> ## 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.

# Predefined Functions

> Built-in library functions in C and their header files

# Predefined Functions in C

**Predefined functions** (also called library functions or built-in functions) are functions that are already written, compiled, and provided by C's standard libraries. You use them by including the appropriate **header file** with `#include`.

***

## Syntax

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

int main() {
    functionName(parameters);
    return 0;
}
```

***

## Common Library Categories

### 1. Input / Output — `<stdio.h>`

| Function    | Description                           |
| ----------- | ------------------------------------- |
| `printf()`  | Prints formatted output to screen     |
| `scanf()`   | Reads formatted input from user       |
| `getchar()` | Reads a single character              |
| `putchar()` | Prints a single character             |
| `fgets()`   | Reads a string (safe, handles spaces) |

```c theme={null}
#include <stdio.h>
int main() {
    printf("Hello, World!");
    return 0;
}
```

***

### 2. String Handling — `<string.h>`

| Function         | Description                               |
| ---------------- | ----------------------------------------- |
| `strlen(s)`      | Returns length of string (excluding `\0`) |
| `strcpy(d, s)`   | Copies string `s` into `d`                |
| `strcat(d, s)`   | Appends `s` to end of `d`                 |
| `strcmp(s1, s2)` | Compares two strings (returns 0 if equal) |
| `strrev(s)`      | Reverses a string (Turbo C)               |

***

### 3. Math Functions — `<math.h>`

| Function    | Description                    |
| ----------- | ------------------------------ |
| `sqrt(x)`   | Square root of x               |
| `pow(x, y)` | x raised to power y            |
| `abs(x)`    | Absolute value of integer x    |
| `fabs(x)`   | Absolute value of float x      |
| `ceil(x)`   | Rounds up to nearest integer   |
| `floor(x)`  | Rounds down to nearest integer |

```c theme={null}
#include <stdio.h>
#include <math.h>
int main() {
    printf("sqrt(25) = %.0f\n", sqrt(25));   // 5
    printf("pow(2,10)= %.0f\n", pow(2, 10)); // 1024
    printf("abs(-7)  = %d\n",   abs(-7));    // 7
    return 0;
}
```

***

### 4. Standard Library — `<stdlib.h>`

| Function             | Description                           |
| -------------------- | ------------------------------------- |
| `malloc(size)`       | Allocates memory at runtime           |
| `calloc(n, size)`    | Allocates and zero-initializes memory |
| `realloc(ptr, size)` | Resizes allocated memory              |
| `free(ptr)`          | Frees allocated memory                |
| `rand()`             | Generates a pseudo-random number      |
| `exit(code)`         | Terminates the program immediately    |
| `atoi(s)`            | Converts string to integer            |

***

### 5. Character Handling — `<ctype.h>`

| Function     | Description                |
| ------------ | -------------------------- |
| `isalpha(c)` | Is the character a letter? |
| `isdigit(c)` | Is the character a digit?  |
| `isupper(c)` | Is it uppercase?           |
| `islower(c)` | Is it lowercase?           |
| `toupper(c)` | Convert to uppercase       |
| `tolower(c)` | Convert to lowercase       |

***

### 6. Time Functions — `<time.h>`

| Function     | Description                                 |
| ------------ | ------------------------------------------- |
| `time(NULL)` | Returns current time as seconds since epoch |
| `clock()`    | Returns processor time used by the program  |

***

## Example: Using Multiple Libraries

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

int main() {
    char name[] = "Vishal";
    printf("Name length: %d\n", strlen(name));   // 6
    printf("sqrt(49)  : %.0f\n", sqrt(49));      // 7
    return 0;
}
```