C in 100 Seconds: Multi-File Projects

Multi-File Projects

Real C projects split code across multiple files. A header file declares functions, an implementation file defines them, and main uses them.

The Header File — math_utils.h

#ifndef MATH_UTILS_H
#define MATH_UTILS_H

int add(int a, int b);
int multiply(int a, int b);

#endif

The #ifndef / #define / #endif pattern is an include guard — it prevents the header from being included twice. The declarations tell the compiler what functions exist without providing the code.

The Implementation — math_utils.c

#include "math_utils.h"

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

int multiply(int a, int b) {
  return a * b;
}

Quotes in #include "math_utils.h" mean look in the current directory. Angle brackets mean look in the system path.

Using It — main.c

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

int main() {
  int sum = add(10, 20);
  int product = multiply(5, 6);
  printf("add(10, 20) = %d\n", sum);
  printf("multiply(5, 6) = %d\n", product);
  return 0;
}

Compiling

Pass all .c files to gcc: gcc main.c math_utils.c -o app. Or use a Makefile to automate it — Make only recompiles files that changed.

Student Code

Try it yourself: episode41/