When allocb(9F) fails and bufcall(9F) is called, a callback is pending until a buffer is actually returned. Since this callback is asynchronous, it must be released before all processing is complete. To release this queued event, use unbufcall(9F).
Pass the id returned by bufcall(9F) to unbufcall(9F). Then close the driver in the normal way. If this sequence of unbufcall(9F) and xxclose is not followed, a situation exists where the callback can occur and the driver is closed. This is one of the most difficult types of bugs to find during the debugging stage.
All bufcall(9F) and timeouts must be canceled in the close routine.