CreateEvent from Windows-7 Logon Screen

Question

I'm asking this question because it turns out that there's some difficulty in writing a screensaver app in Delphi that's capable of running from the Logon screen.

See question: Windows 7 logon screensaver in Delphi

I've narrowed down the problem (or at least one problem) to a particular Win API call CreateEvent.

SyncEvent := CreateEvent(nil, True, False, '');
if SyncEvent = 0 then
  RaiseLastOSError;

This code only fails if called from the Logon screen. And GetLastError returns that access is denied. So clearly the security restrictions on the Logon screen prevent CreateEvent(nil, True, False, ''); from creating the event as desired.

(I don't really see how an Event could be an exploitable security risk.)

So, the question is: Is it possible to create an Event from the Logon screen? Presumably via either:

• Using an appropriate lpEventAttributes
• Or calling CreatingEventEx instead.

Although the problem was experienced in Delphi, this is more about the Win API. So feel free to answer in your language of choice.

Satori

Craig, I'm absolutely no expert at this and maybe I dreamt it, but I'm sure there was a question about this in the Borland groups back in the Win2k or even NT era and the answer at the time was that you had to supply a replacement for Gina.Dll if you wanted to do anything inside the Windows station of the logon screen. Acc to this q, it's different post XP but I imagine you knew that: #1918290 – Fathometer

@Fathometer I must confess much of this is new to me. I just did some investigation because the question about Logon screensavers piqued my interest. However, I have my doubts about GINA being relevant because this question suggests that GINA has been replaced as of Windows Vista. Also, I don't know what the asker of the Logon screensaver question actually intended doing. Perhaps he wanted to write a simple slideshow screensaver. It turns out that of the standard Win-7 screensavers, only the blank screen works due to Direct3D restrictions. – Satori

Try setting the last parameter of CreateEvent() to nil instead of ''. There is a difference between a nil pointer and a pointer to a zero-length string. The documentation does not say anything about a zero-length string being treated any differently than any other named string. If you want an unnamed event, use nil. – Angleaangler

@RemyLebeau Excellent catch! Please go ahead and post as an answer. – Satori

@Anonymous downvoter: Mind sharing what's wrong with the question so I can do better next time? – Satori

Answer 1

Try setting the last parameter of CreateEvent() to nil instead of '' . There is a difference between a nil pointer and a pointer to a zero-length string. The documentation does not say anything about a zero- length string being treated any differently than any other named string. So maybe there is another zero-length-named event that exists somewhere else on your machine that your app does not have access to, thus the Access Denied error when CreateEvent() tries to access the existing event and fails. If you want to create an unnamed event, use nil instead.

Angleaangler

Thanks for that. Now that I'm more consciously aware that PChar is a little richer than string in its ability to differentiate nil and '', it's led me to discover more little mistakes in Delphi. E.g. TSimpleEvent in SyncObjs wraps CreateEvent with the constructor taking a string for the name. So it's impossible to create an unnamed event. Even worse it doesn't check the result of the API call, so if you use TSimpleEvent from a Logon screensaver, you'll quietly get a useless TSimpleEvent instance. – Satori

The T(Simple)Event bug has already been reported to QC, and still exists in XE6: #100175 SyncObjs.TEvent invalid construction – Angleaangler