问题
I'm starting to learn about how to implement in C++ a TCP server/client (on Windows and Linux). At this moment, I'm implementing the server (and testing with the telnet as client). The server application is sending and recieving data like a charm.. (I will implement the client after understand all the server side). But I need to find a programmatic (C++ or O.S API) way to query the TCP socket state (ESTABLISHED, FIN_WAIT_2, CLOSE_WAIT, etc..) without use the write()/recv() to handle the error exception..
For example:
- Server starts, bind the socket and wait a connection
- A client starts and connect on the server
- Here, I have a loop at the server that use the func "ioctlsocket(socket, FIONREAD, pbytes_available)" to determine when I have data to read..
- If the client send some data, then "pbytes_available" will be > 1 and the server use recv() to get.
- If the client doesn't sends anything, then the server can do other tasks and check if there are data on the socket at the next loop.
- If I close the TELNET (client process) and run the "netstat", I'll see that the server socket is with the "CLOSE_WAIT" state, so I need to close the socket at the server side..
And this is the question: How can I query the TCP socket state to determine that I need close this session? (without use the send()/recv(), like the "netstat" do)
Note: I tried the "getsockopt(socket, SOL_SOCKET, SO_ERROR, &optval, &optlen )", but it return 0 when the state is ESTABLISHED/CLOSE_WAIT and the "optval" also doesn't changes.
回答1:
The usual technique with blocking-mode sockets is to dedicate a thread to reading the socket. When recv() returns 0 it means the peer has closed the connection and the port is now in CLOSE_WAIT state.
Alternatively you can use select() and friends to tell you when the socket is readable, which includes the case where recv() will return zero. Using FIONREAD as a polling mechanism is really pretty useless as it doesn't cover this case.
来源：https://stackoverflow.com/questions/21081172/determine-the-state-of-a-tcp-socket