> ## 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.

# Passing Pointers to Functions

> How to pass pointers to functions to modify original values

# Passing Pointers to Functions in C

When you pass a **pointer** to a function, you are sending the **address** of a variable. This allows the function to directly read or **modify the original variable** — unlike normal pass-by-value where only a copy is sent.

***

## Syntax

```c theme={null}
void function_name(int *ptr) {
    // use *ptr to access or modify the original value
}
```

***

## Example: Modify a Variable via Pointer

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

void changeValue(int *p) {
    *p = 100;   // dereference to change original
}

int main() {
    int x = 10;
    printf("Before: x = %d\n", x);

    changeValue(&x);   // pass address of x

    printf("After : x = %d\n", x);
    return 0;
}
```

**Output:**

```
Before: x = 10
After : x = 100
```

***

## Example: Swap Two Numbers

Without pointers, swapping inside a function has no effect on the originals. With pointers:

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

void swap(int *a, int *b) {
    int temp = *a;
    *a = *b;
    *b = temp;
}

int main() {
    int x = 5, y = 10;
    printf("Before: x=%d, y=%d\n", x, y);
    swap(&x, &y);
    printf("After : x=%d, y=%d\n", x, y);
    return 0;
}
```

**Output:**

```
Before: x=5, y=10
After : x=10, y=5
```

***

## Pass by Value vs Pass by Reference

```c theme={null}
void noEffect(int n) {
    n = 999;           // only changes local copy
}

void withEffect(int *n) {
    *n = 999;          // changes original
}

int main() {
    int a = 10, b = 10;
    noEffect(a);       // a is still 10
    withEffect(&b);    // b becomes 999
    printf("%d %d", a, b);   // Output: 10 999
}
```

***

## Key Points

* Pass `&variable` at the call site to send the address
* Use `*ptr` inside the function to access or modify the value
* This is how C achieves **pass-by-reference** behaviour
* Especially useful for functions that need to return multiple values