Page fault on exit

Filippo Carlini -- carlinif@ic8u81.settimo.italtel.it
Monday, May 27, 1996


Env.: Win95, Visual C++ 4.1, linking with MFC DLL.

I have a CDocument derived class that perform a database connection 
in OnOpenDocument and then construct a CRecordset derived object passing 
the database object address as argument.
The DeleteContents of my CDocument, closes the CRecordset, deletes 
and then do the same for CDatabase.
When I close the app with an opened document, sometimes the program 
crashes reporting a page fault.
I tryed to debug the app putting a message box in the destructor of 
my CWinApp and the program reports the access violation with the 
message box opened on the window, pressing the Debug button, I can 
see only the disassembly of the memory that causes the page fault and 
looking at the call stack can't tell me any more.

Any idea ?

The code:

void CLabDoc::DeleteContents()
{
 if (m_pPrjCompSet) {
  if (m_pPrjCompSet->IsOpen())
   m_pPrjCompSet->Close();

  delete m_pPrjCompSet;
 }

 if (m_cdbCurDataBase.IsOpen())
  m_cdbCurDataBase.Close();
}

BOOL CLabDoc::OnOpenDocument(LPCTSTR lpszPathName) 
{
 if (!CDocument::OnOpenDocument(lpszPathName))
  return FALSE;

 CString conn("ODBC;DSN=Data Source;UID=admin;PWD=;DBQ=");

 conn += "c:\\temp\\test.mdb";

 try {
  if (m_cdbCurDataBase.Open(NULL,FALSE,FALSE,conn)) {
   m_pPrjCompSet = new CCompSet(&m_cdbCurDataBase);
   m_pPrjCompSet->Open();
  }
 } catch (CDBException *e) {
  AfxMessageBox(e->m_strError,MB_ICONEXCLAMATION);
  e->Delete();
 }

 return TRUE;
}

---

Mike Ward -- mikew@hilgraeve.com
Friday, May 31, 1996

[Mini-digest: 3 responses]

Yeah, I had the same problem when trying to report memory leaks.  
Turns our you can't call the MFC functions at this point but the old 
Win32 standbys, ::MessageBox() and such work just fine.

-- Mike

-----From: Jay Nabedian 

I had the same problem. This is a bug with the ODBC classes. Microsoft has a
fix for it at their web site.  According to the readme that came with the fix:

  An error can occur when mapping a CString to a SQL_LONGVARCHAR,
  SQL_VARCHAR, or other SQL data type field if a driver returns a large
  precision value from SQLDescribeCol() for the column.

  A bug in the MFC ODBC Database classes results in MFC trying to allocate a
  large chunk of memory to store data that might have this large precision.

  In some cases, such as using a memo field with the FoxPro Desktop ODBC
  driver, a memory allocation of 1 gigabyte may be attempted. Or, as is the
  case with the Visual FoxPro driver, the MFC Database Classes will try to
  allocate a negative number of bytes because the return value of the driver
  is 2 gigabytes and then MFC adds 1, which causes the signed variable to wrap
  into a negative value. In this case, an assertion occurs in
  GetBufferSetLength() on line 447 of Strcore.cpp:

    ASSERT(nNewLength >= 0);

  Mapping a Cstring to a SQL Server text field produces the same result.

  The problem occurs only when mapping CString variables to variable-
  length fields using the RFX_Text() function.

The patch cleared up all of my problems, so I would highly suggest getting it.

============================================================================
========
Jay Nabedian
Quad/Graphics Data Collection
jnabedian@qgraph.com
============================================================================
========

-----From: Vincent Mascart <100425.1337@CompuServe.COM>

If you placed breakpoint in OnOpenDocument() & in DeleteContents(), you should
have noticed that the framework first call OnOpenDocument() where you create
your CRecordset pointer, then call DeleteContents() where you destroy you
CRecordset pointer. So your CRecordset will never be available.

Since you don't reset your pointer to NULL when deleting m_pPrjCompSet, it still
point to invalid memory address for other code in your CDocument derived class.

You should place your cleanup code either in OnCloseDocument() or in the
document Destructor.

Vincent Mascart
100425.1337@compuserve.com

---

| Вернуться в корень Архива |