MSG_DONTWAIT (since Linux 2.2)
Enables nonblocking operation; if the operation would block, the
call fails with the error EAGAIN or EWOULDBLOCK. This provides
similar behavior to setting the O_NONBLOCK flag (via the fcntl(2)
F_SETFL operation), but differs in that MSG_DONTWAIT is a per-call
option, whereas O_NONBLOCK is a setting on the open file description
(see open(2)), which will affect all threads in the calling process
and as well as other processes that hold file descriptors referring
to the same open file description.
