Why does the raw parts not correspond to either endianness?

Question

I'm trying to optimize as much as possible an operation done on slices of u32 from arrays of u8. As such, I'm testing different options (for loops, iterators, using ByteOrder crate, etc.) As part of these tests, I also wanted to check out if I could improve it using from_raw_parts standard function. Here is my code:

use byteorder::{ByteOrder, BigEndian, LittleEndian};
use std::io::Read;

fn main(){
let random_bytes = (0..4).map(|_| { rand::random::<u8>() }).collect::<Vec<u8>>();
let random_bytes = random_bytes.as_slice();

let view = &random_bytes as *const _ as *const u32;
let slice: &[u32] = unsafe { std::slice::from_raw_parts(view, 1) };
println!("{:x?}", slice);
println!("{:x?}", LittleEndian::read_u32(&random_bytes[0..4]));
println!("{:x?}", BigEndian::read_u32(&random_bytes[0..4]));
println!("{:x?}", &random_bytes[0..4]);
}

I would've expected at least one one of the two Little or Big endian to be equal to the first print, but instead this does not seem to be the case, e.g. an example output

[d951db30]
143600ff
ff003614
[ff, 0, 36, 14]

What am I doing wrong?

Please be aware that this code is likely to lead to undefined behavior. See: https://stackoverflow.com/questions/33968870/temporarily-transmute-u8-to-u16 — E_net4, Dec 11 '20 at 10:21

score 5 · Accepted Answer · answered Dec 11 '20 at 10:18

The problem is here:

let view = &random_bytes as *const _ as *const u32;

A slice is a 2-machine-word struct that contains the pointer to data and the element count as members. By doing &random_bytes, you are taking a reference to this slice structure (which itself contains a pointer and length), not acquiring the underlying pointer.

Slices have an as_ptr method that returns the pointer to data itself. When you use it, your code functions correctly:

use byteorder::{ByteOrder, BigEndian, LittleEndian};
use std::io::Read;

fn main(){
    let random_bytes = (0..4).map(|_| { rand::random::<u8>() }).collect::<Vec<u8>>();
    let random_bytes = random_bytes.as_slice();
    
    let view = random_bytes.as_ptr() as *const u32;
    let slice: &[u32] = unsafe { std::slice::from_raw_parts(view, 1) };
    println!("{:x?}", slice);
    println!("{:x?}", LittleEndian::read_u32(&random_bytes[0..4]));
    println!("{:x?}", BigEndian::read_u32(&random_bytes[0..4]));
    println!("{:x?}", &random_bytes[0..4]);
}

Output in playground:

[684a2f5b]
684a2f5b
5b2f4a68
[5b, 2f, 4a, 68]

Damn it should've seen that, thank you very much for you time! — Dominus, Dec 11 '20 at 10:24

Why does the raw parts not correspond to either endianness?

1 Answers1