The Nested Procedure Menace in Oberon

Just stumbled across this article and interview with Niklaus Wirth, from July 2 this year. There are quite a few interesting things in there, and also historical facts which are good to know. I’ve run into Wirth a couple of times when I was doing my master thesis at ETH. In fact, I’ve done my thesis in the group which was originally headed by Wirth and now is headed by Gutknecht (also a well-known name from the article).

That’s also when I detected the following flaw in Oberon. Imagine the following module:

MODULE NestedProcTest;


    PROCEDURE ProcA();
    VAR someLocalVar: LONGINT;

        PROCEDURE NestedProc(val: LONGINT);
            someLocalVar := val
        END NestedProc;

        someLocalVar := 1234H;
        SomeProc := NestedProc;
    END ProcA;

END NestedProcTest.

On line 15 we assign the nested procedure (‘NestedProc’) to the procedure variable ‘SomeProc’. This is valid because the signatures match, right? Wrong. The signature for ‘NestedProc’ is different: Because nested procedures have access to the parent procedures’ variables, their signature is added a hidden parameter, the so called Static Link. If this parameter is not passed to the procedure when it’s called, the procedure is going to read either its local variables or its parents’ variables from a wrong place (depending on the actual implementation of the calling convention). This of course is wrong and can even make this code crash in case we’d read from an unallocated or protected region.

That’s why I recommended disallowing the assignment of nested procedures to procedure variables. Back in 2006 I have also proposed a change for the Oberon compiler which comes with BlueBottle, but I cannot tell at this point whether it has made it in to the release or not.

This and other findings can be found in the report for my master thesis (dated March 2006): Improved Stackmanagement in the Active Object Kernel – Master Thesis, Roger Keller (742.85 kb)

Troubleshooting Authentication Issues with RGS Agent Tab

Sometimes – especially in lab environments – you’ll see issues around user authentication with the RGS Agent Tab of Office Communications Server 2007 R2. This post should help you in determining what could be the issue and how to work around it.

First of all, when the OCS 2007 R2 WebComponents get installed on a machine, by default Integrated Windows Authenticated (IWA) for the RGS parts of WebComponents are enabled. We don’t require IWA, but this is the recommended setting; anything but Anonymous Authentication should work. If Anonymous Authentication is set for the RGS virtual directory in IIS, you’ll find a warning in NT event log about that. In that case, you should turn back on authentication for the virtual directory.

Another problem I’ve seen a couple of times was as follows: Agent A’s credentials are used to sign in with Office Communicator, but the Agent Tab in OC shows the RGS Agent Group memberships of Agent B, or it shows that the “Current User is not an Agent”. In this case, you should start tracing the RgsClientsLib component and either wait until the Agent Tab in OC refreshes automatically (this should happen within 30 – 60 seconds) or you can open the tab URL in IE; it’s typically something like Then, stop tracing and check out the captured traces for RgsClientsLib. You should now find something along the lines of

Authentication type: [Negotiate]
Authenticated user: [CONTOSO\AgentB]
Authenticated user's SID: [S-1-5-21-2278291046-1170081271-1450921830-1285]
Authenticated user's SID maps to: [efa2cabd-462c-49e4-a021-4dd71bd97ce4]

Please note that I left out the less important information like timestamps etc. here. What you see is that instead of AgentA, AgentB is being authenticated. Usually, this happens when the credentials you pass in to OC are different from the credentials you used to log in to Windows. OC uses the IE engine to render the tabs and thus also leaves the authentication for IE. Then, IE performs the authentication based on the “User Authentication” / “Logon” settings for the zone the Agent Tab is in. The default setting for the “Local Intranet Zone” in IE is to automatically try loggin on with the current user’s credentials – i.e. AgentB’s credentials in this case, because AgentB is the currently logged on (Windows) user. Only if authentication for this user fails, IE is going to prompt you for a different set of credentials. To change this behavior, you can set the security settings in IE accordingly:


Setting it to “Prompt for user name and password” will always prompt you for sites in the intranet zone. Once you’ve done that, exit OC and start it again. Now you should be prompted for the credentials to the Agent Tab and you can provide AgentA’s credentials. You then should see the correct list of groups AgentA is a member of.


One of the things I always wanted to do is take a picture of a lightning. Most often when I see them, I either don’t have my camera handy or the view is mainly blocked with buildings, huge trees etc. … But finally, last week, I made it. We got into a thunderstorm around 11pm and here’s what my camera captured.


Do I have to mention that the flashlight of my camera was turned off and that it was otherwise pitch dark outside?