check a receiver socket is alive: tcp^udp

Q: given a TCP receiver socket, how do you tell if it’s connected to a session or disconnected?

Shanyou said when you recv() on the socket but got 0 it means disconnected.

http://man7.org/linux/man-pages/man2/recv.2.html#RETURN_VALUE shows recv() return value of 0 indicates dead connection i.e. disconnected.

https://stackoverflow.com/questions/4142012/how-to-find-the-socket-connection-state-in-c uses getsockopt()

Q: given a UDP multicast receiver socket, how do you tell if it’s still has a live subscription to the multicast group?

%%A: I guess you can use getsockopt() to check socket /aliveness/. If alive but no data, then the group is quiet

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