Unsafe Functions

Calling Unsafe Functions

A function or method can be marked unsafe if it has extra preconditions you must uphold to avoid undefined behaviour:

extern "C" {
    fn abs(input: i32) -> i32;
}

fn main() {
    let emojis = "🗻∈🌏";

    // SAFETY: The indices are in the correct order, within the bounds of the
    // string slice, and lie on UTF-8 sequence boundaries.
    unsafe {
        println!("emoji: {}", emojis.get_unchecked(0..4));
        println!("emoji: {}", emojis.get_unchecked(4..7));
        println!("emoji: {}", emojis.get_unchecked(7..11));
    }

    println!("char count: {}", count_chars(unsafe { emojis.get_unchecked(0..7) }));

    // SAFETY: `abs` doesn't deal with pointers and doesn't have any safety
    // requirements.
    unsafe {
        println!("Absolute value of -3 according to C: {}", abs(-3));
    }

    // Not upholding the UTF-8 encoding requirement breaks memory safety!
    // println!("emoji: {}", unsafe { emojis.get_unchecked(0..3) });
    // println!("char count: {}", count_chars(unsafe {
    // emojis.get_unchecked(0..3) }));
}

fn count_chars(s: &str) -> usize {
    s.chars().count()
}

Writing Unsafe Functions

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

/// 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;

    // SAFETY: ...
    unsafe {
        swap(&mut a, &mut b);
    }

    println!("a = {}, b = {}", a, b);
}
This slide should take about 5 minutes.

Calling Unsafe Functions

get_unchecked, like most _unchecked functions, is unsafe, because it can create UB if the range is incorrect. abs is incorrect for a different reason: it is an external function (FFI). Calling external functions is usually only a problem when those functions do things with pointers which might violate Rust's memory model, but in general any C function might have undefined behaviour under any arbitrary circumstances.

The "C" in this example is the ABI; other ABIs are available too.

Writing Unsafe Functions

We wouldn't actually use pointers for a swap function - 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. This will likely change in a future Rust edition.