BUG: GetRowsEx() Incorrectly Handles Null Field Values

• The DAO SDK, included with:
  • Microsoft Visual C++, 32-bit Editions, versions 4.0, 4.1, 4.2

SYMPTOMS

The DAO SDK defines a function called GetRowsEx that is a method of the DAO recordset object. This function fails to handle null field values correctly for variable-length fields. If a null value exists in a field that is fetched with GetRowsEx(), the buffer doesn't get updated. For example, you may find that the buffer contains the same value from the previous GetRowsEx() call if you specified dbBindSTRING in the DAORSETBINDING struct. Or, if you specified dbBindLPSTRING in the DAORSETBINDING struct, you may find that the pointer points to incorrect data.

CAUSE

This a bug with the GetRowsEx() function.

RESOLUTION

Clear the buffer before calling the GetRowsEx() method. If a null value is encountered, the buffer will remain unchanged for the field. Check the buffer for whether the value has been modified after the GetRowsEx call. If it hasn't changed, the value is null. See the code below for an example of how to handle null values.

STATUS

Microsoft has confirmed this to be a bug in the Microsoft products listed at the beginning of this article. We are researching this bug and will post new information here in the Microsoft Knowledge Base as it becomes available.

MORE INFORMATION

The following sample code demonstrates how to change the DoGetRowsEx() function in the GETROWS DAO SDK sample so that it handles null values. This function is very similar to the old one except the fetching size has been changed to two records at a time, and the buffer where GetRowsEx() stores the data is cleared before each call. Also, a check is performed to see if the data is null and, if it is, the function places the text "null" in the output string.

Sample Code

#define FETCH_SIZE 2
#define MAX_FIRST_NAME_LENGTH 20
// Perform C++ GetRowsEx against the Employee table
void CGetRowsDlg::DoGetRowsEx()
{
    LPEMP           pEmpRows = new EMP[FETCH_SIZE];
    CListBox        *pListBox = (CListBox*)GetDlgItem(IDD_GETROWSLISTEX);
    CString         strLBRow;
    TCHAR           szId[16];
    LONG            lNumRecords;
    LONG            lCount;
    TCHAR           pBuf[MAX_EMP_REC * 15]; // allow avg of 15 chars/name

    while (!m_cEmpRecordSet.GetEOF())
    {

        //Clear values in buffer for variable-length fields
        for (long lCnt = 0; lCnt < FETCH_SIZE; lCnt++)
        {
          _strnset(pEmpRows[lCnt].strFirstName,'\0',
              MAX_FIRST_NAME_LENGTH);
          pEmpRows[lCnt].strLastName=NULL;
        }

       try
        {
        lNumRecords = m_cEmpRecordSet.GetRowsEx(pEmpRows, sizeof(EMP),
                        &Bindings[0], sizeof(Bindings) / 
                        sizeof(DAORSETBINDING),
                        pBuf, sizeof(pBuf),
                        FETCH_SIZE);
        }
    catch (CdbException e)
        {
        //Differentiate between GetRowsEx Errors and other CdbExceptions
        // see defines in DAOGETRW.H
        if( e.m_hr == E_ROWTOOSHORT ||
            e.m_hr == E_BADBINDINFO ||
            e.m_hr == E_COLUMNUNAVAILABLE )
            {
                AfxMessageBox(_T("Error in GetRowsEx call."));
            }
        else
            {
                AfxMessageBox(_T("General CdbException"));
            }
        delete [] pEmpRows;
        return;
        }

    //Step through the returned rows
    for (lCount = 0; lCount < lNumRecords; lCount++)
        {
        strLBRow.Empty();
        wsprintf(szId, _T("%d,  "), pEmpRows[lCount].lEmpId);
        strLBRow += szId;
        if (pEmpRows[lCount].strLastName!=NULL)
            strLBRow += pEmpRows[lCount].strLastName;
        else
            strLBRow += "null";

        strLBRow += _T(", ");
        if (pEmpRows[lCount].strFirstName[0]=='\0')
            strLBRow += "null";
        else
            strLBRow += (LPCTSTR) pEmpRows[lCount].strFirstName;
        pListBox->AddString(strLBRow);
        }

      if (!m_cEmpRecordSet.GetEOF())
         m_cEmpRecordSet.MoveNext(); // Need to move to next unread
                                     // record before fetching the next
                                     // set of records.
    }

    delete [] pEmpRows;
} 

Additional query words: kbVC400bug 4.00 4.10 4.20

Keywords : kbDAO
Version : 4.00 4.10 4.20
Platform : NT WINDOWS
Issue type :