Header Files

As C programs grow larger, keeping all code in a single file becomes unmanageable. Header files provide a way to organize your code by separating what a function does from how it does it.

A header file (with the .h extension) contains function declarations (also called prototypes). These declarations tell the compiler that a function exists and what parameters it takes, without providing the actual implementation.

// math_utils.h
int add(int a, int b);

To use declarations from a header file, you include it with the #include directive. For your own headers, use double quotes instead of angle brackets:

// main.c
#include "math_utils.h"

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

int main() {
    int result = add(5, 3);
    return 0;
}

The key difference: #include <stdio.h> searches system directories, while #include "math_utils.h" first searches your project folder. This separation allows multiple files to share the same function declarations, making your codebase easier to navigate and maintain.