Previously, using -U with -u was an error that was not documented in the man page. Now it will use a unix socket in datagram mode. Bidirectional unix datagram communication requires a socket at both ends, so in client mode (without -l), a temporary socket is created so that responses from the server can be received. If -s is specified with -U and -u, it specifies the location of the temporary socket to create. This was mostly written way back in 2007. Since then, various improvements implemented based on suggestions from guenther@, tedu@, and nicm@. Man page help from nicm@ and jmc@. Unix datagram support requires a small change to atomicio.c in order to function correctly, this will be committed separately shortly. OK nicm@