newbie question - how to overlay a struct onto the packet bytes

[magicmouse]

when you get a packet, it is of type &[u8], an array of bytes.
I define a structure, like the ethernet header, and i want to then take a pointer to a structure, set to an offset in the packet bytes, and then access the fields of the struct. Rust doesn't let me do this because it is a violation of type matching. So i tried to use an unsafe{} block, but it still complains. I am sure there is a simple way to do this without copying the bytes in Rust, but it sure isn't obvious. When you are decoding packets this will be very common.

pub struct Ethernet {
    ether_dst_mac : [u8;6],
    ether_src_mac : [u8;6],
    ether_type : u16,
}

...
let packet = cap.next_packet().unwrap();
let mut ether_ptr : &Ethernet;
unsafe {
            ether_ptr = &*packet.data as &Ethernet;            
        }
println!("ethernet type is {}", ether_ptr.ether_type);

The above code doesn't compile, because apparently you can't put a record at a certain offset in a byte array, which in C is a trivial task. I will also need to convert to big endian the ether_type field, but i am sure there are std library functions in rust that convert a u16 to big endian. Where i am stuck is how to force Rust to allow me to point the struct into an array of bytes.

[Stargateur]

you could use an union but it's hard to advice with no MCVE.

[stappersg]

TIL: "minimal, complete and verifiable example" (MCVE)

[magicmouse]

The decoding of buffers is an extremely common task. The buffers come in an array of bytes, in this case above, the next_packet function is going to return a data buffer of some slice, which is an array of bytes.

As my code traverses across this buffer, i will need to view the next 10 or 20 bytes as some kind of record, maybe an ethernet record, maybe a TCP/IP header, etc., then there will be payload. In C, i can take the address of buf[10], and cast that as a pointer to a struct, and then reference the fields of the struct. zero overhead, and the only risk is going off the end of the array which i can check for externally. Why is Rust fighting me so hard in trying to cast a raw pointer to a pointer to a struct? This is such a basic task. In most languages you can tell the compiler, this is a pointer, convert it to a raw address and treat it as if it points to this kind of struct... there is a function in std::mem:: called transmute, but i can't seem to get it to work.

The Unsafe blocks don't seem to relax the compiler's fanaticism.

[Stargateur]

This is extensive way to do thing in rust, it's hard to advice one without know how you would use it exactly, here a SO ressource to help you https://stackoverflow.com/questions/42499049/how-to-transmute-a-u8-buffer-to-struct-in-rust, but be aware that I wouldn't advice it to a newbie, I would advice parsing like https://crates.io/crates/pktparse crates.