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I'm using the Boost::asio to implement a client/server applicaion. The client code below is used to connect to the remote server . 

   try
    {
        boost::asio::io_service         m_io_service;
        boost::asio::ip::tcp::socket    m_socket(m_io_service);
        boost::asio::ip::tcp::endpoint endpoint(boost::asio::ip::address::from_string("127.0.0.1"), 17);
        m_socket.connect(endpoint);
    }
    catch (std::exception& e)
    {
        std::cerr << e.what() << std::endl;
    }

At the client side, I want to check if the connection is live. The function "m_socket.is_open();" doesn't work. When the server socket is closed, the "m_socket.is_open();" still returns true at client side. Is there any way to check the connection?

Sam Miller
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Jeffrey
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    I had a similar doubt here: http://stackoverflow.com/q/1511129/174605 and there is a fine answer by joshperry – coelhudo Mar 29 '12 at 19:47

2 Answers2

7

Due to the limitation of the underlying socket interface, there is no way to implement the function like isConnected to check the TCP connection status at the socket level. I think out a workaround about it.

In my implementation, I cache a connection status flag (bool m_IsConnected) in my application. This flag is used to designate the connection status. It assumes that if there is no error from socket, the TCP connection is alive.

The flag will be updated every time when use the socket. If there are errors when send and read data, that means the connection is disconnected. Then change the flag correspondingly. If the TCP connection is idle for a long time. This flag doesn't reflect the actual status of the TCP connection until using the socket. For example, the socket is idle for a long time. It is disconnected due to the poor network. In this case, the m_IsConnected is still true since we don't get any callback regarding the disconnection event. When try to send data through this socket, there will be error. And now we know that the connection is disconnected.

Jeffrey
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1

This is a limitation of the underlying socket interface (I think both for Winsock/Bekerly). You could invent a message specifically for this purpose, that the server answers on if it is alive. Else if you get a timeout, means connection is down.

EDIT: As Joachim pointed out, trying to read the socket might be a better way.

Rolle
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    Trying to send to a connection that has been closed will give give an error, even if it was a clean and proper close. Trying to _read_ from the socket will give a better clue about how the socket was closed (i.e. `read` or `recv` returns zero for a clean close or an error when there is an error.) – Some programmer dude Mar 29 '12 at 06:54
  • Ah yes you are right, I forgot you could read it as well. Will update the answer above. – Rolle Mar 29 '12 at 06:58
  • The free function boost::asio::read can be used to read the socket. I think there is side-effect if using the read function the detect the connection status. It will read the data from the socket stream, which is I don't want in a status detection function. Furthermore, if the connection is alive, the read function blocks. – Jeffrey Mar 29 '12 at 07:36
  • You could use a read with a very short timeout. And if you read on the right times, that is when you know there will be no data cause you havent requested anything from the server, it should not get in the way. Anyway, maybe it works better in your case to do what I suggested first. – Rolle Mar 29 '12 at 08:49