Comment by knorker

Sure, I would strongly recommend against doing something like that. But I would expect it to work in the obvious way, and not be undefined behavior.

E.g. if UdpSocket were to dup() internally its fd A into a fd B, and as_fd() returned B, but all actual recv/send is on fd A, then that would cause worse problems than this.

But say an OS has a sockopt that turns an IPv4 UDP socket into a IPv6 UDP socket. Would it be OK for me to call that on UdpSocket's underlying fd? I'd say yes.

Now if I closed the fd for a UdpSocket from underneath it, I would expect that to be basically UB, if not by spec, then in practice impossible to reason about.

As for unsound, sure. I could be convinced of calling the dup2 thing unsound. Not sure unsound is well enough defined, but basically: don't do it.

Is it unsound to create a UdpSocket from a non-UDP file descriptor? Not in a way that can trigger unsafe, no.