Remko Weijnen's Blog (Remko's Blog)

About Virtualization, VDI, SBC, Application Compatibility and anything else I feel like

Archive for the ‘Citrix’ Category

Using WTSWaitSystemEvent

If you develop an application for Terminal Server you might want to react on session events. This means that your application is notified when a user logs on, logs off or becomes idle. This can be done with the WTSWaitSystemEvent function. Implementing it is rather simple and could look something like this:

Notice that you would probably do this from a seperate thread otherwise you will block the main thread. To stop waiting for Events you send a special event:

Please note that there are at least 2 issues with this API, one with Windows 2000 and one with Windows Vista. On Windows 2000 events are reported twice for each actual event. Microsoft’s resolution?

The application should expect the event twice, and filter out the second occurrence.

Now how do we solve this? I would suggest introducing a small delay after an event trigger, this way you will probably not receive the duplicate event.

On Windows Vista there’s another issue: After you set the value of the EventMask parameter to WTS_EVENT_FLUSH in the WTSWaitSystemEvent function, no pending calls to the function return on a Windows Vista-based computer. Now what does this mean? It means that after sending WTS_EVENT_FLUSH the thread never responds! So there’s actually no nice way to end the thread, the only escape is a call to TerminateThread.

Microsoft does offer a hotfix, so my suggestion is a check on startup that will notify the user that he/she needs to install the hotfix. A version check can be done on winsta.dll, the version before the fix is 6.0.6000.16386. Hotfix version is 6.0.6000.20664. According to this article the fix will be included in Vista SP1.


Undocumented API’s from Utildll

Several of Microsoft’s Terminal Server tools use undocumented API’s from Utildll.dll. For instance Terminal Server Admin uses it to get a localised connect state string and to format time strings like idle time, logon time etc.

Functions below seems to be the most usefull ones, I will add those to the JwaWinsta unit:

  • function StrConnectState (returns localised string of the given ConnectState)
  • DateTimeString (returns formatted date timestring according to user’s timesettings)
  • function CurrentDateTimeString (like the name says)
  • function ElapsedTimeString (returns formatted string with elapsed time as in TSAdmin)
  • function CalculateElapsedTime (returns elapsed time in seconds)
  • function CalculateDiffTime (returns time difference in seconds)
  • function GetUnknownString (returns localised “unknown” string)

Citrix has it’s own version of this DLL called CUtildll.dll which is similar but uses (Citrix) MUI for localising strings.

Delphi and Terminal Server Aware

When an application is not Terminal Server aware (also known as a legacy application), Terminal Server makes certain modifications to the legacy application to make it work properly in a multiuser environment. For example, Terminal Server will create a virtual Windows folder, such that each user gets a Windows folder instead of getting the system’s Windows directory. This gives users access to their own INI files. In addition, Terminal Server makes some adjustments to the registry for a legacy application. These modifications slow the loading of the legacy application on Terminal Server and require up to 8 MegaBytes extra memory. This behaviour can be avoided if the TSAware flag is present in the PE header of an executable as can be read here at MSDN.

But how do we set this property in Delphi?

In Windows.pas we can see that the constant is defined:

But how to use this in your application?
Add the line

somewhere below the uses clause and we’re done!

Offcourse you are now responsible for making your application Terminal Server compliant which according to Microsoft means: If an application is Terminal Server aware, it must neither rely on INI files nor write to the HKEY_CURRENT_USER registry during setup.

More undocumented Terminal Server API’s

I added some more undocumented API’s to my Jwawinsta unit, the unit is now becoming a collection of the undocumented API’s in winsta.dll.

These are the functions I added:

  • WinStationDisconnect
  • WinStationGetProcessSid
  • CachedGetUserFromSid (exported by utildll.dll)

I also added some more parts of the undocumented structure returned by WinStationQueryInformationW, it now contains:

  • Session State
  • WinStationName
  • SessionId
  • ConnectTime
  • DisconnectTime
  • LastInputTime
  • LogonTime
  • OutgoingFrames
  • OutgoingBytes
  • OutgoingCompressedBytes
  • IncomingCompressedBytes
  • IncomingFrames
  • IncomingBytes
  • Domain
  • Username
  • CurrentTime

Citrix Solutions Conference 2007

Yesterday I attended the Citrix Solutions Conference in Antwerp. Brad Pedersen (Chief Architect and Senior Fellow at  Citrix Systems) held an interesting speech about The End User Experience. I liked especially liked the part about the history of Citrix and the early versions of their products like Wincredible and Winframe. Since Brad wrote the original code for Citrix and thus Terminal Server (the stuff that is in winsta.dll now) I hoped he could share some info with me on the undiscovered parts of winsta.dll. Unfortunately Brad could not do this because of a non disclosure agreement with Microsoft. He did tell me that Citrix is pushing Microsoft to make more Terminal Server API’s public. I presume that’s why Vista and Windows 2008 offer some new API’s which I wrote about earlier.

Command Prompt IconA little while ago I wrote an article on launching a process in another Terminal Session (

The article didn’t have a demo app yet so I’ve attached it here. (more…)

RDP Clipboard Fix

Did you ever loose Clipboard functionality (copy/paste) while working with several Terminal Server sessions? I think everyone that works a lot with Terminal Server has experienced this from time to time.

It’s caused by badly behaving applications. Dimitry Vostokov wrote a tool to fix this issue for Citrix (RepairCBDChain.exe), he explains the issue very well on his blog:

Windows has a mechanism to notify applications about clipboard changes. An application interested in such notifications has to register itself in the so called clipboard chain. Windows inserts it on top of that chain and that application is responsible to propagate changes down the chain:


If 3rd-party application forgets to forward notifications down then we have a broken clipboard chain and clipboard changes are not sent via ICA protocol:

Read more at Dimitry’s Blog:

So how can we fix this for Terminal Server then?