> There's still the problem of sending to multiple destinations: OK sendmmsg() can send multiple datagrams, but for a given socket.

Hmm? sendmsg takes the destination address in the `struct msghdr` structure, and sendmmsg takes an array of those structures.

At the same time, the discussion of efficiency is about UDP vs. TCP. TCP writes are per socket, to the connected peer, and so UDP has the upper hand here. The concerns were about how TCP allows giving a large buffer to the kernel in a single write that then gets sliced into smaller packets automatically, vs. having to slice it in userspace and call send more, which sendmmsg solves.

(You can of course do single-syscall or even zero-syscall "send to many" with io_uring for any socket type, but that's a different discussion.)