socket: how to detect peer's shutdown(SD_RECEIVE) by select?

When server calls shutdown(SD_SEND), client can detect it by select and recv(returns 0). But how can client detect shutdown(SD_RECEIVE)?

2 answers

  • answered 2022-01-23 02:31 Remy Lebeau

    If the server shuts down its receive channel, it can't receive any further data anymore. There is no way to detect this condition with select(), except that the socket simply won't be reported as writable anymore. There is no signal sent to the client in this situation (unlike shutting down the send channel, which sends a FIN packet indicating no more data will be sent). So, any attempts by the client to send data afterwards will either be buffered indefinitely while waiting for the server to acknowledge there is room to receive, or more likely will simply fail with an error.

  • answered 2022-01-23 02:31 paulsm4

    Short answer: there's no magic API the client can use to detect if the (Microsoft!) server happened to call shutdown(SD_RECV). However, all subsequent sends from the client to the server on that socket will fail.

    Longer answer:

    • shutdown(SD_RECV) is a Windows thing. It doesn't necessarily pertain to sockets in general, and certainly not to TCP/IP itself. There is no TCP-level FIN or RST. There's no "message" to the client.

    • From the documentation:

    https://docs.microsoft.com/en-us/windows/win32/api/winsock/nf-winsock-shutdown

    The shutdown function does not close the socket. Any resources attached to the socket will not be freed until closesocket is invoked.

    Another method to wait for notification that the remote end has sent all its data and initiated a graceful disconnect uses overlapped receive calls follows :

    1. Call shutdown with how=SD_SEND.
    2. Call recv or WSARecv until the function completes with success and indicates zero bytes were received. If SOCKET_ERROR is returned, then the graceful disconnect is not possible.
    3. Call closesocket.

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