comprehensive-rust/src/unsafe/writing-unsafe-functions.md

# Writing Unsafe Functions

You can mark your own functions as `unsafe` if they require particular conditions to avoid undefined
behaviour.

```rust,editable
/// Swaps the values pointed to by the given pointers.
///
/// # Safety
///
/// The pointers must be valid and properly aligned.
unsafe fn swap(a: *mut u8, b: *mut u8) {
    let temp = *a;
    *a = *b;
    *b = temp;
}

fn main() {
    let mut a = 42;
    let mut b = 66;

    // Safe because ...
    unsafe {
        swap(&mut a, &mut b);
    }

    println!("a = {}, b = {}", a, b);
}
```

<details>

We wouldn't actually use pointers for this because it can be done safely with references.

Note that unsafe code is allowed within an unsafe function without an `unsafe` block. We can
prohibit this with `#[deny(unsafe_op_in_unsafe_fn)]`. Try adding it and see what happens.

</details>