Header File Libraries In C

C Header File Libraries – Complete Guide

1. What is a Header File?

A header file in C contains:

• Function declarations (prototypes)
• Macros and constants
• Type definitions

They allow you to use pre-written code without rewriting it.

💡 Think of it as a toolbox — it gives you ready-made tools (functions) so you don’t have to build them yourself.

---

2. Common C Header Files

Header File	Purpose & Common Functions
<stdio.h>	Standard input/output → printf(), scanf(), fopen(), fclose()
<stdlib.h>	Memory management → malloc(), free(), exit()
<string.h>	String handling → strlen(), strcpy(), strcmp()
<math.h>	Math functions → sqrt(), pow(), sin(), cos()
<ctype.h>	Character handling → toupper(), isdigit(), isalpha()
<time.h>	Date & time → time(), clock(), difftime()
<stdbool.h>	Boolean type → true, false

---

3. Creating a Custom Header File

You can make your own header file to organize code better.

Step 1 – Create a header file (myheader.h)

#ifndef MYHEADER_H   // Prevents multiple inclusion
#define MYHEADER_H

void sayHello();     // Function prototype

#endif

---

Step 2 – Create the implementation file (myheader.c)

#include <stdio.h>
#include "myheader.h" // Include your custom header

void sayHello() {
    printf("Hello! I am your custom function!\n");
}

---

Step 3 – Create the main program (main.c)

#include <stdio.h>
#include "myheader.h" // Use the custom header

int main() {
    sayHello(); // Call the function
    return 0;
}

---

Step 4 – Compile and Run

gcc main.c myheader.c -o main
./main

✅ Output:

Hello! I am your custom function!

---