On a connected socket sendmsg_x sends a whole batch in one syscall;
msg_name is unsupported there, so batched sends require connecting to
the peer, which loses roaming and is therefore opt-in via
SetSinglePeerMode for single fixed-peer binds. recvmsg_x fills in
per-message source addresses, so batched receive works on unconnected
sockets too. Any unexpected errno permanently falls back to the generic
paths. iOS is excluded: in the Network Extension recvmsg_x on
unconnected UDP sockets delivers no data, and connected sockets stop
passing traffic after a rebind.
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