How to unsafely increase the size of a mutable slice of bytes?

Question

I have this function:

use std::io;

pub fn recv(mut buf: &mut [u8]) -> io::Result<usize> {
    let size_to_extend = 50; // I want to increase the size of "buf" by 50
    unsafe {
        /* ??? */
    }
}

How can I adjust the size of the array buf, even though it's a parameter? It is necessary that this method needs to happen.

Answer 1

What you are attempting to do is all-but-guaranteed to cause undefined behavior. Find a better API.

---

If you want the changes to be reflected outside of the function, you are out of luck. There's no way that this function signature will allow for that to happen, for the same reason that fn foo(x: i32) won't allow you to change the value passed in as observed by the caller.

If you just need this inside of the function, use slice::from_raw_parts. I've marked the entire function as unsafe because certain inputs will cause undefined behavior and this code cannot possibly guard against it:

use std::slice;

pub unsafe fn recv(buf: &mut [u8]) {
    let size_to_extend = 50;

    let ptr = buf.as_mut_ptr();
    let len = buf.len();
    let bad_idea = slice::from_raw_parts_mut(ptr, len + size_to_extend);

    for b in bad_idea.iter_mut() {
        *b = 10;
    }
}

If you can change the API, something like this works to expose the change outside the function:

pub unsafe fn recv(buf: &mut &mut [u8]) {
    let size_to_extend = 50;

    let ptr = buf.as_mut_ptr();
    let len = buf.len();
    let bad_idea = slice::from_raw_parts_mut(ptr, len + size_to_extend);

    for b in bad_idea.iter_mut() {
        *b = 10;
    }

    *buf = bad_idea;
}

See also:

• How can I get an array or a slice from a raw pointer?