FIX: MFC ODBC Exceptions Using the SQL Server 6.5 Driver

The information in this article applies to:

• The Microsoft Foundation Classes (MFC), included with: Microsoft Visual C++, 32-bit Edition, versions 2.0, 2.1, 2.2, 4.0, 4.1

SYMPTOMS

There are two bugs in the MFC code that are exposed when using the MFC ODBC classes with the Microsoft SQL Server ODBC driver version 2.65.0201 that ships with Microsoft SQL Server 6.5. Setting synchronous mode works around this problem.

The bugs mentioned above can lead to exceptions being thrown for no valid reason. Specifically, CDBExceptions can be thrown that contain no information about what led to the exception. With DB tracing enabled, one of the following trace messages may be displayed in the output window of Visual C++/MSDEV:

   Error: SQLNumResultCols failed during IsOpen()
   Error: failure updating record.

CAUSE

Version 2.65.0201 of the Microsoft SQL Server ODBC driver can return SQL_STILL_EXECUTING from two ODBC API functions that previous versions of the driver implemented as synchronous. This change in the driver has led to the exposure of two bugs in the MFC ODBC classes. MFC wraps ODBC API function calls in one of two wrapper macros: AFX_SQL_SYNC for those calls that are expected to return only when completed; and AFX_SQL_ASYNC for those calls that can return SQL_STILL_EXECUTING. There are two places in the MFC ODBC classes where the wrong macro is used (Visual C++ 4.1 source code shown):

  CRecordset::IsOpen()
  {
    ...
    AFX_SQL_SYNC(::SQLNumResultCols(m_hstmt, &nCols));
    if (!Check(nRetCode))
    {
      ...
      {
  #ifdef _DEBUG
        TRACE0("Error: SQLNumResultCols failed during IsOpen().\n");
    ...
  }

  CRecordset::ExecuteSetPosUpdate();
  {
    ...
    AFX_SQL_SYNC(::SQLSetPos(m_hstmt,1,wPosOption,SQL_LOCK_NO_CHANGE));
    if (!Check(nRetCode))
    {
      TRACE0("Error: failure updating record.\n");
      AfxThrowDBException(nRetCode, m_pDatabase, m_hstmt);
    }
    ...
  }

RESOLUTION

It is recommended that you set the synchronous mode to "on" for all connections that use this version of the SQL Server ODBC driver. Since most applications do not actually need to do asynchronous processing, this is the most direct solution. By setting synchronous mode, you guarantee that every ODBC API function will return only when completed.

To set the synchronous mode to on, use the CDatabase::SetSynchronousMode() function or make a call to the ODBC API function SQLSetStmtOption(). The MFC database classes set the synchronous mode to "off" by default, so you must be careful to avoid conflicts with MFC. Two suggested ways to set synchronous mode to on are:

• Create a CDatabase object, open it on the desired datasource, set the synchronous mode to on using CDatabase::SetSynchronousMode(TRUE), and pass a pointer to this CDatabase object to the constructors of your recordsets. Only those recordsets that are created off this CDatabase object will inherit its synchronous nature. Note that you must manage the cleanup of this database object yourself.

  -or-

• Override the virtual CRecordset::OnSetOptions() to set the synchronous mode to on by calling SQLSetStmtOption() following the call to the base class OnSetOptions(). Do not set the mode prior to the base class OnSetOptions where synchronous mode is set to off.

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

Note that the responsibility for this problem does not lie with the SQL Server ODBC driver, but with the MFC ODBC classes. The ODBC 2.0 specification states that SQLSetPos and SQLNumResultCols can return SQL_STILL_EXECUTING.

REFERENCES

For more information, please see: ODBC 2.0 Programmer's Reference and SDK Guide (available Online).