Multi-File Programs & Header Files

A real project splits code across multiple .c files, sharing declarations through a header:

// math_utils.h
#ifndef MATH_UTILS_H
#define MATH_UTILS_H

int add(int a, int b);

#endif

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

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

// main.c
#include <stdio.h>
#include "math_utils.h"

int main(void) {
    printf("%d\n", add(2, 3));
    return 0;
}

gcc main.c math_utils.c -o myprogram

The #ifndef/#define/#endif header guard stops the same header being pasted in twice if it's #included from more than one place — without it, the compiler sees int add(int a, int b); declared twice and errors out.

Common mistakes

• Forgetting to list every .c file on the gcc command line — unlike some languages, nothing "discovers" source files automatically.