Simple programs are often put during a 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 seems like a traditional C file, except it ends with .h rather than .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 once you first used the printf() function, or
other I/O function, and you had to type:

#include<stdio.h>

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"

This program calculates the years since a given year example

#include <stdio.h>
int calculateAge(int year) {
const int CURRENT_YEAR = 2020;
return CURRENT_YEAR - year;
}
int main(void) {
printf("%u", calculateAge(1993));
}

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.

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