6.2 Multi-file Builds and Headers

As programs grow, put declarations in .h header files and implementations in .c source files.

// geometry.h
#ifndef GEOMETRY_H
#define GEOMETRY_H

double circle_area(double radius);

#endif

// geometry.c
#include "geometry.h"

#define PI 3.1415926

double circle_area(double radius) {
    return PI * radius * radius;
}

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

int main(void) {
    printf("%.2f\n", circle_area(5));
    return 0;
}

Compile the related source files together:

bash

gcc -Wall geometry.c main.c -o area

The #ifndef / #define / #endif lines in a header are called an include guard; they stop the same header from being included twice.

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