Since neither of the functions is declared in ``main.c'', the compiler
has no way of checking that we are actually using them properly. If
we make a mistake (for example, use too many arguments, too few
arguments, or the wrong type of arguments) we won't find out until the
program gives us incorrect results or even crashes at runtime. If we
are to enjoy the benefits of type checking, we must be sure that
everything is declared.
Return to lesson.