Function Prototypes

Part of the Fundamentals section of Coddy's C journey — lesson 51 of 63.

In C, we can declare a function before we use it. A function prototype tells the compiler about a function's name, parameters, and return type before the actual function is defined.

Create a function prototype for a function that adds two integers:

int add(int a, int b);

Then define the function:

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

The prototype is usually above the main, while the function itself is below the main, so the code looks cleaner.

Now you can use this function in the main:

int main() {
    int result = add(5, 3);
    printf("%d", result);
    return 0;
}

Without the prototype, if you call the function before its definition, you'll get a compiler error.

Challenge

Easy

Create a program that:

Declares a function prototype for a function named calculateArea that takes two integers (length and width) and returns an integer.
Implements the function to calculate and return the area (length × width).
In the main function, it gets two integers from the user, calls calculateArea, and prints the result.

Cheat sheet

A function prototype declares a function before it's defined, telling the compiler about the function's name, parameters, and return type:

int add(int a, int b);

Then define the function:

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

The prototype is usually placed above main, while the function definition goes below main:

int add(int a, int b);  // prototype

int main() {
    int result = add(5, 3);
    printf("%d", result);
    return 0;
}

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

Without the prototype, calling a function before its definition will cause a compiler error.

Try it yourself

#include <stdio.h>

// Write your function prototype here

int main() {
    // Declare and read variables
    
    // Call your function and print the result here
    
    return 0;
}

// Implement your function here

Test yourself

This lesson includes a short quiz. Start the lesson to answer it and track your progress.

All lessons in Fundamentals

1Introduction

C Language Hello World!Comments Basic Program Structure

4Control Flow

If Statement If - Else Else-If Switch Case Ternary Conditional Operator Recap Challenge Nested If - Else

7Functions

Declare a Function Return Types Parameters Recap Challenge #1 Recursion BasicsFunction PrototypesRecap Challenge #2