standards describe the same language), it's legal to call a function with no visible declaration; an implicit declaration is provided. If the implicit declaration is incompatible with the actual definition (say, calling sqrt("foo"), then the behavior is undefined. Neither this implicit declaration nor a non-prototype declaration can be compatible with a variadic function, so any call to a variadic function (like printf or scanf) must have a visible prototype.

C99 dropped implicit declarations. Any call to a function without a visible declaration is a constraint violation, requiring a compiler diagnostic. But that declaration is still not required to be a prototype; it can be an old-style declaration that doesn't specify parameter types.

C11 made no significant changes in this area.

So even as of the 2011 ISO C standard, old-style function declarations and definitions (which have been "obsolescent" since 1989) are still permitted in conforming code.

For all versions of C going back to 1989, as a matter of style, there is very little reason not to use prototypes for all functions. Old-style declarations and definitions are kept only to avoid breaking old code.