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FIX: Access Violation When Calling COleVariant::Clear()

This article was previously published under Q177912
This article has been archived. It is offered "as is" and will no longer be updated.
When calling COleVariant::Clear(), an access violation occurs when thevariant type is VT_DECIMAL.
There is special code in COleVariant::Clear() that checks the type of thevariant and calls SysFreeString() if it is equal to VT_BSTRT (14). Here isthe code:
   #ifndef _UNICODE      if (vt == VT_BSTRT)             ::SysFreeString(bstrVal);      else   #endif      {             VERIFY(::VariantClear(this) == NOERROR);      }				
VT_BSTRT is used to identify ANSI BSTRs and is a special type that MFC usesinternally. The problem is that OLE also defines VT_DECIMAL to be 14.

Thus, if you call Clear() on a COleVariant, which has a variant type ofVT_DECIMAL, an access violation will occur because the function callsSysFreeString() on a non-existent string.
Don't call COleVariant::Clear(); instead, call VariantClear() and pass inthe COleVariant.
Microsoft has confirmed this to be a bug in the Microsoft products listedat the beginning of this article. This problem was corrected in Visual C++version 6.0 for Windows.
The GetRows() ADO sample, which is provided with the OLE DB 1.1 SDK,contains code that can cause the access violation when calling Clear().

Here is the DoGetRows() function from GETRDLG.CPP:
   HRESULT CGetRowsDlg::DoGetRows()   {      HRESULT         hr;      COleVariant      vBookmark, rgvFields;      COleVariant      cRows;      COleVariant      varField, varNewField;      CString         strLBRow;      LONG         lNumOfCol, lNumRecords;      LONG         lIndex[2];      CListBox      *pListBox =                               (CListBox *)GetDlgItem(IDD_GETROWSLIST);      //Perform GetRows on Employee table.      //Start from the current place      vBookmark.vt = VT_ERROR;      vBookmark.scode = DISP_E_PARAMNOTFOUND;      //Get all columns.      rgvFields.vt = VT_ERROR;      rgvFields.scode = DISP_E_PARAMNOTFOUND;   if (!m_piEmpRecordSet)      return E_NOINTERFACE;      hr = m_piEmpRecordSet->GetRows(   adGetRowsRest,                                           vBookmark,                                           rgvFields,                                           &cRows                                         );      if (FAILED(hr)) goto ErrorExit;      //Find out how many records were actually retrieved,      //(SafeArrays are 1-based)      lNumOfCol = 2;      SafeArrayGetUBound(cRows.parray, 2, &lNumRecords);      //Clear the listbox      pListBox->ResetContent();      for (lIndex[1] = 0; lIndex[1] <= lNumRecords; lIndex[1]++)      {             strLBRow.Empty();//Clear the string             for (lIndex[0] = 0; lIndex[0] <= lNumOfCol; lIndex[0]++)             {               SafeArrayGetElement(cRows.parray, &lIndex[0], &varField);               hr = VariantChangeType(&varNewField, &varField, 0, VT_BSTR);               if(hr == S_OK)               {                 strLBRow += (LPCWSTR)varNewField.bstrVal;                 strLBRow += _T(", ");               }               varField.Clear();               varNewField.Clear();             }             pListBox->AddString(strLBRow);           }      return hr;   ErrorExit:      TCHAR szBuf[256];      wsprintf(szBuf, _T("Error: %d \n"), hr);      AfxMessageBox(szBuf);      return hr;   }				
NOTE: If the column type returned from the ADO source is VT_DECIMAL, thevarField.Clear() function will crash because COleVariant::Clear() callsSysFreeString() on data that is not a string.

To reproduce the error, add the following lines of code to the functionCGetRowsDlg::DoGetRows():
   COleVariant var10;   hr = VariantChangeType(&var10, &varField, 0, VT_DECIMAL);   if(hr!=S_OK)        AfxMessageBox("Can't VariantChangeType to decimal");   else       var10.Clear();				
Add the code just before the line:
bug AV GPF

Article ID: 177912 - Last Review: 10/07/2013 16:13:54 - Revision: 1.0

Microsoft Visual C++ 5.0 Enterprise Edition

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