Creating and using your own header file in C

mainProgram.c

// Instructions: 

// Compilation Command: gcc mainProgram.c myLibrary.c
// Creates required object files.

// Execution Command: ./a.out

#include <stdio.h>

// Including my library
#include "myLibrary.h"

int main()
{
  int a = 4, b = 5;
  int product = multiply(a, b);
  printf("Product: %d x %d = %d\n", a, b, product);
  return 0;
}

myLibrary.c

// This file contains definitions of functions 
// declared in included header files.
// In this case, for "myLibrary.h".

#include "myLibrary.h"

// Takes 2 integers as arguments and returns their product.
int multiply(int a, int b) {
  return a*b;
}

myLibrary.h

// Declaration of my function: multiply

int multiply(int, int);