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

If you don't want to copy the data to the struct but instead leave it in place, you can use slice::align_to. This creates a &MyStruct instead:

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

fn main() {
    let v = vec![1u8, 2, 3];

    // I copied this code from Stack Overflow
    // without understanding why this case is safe.
    let (head, body, _tail) = unsafe { v.align_to::<MyStruct>() };
    assert!(head.is_empty(), "Data was not aligned");
    let my_struct = &body[0];

    println!("{:?}", my_struct);
}

Here, it's safe to use align_to to transmute some bytes to MyStruct because we've used repr(C, packed) and all of the types in MyStruct can be any arbitrary bytes.

See also:

• How to read a struct from a file in Rust?
• Can I take a byte array and deserialize it into a struct?

Answer 2

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.

Answer 3

I gave up on the transmute stuff. *mut (raw pointers) in Rust are pretty similar to C pointers, so this was easy:

#[repr(C, packed)] // necessary
#[derive(Debug, Copy, Clone)] // not necessary
struct MyStruct {
    foo: u16,
    bar: u8,
}

fn main() {
    let v: Vec<u8> = vec![1, 2, 3];
    let buffer = v.as_slice();
    let mut s_safe: Option<&MyStruct> = None;
    let c_buf = buffer.as_ptr();
    let s = c_buf as *mut MyStruct;
    unsafe {
        let ref s2 = *s;
        s_safe = Some(s2);
    }
    println!("here is the struct: {:?}", s_safe.unwrap());
}

The unsafe tag there is no joke, but the way I'm using this, I know my buffer is filled and take the proper precautions involving endianness later on.

Answer 4

I am currently using:

unsafe { transmute::<[u8; 0x70], Header>(header_data) };

It's unsafe ofcourse, but works well.

let mut header_data = [0; 0x70];
reader.seek(SeekFrom::Start(0))?;
reader.read_exact(&mut header_data)?;
let header = unsafe { transmute::<[u8; 0x70], Header>(header_data) };