BUG: Extra CN_RECEIVE/CN_TRANSMIT Events

• Microsoft Windows Software Development Kit (SDK) for Windows, version 3.1

SYMPTOMS

Using EnableCommNotification() to enable WM_COMMNOTIFY messages for CN_RECEIVE or CN_TRANSMIT events can cause spurious WM_COMMNOTIFY messages. At higher baud rates, this problem can cause a system to crash and reboot.

WM_COMMNOTIFY messages are received with a 0 (zero) value for the NotifyStatus parameter. The system crashes and reboots while receiving or transmitting data.

RESOLUTION

If the application is experiencing these symptoms, possible solutions are:

• Extra WM_COMMNOTIFY messages with NotifyStatus=0 can be ignored.
• If the system is crashing, change the application to NOT enable CN_RECEIVE and CN_TRANSMIT events.
• Enable the CN_EVENT notification for the EV_RXCHAR event by calling SetCommEventMask() then EnableCommNotification(). EV_RXCHAR is generated when any character is received and put into the queue. Make sure that EnableCommNotification() is called with -1 in both the cbWriteNotify and cbOutQueue parameters, such that the CN_RECEIVE and CN_TRANSMIT events are not enabled. This method is demonstrated in the TTY sample in the Windows Software Development Kit (SDK).
• Instead of using EnableCommNotification, the application could poll for data. This could be implemented in the message loop of the application, or using a timer.
• Modify COMM.DRV to work around this problem.

STATUS

This problem has been reported as a bug. There will not be a fix for COMM.DRV in Windows version 3.1.

MORE INFORMATION

CN_RECEIVE events are generated when the number of bytes in the receive queue exceeds the threshold (cbWriteNotify) set in EnableCommNotification() or when a time-out occurs. After a CN_RECEIVE has been generated for exceeding the cbWriteNotify threshold, another CN_RECEIVE should not be generated until the number of bytes drops below cbWriteNotify then exceeds it again.

CN_TRANSMIT events are generated similarly to CN_RECEIVE. The notification threshold of the output queue is set in the cbOutQueue parameter of EnableCommNotify. If the number of bytes in the output queue drops below cbOutQueue, a CN_TRANSMIT should be generated. A new CN_TRANSMIT should not be generated until the output queue exceeds, then drops below cbOutQueue again.

However, if interrupts arrive fast enough, extra "threshold" CN_RECEIVEs (or, similarly, CN_TRANSMITs) may be generated before the size of the output queue drops below then again exceeds cbWriteNotify. These extra events can be ignored; however, this characteristic may cause the system to reboot, especially at higher baud rates.