MSG_ [constants]
message flag constants
- MSG_CONFIRM - confirm path validity
- MSG_DONTROUTE - bypass gateway for packet transfer
- MSG_DONTWAIT - non-blocking I/O
- MSG_EOR - end of record
- MSG_MORE - sender will send more
- MSG_NOSIGNAL - do not generate SIGPIPE
- MSG_OOB - out-of-band data