How to properly use BytesMut with UnixDatagram recv/recv_from?

[xonatius]

I was wondering what is the correct way of recv-ing data into BytesMut from UnixDatagram. The only safe way to do it seems to be resize-ing the buffer to its capacity and then truncate-ing till the received length, but that will add a cost of initializing data which is not going to be read anyway. The other approach seems to be to use unsafe block and use set_len to buf capacity and then set_len to the actual amount of bytes read. The later approach avoids unnecessary initialization, but is it safe to do?

[Darksonn]

This is generally a problem with all IO sources. If you don't want to zero the memory first, the correct way to do it is to call BufMut::bytes_mut to obtain a reference into the extra capacity, then unsafely cast it to an &mut [u8], which you can then give to the datagram. You can then call BufMut::advance_mut after you know how much data was written into the slice.

[xonatius]

Thanks! As for set_len - is there any issue with using that approach?

[Darksonn]

The advance_mut method just calls set_len internally.

[xonatius]

the correct way to do it is to call BufMut::bytes_mut to obtain a reference into the extra capacity, then unsafely cast it to an &mut [u8],

Hm, that did not work for me, as I can't spawn a task with recv then as *mut [u8] is not send-able:

error[E0277]: `*mut [u8]` cannot be sent between threads safely

[Darksonn]

That error message does not seem related to BufMut::bytes_mut.