common:
    IP sockets => `man 7 ip`
    UNIX sockets => `man 7 unix`
    check both structs and implement both, then use #ifdef to selectively use one or the other
server:
    what to do when client disconnects?
client:
    put mutex in shared memory (it is used by processes, not by threads)
    make sure mutex lock for Listener is correct (look at ClientListener)