Function prototypes

Learn function prototypes (declarations) and ordering.

Function prototype

A prototype is a line that declares a function's signature in advance. It tells the compiler "this function is defined further down".

int add(int, int);  /* prototype */

int main(void) {
    int s = add(2, 3);
    return 0;
}

int add(int a, int b) {  /* definition */
    return a + b;
}

Without a prototype, if the definition comes after main, the compiler warns. In modern C, headers included with #include also contain prototypes.

Headers and prototypes

Lines like #include <stdio.h> add prototypes of library functions to your file. This is why you can call printf without defining it.

#include <stdio.h>

int main(void) {
    printf("%d\n", 42);  /* prototype comes from stdio.h */
    return 0;
}

When using standard library functions, you must include the relevant header; otherwise the compiler may assume a wrong return type.

Exercise Prototype and definition together

Write the int square(int); prototype before main. In main, call printf("%d\n", square(5));. After main, define int square(int x) { return x * x; }. Output should be 25.

Show example solution

#include <stdio.h>

int square(int);

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

int square(int x) {
    return x * x;
}

Test yourself

What does a function prototype do?
- Tells the compiler the signature in advance
- Speeds up the function
- Makes the function callable
- Renames the function
Show answer

Tells the compiler the signature in advance — A prototype tells the compiler the function signature in advance.

Prototype line

int add(int, ____);  /* prototype */

Show answer

int add(int, int);  /* prototype */

Match prototype concepts

int add(int, int);

prototype
int add(int a, int b) {

definition start
add(2, 3);

call
#include <stdio.h>

header

Next lesson: Recursion

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