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Q147948: FIX: Errors Occur Using DAO CLongBinary Data from Database

Article: Q147948
Product(s): Microsoft C Compiler
Version(s): 4.00 4.10
Operating System(s): 
Keyword(s): kbDAOsearch kbDatabase kbMFC kbVC kbVC420fix
Last Modified: 03-AUG-2001

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

- The Microsoft Foundation Classes (MFC), included with:
   - Microsoft Visual C++, 32-bit Editions, versions 4.0, 4.1 
-------------------------------------------------------------------------------

SYMPTOMS
========

Running an application that uses the MFC DAO classes from Visual C++ version 4.0
or 4.1 to retrieve CLongBinary data from a database may result in one or more of
the following behaviors when performing a recordset move or requery:

  Access violations

-or-

  "Out of memory" exceptions

CAUSE
=====

There is a programming error in the MFC code that can lead to this behavior:
GlobalReAlloc is incorrectly used. The return value of GlobalReAlloc is not
used, so the global handle embedded in the CLongBinary recordset member variable
becomes invalid if GlobalReAlloc moves the virtual memory block. For more
information, refer to the MFC source code in Daodfx.cpp of the Mfc\Src
directory, and look at the use of GlobalReAlloc in the AllocLongBinary()
function.

RESOLUTION
==========

There are two steps to resolving this problem. Both must be performed in order
to reliably work around this issue.

Step 1
------

Make certain you have caching disabled in all DFX_LongBinary function calls.
Caching is disabled if there is no fifth parameter specified in the
DFX_LongBinary function call or if the value of the fifth parameter is the
symbol AFX_DISABLE_FIELD_CACHE or the equivalent numeric value 0. This only
applies to DFX_LongBinary function calls; no other DFX function calls need to be
modified.

Enabling caching for long binary fields is generally unwise anyway, and can be
fatal due to the GlobalReAlloc misuse. Additionally, it is not a simple matter
to work around the GlobalReAlloc issue as it pertains to caching, so the best
solution is to simply not use caching for long binary fields.

Step 2
------

A new implementation of DFX_LongBinary that avoids the defect in the MFC code is
available to assist you in correcting this problem. Please note that this code
does not fix caching for long binary fields. As Step 1 indicates, caching of
long binary fields must be disabled.

The following file is available for download from the Microsoft Software
Library:

  Daoclb.exe
  (http://support.microsoft.com/download/support/mslfiles/Daoclb.exe)

For more information about downloading files from the Microsoft Software Library,
please see the following article in the Microsoft Knowledge Base:

  Q119591 How to Obtain Microsoft Support Files from Online Services


STATUS
======

Microsoft has confirmed this to be a bug in the Microsoft products listed at the
beginning of this article. This problem was corrected in Visual C++, 32-bit
Edition, version 4.2.

Additional query words: kbVC400bug 4.00 4.10 4.20 softlib software library

======================================================================
Keywords          : kbDAOsearch kbDatabase kbMFC kbVC kbVC420fix 
Technology        : kbAudDeveloper kbMFC
Version           : 4.00 4.10
Solution Type     : kbfix

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

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