Re: Threading problem

"Eric Margheim" <NOSPAM******NOSPAM>
Tue, 4 Jul 2006 17:00:45 -0500

Note that if you manage to delete the form while this loop is running,
this->GetSafeHwnd() is not defined, and might return anything.
GetSafeHwnd() essentially
does very little, and doesn't check to see if this() is actually a valid
pointer (it works
correctly if this is NULL, but if this is a pointer to free storage on the
heap it works
just fine!) So you have a fundamental race condition.

Key here is that you must not allow the form to be closed if there are any
active threads.
This means you must intercept the OnClose handler and defer its actual
working, e.g.,

void CMyForm::OnClose()
        { /* have to wait */
         closing = TRUE;
        } /* have to wait */
now you can set it up so that it disables all controls (e.g., I would call
updateControls() method which would notice the closing boolean is now TRUE
and disable all
the controls) and the thread loop would say

for(...process recordset)
      { /* shut thread down */
      } /* shut thread down */


then the thread-done handler looks like

    return 0;

[note that you might want to do more than this bare outline, this is just
a sketch]

if you have many threads, the simplest way to handle this is do an
InterlockedIncrement of
the thread counter when you start a thread, and an InterlockedDecrement as
each thread
sends its completion notification, and close the form when the reference
count goes to

   long count = InterlockedDecrement(&threadcount);
   if(closing && count == 0)
  return 0;

this is an asynchronous shutdown, which is pretty much mandatory if you
can't abort the
worker thread (such as it is blocked on doing the database access).

Actually, I'm amazed it ever worked; it is probably just good luck that it
gave the
illusion of working. Sounds like VS8 has a more stringent protocol on its
allocator which makes the existing error a bit more obvious.

If you recall this is one of the issues I was dealing back years ago when
you came to visit. There has to be a way to handle this. From a end user
perspective it's really bad to force that form to be open. I think I tried
hiding the window while it's cleaning up so from the user perspective it is
closed. Is that what you are alluding to? Perhaps I should set a flag in
the thread instead of relying on the GetSafeHwnd() call. I am nulling out
the pointer to the parent wnd, perhaps I should be referencing that
instead.... or is the problem because the memory space used for the
function is part of the CFormView class and once it's closed I'm hosed?

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