the listen() function

This is a discussion on the listen() function within the Networking/Device Communication forums, part of the General Programming Boards category; Is the listen() function usually blocking? And if I choose to unblock it, and do a WSAAsyncSelect on the used ...

  1. #1
    Politics&Cpp geek Da-Nuka's Avatar
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    the listen() function

    Is the listen() function usually blocking?
    And if I choose to unblock it, and do a WSAAsyncSelect on the used socket, how can I later continue the accept process?



    Or even shorter and simpler:
    How can I easily listen for a client, and connect to it, unblocking?

    (Im expecting just a quick and little example)

  2. #2
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    I haven't done any thing with Winsock (only a little bit with *nix sockets actually). Basicly with nonblocking sockets, you pretty much just need to keep checking if a client has requested, which means looping. I can't say any thing more then that, as I truthfully don't know where to got from there (heck that is probably a bit off any how), but it might start giving you an idea.
    If any part of my post is incorrect, please correct me.

    This post is not guarantied to be correct, and is not to be taken as a matter of fact, but of opinion or a guess, unless otherwise noted.

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    Is the listen() function usually blocking?
    No, it isn't.

  4. #4
    carry on JaWiB's Avatar
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    >>And if I choose to unblock it, and do a WSAAsyncSelect on the used socket, how can I later continue the accept process?

    With async sockets, handle the FD_ACCEPT message, ie:
    Code:
    //set up async server socket
    if (WSAAsyncSelect(hSocket, hwnd, WM_WSAASYNC, FD_READ |  
                                            FD_ACCEPT | FD_CLOSE)!=0)
    {
      //handle error
    }
    sockaddr_in	sockAddr = {0};
    SetServerSockAddr(&sockAddr, SERVER_PORT);
    if (bind(hSocket, reinterpret_cast<sockaddr*>(&sockAddr), sizeof(sockAddr))!=0)
      //handle error
    if (listen(hSocket, SOMAXCONN)!=0) //set socket to listen for incoming data
      //handle error
    Code:
    //in message loop
     case FD_ACCEPT:
      if (WSAGETSELECTERROR(lParam))
      {
        MessageBox(NULL,"error accepting connection","FD_ACCEPT Error",MB_OK);
      }  
      //accept connection
    break;
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    listen() can only hold up to five requests in a queue, so if there are many requests at one time, listen() deny everybody but the first five, right? Do web servers, like Apache, use listen()?

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    No. Some BSD implementations limit the values passed to listen() to 5, but most *nix systems use the value SO_MAXCONN to determine the maximum listen value (128 on my implementation).

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    Is that a no to my first or second question?

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    That is a no to your comment that listen() can only hold up to five requests in a queue.

    Apache does use the listen function.

  9. #9
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    Thanks.

    I've heard that Apache can handle many incoming connections at the same time. How does it get around the limited queue size?

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    listen() doesn't put a cap on incoming connections, but rather puts a cap on sockets waiting to be accepted. A fast server would be hard pressed to let more than a couple sockets pile up in the queue, so it's not really a problem. Applications like apache, usually have memory pre-allocated for new connections as well, so it allows the program to accept new connections ever faster.

    Also, if the queue is full, and a new connection attempt fails (receiving ECONNREFUSED), it can just silently attempt a reconnect.

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