PRB: WSAAsyncSelect() Notifications Stop Coming

• Microsoft Win32 Software Development Kit (SDK), versions 3.1, 3.5, 3.51, and 4.0

SYMPTOMS

I have set a WSAAsyncSelect() call to notify me of read (FD_READ) and disconnection (FD_CLOSE). When a read call is posted on my message queue, I continually read from the socket until there are no more characters waiting. After each read, I use a select() call to determine if more data needs to be read. However, after a while, the notifications stop coming. Why is this?

CAUSE

The message queue must be cleared of extraneous notification messages for each read notification message.

RESOLUTION

Call WSAAsyncSelect( sockt, hWnd, 0, 0) to clear the message queue for each read notification.

MORE INFORMATION

Sample Code

WSA_READCLOSE:

  if (WSAGETSELECTEVENT( lParam ) == FD_READ) {

    FD_ZERO( &readfds );
    FD_SET( sockt, &readfds);

    timeout.tv_sec = 0;
    timeout.tv_usec = 0;

    /* Clear the queue of any extraneous notification messages. */

    WSAAsyncSelect( sockt, hWnd, 0, 0);

    while (select(0, &readfds, NULL, NULL, &timeout) != 0) {
      recv(sockt, &ch, 1, 0);
      }

    /* Reset the message notification. */

    WSAAsyncSelect( sockt, hWnd, WSA_READCLOSE, FD_READ | FD_CLOSE);
    }