Q235507: FIX: DAO 3.6 Causes Debug Errors in MFC DAO Non-Unicode Builds
Article: Q235507
Product(s): Microsoft C Compiler
Version(s): 6.0
Operating System(s):
Keyword(s): kbDatabase kbMFC kbVC600 kbGrpDSVCDB kbDAO360 kbDAO360bug
Last Modified: 07-MAY-2001
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The information in this article applies to:
- Microsoft Visual C++, 32-bit Enterprise Edition, version 6.0
- Microsoft Visual C++, 32-bit Professional Edition, version 6.0
- Microsoft Visual C++, 32-bit Learning Edition, version 6.0
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SYMPTOMS
========
When you convert a Microsoft Foundation Classes (MFC) Data Access Objects (DAO)
application to use DAO 3.6, the application can produce a debug error. The exact
error message is as follows:
DAMAGE: after Normal block (#204) at <hex address>
CAUSE
=====
DAO 3.6 incorrectly overwrites the character buffer supplied to it by MFC when
it fetches text data. It first copies the Unicode string into the buffer that is
twice as long as the length of the buffer, and then converts the string to an
Ansi string.
RESOLUTION
==========
With the exception of one case where the error still occurs, this problem has
been fixed in DAO 3.60 version 3.60.3714.5. To obtain the updated DAO 3.60 dll,
please install Microsoft Jet 4.0 Service Pack 4 (SP4) or later. For additional
information, see the following article:
Q239114 ACC2000: Updated Version of Microsoft Jet 4.0 Available in Download
Center
Even after you apply this fix, the problem may still occur when the table you are
accessing through DAO contains a text field that is defined to be only one
character in length. If the field definition allows more than one character in
the field, the problem should be resolved by applying the fix described above.
If your table must contain a single-character text field, you need to use the
code workaround in the More Information section.
STATUS
======
Microsoft has confirmed this to be a problem in the Microsoft products that are
listed at the beginning of this article.
This problem was corrected in Microsoft Jet 4.0 Service Pack 4, with the one
exception noted in the Resolution section of this article.
MORE INFORMATION
================
Steps to Reproduce Behavior
---------------------------
1. Create a new MFC application with database support using DAO and pointing to
the Northwind.mdb sample database. Use the Employees table to base your
recordset class.
a. Select Multiple Documents if necessary on step one.
b. Select Database View without file support on step two.
c. Click Data Source.
d. In the Database Options dialog box select DAO and specify the
Northwind.mdb file in the corresponding edit box. Click OK on the Database
Options dialog.
e. Select the Employees table from the list and click OK in the Select
Database Tables dialog box.
f. Click Finish on step two of the wizard to accept default options and
finish creating your project.
2. Insert the following line as the first line in your application's
InitInstance() method. This will load DAO 3.6 instead of DAO 3.5.
AfxGetModuleState()->m_dwVersion = 0x0601;
3. Build your application in the Win32 Debug configuration.
4. Run your application.
5. Click Move Next a few times. You will get a debug error similar to the one
described above.
Workaround
----------
1. Declare your own callback function in your application header file. It should
be in the same file where your application class is declared.
STDAPI MyDaoStringAllocCallback(DWORD dwLen, DWORD pData, void** ppv);
2. Define the function in the implementation file where your application class
is defined. This function is very similar to the function that MFC defines
except it reallocates the CString internal buffer to twice the size that is
needed.
STDAPI MyDaoStringAllocCallback(DWORD dwLen, DWORD pData, void** ppv)
{
LPTSTR lpsz;
CString* pstr = (CString*)pData;
dwLen++;
TRY
{
//Allocate twice the space needed so that DAO does not overwrite the buffer
lpsz = pstr->GetBufferSetLength(2*dwLen/sizeof(TCHAR));
*ppv = (void*)(dwLen > 0 ? lpsz : NULL);
}
CATCH_ALL(e)
{
e->Delete();
return E_OUTOFMEMORY;
}
END_CATCH_ALL
return S_OK;
}
3. In your DoFieldExchange function for your recordset class, add the code
below. This modifies the DAO binding structure for each field that is a text
field so that the callback function is called instead of the MFC function.
Insert the code after the calls to the RFX functions.
if (pFX->m_nOperation == CDaoFieldExchange::BindField)
{
for (int i=0;i<this->m_nFields;i++)
{
LPDAOCOLUMNBINDING pcb = &m_prgDaoColBindInfo[i];
if (pcb->dwDataType == DAO_CHAR)
{
pcb->cbDataOffset = (DWORD)MyDaoStringAllocCallback;
}
}
}
4. Compile your application.
5. Run the application.
6. Moving through the recordset does not produce errors.
REFERENCES
==========
Q239114 ACC2000: Updated Version of Microsoft Jet 4.0 Available in Download
Center
Q236991 PRB: Unrecognized Database Format Error with Access 2000 Database
Q236056 FIX: DAO 3.6 Overruns Buffers in GetRowsEx Method
Additional query words: DAO AppWizard MFC MoveNext
======================================================================
Keywords : kbDatabase kbMFC kbVC600 kbGrpDSVCDB kbDAO360 kbDAO360bug
Component : dao
Technology : kbVCsearch kbAudDeveloper kbVC600 kbVC32bitSearch
Version : :6.0
Issue type : kbbug
Solution Type : kbfix
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