PRB: Access Violation in ODBC Cursor Library in MFC Application
ID: Q185864
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The information in this article applies to:
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Microsoft Visual C++, 32-bit Editions, version 5.0
SYMPTOMS
An MFC application that uses Open Database Connectivity (ODBC) may crash
randomly on a PUSH DWORD PTR instruction in ODBC Cursor Library version
3.50.36.9. The application error is:
The instruction at "0x6a9b7c1a" referenced memory at "0x026a6000".
The memory could not be "read".
When stepping through the assembly code in the debugger, the access
violation is on a PUSH DWORD PTR instruction in Odbccr32.dll. This access
violation occurs at random times.
CAUSE
An internal Cursor Library (Odbccr32.dll) function is performing a PUSH
DWORD PTR instruction instead of acquiring a byte. As a result three more
bytes are being read than necessary. This occurs as parsing is performed on
the SQL string that was passed to the cursor library.
RESOLUTION
If you use the MFC database classes, there are two possible workarounds:
- Do not use the cursor library.
- Override the MFC code to add three spaces to the end of each SQL
statement before calling the ODBC SQLPrepare() function.
It might not be necessary to use the ODBC Cursor Library. If you mainly
add, update, or delete records then you should use dynasets because
dynasets do not use the Cursor Library and they give the best performance
for these scenarios. If you are fetching data frequently, then you can use
snapshots without using the Cursor Library. You can avoid using the Cursor
Library by specifying FALSE for the last argument (bUseCursorLib) of
CDatabase::Open(), or use the CDatabase::OpenEx() member function which
doesn't load the Cursor Library by default.
Use the Cursor Library where you need to modify records and need the
fetching advantages of a snapshot. To use the Cursor Library, you must
override the two areas in MFC where SQLPrepare() is called. You need to add
three spaces to the end of the SQL string that gets passed to SQLPrepare.
To do this, create a new CRecordset-derived class and perform the following
steps:
- Copy the code in the CRecordset class (present in dbcore.cpp) for the
following member functions into your new CRecordset-derived class:
CRecordset::Open()
CRecordset::Update()
CRecordset::PrepareAndExectue()
CRecordset::BuildUpdateSQL()
CRecordset::UpdateInsertDelete()
CRecordset::Delete()
- Make the following modifications to BuildUpdateSQL() and
PrepareAndExecute() in the new CRecordset class.
- For BuildUpdateSQL(), replace this statement:
AFX_ODBC_CALL(::SQLPrepare(m_hstmtUpdate,
(UCHAR*)T2A((LPTSTR)(LPCTSTR)(m_strUpdateSQL), SQL_NTS));
with this code:
AFX_ODBC_CALL(::SQLPrepare(m_hstmtUpdate,
(UCHAR*)T2A((LPTSTR)(LPCTSTR)(m_strUpdateSQL+" ")),
SQL_NTS));
This adds three spaces to the SQL statement.
- For PrepareAndExectue(), replace this statement:
LPCSTR lpszWSQL = T2CA(m_strSQL);
with this code:
CString strSQLTmp = m_strSQL + " "; // Added three spaces.
LPCSTR lpszWSQL = T2CA(strSQLTmp);
STATUS
Microsoft is researching this problem and will post new information here in
the Microsoft Knowledge Base as it becomes available.
Additional query words:
gpf general protection fault snapshot VC++ VC kbDatabase kbODBC kbMFC kbVC500 kbDSupport
Keywords : kberrmsg
Version : WINNT:5.0
Platform : winnt
Issue type : kbprb