Terminology: Forward Declaration versus Function Prototype
struct Foo;

char *foo();  // NOT the same as char *foo(void)

int foo(char *str_one, char *str_two, char *str_three);

int foo(char *str_one, char *str_two);

int foo(int a, char *b) {...}

int foo();             // a declaration, but not a prototype
int foo(a, b);         // a declaration, but not a prototype
int foo(int, char *);  // a declaration and a prototype
int foo(int a, char *b);  // a declaration and a prototype