A header file in C is a file with a .h extension that typically contains function declarations, macro definitions, and type definitions. Its purpose is to share this common information across multiple source files (.c files) in a project.

Think of a header file as a table of contents for a library of code. It tells the compiler what functions are available to use, but the actual implementation (the code that makes the functions work) is stored elsewhere, usually in a corresponding .c source file or a pre-compiled library.

The #include Preprocessor Directive

You use a header file in your program with the #include preprocessor directive. This directive tells the C preprocessor to find the specified file and essentially copy and paste its entire content into the current source file before the actual compilation begins.

There are two ways to use #include:

1.   #include <filename.h> (using angle brackets)

o    This is used for standard library header files.

o    It tells the preprocessor to look for the file in the standard system directories where all the standard library files are stored.

o    Example: #include <stdio.h>

2.   #include "filename.h" (using double quotes)

o    This is used for your own custom header files that you create as part of your project.

o    It tells the preprocessor to look for the file in the current directory first. If it's not found there, it will then search the standard system directories.

o    Example: #include "my_functions.h"

Common Standard Library Header Files

C comes with a rich set of standard libraries, each with a corresponding header file that provides declarations for useful functions.

Example: Creating and Using a Custom Header File

Creating your own header files is essential for organizing larger projects. Here’s how you do it in three steps.

Step 1: Create the Header File (mymath.h)

This file contains the function prototype (declaration). We also add include guards (#ifndef, #define, #endif) to prevent errors if the header is included more than once in the same file.

C

// mymath.h

#ifndef MYMATH_H  // If MYMATH_H has not been defined...

#define MYMATH_H  // ...then define it.

// Function prototype for adding two integers

int add(int a, int b);

#endif // End of the include guard block

Step 2: Create the Implementation File (mymath.c)

This source file contains the actual function definition (the code that makes the function work).

C

// mymath.c

#include "mymath.h" // Include the header to link the declaration

// Function definition

int add(int a, int b) {

    return a + b;

}

Step 3: Use the Function in Your Main Program (main.c)

Now, you can include your custom header in your main program and use the add function as if it were a standard library function.

C

// main.c

#include <stdio.h>

#include "mymath.h" // Include your custom header file

int main() {

    int x = 10;

    int y = 20;

    int sum;

    // Call the function from your library

    sum = add(x, y);

    printf("The sum of %d and %d is: %d\n", x, y, sum);

    return 0;

}

To compile this project, you would need to compile both .c files together: gcc main.c mymath.c -o myprogram