How to pass a socket connection between threads in an MFC application in Visual C++

MORE INFORMATION

Server

The server illustrates using sockets in multiple threads in an MFC application. The server listens for connections. When a new connection is requested, the server accepts the connection and then creates the thread to handle the connection. When the server receives a message, it reverses the message and sends it back to the client.

In MFC when you have a new connection it is necessary to accept the connection in the thread that the listening socket is in. The Accept call requires that you pass in a CAsyncSocket object. MFC then sets up everything correctly so that the connection can be handle in the thread. If you want to handle this connection in a different thread, just passing the MFC object to the thread will not work correctly. To correctly set up everything for the connection to be handled in a different thread, the following steps are required:

1. Detach the socket handle from the CAsyncSocket object in the thread where the connection was accepted.
2. Pass the socket handle to the thread.
3. In the thread, attach this handle to a CAsyncSocket derived object.

The Attach call in the thread sets up everything correctly for the socket notifications to be sent to the thread you want to handle the connection.

The following code from the Server project illustrates this:

OnAccept for the listening socket.

   void CListensoc::OnAccept(int nErrorCode)
   {
      // New connection is being established

      CSocket soc;

      // Accept the connection using a temp CSocket object.
      Accept(soc);

      // Create a thread to handle the connection.
      // The thread is created suspended so that we can
      // set variables in CConnectThread before it starts executing.
      CConnectThread* pThread =
         (CConnectThread*)AfxBeginThread(
            RUNTIME_CLASS(CConnectThread),
            THREAD_PRIORITY_NORMAL,
            0,
            CREATE_SUSPENDED);
   ...
      // Pass the socket to the thread by passing the socket handle.
      // You cannot pass a CSocket object across threads.
      pThread->m_hSocket = soc.Detach();

      // Now start the thread.
      pThread->ResumeThread();

      CAsyncSocket::OnAccept(nErrorCode);
   }
				

InitInstance of the thread.

   BOOL CConnectThread::InitInstance()
   {
   ...
      // Attach the socket handle to a CSocket object.
      // This makes sure that the socket notifications are sent
      // to this thread.
      m_socket.Attach(m_hSocket);
   ...
   }
				

The above code makes sure that the socket is set up correctly in the secondary thread.

Client

The client accepts a host name to connect to. Once the connection is made, the client allows you to send messages to the server. The client then displays the message returned from the server.

The server listens on port 9000. The client tries to connect to this port on the specified host.

The projects were created with Visual C++ version 5.0. However the code in the samples should apply to the versions of MFC mentioned above.

(c) Microsoft Corporation 1997, All Rights Reserved. Contributions by Sridhar S Madhugiri, Microsoft Corporation