22.1.2 Forward Function Declarations

The order of the function definitions in the source code makes no difference, except that each function needs to be defined or declared before code uses it.

The definition of a function also declares its name for the rest of the containing scope. But what if you want to call the function before its definition? To permit that, write a compatible declaration of the same function, before the first call. A declaration that prefigures a subsequent definition in this way is called a forward declaration. The function declaration can be at top level or within a block, and it applies until the end of the containing scope.

See Function Declarations, for more information about these declarations.