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Note

C: Function Prototypes

Declaring a function prototype so a function can be defined anywhere, not just before its first use.

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Though you have to define a function before using it, it’s not entirely true. You can notify the compiler in advance that you’ll be using a function of a certain type that has a certain parameter list, and that way the function can be defined anywhere at all — as long as the function prototype has been declared first.

#include <stdio.h>


int foo(void); /* this is the prototype */


int main(void){
    int i;


    i = foo();


    return 0;
}


int foo(void){ /* this is the definition, just like the prototype*/
    return 45;
}