Header Files in C

What is a Header File?

A header file is a file with the extension .h that contains:

• Function declarations (prototypes)
• Macro definitions
• Constant values
• Structure and type definitions
• External variable declarations

Header files are included in a C program using:

#include <filename.h>

Types of Header Files

• Standard Header Files (provided by compiler)
• User-Defined Header Files (created by the programmer)

1. Standard Header Files

These are built-in and already available with your C compiler:

• <stdio.h> – input/output functions
• <stdlib.h> – general utilities
• <string.h> – string handling functions
• <math.h> – mathematical functions
• <ctype.h> – character handling
• <time.h> – time/date functions
• <stdbool.h> – boolean type

Example: Using a Standard Header File

#include <stdio.h>

int main() {

    printf("Using stdio.h for input/output");

    return 0;
}

2. User Defined Header Files

You can create your own header files to separate reusable code. Example: myfunc.h + myfunc.c

Step 1 — Create the Header File (myfunc.h)

void greet();

Step 2 — Create the Function File (myfunc.c)

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

void greet() {
    printf("Hello from user-defined header!");
}

Step 3 — Include in Main Program

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

int main() {

    greet();

    return 0;
}

How Header Files Work Internally?

The line:

#include <stdio.h>

is expanded by the **preprocessor**, which literally copies the contents of stdio.h into your program before compilation.

Common Mistakes with Header Files

• Including same header multiple times → FIX: use include guards
• Declaring variables inside header files (bad practice)
• Using <> for user-defined headers
• Creating circular dependencies

Header Guards (#ifndef / #define)

Used to prevent multiple inclusions.

#ifndef MYFUNC_H
#define MYFUNC_H

void greet();

#endif

Real Life Uses of Header Files

• Organizing large projects
• Reusing code across multiple files
• Modular programming
• API function declarations
• Managing global constants

Practice Questions

1. What is a header file? Explain with example.
2. Difference between standard and user-defined headers?
3. Why do we use header guards?
4. List five commonly used header files and their purpose.
5. Explain how #include works internally.

Practice Task

5 Complete Examples

/* Example 1 – Using stdio.h */
#include <stdio.h>
int main() {
    printf("Hello Header Files");
    return 0;
}

/* Example 2 – Using math.h */
#include <stdio.h>
#include <math.h>

int main() {
    printf("%.2f", sqrt(25));
    return 0;
}

/* Example 3 – Using string.h */
#include <stdio.h>
#include <string.h>

int main() {
    printf("%lu", strlen("Header"));
    return 0;
}

/* Example 4 – User-defined header */
#include <stdio.h>
#include "myfunc.h"
int main() {
    greet();
    return 0;
}

/* Example 5 – Header guard demonstration */
#ifndef TEMP_H
#define TEMP_H
void hello();
#endif