Explain errors from GetKeyState / GetCursorPos

Question

Sometimes I get bug reports from customers, that I can't explain. After Application.Run() in Delphi I get the following errors:

 EOSError: System error: Code:_5 Access denied

 Call Stack Information:
-------------------------------------------------------------------
|Address |Module     |Unit       |Class |Procedure       |Line    |
-------------------------------------------------------------------
|Running Thread: ID=4352; Priorität=0; Klasse=; [Main Thread]     |
|-----------------------------------------------------------------|
|772B291F|USER32.dll |           |      |GetKeyState     |        |
|772B7B96|USER32.dll |           |      |GetPropA        |        |
|772B7B5A|USER32.dll |           |      |GetPropA        |        |
|772A7BC5|USER32.dll |           |      |DispatchMessageA|        |
|772A7BBB|USER32.dll |           |      |DispatchMessageA|        |
|00A6D804|Program.exe|Program.dpr|      |                |803[369]|  // Application.Run
-------------------------------------------------------------------

and

EOsError: A call to an OS function failed

Call Stack Information:
-------------------------------------------------------------------
|Address |Module     |Unit       |Class |Procedure       |Line    |
-------------------------------------------------------------------
|Running Thread: ID=2712; Priorität=0; Klasse=; [Main Thread]     |
|-----------------------------------------------------------------|
|7E379758|USER32.dll |           |      |GetCursorPos    |        |
|7E379ED9|USER32.dll |           |      |GetKeyState     |        |
|7E37B3FC|USER32.dll |           |      |CallNextHookEx  |        |
|7E380078|USER32.dll |           |      |GetPropA        |        |
|7E380042|USER32.dll |           |      |GetPropA        |        |
|7E3696C2|USER32.dll |           |      |DispatchMessageA|        |
|7E3696B8|USER32.dll |           |      |DispatchMessageA|        |
|00A6E823|Program.exe|Program.dpr|      |                |803[369]|  //Application.Run
-------------------------------------------------------------------

In both cases the screenshot submitted from Eurekalog is completely black.

Can anybody explain, what can cause GetCursorPos or GetKeyState to fail this way?

Answer 1

The documentation for GetCursorPos says:

The input desktop must be the current desktop when you call GetCursorPos. Call OpenInputDesktop to determine whether the current desktop is the input desktop. If it is not, call SetThreadDesktop with the HDESK returned by OpenInputDesktop to switch to that desktop.

You can fall foul of this, most commonly when unlocking a workstation. In my code I replace GetCursorPos with this variant:

function GetCursorPos(var lpPoint: TPoint): BOOL; stdcall;
(* The GetCursorPos API in user32 fails if it is passed a memory address >2GB 
   which breaks LARGEADDRESSAWARE apps.  We counter this by calling GetCursorInfo
   instead which does not suffer from the same problem.

   In addition we have had problems with GetCursorPos failing when called 
   immediately after a password protected screensaver or a locked workstation 
   re-authenticates. This problem initially appeared with XP SP1 and is brought 
   about because TMouse.GetCursorPos checks the return value of the GetCursorPos 
   API call and raises an OS exception if the API has failed.
*)
var
  CursorInfo: TCursorInfo;
begin
  CursorInfo.cbSize := SizeOf(CursorInfo);
  Result := GetCursorInfo(CursorInfo);
  if Result then begin
    lpPoint := CursorInfo.ptScreenPos;
  end else begin
    lpPoint := Point(0, 0);
  end;
end;

You can use your favourite code hooking mechanism to replace GetCursorPos. I do it like so:

RedirectProcedure(@Windows.GetCursorPos, @CodePatcher.GetCursorPos);

with RedirectProcedure as described here: Patch routine call in delphi

It turned out for my particular scenario, that GetCursorPos would fail, but GetCursorInfo would not fail. But as has been pointed out in the comments, there are scenarios where GetCursorInfo will also fail. In that case you might find it expedient to arrange that the patched function does not return False.

As for GetKeyState, I'm not sure about that one. It's quite possibly similar but GetKeyState is an API that I am not personally familiar with.

Answer 2

For reference: Thanks to the answer and comments I found the following situations which might likely trigger these errors:

• Login after user switching in Windows
• Login from password protected screensavers
• VNC / Remote Desktop Services

I will ignore the errors in a global excpetion handler as follows:

procedure MyGlobalExceptionHandler(Sender: TObject; E: Exception);
var
    TopCallStackFunction: string;
begin
    if E is EOSError then
    begin
        TopCallStackFunction := GetEurekalogTopCallStackFunction();

        //EOSError: System error: Code: 5 Access denied, 
        //caused by GetKeyState or EndPaint or GetCursorPos
        if ((E as EOSError).ErrorCode = Windows.ERROR_ACCESS_DENIED) 
          and ((TopCallStackFunction = 'GetKeyState') 
          or (TopCallStackFunction = 'EndPaint')
          or (TopCallStackFunction = 'GetCursorPos')) then
            Exit;

        //EOsError: A call to an OS function failed, caused by GetCursorPos 
        if ((E as EOSError).ErrorCode = 0) 
          and (TopCallStackFunction = 'GetCursorPos') then
            Exit;
    end;

    ... //other error handling
end;