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: -------------------------------------------------------…

Accepted Answer

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.

Explain errors from GetKeyState / GetCursorPos

Solution

Problem

Related problems