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

Yes, every function must have a prototype, but that prototype may appear either in a separate declaration or as part of the function's definition. Function definitions written in C89 and up naturally have prototypes, but if you write things in classic K&R style, thus:

main (argc, argv)

  int argc;
  char **argv;

{
  ...
}

then the function definition has no prototype. If you write ANSI C (C89) style, thus:

main (int argc, char **argv) { ... }

then the function definition has a prototype.