The flags argument to send and recv is a bit
mask. You can bitwise-OR the values of the following macros together
to obtain a value for this argument. All are defined in the header
file sys/socket.h.
Look at the data but don't remove it from the input queue. This is
only meaningful with input functions such as recv, not with
send.
— Macro: int MSG_DONTROUTE
Don't include routing information in the message. This is only
meaningful with output operations, and is usually only of interest for
diagnostic or routing programs. We don't try to explain it here.