Function Prototypes

Why prototypes?

C reads top to bottom. If main calls a function defined later in the file, the compiler needs a prototype — the function's signature ending in ; — declared earlier.

Declare, then define

The prototype's parameter names are optional and only for documentation.

#include <stdio.h>

int square(int n);   // prototype

int main(void) {
    printf("%d\n", square(5));
    return 0;
}

int square(int n) {  // definition
    return n * n;
}

Output:

25

Header files hold prototypes

<stdio.h> and friends are just files full of prototypes for functions like printf, so the compiler trusts your calls before linking.