Simple programs can be put in a single file, but when your program grows larger, it’s impossible to keep it all in just one file.

You can move parts of a program to a separate file, then you create a header file.

A header file looks like a normal C file, except it ends with .h instead of .c, and instead of the implementations of your functions and the other parts of a program, it holds the declarations.

You already used header files when you first used the printf() function, or other I/O function, and you had to type:

#include <stdio.h>

to use it.

#include is a preprocessor directive.

The preprocessor goes and looks up the stdio.h file in the standard library, because you used brackets around it. To include your own header files, you’ll use quotes, like this:

#include "myfile.h"

The above will look up myfile.h in the current folder.

You can also use a folder structure for libraries:

#include "myfolder/myfile.h"

Let’s make an example. This program calculates the years since a given year:

#include <stdio.h>

int calculateAge(int year) {
  const int CURRENT_YEAR = 2020;
  return CURRENT_YEAR - year;
}

int main(void) {
  printf("%u", calculateAge(1983));
}

Suppose I want to move the calculateAge function to a separate file.

I create a calculate_age.c file:

int calculateAge(int year) {
  const int CURRENT_YEAR = 2020;
  return CURRENT_YEAR - year;
}

And a calculate_age.h file where I put the function prototype, which is same as the function in the .c file, except the body:

int calculateAge(int year);

Now in the main .c file we can go and remove the calculateAge() function definition, and we can import calculate_age.h, which will make the calculateAge() function available:

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

int main(void) {
  printf("%u", calculateAge(1983));
}

Don’t forget that to compile a program composed by multiple files, you need to list them all in the command line, like this:

gcc -o main main.c calculate_age.c

And with more complex setups, a Makefile is necessary to tell the compiler how to compile the program.