Implicitly Declared Functions

Implicitly Declared Functions

It is always risky to call a function without an explicit declaration in scope. Furthermore, be sure to declare with a compatible prototype any function defined with a prototype. Problems arise when mixing prototype and nonprototype declarations for the same function. For example, suppose you call a function (defined with a prototype to take a variable number of arguments) in a scope without a prototyped declaration. You may get unexpected results if a floating point argument is passed to it. This is a typical problem with calls to printf and after stdio.h routines. Therefore, always include stdio.h in any context where you use stdio.h facilities.