Re: Access violation on calling GetSafeHwnd()

From:

"Doug Harrison [MVP]" <dsh@mvps.org>

Newsgroups:

microsoft.public.vc.mfc

Date:

Thu, 01 Mar 2007 17:35:24 -0600

Message-ID:

<usneu2dscg893lp5i17barp1jli40p6log@4ax.com>

On Wed, 28 Feb 2007 06:26:02 -0800, Testkr
<Testkr@discussions.microsoft.com> wrote:

Hi Scott,

Here is the piece of code -

BOOL CZedObjectFrame::OnCmdMsg( UINT nID, int nCode, void * pExtra,
AFX_CMDHANDLERINFO* pHandlerInfo)
{
  // call up, does menu and toolbar updates
  BOOL rc = FALSE;

  if (!CanUserModify() && m_pToolBar != NULL)
  {
     CToolBarCtrl & ctrlFramework = m_pToolBar->GetToolBarCtrl();
     if(ctrlFramework.IsButtonEnabled(nID) == 0)
     {
        return rc;
     }
  }

  rc = CFrameWnd::OnCmdMsg(nID, nCode, pExtra, pHandlerInfo);

access violation here >>>> HWND viewHandle = GetSafeHwnd();
  if(viewHandle != NULL && ::IsWindow(viewHandle))
  {

If the command message caused the window to be closed, I could (barely) see
this faulting. If this is not the case, then you entered this member
function on an invalid window, or something has caused damage to the object
it was called on or perhaps the thread's stack, e.g. a buffer overrun, or
maybe the register used to hold the "this" pointer wasn't properly saved
and restored across the function call. I don't see any other possibilities,
since GetSafeHwnd amounts to the following in this context:

HWND viewHandle = (this != 0) ? m_hWnd : 0;

I have to say, it's strange you'd get an access violation there and not
earlier. Even if the window were closed and the memory freed, it would have
to be decommitted from your address space to cause an access violation, and
AFAIK, that happens rarely if ever for the C++ heap. What do you observe in
the debugger?

--
Doug Harrison
Visual C++ MVP

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