Q153216: BUG: GetRowsEx() Incorrectly Handles Null Field Values

Article: Q153216
Product(s): Microsoft C Compiler
Version(s): 4.00 4.10 4.20
Operating System(s): 
Keyword(s): kbDAOsearch
Last Modified: 04-AUG-2001

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

- The DAO SDK, included with:
   - Microsoft Visual C++, 32-bit Editions, versions 4.0, 4.1 
   - Microsoft Visual C++, 32-bit Enterprise Edition, version 4.2 
   - Microsoft Visual C++, 32-bit Professional Edition, version 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          : kbDAOsearch 
Technology        : kbAudDeveloper kbDAOsearch kbSDKDAOSearch kbSDKSearch
Version           : 4.00 4.10 4.20

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

THE INFORMATION PROVIDED IN THE MICROSOFT KNOWLEDGE BASE IS PROVIDED "AS IS" WITHOUT WARRANTY OF ANY KIND. MICROSOFT DISCLAIMS ALL WARRANTIES, EITHER EXPRESS OR IMPLIED, INCLUDING THE WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE. IN NO EVENT SHALL MICROSOFT CORPORATION OR ITS SUPPLIERS BE LIABLE FOR ANY DAMAGES WHATSOEVER INCLUDING DIRECT, INDIRECT, INCIDENTAL, CONSEQUENTIAL, LOSS OF BUSINESS PROFITS OR SPECIAL DAMAGES, EVEN IF MICROSOFT CORPORATION OR ITS SUPPLIERS HAVE BEEN ADVISED OF THE POSSIBILITY OF SUCH DAMAGES. SOME STATES DO NOT ALLOW THE EXCLUSION OR LIMITATION OF LIABILITY FOR CONSEQUENTIAL OR INCIDENTAL DAMAGES SO THE FOREGOING LIMITATION MAY NOT APPLY.

Copyright Microsoft Corporation 1986-2002.