data structures - Transmuting u8 buffer to struct in Rust

Question

I have a byte buffer of unknown size, and I want to create a local struct variable pointing to the memory of the beginning of the buffer. Following what I'd do in C, I tried a lot of different things in Rust and kept getting errors. This is my latest attempt:

use std::mem::{size_of, transmute};

#[repr(C, packed)]
struct MyStruct {
    foo: u16,
    bar: u8,
}

fn main() {
    let v: Vec<u8> = vec![1, 2, 3];
    let buffer = v.as_slice();
    let s: MyStruct = unsafe { transmute(buffer[..size_of::<MyStruct>()]) };
}

I get an error:

error[E0277]: the size for values of type `[u8]` cannot be known at compilation time
   --> src/main.rs:12:42
    |
12  |     let s: MyStruct = unsafe { transmute(buffer[..size_of::<MyStruct>()]) };
    |                                          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ doesn't have a size known at compile-time
    |
    = help: the trait `std::marker::Sized` is not implemented for `[u8]`
    = note: to learn more, visit <https://doc.rust-lang.org/book/ch19-04-advanced-types.html#dynamically-sized-types-and-the-sized-trait>

Answer 1

You can use methods on raw pointers and functions in std::ptr to directly read/write objects in place.

• std::ptr::read
• std::ptr::read_unaligned
• std::ptr::write
• std::ptr::write_unaligned

In your case:

fn main() {
    let v: Vec<u8> = vec![1, 2, 3];
    let s: MyStruct = unsafe { std::ptr::read(v.as_ptr() as *const _) };
    println!("here is the struct: {:?}", s);
}

I would encourage you to wrap this in a reusable method and perform a length check on the source buffer before attempting the read.