Splitting a Program into Files

Real programs have thousands of lines. Putting them all in one file would be a mess, so C programs are split into several files:

• .h files (headers) list what is available: prototypes, structs, #defines.
• .c files hold the actual code.

Think of it like this: A header file is like the menu in a restaurant: it tells you what dishes exist. The .c file is the kitchen where they are really made.

An example with three files

mathutils.h – the menu:

#ifndef MATHUTILS_H
#define MATHUTILS_H

int square(int n);
int cube(int n);

#endif

mathutils.c – the kitchen:

#include "mathutils.h"

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

int cube(int n) {
    return n * n * n;
}

main.c – uses the functions:

#include <stdio.h>
#include "mathutils.h"

int main() {
    printf("%d %d\n", square(4), cube(3));
    return 0;
}

Compile both .c files together:

gcc main.c mathutils.c -o app
./app

• #include "mathutils.h" uses double quotes for your own files, and < > for C's built-in ones.
• #ifndef / #define / #endif is an include guard: it stops the same header being pasted in twice.

Remember: Prototypes and types go in .h files; code goes in .c files.