Access violation when mouse enters custom TPanel

Question

I have a custom TPanel that handles WMNCHitTest. At runtime, when mouse enters the panel, I get the following AV:

Project ClientDatasetEditor.exe raised exception class $C0000005 with message 'access violation at 0x00405219: read of address Ox0000000c'

The Call Stack shows:

> :00405219 SysGetMem + $30
• :0040b7c6 @DynArraySetLength + $A
• :005394e7 TWinControl.MainWndproc + $2F
• :004c945a StdWndProc + $16
• :76ff7694 user32.CaIINextt-iookEx + 0x97
• :76ff8baa; C: \WIND0WSysWOW64iser32.dII
• :76ff8468; C:\WINDOWSysW0W64iser32.dII
• :76ff9c40 user32.DispatchMessageW + Ox1O

Follows WMNCHitTest handle:

procedure tExGraphicControl.WMNCHitTest(var Message: tWMNCHitTest);
begin
  inherited;

  if not (csDesigning in  ComponentState) or
         (msOver      in fMouseStates   )    then Exit;

  if  Message.Msg = WM_NCRButtonUp then begin
      fMouseStates := [msLeftUp];
      Exit                      ;
  end;

  if  Message.Msg = WM_NCMBUTTONUP then begin
      fMouseStates := [msLeftUp];
      Exit                      ;
  end;

  if IsMouseOverControl (Self) then begin
     fMouseStates := [msOver, msMove];
     if  Assigned (fOnMouseEvent) then
          fOnMouseEvent (Self, fMouseStates);
  end;
end;

Any idea about what's causing the AV?

I've noticed that the AV is raised when I include the `inherited` clause. But if I take it off, then the `OnClick` event stops responding. — user2383818, Feb 22 '14 at 16:17
Interesting that it looks incomplete. You have to put a breakpoint on inherited and trace into it, I think. — Sertac Akyuz, Feb 22 '14 at 16:44

David Heffernan · Accepted Answer · 2014-02-22T16:18:17.693

0

Any idea what is causing the AV?

An access violation in GetMem is caused by a heap corruption. You have a defect in your program that is corrupting the heap.

The best way for you to track the fault down is to use the FastMM memory manager in full debug mode. This should, hopefully, tell you the point in the code that corrupts the heap.

edited Feb 22 '14 at 16:18

answered Feb 22 '14 at 16:10

David Heffernan

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Thanks for your comment. If it is so, how can I detect what is corrupting the heap? Any suggestion? – user2383818 Feb 22 '14 at 16:20
Follow the advice in the second paragraph of my answer. – David Heffernan Feb 22 '14 at 16:21
The address looks like accessing something of a nil something, but maybe you're right. – Sertac Akyuz Feb 22 '14 at 16:51
@Sertac It's in GetMem. Given that we take as given that GetMem is correct, what else could it be. What causes GetMem to AV other than heap corruption? – David Heffernan Feb 22 '14 at 16:55
@David - that's ok, but I suspect the call stack. – Sertac Akyuz Feb 22 '14 at 17:00
It's dubious @Sertac, I agree. – David Heffernan Feb 22 '14 at 17:11

Access violation when mouse enters custom TPanel

1 Answers1