如何使用Winsock API控制连接超时？

Question

I'm writing a program using the Winsock API because a friend wanted a simple program to check and see if a Minecraft server was running or not. It works fine if it is running, however if it is not running, the program freezes until, I'm assuming, the connection times out. Another issue is, if I have something like this (pseudo-code):

void connectButtonClicked()
{
     setLabel1Text("Connecting");
     attemptConnection();
     setLabel1Text("Done Connecting!");
}

it seems to skip right to attemptConnection(), completely ignoring whats above it. I notice this because the program will freeze, but it wont change the label to "Connecting".

Here is my actual connection code:

bool CConnectionManager::ConnectToIp(String^ ipaddr)
{
    if(!m_bValid)
        return false;

    const char* ip = StringToPConstChar(ipaddr);
    m_socket = socket(AF_INET, SOCK_STREAM, IPPROTO_TCP);

    if(isalpha(ip[0]))
    {
        ip = getIPFromAddress(ipaddr);
    }
    sockaddr_in service;
    service.sin_family = AF_INET;
    service.sin_addr.s_addr = inet_addr(ip);
    service.sin_port = htons(MINECRAFT_PORT);
    if(m_socket == NULL)
    {
        return false;
    }
    if (connect(m_socket, (SOCKADDR*)&service, sizeof(service)) == SOCKET_ERROR)
    {
        closesocket(m_socket);
        return false;
    }
    else
    {
        closesocket(m_socket);
        return true;
    }

    return true;
}

There is also code in the CConnectionManager's contructor to start up Winsock API and such.

So, how do I avoid this freeze, and allow me to update something like a progress bar during connection? Do I have to make the connection in a separate thread? I have only worked with threads in Java, so I have no idea how to do that :/

Also: I am using a CLR Windows Form Application
I am using Microsoft Visual C++ 2008 Express Edition

Answer 1

Your code does not skip the label update. The update simply involves issuing window messages that have not been processed yet, that is why you do not see the new text appear before connecting the socket. You will have to pump the message queue for new messages before connecting the socket.

As for the socket itself, there is no connect timeout in the WinSock API, unfortunately. You have two choices to implement a manual timeout:

1) Assuming you are using a blocking socket (sockets are blocking by default), perform the connect in a separate worker thread.

2) If you don't want to use a thread then switch the socket to non-blocking mode. Connecting the socket will always exit immediately, so your main code will not be blocked, then you will receive a notification later on if the connection was successful or not. There are several ways to detect that, depending on which API you use - WSAAsyncSelect(), WSAAsyncEvent(), or select().

Either way, while the connect is in progress, run a timer in your main thread. If the connect succeeds, stop the timer. If the timer elapses, disconnect the socket, which will cause the connect to abort with an error.

Answer 2

Maybe you want to read here:

To assure that all data is sent and received on a connected socket before it is closed, an application should use shutdown to close connection before calling closesocket. http://msdn.microsoft.com/en-us/library/ms740481%28v=VS.85%29.aspx

Since you are in the blocking mode there still might be some data...