> ## Documentation Index
> Fetch the complete documentation index at: https://learn.acadlink.app/llms.txt
> Use this file to discover all available pages before exploring further.

# Using #include

> How to include header files in C programs

# `#include` Directive in C

The `#include` directive tells the preprocessor to **copy the contents of another file** into the current source file before compilation. This is how you bring standard library functions and your own header files into a program.

***

## Syntax

**For standard (system) headers — use angle brackets:**

```c theme={null}
#include <header_file.h>
```

**For your own (user-defined) headers — use double quotes:**

```c theme={null}
#include "my_header.h"
```

The difference: angle brackets search the system include directories; double quotes search the current directory first, then system directories.

***

## Common Standard Header Files

| Header        | Contents                                           |
| ------------- | -------------------------------------------------- |
| `<stdio.h>`   | `printf()`, `scanf()`, `fopen()`, file I/O         |
| `<stdlib.h>`  | `malloc()`, `free()`, `exit()`, `rand()`           |
| `<string.h>`  | `strlen()`, `strcpy()`, `strcat()`, `strcmp()`     |
| `<math.h>`    | `sqrt()`, `pow()`, `abs()`, `ceil()`, `floor()`    |
| `<ctype.h>`   | `isalpha()`, `isdigit()`, `toupper()`, `tolower()` |
| `<time.h>`    | `time()`, `clock()`                                |
| `<stdbool.h>` | `bool`, `true`, `false` (C99+)                     |

***

## Example: Including Multiple Headers

```c theme={null}
#include <stdio.h>
#include <stdlib.h>
#include <math.h>
#include <string.h>

int main() {
    // stdio.h
    printf("Hello!\n");

    // math.h
    printf("sqrt(16) = %.0f\n", sqrt(16));

    // string.h
    char name[] = "Vishal";
    printf("Length = %d\n", strlen(name));

    // stdlib.h
    int r = rand() % 100;
    printf("Random = %d\n", r);

    return 0;
}
```

***

## Creating Your Own Header File

**`mymath.h`:**

```c theme={null}
#ifndef MYMATH_H
#define MYMATH_H

int square(int x);
int cube(int x);

#endif
```

**`mymath.c`:**

```c theme={null}
#include "mymath.h"

int square(int x) { return x * x; }
int cube(int x)   { return x * x * x; }
```

**`main.c`:**

```c theme={null}
#include <stdio.h>
#include "mymath.h"   // user header — double quotes

int main() {
    printf("Square of 4 = %d\n", square(4));
    printf("Cube   of 3 = %d\n", cube(3));
    return 0;
}
```

***

## Include Guards

Header files use include guards to prevent being included more than once (which would cause redefinition errors):

```c theme={null}
#ifndef MY_HEADER_H
#define MY_HEADER_H

// declarations here

#endif
```

Or the modern equivalent (most compilers support it):

```c theme={null}
#pragma once
```