June, 2005

  • The Old New Thing

    The Date/Time control panel is not a calendar


    Although many people use the Date/Time control panel to flip through a calendar, that's not what it is for. In fact, if you use it that way, you can create all sorts of havoc!

    In its original incarnation in Windows 95, the Date/Time control panel changed your date and time. If you clicked through the calendar to see next month, you actually changed your system clock to next month. If you changed your mind and clicked "Cancel", the Date/Time control panel undid its changes and restored the date to the original date.

    In other words, here's what happened, step by step:

    • On April 1, you open the Date/Time control panel.
    • You change the month to May. The Date/Time control panel changes your system date to May 1. If you are running an appointment calendar program, all appointments from the month of April will fire. (For example, your April 15th alarm to remind you to file your income taxes.) You are annoyed by all these alerts and you cancel them.
    • You decide you didn't want to change the month to May after all and click Cancel.
    • The Date/Time control panel changes the date back to April 1.
    • On April 15th, your income tax alarm fails to fire because you cancelled it, remember?

    In other words, the Date/Time control panel was not designed for letting you flip through a calendar. It was designed for changing the system date and time.

    Unaware of its design, people have been using the Date/Time control panel as if it were a calendar, not realizing that it was doing all sorts of scary things behind the scenes. It's like using a cash register as an adding machine. Sure, it does a great job of adding numbers together, but you're also messing up the accounting back at the main office!

    For Windows 2000, in reluctant recognition of the way people had been mis-using the Date/Time control panel, it was rewritten so that it doesn't change the system time until you hit the Apply button.

    Aaron Margosis shows you how to grant a user permission to change the system date and time without having to make them a full administrator.

  • The Old New Thing

    Google is the cute two-year-old girl with curly blond hair that gets all the attention


    Let's see, Google Maps adds the world outside the United States, Canada and the UK, and people go ga-ga. Nevermind that Google's new "maps" have nothing beyond country boundaries. "Aww, look at Google, she's so cute and adorable!"

    I'm sure the people at the existing online map services like MapQuest and MSN MapPoint are sitting there like older siblings, wondering when exactly they turned into chopped liver. MSN MapPoint has actual maps of most of Europe, and MapQuest's library of maps is larger still. (Kathmandu anyone?) Both sites provide documentation on how to link directly to them. Yet they don't get drooled over.

    Somebody at MapQuest should take out a full page ad that goes something like this:

    Dear Google Maps,

    Welcome to the rest of the world! If you ever need driving directions, don't hesitate to ask.

    Love ever,

  • The Old New Thing

    Why does the CreateProcess function do autocorrection?


    Programs that weren't designed to handle long file names would make mistakes like taking the path to the executable and writing it into the registry, unaware that the path might contain a space that needs quoting. (Spaces—while technically legal—were extremely rare in SFN paths.) The CreateProcess function had to decide whether to "autocorrect" these invalid paths or to let those programs simply stop working.

    This is the battle between pragmatism and purity.

    Purity says, "Let them suffer for their mistake. We're not going to sully our beautiful architecture to accomodate such stupidity." Of course, such an attitude comes with a cost: People aren't going to use your "pure" system if it can't run the programs that they require.

    Put another way, it doesn't matter how great your 1.0 version is if you don't survive long enough to make a 2.0.

    Your choice is between "being pure and unpopular" or "being pragmatic and popular". Look at all the wonderful technologies that died for lack of popularity despite technical superiority. Sony Betamax. Mattel Intellivision. (And, in the United States: The metric system.)

    Electric cars are another example. As great as electric cars are, they never reached any significant market success. Only after conceding to popularity and "sullying" their "purity" by adding a gasoline hybrid engine did they finally gain acceptance.

    I see this happening over and over again. A product team that, hypothetically, makes automated diagramming software, says, "I can't believe we're losing to Z. Sure, Z's diagrams may be fast and snazzy, but ours gets <subtle detail> right, and when you go to <extreme case> their diagrams come out a little distorted, and they're faster only because they don't try to prevent X and Y from overlapping each other in <scenario Q>. We're doing all those things; that's why we're slower, but that's also why we're better. Those people over at Z just don't 'get it'."

    Guess what. People are voting with their wallets, and right now their wallets are saying that Z is better in spite of all those "horrible flaws". Whatever part of "it" they don't get, it's certainly not the "make lots of people so happy that they send you money" part.

  • The Old New Thing

    What if two programs did this?


    The thought experiment "Imagine if this were possible" is helpful in thinking through whether Windows lets you do something or other. (A special case of this is "When people ask for security holes as features.") If the possibility leads to an obvious contradiction or the violation of generally-accepted rules of metaphysics, then you can be pretty sure that Windows doesn't support it. (Of course, the absence of such a contradiction doesn't prove that Windows does support it. But you can use it to rule out obvious bad ideas.)

    The question "What if two programs did this?" is also helpful in evaluating a feature or a design request. Combining this with "Imagine if this were possible" leads to an impressive one-two punch. Here are a few examples:

    "How do I create a window that is never covered by any other windows, not even other topmost windows?"

    Imagine if this were possible and imagine if two programs did this. Program A creates a window that is "super-topmost" and so does Program B. Now the user drags the two windows so that they overlap. What happens? You've created yourself a logical impossibility. One of those two windows must be above the other, contradicting the imaginary "super-topmost" feature.

    "How do I mark my process so that it always the first/last to receive the system shutdown notification? I want to do something before/after all other programs have shut down."

    Imagine if this were possible and imagine if two programs did this. You now have two programs both of which want to be first/last. But you can't have two first or two last things. One of them must lose. (This of course generalizes to other things people might want to be first or last.)

    "How do I make sure that my program is always the one that runs when the user double-clicks an .XYZ file?"

    Imagine if this were possible and imagine if two programs did this. Now the user double-clicks an .XYZ file. Which program runs?

    In this case, the solution is to leave the user in charge of their file associations; if they decide that they want your competitor's program to be used for .XYZ files, then that's their decision and you should respect it.

    My colleague Zeke [link fixed 11am], who is responsible, among other things, for the way file associations work in Explorer, provides a few alternatives:

    Here is the entry point in MSDN to the documentation on file associations in Explorer.

    For many of these "I want to be the X-est"-type questions, you can often come up with some sort of hack, where you run a timer that periodically checks whether you are still X, and if not, pushes you back into the X-position. And then you stop and think, "What if two programs did this?" and realize that it's a bad idea. At least I hope you do.

    Even with this explanation, some people still don't get it. I'll ask them to consider, "What if two programs did this? They'll be fighting back and forth," and the answer I get back is, "Then I can have the second program check if the first program is already running." They don't understand that they didn't write the second program.

    When two programs "duke it out" like this, you can't predict which one will win, but you can predict with 100% certainty who will lose: The user.

    I remember well when one of my colleagues called me into his office to show me two very popular commercial programs that both wanted to guarantee that they were the program that ran when the user double-clicked an .XYZ document. Since there is no such guarantee, they faked it with the timer hack.

    You installed the first program, it set itself as the .XYZ file handler, and everything seemed normal. You then installed the second program, it set itself as the new .XYZ file handler, and the first program noticed and said, "Uh-uh, I'm the program that runs .XYZ files", and changed things back. Then the second program said, "No way, I'm the program that runs .XYZ files" and set itself back.

    This childish game of "Nuh-uh/Yuh-huh!" went on while the user sat there dumbfounded and helpless, watching the icon for their .XYZ files flicker back and forth between the two programs, both of whom egotistically believed they were doing the user a "favor" by insisting on being the program that runs .XYZ files.

  • The Old New Thing

    Why don't control panel programs and property sheets show up in the taskbar?


    Control panel programs and property sheets don't show up in the taskbar. Why not?

    As I recall, the explanation was that control panel programs and property sheets aren't applications. They are auxiliary helper windows that assist you with a task, but they aren't a program in their own right. Therefore, they don't get a taskbar button.

    I've always been kind of suspicious of that explanation, but there it is, make of it what you will. (I don't mind the behavior—putting them in the taskbar just creates clutter—but the explanation I found kind of wanting.)

  • The Old New Thing

    I'll see (some of) you in Los Angeles in September


    Jeremy Mazner has asked me to put together a 400-level session at this year's PDC. I came up with the title "Five(ish) things every Win32 developer should know (but likely doesn't)". Of course, now I have to think of five things! Here are some ideas I've been kicking around.

    • The memory scene: Physical address space != physical memory != virtual memory != virtual address space
    • Consequences of the way CPUs work: How my O(n) algorithm can run circles around your O(log n) algorithm; why much of what you learned in school simply doesn't matter
    • Parent vs. owner windows
    • Asynchronous input queues, the hazards of attaching thread input, and how it happens without your knowledge
    • Dialog units, DPI and the overloaded term "large fonts"

    Would you go to a talk that covered these topics? If not, what topics would you rather hear me talk about?

    Follow-up: The talk will be a little over an hour. (And fixed the title. Thanks, Dave.)

  • The Old New Thing

    If strncpy is so dangerous, why does Visual Studio 2005 still support it?


    In response to the news that strncpy is so dangerous, at least one person has called for Visual Studio to revoke support for such a dangerous function, considering the continued support for the function grounds for holding the compiler manufacturer liable for any defects in programs compiled with that compiler.

    Well, for one thing, while it's true that strncpy is dangerous if used improperly, it is still a valid function, and my original discussion explained the history behind strncpy and the very specific scenario in which it is still useful. It just so happens that most people don't use the function in the manner it was intended, but instead treat it as a sort of "copy string with a character limit" function, which it isn't really.

    For another thing, just because something is dangerous doesn't mean it shouldn't be supported. Pointers and casts are dangerous, but I don't see them disappearing from C or C++ any time soon.

    Third, support for strncpy is mandated by the C standard. If you removed it, you couldn't call yourself a C compiler any more. (Not to mention breaking compatibility with existing source code that uses the strncpy function. How would you like it if you bought a so-called C compiler and found that it couldn't compile a large class of valid C programs?)

  • The Old New Thing

    Why can't the default drag/drop behavior be changed?


    A common reaction to my explanation of whether dragging a file will result in a move or copy was that there should be a setting that lets you change the algorithm by which Explorer decides whether you want to move or copy.

    There are a few reasons why this is a bad idea.

    First, if there were such a setting, then it removes some of the predictability from the user interface. One of the benefits of a common user interface is that once you learn it, you can apply the rules generally. But if each user could customize how drag/drop works, then the knowledge you developed with drag/drop wouldn't transfer to other people's machines.

    Some people view infinite customizability as a good thing. But each added bit of customizability increases the possibility that a particular combination of settings won't get tested as heavily as perhaps it should. ("What do you mean, this doesn't work if you have the notification icons set to hide after only 5 seconds, the taskbar's auto-hide delay customized to a value larger than 5 seconds, the taskbar buttons customized to a boldface font larger than 14pt, and drag/drop operations defaulting always to move? How could you have missed that combination in your testing? Surely you should have anticipated the interaction between an auto-hide delay longer than the notification auto-hide delay combined with a nondefault drag on a network with fewer than 50 machines!")

    Infinite customizability also means that you can't just sit down in front of somebody's machine and start using it. You first have to learn how they customized their menus, button clicks, default drag effects, and keyboard macros. "Oh, on this machine, you paste by shift-right-clicking. Sorry. On my machine, I use ctrl-alt-middle-click to paste." Imagine if everybody could easily customize the order of the clutch, brake, and gas pedals in their car to suit their fancy.

    There is also the branding element. Like the Mac, Windows tries to cultivate a specific "look" that makes people say, "Hey, this computer is running Windows; I know how to use it!" My DVD player and my car both show the manufacturer's logo when they are booting up. So too does Windows.

    Even if the "change the default drag/drop behavior" option passed "settings court" and was deemed worth the additional test cost, you still have the problem that it affects only Explorer. Other programs would continue to use the old algorithm, at least until you found their settings to change how they perform default drag/drop as well, if such a setting existed at all. Imagine the confusion if Windows Explorer followed one set of rules, but Microsoft Outlook followed a different set of rules. "Oh right, this is a mail message I'm dragging; the default operation is going to be a move, not a copy."

  • The Old New Thing

    Why does Explorer eject the CD after you finish burning it?


    Partly as a convenience, but partly to work around buggy hardware. The developer responsible for CD burning explained it to me.

    Most CD drives cache information about the disc in their internal memory to improve performance. However, some drives have a bug where they fail to update the cache after the CD has been written to. As a result, you can write some data to a CD, then ask the CD drive for the data you just wrote, and it won't be there! The drive is returning the old cached data instead of the new data. For most drives, ejecting and reinserting the CD is enough to force the drive to update its internal cache.

    "But wait, it gets worse!" I'm told.

    Some drives are "smart" and realize you've reinserted the same media, and then don't update. These drives require that you put in another type of media (or pressed CD-ROM media) to force them to update. These drives were manufactured around 2002, and new drives don't have it this bad, but still have the above problem requiring an eject/insertion cycle.

    So there's your tip for the day. If you are burning data to a CD and you find the data isn't there, try ejecting the disc and reinserting it. If your drive is particularly buggy, you'll have to eject the disc, insert a different type of disc, then eject that second disc and reinsert the first one.

  • The Old New Thing

    Beware of roaming user profiles


    One of the less-known features of Windows is the roaming user profile. I know that this is not well-known because I often see suggestions that fail to take the roaming user profile scenario into account. Indeed, if your program behaves badly enough, you can cause data loss. (More on this later.)

    What is a roaming user profile?

    Well, your user profile is the collection of things that reside under your %USERPROFILE% directory. (This is not quite true, but it's a good enough approximation for the purpose of this discussion. An important exception will be noted next time.) Your per-user registry is kept in %USERPROFILE%\ntuser.dat, so your per-user registry is part of your user profile.

    In highly managed environments (corporations), system administrators can set up user profiles on a centralized server, so that users log onto any machine and have available their files and settings. This is accomplished by copying the user profile from the server when the user logs on and copying it back to the server when the user logs off. (Of course, there is also caching involved to save time if the user logs back onto the same machine.)

    What does this mean for you, the programmer?

    For one thing, it means that the path to the user's profile can change from one logon session to the next. If the user runs your program from Computer A, their user profile directory might be C:\Documents and Settings\Fred, but when they log off from Computer A and log on to Computer B, the directory to their user profile might change to C:\WINNT\Profiles\Fred. In particular, that file that used to be at C:\Documents and Settings\Fred\My Documents\Proposal.txt has moved to C:\WINNT\Profiles\Fred\My Documents\Proposal.txt. If your program has a feature where it offers a list of recently-used files (or auto-opens the most recently used file), you may find that the file no longer exists at its old location. The solution is to use profile-relative paths, or even better, shell virtual folder-relative paths (e.g., recording the path relative to CSIDL_MYDOCUMENTS), so that when the profile roams to a machine with a different user profile path, your program can still find its files.

    For another thing, you cannot just cruise through the HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion\ProfileList registry key expecting to find all the user profiles and possibly even modify them, because the copy of the user profile on the local computer might not be the authoritative one. If the profile is a cached roaming profile, then any changes you make will either (1) be lost when the user roams back to the computer after using another computer, or (2) cause the local profile to be considered newer than the master copy on the server, causing the changes the user made to the copy on the server to be lost! (Which of the two bad scenarios you find yourself in depends on the time you change the cached profile and the time the target user logs off that other computer.)

    Another consequence of roaming user profiles is that your program can effectively see itself changing versions constantly. If Computer A has version 1.0 of your program and Computer B has version 2.0, then as the profile roams between the two computers, both versions 1.0 and 2.0 will be operating on the user profile in turn. If versions 1.0 and 2.0 use the same registry keys to record their settings, then your registry formats had better be both upward- and downward-compatible. This is a particularly painful requirement for operating system components, which consequently need to maintain bidirectional registry format compatibility with systems as old as Windows NT 4. (Windows NT 3.51 had a different model for roaming user profiles.)

    Yet another consequence of roaming user profiles applies to services. Prior to Windows XP, if a service holds a registry key open after the user logged off, then the registry hive cannot be unloaded and consequently (1) consumes memory for that profile even though the user is no longer logged on, and (2) prevents the user's local registry changes from being copied back to the server. This "hive leakage" problem was so rampant that in Windows XP, the profile unload code takes a more aggressive stance against services that hold keys open too long. You can read more about the changes to registry hive roaming in the Resource Kit article linked at the top of this entry.

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