> ## 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.

# User-Defined Functions

> Creating and using your own functions in C

# User-Defined Functions in C

A **function** is a block of code that performs a specific task and can be reused whenever needed. Functions help break a large program into smaller, manageable parts and eliminate code repetition.

**Types of functions in C:**

1. **Predefined Functions** — Built-in, e.g. `printf()`, `scanf()`, `sqrt()`
2. **User-Defined Functions** — Created by the programmer

***

## Structure of a Function

### 1. Function Declaration (Prototype / Signature)

Tells the compiler the function's name, return type, and parameter types — without a body. Placed before `main()`.

```c theme={null}
returnType functionName(parameter list);

// Example:
int add(int a, int b);
```

### 2. Function Definition

The actual function body — what the function does.

```c theme={null}
returnType functionName(parameter list) {
    // function body
}

// Example:
int add(int a, int b) {
    return a + b;
}
```

### 3. Function Call

Executes the function from somewhere in the program.

```c theme={null}
int result = add(5, 3);
```

***

## Complete Example

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

// Declaration
int add(int a, int b);

int main() {
    int result = add(10, 20);
    printf("Sum = %d", result);
    return 0;
}

// Definition
int add(int a, int b) {
    return a + b;
}
// Output: Sum = 30
```

***

## Types of Function Based on Arguments and Return

| Type                       | Arguments | Returns |
| -------------------------- | --------- | ------- |
| No argument, no return     | None      | None    |
| No argument, with return   | None      | Yes     |
| With argument, no return   | Yes       | None    |
| With argument, with return | Yes       | Yes     |

**Examples:**

```c theme={null}
void greet();              // no argument, no return
int  getYear();            // no argument, returns a value
void display(int x);       // takes argument, returns nothing
int  add(int a, int b);    // takes arguments, returns a value
```

***

## Why Use Functions?

* **Reusability** — Write once, use anywhere in the program
* **Modularity** — Break complex programs into smaller pieces
* **Readability** — Code becomes easier to understand
* **Easy debugging** — Isolate and fix bugs in specific functions
* **Avoid repetition** — No need to rewrite the same logic

***

## Key Rules

* Arguments in C are passed **by value** by default (changes inside the function don't affect the original unless pointers are used)
* A function without a `return` statement that has a non-void return type will return a **garbage value**
* Function prototypes allow defining the function **after** `main()`