If one doesn’t specify the function prototype, the behavior is specific to C standard (either C90 or C99) that the compilers implement. Up to C90 standard, C compilers assumed the return type of the omitted function prototype as int. And this assumption at compiler side may lead to unspecified program behavior.
Later C99 standard specified that compilers can no longer assume return type as int. Therefore, C99 became more restrict in type checking of function prototype. But to make C99 standard backward compatible, in practice, compilers throw the warning saying that the return type is assumed as int. But they go ahead with compilation. Thus, it becomes the responsibility of programmers to make sure that the assumed function prototype and the actual function type matches.
To avoid all this implementation specifics of C standards, it is best to have function prototype.