Header file

A header file is a file containing C declarations and macro definitions to be shared between several source files.

Header can be the system header file or it can be created by user.

Syntax for System Header File :

#include <file>

Example :

#include <stdio.h>

Explanation :

It searches for a file named ‘stdio.h’ in a standard list of system directories.

Syntax for User defined header file :

#include "file"

Explanation :

It searches for a file named 'file' in the directory containing the current file.

Example :

Create a header file in demoheader.h, that contains add() function

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

Main program in demo.c

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

void main()
{
    int num1 = 20, num2 = 30, num3;
    num3 = add(num1, num2);
    printf("Addition of Two numbers : %d", num3);
}

Output :

Addition of Two numbers : 50

Explanation :

The compiler will see the same token stream as it would, if demo.c read.

#include<stdio.h>
int add(int a,int b)
{
    return(a+b);
}
void main()
{
    int num1 = 20, num2 = 30, num3;
    num3 = add(num1, num2);
    printf("Addition of Two numbers : %d", num3);
}

NOTE :

• 1. A header file is a file with extension .h
• 2. To use header file in the program by including it with the C preprocessing directive #include