when trying to establish a connection on a socket which already has one, or when trying to send a datagram with the destination address specified and the socket is already connected;
[ENOTCONN]
when trying to send a datagram, but no destination address is specified, and the socket has not been connected;
[ENOBUFS]
when the system runs out of memory for an internal data structure;
[EADDRINUSE]
when an attempt is made to create a socket with a port which has already been allocated;
[EADDRNOTAVAIL]
when an attempt is made to create a socket with a network address for which no network interface exists.
(udp.checksum) Enable udp checksums (enabled by default).
UDPCTL_MAXDGRAM
(udp.maxdgram) Maximum outgoing UDP datagram size
UDPCTL_RECVSPACE
(udp.recvspace) Maximum space for incoming UDP datagrams
udp.log_in_vain
For all udp datagrams, to ports on which there is no socket listening, log the connection attempt (disabled by default).
udp.blackhole
When a datagram is received on a port where there is no socket listening, do not return an ICMP port unreachable message. (Disabled by default. See blackhole 4.)
