Invalid pointer operation in TMonitor.Destroy

I'm currently working on porting an existing Delphi 5 application to Delphi 2010.

It's a multithreaded DLL (where the threads are spawned by Outlook) that loads into Outlook. When compiled through Delphi 2010, whenever I close a form I run into an "invalid pointer operation" inside TMonitor.Destroy... the one in system.pas, that is.

As this is an existing and kinda complex application, I have a lot of directions to look into, and the delphi help ~~doesn't even document~~ barely documents this particular TMonitor class to begin with (I traced it to some Allen Bauer posts with additional information) ... so I figured I'd first ask around if anyone had encountered this before or had any suggestions on what could cause this problem. For the record: I am not using the TMonitor functionality explicitly in my code, we are talking a straight port of Delphi 5 code here.

Edit Callstack at the moment the problem occurs:

System.TMonitor.Destroy
System.TObject.Free
Forms.TCustomForm.CMRelease(???)
Controls.TControl.WndProc(???)
Controls.TWinControl.WndProc((45089, 0, 0, 0, 0, 0, 0, 0, 0, 0))
Forms.TCustomForm.WndProc(???)
Controls.TWinControl.MainWndProc(???)
Classes.StdWndProc(15992630,45089,0,0)
Forms.TApplication.ProcessMessage(???)

Solution

The pointer to the System.Monitor instance of each object is stored after all the data fields. If you write too much data to the last field of an object it could happen that you write a bogus value to the address of the monitor, which would most probably lead to a crash when the destructor of the object attempts to destroy the bogus monitor. You could check for this address being nil in the BeforeDestruction method of your forms, for a straight Delphi 5 port there shouldn't be any monitors assigned. Something like

procedure TForm1.BeforeDestruction;
var
  MonitorPtr: PPMonitor;
begin
  MonitorPtr := PPMonitor(Integer(Self) + InstanceSize - hfFieldSize + hfMonitorOffset);
  Assert(MonitorPtr^ = nil);
  inherited;
end;

If this is a problem in your original code you should be able to detect it in the Delphi 5 version of your DLL by using the FastMM4 memory manager with all checks activated. OTOH this could also be caused by the size increase of character data in Unicode builds, and in that case it would only manifest in DLL builds using Delphi 2009 or 2010. It would still be a good idea to use the latest FastMM4 with all checks.

Edit:

From your stack trace it looks like the monitor is indeed assigned. To find out why I would use a data breakpoint. I haven't been able to make them work with Delphi 2009, but you can do it easily with WinDbg.

In the OnCreate handler of your form put the following:

var
  MonitorPtr: PPMonitor;
begin
  MonitorPtr := PPMonitor(Integer(Self) + InstanceSize - hfFieldSize + hfMonitorOffset);
  MessageDlg(Format('MonitorPtr: %p', [pointer(MonitorPtr)]), mtInformation,
    [mbOK], 0);
  DebugBreak;
  // ...

Now load WinDbg and open and run the process that calls your DLL. When the form is created a message box will show you the address of the monitor instance. Write down the address, and click OK. The debugger will come up, and you set a breakpoint on write access to that pointer, like so:

ba w4 A32D00

replacing A32D00 with the correct address from the message box. Continue the execution, and the debugger should hit the breakpoint when the monitor gets assigned. Using the various debugger views (modules, threads, stack) you may get important information about the code that writes to that address.