DOCUMENT:Q108309  03-NOV-1999  [win16ddk]
TITLE   :FIX: Posting WM_COMMNOTIFY Messages May Fail
PRODUCT :Microsoft Windows Device Driver Kit
PROD/VER::3.1
OPER/SYS:
KEYWORDS:

======================================================================
-------------------------------------------------------------------------------
The information in this article applies to:

 - Microsoft Windows Device Development Kit (DDK) for Windows, version 3.1 
-------------------------------------------------------------------------------

SYMPTOMS
========

When using WM_COMMNOTIFY messages in a communications application, these
messages may appear to have stopped being posted.

CAUSE
=====

When COMM.DRV calls PostMessage(), it does not check the return value. Even if
PostMessage() fails, it appears to COMM.DRV that the message was sent.

For CN_EVENT notifications, the event must be cleared using GetCommEventMask()
before a new CN_EVENT notification will be sent. If the application never
receives the CN_EVENT notification, the event never gets cleared, so COMM.DRV
will not send a new CN_EVENT notification. If this happens, the notification
process appears to have stopped.

Similarly, for CN_RECEIVE and CN_TRANSMIT notifications, if these notifications
are not received, the application may not know to do a ReadComm() or
WriteComm(). If ReadComm() or WriteComm() is never called, the character level
will never cross the buffer threshold to create a new notification. For more
information on how CN_RECEIVE and CN_TRANSMIT notifications are generated, query
on the following words in the Microsoft Knowledge Base:

   " docerr and CN_RECEIVE and events and generated " (without the quotation
   marks)


RESOLUTION
==========

Implement a polling strategy to determine communications status, in addition to
using WM_COMMNOTIFY messages. This could be implemented in the message loop of
the application, or by using a timer.

STATUS
======

Microsoft has confirmed this to be a problem in Windows version 3.1. This bug
has been corrected in later versions of Microsoft Windows.

Additional query words: 3.10 buglist3.10

======================================================================
Keywords          :  
Technology        : kbAudDeveloper kbWin3xSearch kbWinDDKSearch kbWinDDK310
Version           : :3.1
Solution Type     : kbfix

=============================================================================

THE INFORMATION PROVIDED IN THE MICROSOFT KNOWLEDGE BASE IS
PROVIDED "AS IS" WITHOUT WARRANTY OF ANY KIND.  MICROSOFT DISCLAIMS
ALL WARRANTIES, EITHER EXPRESS OR IMPLIED, INCLUDING THE WARRANTIES
OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE.  IN NO
EVENT SHALL MICROSOFT CORPORATION OR ITS SUPPLIERS BE LIABLE FOR
ANY DAMAGES WHATSOEVER INCLUDING DIRECT, INDIRECT, INCIDENTAL,
CONSEQUENTIAL, LOSS OF BUSINESS PROFITS OR SPECIAL DAMAGES, EVEN IF
MICROSOFT CORPORATION OR ITS SUPPLIERS HAVE BEEN ADVISED OF THE
POSSIBILITY OF SUCH DAMAGES.  SOME STATES DO NOT ALLOW THE EXCLUSION
OR LIMITATION OF LIABILITY FOR CONSEQUENTIAL OR INCIDENTAL DAMAGES
SO THE FOREGOING LIMITATION MAY NOT APPLY.

Copyright Microsoft Corporation 1999.