FIX: DFX_Bool Calls Default() with AFX_RFX_LONGBINARY

ID: Q150789

The information in this article applies to:

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

SYMPTOMS

Memory adjacent to the BOOL variable gets overwritten when working with a BOOL CDaoRecordset field data member.

CAUSE

The DFX_Bool function delegates the Fixup case to CDaoFieldExchange::Default and incorrectly passes AFX_RFX_LONGBINARY as the field data type.

RESOLUTION

To resolve this problem, copy the DFX_Bool function to the project and give it a different name. For example, rename it DFX_Bool2. In the call to CDaoFieldExchange::Default, change the field data type from AFX_RFX_LONGBINARY to AFX_RFX_BOOL by using the following code:

Change this code


   case CDaoFieldExchange::Fixup:
      // Convert BOOL value from AFX_DAO_TRUE/FALSE to TRUE/FALSE
      value = (value != AFX_DAO_FALSE);

      pFX->Default(lpszName, (void*)&value,
         AFX_RFX_LONGBINARY, dwBindOptions);
      return;

to this code:


   case CDaoFieldExchange::Fixup:
      // Convert BOOL value from AFX_DAO_TRUE/FALSE to TRUE/FALSE
      value = (value != AFX_DAO_FALSE);

      pFX->Default(lpszName, (void*)&value,
         AFX_RFX_BOOL, dwBindOptions);       // <<-- Change to AFX_RFX_BOOL
      return;

Then, change the class of the CDaoRecordset-derived DoFieldExchange function to call this new function in place of calling DFX_Bool.

STATUS

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

MORE INFORMATION

The code for the Fixup case in CDaoFieldExchange::Default is:


   case Fixup:
      if (m_prs->GetFieldLength(m_nField-1) == DAO_NULL)
      {
         // Set the value to PSEUDO NULL and mark the status NULL
         SetNullValue(pv, dwColumnType);
         m_prs->SetNullFieldStatus(m_nField-1);
      }
      return;

The dwColumn type is the variable that is incorrectly set to AFX_RFX_LONGBINARY in DFX_Bool. Note that SetNullValue is called with the value passed in the second parameter. The code for SetNullValue (showing relevant cases) is:


   void CDaoFieldExchange::SetNullValue(void* pv, DWORD dwDataType)
   {
      switch (dwDataType)
      {
         .
         .
         .

      case AFX_RFX_LONGBINARY:
         ((CLongBinary*)pv)->m_dwDataLength = 0;
         break;

      case AFX_RFX_BOOL:
         *(BOOL*)pv = AFX_RFX_BOOL_PSEUDO_NULL;
         break;

        .
        .
        .
    }
}

Note that the AFX_RFX_BOOL case (the one that should be called) sets the BOOL variable to the constant AFX_RFX_BOOL_PSEUDO_NULL. The AFX_RFX_LONGBINARY case (incorrectly called in the MFC source) sets the m_dwDataLength member of a CLongBinary object (at an offset in the object beyond the size of a BOOL) to zero. Whatever is adjacent to the BOOL variable has 4 bytes of its memory overwritten. The symptoms of this memory overwrite depend on what that variable was used for. There can also be no symptoms.

Additional query words: kbVC400bug 4.00 4.10 4.20 vcfixlist420 MfcDAO

Keywords : kbDAO kbDatabase kbMFC kbVC
Version : winnt:4.0,4.1; :
Platform : winnt
Issue type :