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Q101420: BUG: Extra CN_RECEIVE/CN_TRANSMIT Events

Article: Q101420
Product(s): Microsoft Windows Software Development Kit
Version(s): WINDOWS:3.1
Operating System(s): 
Keyword(s): 
Last Modified: 06-NOV-1999

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The information in this article applies to:

- Microsoft Windows Software Development Kit (SDK) 3.1 
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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.

Additional query words: buglist3.10 3.10

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Keywords          :  
Technology        : kbAudDeveloper kbWin3xSearch kbSDKSearch kbWinSDKSearch kbWinSDK310
Version           : WINDOWS:3.1

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