DOCUMENT:Q65928  03-NOV-1999  [win16ddk]
TITLE   :FIX: COMM.DRV Locks Up with Full Duplex Communications
PRODUCT :Microsoft Windows Device Driver Kit
PROD/VER::3.0
OPER/SYS:
KEYWORDS:

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

 - Microsoft Windows Device Development Kit (DDK) for Windows, version 3.0 
-------------------------------------------------------------------------------

SYMPTOMS
========

When it is subjected to a heavy load during full-duplex communications, the
transmit portion of the Microsoft Windows communications driver (COMM.DRV) stops
working (no characters are sent). This problem occurs in real, standard, and
enhanced modes. The receive portion of the communications driver is unaffected.

CAUSE
=====

COMM.DRV encounters a "deadlock" situation during full-duplex operation.

RESOLUTION
==========

To work around this problem, an application must detect whether deadlock has
occurred. This can be done by setting a timer in Windows that sends a message to
the application every one or two seconds. When the application receives timer
message, check the txQueue count in COMM.DRV. If the count has not changed since
the last message (indicating that no characters have been sent), the application
calls the FlushComm function and resends the data that was not transmitted. The
alternative, which is not recommended, is to close the COM port and then reopen
it. No ill side effects, other than some performance degradation, have been
observed from using these techniques.


For applications that are very transmission intensive -- for example, sending
data through a full-duplex communications line for an extended period of time --
the 1 to 2 second wait might be considered too long. For these applications,
shorten the checking interval as desired. However, if the interval is too short,
the application may detect deadlock when it has not actually occurred.
Unnecessary calls to FlushComm and resending data would degrades the performance
of the application.

STATUS
======

Microsoft has confirmed this to be a problem in Windows version 3.0. This
problem was fixed in the Windows for Workgroups 3.11 COMM.DRV and above.


Additional query words: 3.00 hangs DDKCOMM buglist3.00

======================================================================
Keywords          :  
Technology        : kbAudDeveloper kbWin3xSearch kbWinDDKSearch kbWinDDK300
Version           : :3.0
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.