• The Old New Thing

    Your exception handler can encounter an exception


    Consider the following code, written in C# just for kicks; the problem is generic to any environment that supports exception handling.

    void ObliterateDocument()
     try {
      try {
      } finally {
     } finally {

    Some time later, you find yourself facing an assertion failure from document.Destroy() claiming that you are destroying the document while there are still active plugins. But there is your call to document.DestroyPlugins(), and it's in a finally block, and the whole point of a finally block is that there is no way you can escape without executing it.

    So why didn't document.DestroyPlugins() execute?

    Because your exception handler itself encountered an exception.

    The exception handler is not active during its own finally clause. As a result, if an exception is thrown during document.Close(), the exception handler search begins at the block outside the finally block.

    (That the exception handler is not active during its own finally clause should be obvious. It would mean that if an exception were to occur during the finally clause, the program would go into an infinite loop. And it also wouldn't be possible to rethrow a caught exception; your throw would end up caught by yourself!)

    In this case, the exception was caught by some outer caller, causing the remainder of the first finally block to be abandoned. The other finally blocks do run since they contain the one that died.

    (This bug also exists in the proposed alternative to error-checking code posted by an anonymous commenter.)

  • The Old New Thing

    Competing to be the worst-dressed couple in America


    The U.S. cable network TLC is putting on a special episode of What Not to Wear devoted to identifying the worst-dressed couple in America. It so happens that one of my friends knows one of the finalists, so we'll be rooting for them. Or is it against them? Are you supposed to hope that your favorite is in fact the worst-dressed? Or should you be relieved that they're only "sort of badly dressed but at least not the worst I've seen"?

    Yes, the show is a rip-off of the original BBC show. It's interesting how we yanks look to our British neighbors as examples of quality television (What Not to Wear, Changing Rooms, The Office) and bemoan how U.S. television producers can only come up with drivel. Meanwhile, the Brits look right back (Hill Street Blues, Cheers, 24) and bemoan the same thing about their own television industry.

    The other guy's stuff is always better.

  • The Old New Thing

    The great Alaskan ice sculpture


    NPR interviewed John Reeves, the artist behind a 160-foot-tall mountain of ice in Alaska. The man has a down-home aw-shucks kind of demeanor that I found quite charming.

    I'm a middle-aged guy that has a lot of time in the winter and a little bit of extra money to play with, so my hobby was to see how big an ice hill I could grow. I started last year as just a way to amuse myself just to see what would happen if you left the water running all winter, and last year's model was so interesting and so much fun that this year I decided to continue doing it.

    Well, a long winter in Alaska will do that to you, I guess.

    Yes it will. A long, cold, dark... lengthy, cold... did I mention cold?

  • The Old New Thing

    Windows NT Security in Theory and Practice


    Today, I'm not writing anything new. Instead, I'm referring you to the series of articles by Ruediger Asche starting with Windows NT Security in Theory and Practice. These articles are quite old but the principles are still sound. Just bear in mind that the newer stuff won't be covered.

  • The Old New Thing

    Windows are not cheap objects


    Although Windows is centered around, well, windows, a window itself is not a cheap object. What's more, the tight memory constraints of systems of 1985 forced various design decisions.

    Let's take for example the design of the list box control. In a modern design, you might design the list box control as accepting a list of child windows, each of which represents an entry in the list. A list box with 20,000 items would have 20,000 child windows.

    That would have been completely laughable in 1985.

    Recall that Windows was built around a 16-bit processor. Window handles were 16-bit values and internally were just near pointers into a 64K heap. A window object was 88 bytes (I counted), which means that you could squeeze in a maximum of 700 or so before you ran out of memory. What's more, menus hung out in this same 64K heap, so the actual limit was much lower.

    Even if the window manager internally used a heap larger than 64K (which Windows 95 did), 20,000 windows comes out to over 1.5MB. Since the 8086 had a maximum address space of 1MB, even if you devoted every single byte of memory to window objects, you'd still not have enough memory.

    Furthermore, making each list box item a window means that every list box would be a variable-height list box, which carries with it the complexity of managing a container with variable-height items. This goes against two general principles of API design: (1) simple things should be simple, and (2) "pay-for-play", that if you are doing the simple thing, you shouldn't have to pay the cost of the complex thing.

    Filling a list box with actual windows also would have made the "virtual list box" design significantly trickier. With the current design, you can say, "There are a million items" without actually having to create them.

    (This is also why the window space is divided into "client" and "non-client" areas rather than making the non-client area consist of little child windows.)

    To maintain compatibility with 16-bit Windows programs (which still run on Windows XP thanks to the WOW layer), there cannot be more than 65536 window handles in the system, because any more than that would prevent 16-bit programs from being able to talk meaningfully about windows. (Once you create your 65537'th window, there will be two windows with the same 16-bit handle value, thanks to the pigeonhole principle.)

    (And yes, 16/32-bit interoperability is still important even today.)

    With a limit of 65536 window handles, your directory with 100,000 files in it would be in serious trouble.

    The cost of a window object has grown over time, as new features get added to the window manager. Today it's even heftier than the svelte 88 bytes of yesteryear. It is to your advantage not to create more windows than necessary.

    If your application design has you creating thousands of windows for sub-objects, you should consider moving to a windowless model, like Internet Explorer, Word, list boxes, treeview, listview, and even our scrollbar sample program. By going windowless, you shed the system overhead of a full window handle, with all the baggage that comes with it. Since window handles are visible to all processes, there is a lot of overhead associated with centrally managing the window list. If you go windowless, then the only program that can access your content is you. You don't have to worry about marshalling, cross-process synchronization, Unicode/ANSI translation, external subclassing, hooks... And you can use a gigabyte of memory to keep track of your windowless data if that's what you want, since your windowless controls don't affect any other processes. The fact that window handles are accessible to other processes imposes a practical limit on how many of them can be created without impacting the system as a whole.

    I believe that WinFX uses the "everything on the screen is an element" model. It is my understanding that they've built a windowless framework so you don't have to. (I'm not sure about this, though, not being a WinFX person myself.)

  • The Old New Thing

    Dot-Con Job: How InfoSpace took its investors for a ride


    The Seattle Times ran an excellent series last week on the rise and fall of InfoSpace and its charismatic leader, Naveen Jain, who at one point even used the phrase "cult leader" to refer to himself.

    To set the tone, and perhaps to serve as a reference while you read the series, here's a list of reported Infospace earnings per share (EPS), both pro-forma and following Generally Accepted Accounting Principles (GAAP), as reported in their SEC filings and press releases, illustrated with quotes from contemporary press releases (attributed to Naveen Jain unless otherwise noted).

    Period Pro-forma GAAP Quote from press release Remarks
    1999 Q1   -3¢ We executed flawlessly on our plan. Our performance this quarter is a clear demonstration of our undisputed position as a leader in providing Internet infrastructure services.  
    1999 Q2 +1¢
    −8¢ This demonstrates that growth and profitability are not mutually exclusive. Achieving profitability two quarters ahead of Wall Street's expectations demonstrates the strength of our business model. This press release begins the policy of not even mentioning GAAP results.
    1999 Q3 +6¢
    +2¢ We had a totally awesome quarter.  
    1999 Q4 +10¢
    InfoSpace is now synonymous with wireless Internet services. They did not appear to file a 10-Q with the SEC, so I couldn't find their contemporary GAAP EPS. A year later they claimed it was −3¢.
    2000 Q1 +1¢ −38¢ InfoSpace is leading the convergence of the two fastest growing industry segments in history—wireless and the Internet—creating a new industry: the wireless Internet.  
    2000 Q2 −1¢ −14¢ Today marks another historic milestone in the history of the rapid evolution of InfoSpace.  
    2000 Q3 +1¢ −30¢ This was an excellent quarter for InfoSpace, as we continued to build upon our market leadership in the globally-expanding wireless sector. Quote comes from Arun Sarin, CEO.
    2000 Q4 +4¢ −17¢ InfoSpace continues to expand its relationships and deliver value to wireless carriers proven by the significant revenue growth in our wireless business and the more than 1.5 million wireless subscribers.  
    2001 Q1 −2¢ −37¢ InfoSpace continues to demonstrate its strength and ability to generate new business and pursue favorable market opportunities.  
    2001 Q2 +1¢ −22¢ Our return to pro-forma profitability this quarter reconfirms the strength of our business model. I like how they're proud that they are profitable "once you ignore all that accounting stuff".
    2001 Q3 −3¢ −63¢ InfoSpace's performance this quarter underscores our continuing success in focusing on our high growth areas of wireless and merchant.  
    2001 Q4 −2¢ −32¢ We are feeling better about our near and long-term prospects and believe we have turned a corner in our business. In response to the Enron scandal, GAAP numbers are once again being reported.
    2002 Q1 −2¢ −77¢ Results this quarter demonstrate our ability to continue executing on our strategy.  
    2002 Q2 −2¢ −4¢ We are pleased to post another quarter of encouraging results.  
    2002 Q3 −0.2¢ −8.6¢ I'm pleased to report that our focus on profitability is paying off.  
    2002 Q4 +0.5¢ −20.7¢ Our team made significant progress this past year aligning costs with revenues. Quote comes from Jim Voelker, CEO.

    The stock underwent a 10-1 reverse split in September 2002; values have been adjusted to pre-split values for comparison purposes.

    Some boxes list two numbers. The top number is the value reported at the time of the press release. The bottom number is the value reported the following year. For example, in 1999 Q2, the press release claimed that they earned 1¢/share pro forma, but one year later, in the 2000 Q2 filing, they reported a 2¢ loss per share for 1999 Q2. [Years fixed, 10pm.] Why change the amount? Because it makes the 2000 Q2 results look better when compared to the "same period last year". I have no idea which set of numbers (if any!) is correct.

  • The Old New Thing

    A subtlety in restoring previous window position


    A common feature for many applications is to record their screen location when they shut down and reopen at that location when relaunched. If implemented naively, a program merely restores from its previous position unconditionally.

    You run into usability problems with this naive implementation. If a user runs two copies of your program, the two windows end up in exactly the same place on the screen. Unless the user paid close attention to their taskbar, it looks like running the second copy had no effect. Now things get interesting.

    Depending on what the program does, the second copy may encounter a sharing violation, or it may merely open a second copy of the document for editing, or two copies of the song may start playing, resulting in a strange echo effect since the two copies are out of sync. Even more fun is if the user hits the Stop button and the music keeps playing! Why? Because only the second copy of the playback was stopped. The first copy is still running.

    I know one user who not infrequently gets as many as four copies of a multimedia title running, resulting in a horrific cacophany as they all play their attract music simultaneously, followed by mass confusion as the user tries to fix the problem, which usually consists of hammering the "Stop" button on the topmost copy. This stops the topmost instance, but the other three are still running...

    If a second copy of the document is opened, the user may switch away from the editor, then switch back to the first instance, and think that all the changes were lost. Or the user may fail to notice this and make a conflicting set of changes to the first instance. Then all sorts of fun things happen when the two copies of the same document are saved.

    Moral of the story: If your program saves and restores its screen position, you may want to check if a copy of the program is already running at that screen position. If so, then move your second window somewhere else so that it doesn't occupy exactly the same coordinates.

  • The Old New Thing

    VegFest 2005 this weekend - and - vegetarian is as vegetarian does


    The weekend of March 12th and 13th, Vegetarians of Washington is hosting VegFest 2005, a festival of vegetarian food.

    This reminds me of a Time Magazine cover story from July 2002, wherein it was revealed that...

    In a survey of 11,000 individuals, 37% of those who responded "Yes, I am a vegetarian" also reported that in the previous 24 hours they had eaten red meat; 60% had eaten meat, poultry or seafood.
  • The Old New Thing

    Performance gains at the cost of other components


    In the operating systems group, we have to take a holistic view of performance. The goal is to get the entire system running faster, balancing applications against each other for the greater good.

    Applications, on the other hand, tend to have a selfish view of performance: "I will do everything possible to make myself run faster. The impact on the rest of the system is not my concern."

    Some applications will put themselves into the Startup group so that they will load faster. This isn't really making the system run any faster; it's just shifting the accounting. By shoving some of the application startup cost into operating system startup, the amount of time between the user double-clicking the application icon and the application being ready to run has been reduced. But the total amount of time hasn't changed.

    For example, consider the following time diagram. The "*" marks the point at which the user turns on the computer, the "+" marks the point at which Explorer is ready and the user double-clicks the application icon, and the "!" marks the point at which the application is ready.

    * OS Startup + Application Startup !

    The application developers then say, "Gosh, that pink 'Application Startup' section is awfully big. What can we do to make it smaller? I know, let's break our application startup into two pieces...

    * OS Startup + Application Startup 1 Application Startup 2 !

    "... and put part of it in the Startup group.

    * OS Startup Application Startup 1 + Application Startup 2 !

    "Wow, look, the size of the pink bar (which represents how long it takes for our application to get ready after the user double-clicks the icon) is much shorter now!"

    The team then puts this new shorter value in their performance status report, everybody gets raises all around, and maybe they go for a nice dinner to celebrate.

    Of course, if you look at the big picture, from the asterisk all the way to the exclamation point, nothing has changed. It still takes the same amount of time for the application to be ready from a cold start. All this "performance" improvement did was rob Peter to pay Paul. The time spent doing "Application Startup 1" is now charged against the operating system and not against the application. You shuffled numbers around, but the end user gained nothing.

    In fact, the user lost ground. For the above diagrams assume that the user wants to run your application at all! If the user didn't want to run your application but instead just wanted to check their email, they are paying for "Application Startup 1" even though they will reap none of the benefits.

    Another example of applications having a selfish view of performance came from a company developing an icon overlay handler. The shell treats overlay computation as a low-priority item, since it is more important to get icons on the screen so the user can start doing whatever it is they wanted to be doing. The decorations can come later. This company wanted to know if there was a way they could improve their performance and get their overlay onto the screen even before the icon shows up, demonstrating a phenomenally selfish interpretation of "performance".

    Performance is about getting the user finished with their task sooner. If that task does not involve running your program, then your "performance improvement" is really a performance impediment. I'm sure your program is very nice, but it would also be rather presumptuous to expect that every user who installs your program thinks that it should take priority over everything else they do.

  • The Old New Thing

    Why does SystemParametersInfo hang when I pass the SPIF_SENDCHANGE flag?


    If you pass the SPIF_SENDCHANGE flag to the SystemParametersInfo function, it will broadcast the WM_SETTINGCHANGE message with the wParam equal to the system parameter code you passed. For example, if you call


    then the system will broadcast the message

                SPI_SETDOUBLECLICKTIME, 0);

    If there is a window that isn't responding to messages, then this broadcast will hang until that unresponsive window finally resumes responding to messages or is killed.

    If you'd rather not be victimed by unresponsive windows, you have a few options, but it also may affect your program's expectations.

    You could issue the SystemParametersInfo call on a background thread. Then your background thread is the one that blocks instead of your UI thread.

    With this message, the background thread can notify the main thread when the broadcast finally completes, at which point your program now knows that all windows have received their notifications and are on board with the new setting.

    You could issue the SystemParametersInfo call without the SPIF_SENDCHANGE flag, then manually broadcast the change via

    DWORD dwResult;
                SPI_SETDOUBLECLICKTIME, 0,
                5000, &dwResult);

    This does mean that unresponsive windows will not receive the notification that a system parameter has changed. This is acceptable if you decide that your change in settings was minor enough that a program missing the notification is no big deal. In other words, when the unresponsive program finally wakes up, it will not know that the setting has changed since it missed the notification.

    You can combine the above two methods: Use a background thread and send the message with a timeout.

    Perhaps the best technique is to use the SendNotifyMessage function. As we learned earlier, the SendNotifyMessage function is like SendMessage except that it doesn't wait for a response. This lets your program get back work while not messing up programs that were momentarily unresponsive when you decided to broadcast the notification.

                SPI_SETDOUBLECLICKTIME, 0);

    The downside is that you don't know when all windows have finally received and processed the notification. All you know is that someday, they will eventually find out. Usually you don't care about this aspect of the broadcast, so this lack of information is not an impediment.

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