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 nice tip when writing new functions is to write them upside-down with main at the bottom so when you change your mind about the function's args or return type you don't have to fix the prototype too. Constantly fixing prototypes, and dealing with all the compiler's warnings when they are out of date gets really tedious.

Once you have your functions working smoothly together move the code to a well-named module and put the prototypes in a .h file of the same name. It saves serious time. The biggest productivity aid I've found in 5 years.