May, 2005

  • The Old New Thing

    How to query properties of the taskbar


    Occasionally, people want to query properties of the taskbar. I don't quite understand why; you should just get on with your life and let the taskbar get on with its life. After all, there might not even be a taskbar, as we discussed last time.

    But if you really want to know (perhaps you're collecting usability data), here's how:

    #include <stdio.h>
    #include <windows.h>
    #include <shellapi.h>
    int __cdecl main(int argc, const char* argv[])
     APPBARDATA abd = { sizeof(abd) };
     UINT uState = (UINT)SHAppBarMessage(ABM_GETSTATE, &abd);
     printf("Taskbar on top? %s\n",
            (uState & ABS_ALWAYSONTOP) ? "yes" : "no");
     printf("Taskbar autohide? %s\n",
            (uState & ABS_AUTOHIDE) ? "yes" : "no");
     return 0;

    This little program uses the ABM_GETSTATE message of the SHAppBarMessage function to get the "Always on top" and "Auto-hide" properties of the taskbar.

    Since you're using the SHAppBarMessage function, if a future version of Windows changes the way it maintains the taskbar state (or perhaps even changes the name of the taskbar to something else), your program will still work because the SHAppBarMessage function will be kept in sync with whatever changes happen to the taskbar.

    You can also use the ABM_SETSTATE message to change these states. Note that doing so from a program is discouraged; these state bits belong to the user's preferences. A program shouldn't be messing with the user's preferences. (Well, unless the whole point of the program is to change the user's preferences, of course. But the frequency with which I see this question makes me wonder whether there really are that many settings-tweaking programs out there. I suspect people are using this power for evil, not for good.)

    And to stave off follow-up questions: These are the only two properties of the taskbar that are programmable. Exposing a programmable interface for something as highly visible as the taskbar is a very sensitive issue, because once you grant programmatic access to something, there is a very strong temptation for programs to start abusing it.

  • The Old New Thing

    How do I cover the taskbar with a fullscreen window?


    For some reason, people think too hard. If you want to create a fullscreen window that covers the taskbar, just create a fullscreen window and the taskbar will automatically get out of the way. Don't go around hunting for the taskbar and poking it; let it do its thing.

    As always, start with the scratch program and add the following:

    HWND CreateFullscreenWindow(HWND hwnd)
     HMONITOR hmon = MonitorFromWindow(hwnd,
     MONITORINFO mi = { sizeof(mi) };
     if (!GetMonitorInfo(hmon, &mi)) return NULL;
     return CreateWindow(TEXT("static"),
           TEXT("something interesting might go here"),
           WS_POPUP | WS_VISIBLE,
           mi.rcMonitor.right - mi.rcMonitor.left,
           mi.rcMonitor.bottom -,
           hwnd, NULL, g_hinst, 0);
    void OnChar(HWND hwnd, TCHAR ch, int cRepeat)
     if (ch == TEXT(' ')) {
        HANDLE_MSG(hwnd, WM_CHAR, OnChar);

    Note that this sample program doesn't worry about destroying that fullscreen window or preventing the user from creating more than one. It's just a sample. The point is seeing how the CreateFullScreenWindow function is written.

    We use the MonitorFromWindow function to figure out which monitor we should go fullscreen to. Note that in a multiple monitor system, this might not be the same monitor that the taskbar is on. Fortunately, we don't have to worry about that; the taskbar figures it out.

    I've seen people hunt for the taskbar window and then do a ShowWindow(hwndTaskbar, SW_HIDE) on it. This is nuts for many reasons.

    First is a mental exercise you should always use when evaluating tricks like this: "What if two programs tried this trick?" Now you have two programs both of which think they are in charge of hiding and showing the taskbar, neither of which is coordinating with the other. The result is a mess. One program hides the taskbar, then the other does, then the first decides it's finished so it unhides the taskbar, but the second program wasn't finished yet and gets a visible taskbar when it thought it should be hidden. Things only go downhill from there.

    Second, what if your program crashes before it gets a chance to unhide the taskbar? The taskbar is now permanently hidden and the user has to log off and back on to get their taskbar back. That's not very nice.

    Third, what if there is no taskbar at all? It is common in Terminal Server scenarios to run programs by themselves without Explorer [archived]. In this configuration, there is no Explorer, no taskbar. Or maybe you're running on a future version of Windows that doesn't have a taskbar, it having been replaced by some other mechanism. What will your program do now?

    Don't do any of this messing with the taskbar. Just create your fullscreen window and let the taskbar do its thing automatically.

  • The Old New Thing

    When people ask for security holes as features: Stealing passwords


    Sometimes people ask for features that are such blatant security holes I don't know what they were thinking.

    Is there a way to get the current user's password? I have a program that does some stuff, then reboots the system, and I want to have the current user's password so I can log that user back in when I'm done, then my program can resume its operation.

    (Sometimes they don't bother explaining why they need the user's password; they just ask for it.)

    Imagine the fantastic security hole if this were possible. Anybody could write a program that steals your password without even having to trick you into typing it. They would just call the imaginary GetPasswordOfCurrentUser function and bingo! they have your password.

    For another angle on credential-stealing, read Larry Osterman's discussion of why delegation doesn't work over the network.

    Even if you didn't want the password itself but merely some sort of "cookie" that could be used to log the user on later, you still have a security hole. Let's call this imaginary function GetPasswordCookieOfCurrentUser; it returns a "cookie" that can be used to log the user on instead of using their password.

    This is just a thinly-disguised GetPasswordOfCurrentUser because that "cookie" is equivalent to a password. Log on with the cookie and you are now that person.

  • The Old New Thing

    Managing the UI state of accelerators and focus rectangles


    Starting with Windows 2000, keyboard indicators such as underlined accelerators and focus rectangles (collectively known as "keyboard cues") are hidden by default, and are revealed only when you start using the keyboard. You can control this behavior from the Desktop Control Panel, under Appearance, Effects, "Hide underlined letters for keyboard navigation until I press the Alt key".

    Note that this setting actually controls both underlined letters and focus rectangles, even though the text describes only one of the effects. Underlines are hidden until you press the Alt key, and focus rectangles are hidden until you either press the Alt key or press the Tab key.

    Here's how it works.

    There are three UI state mesages: WM_CHANGEUISTATE, WM_QUERYUISTATE and WM_UPDATEUISTATE. The third one is, in my opinion, a misnomer. It really should be called something like WM_UISTATECHANGED since it is a notification that something has happened, not a message that you send to cause something to happen.

    When a dialog box or menu is displayed via a mouse click, keyboard cues are hidden; if the dialog box or menu was displayed via a keypress, then keyboard cues are visible. This decision is made by sending a WM_CHANGEUISTATE message to the root window with the UIS_INITIALIZE flag. This is done automatically by the dialog manager, but if you're doing your own custom windows, you'll have to send it yourself.

    The WM_CHANGEUISTATE message bubbles up to the top-level window, which changes the window UI state accordingly, then broadcasts a WM_UPDATEUISTATE message to all its child windows to notify them that the state has changed. (Of course, if the WM_CHANGEUISTATE message has no effect—for example, hiding something that is already hidden—then the WM_UPDATEUISTATE message is optimized out since the entire operation is a no-op.)

    When a window that draws keyboard cues receives a WM_UPDATEUISTATE message, it typically invalidates itself so that the cues can be redrawn/erased, depending on the new state.

    At drawing time, a window that draws keyboard cues can use the WM_QUERYUISTATE message to determine which keyboard cues are visible and which are hidden, and draw its content accordingly. If focus rectangles are hidden, then the window should skip the call to the DrawFocusRect function. If keyboard underlines are hidden, then the window suppresses underlines in its text drawing. If the window uses the DrawText function, it can pass the DT_HIDEPREFIX flag to suppress the underlines. If you are responding to the WM_DRAWITEM message, then you should check for the ODS_NOACCEL and ODS_NOFOCUSRECT flags to determine you should draw an underline accelerator or a focus rectangle.

    Finally, during execution you may discover that the user has used the keyboard to perform navigation within your control. For example, the listview control may have noticed that the user has used the arrow keys to change the selected item. When this happens, the control sends itself a WM_CHANGEUISTATE specifying which keyboard cues should be revealed. As noted above, the WM_CHANGEUISTATE message eventually causes all the windows in the window tree to receive a WM_UPDATEUISTATE message if their states need to change.

    The IsDialogMessage function sends WM_CHANGEUISTATE messages as appropriate, so dialog boxes and anybody else who uses IsDialogMessage gets keyboard-cues tracking for free.

  • The Old New Thing

    Another dead computer: My personal laptop


    I'm kind of surprised at how much people reacted to my previous dead computer story. I guess there's an audience for stories about dead computers.

    Today's dead computer is my Sony Vaio PCG-Z505LE laptop, with a 600MHz processor and 192MB of RAM. Certainly a big step up from that 486/50 with 12MB of RAM.

    Laptop computers have a comparatively short lifetime. (Hardware vendors must love them.) I've learned that the best value comes from buying used laptops. You have to accept being a bit behind the curve, but the way I use my laptop, it needs to do only a small number of things:

    • surf the web,
    • read e-mail,
    • use Remote Desktop Connection to access my desktop machine,
    • download pictures from my digital camera, and
    • compile the occasional program.

    Only that last operation requires a hefty processor, and I do it so rarely that it doesn't bother me that it's kind of slow. (I just run the command line version of the compiler, so that at least takes the IDE overhead out of the picture.)

    I bought this laptop two years ago, used, and it ran just fine until a couple months ago when the internal power supply burnt out. I was ready to abandon the model line and give away the accessories I had bought, including a $200+ double-capacity battery.

    Allow me to digress on laptop batteries. Observe that batteries for old-model laptops cost almost as much as the laptops themselves. That's because the battery is the only real consumable in a laptop computer. The other components will run practically indefinitely if you don't drop them or douse them in soda, but batteries just plain wear out. That's where the money is.

    This means that many ads for used laptops will mention "needs new battery" at the end. And those are the ones I sought out. Because I have a working battery! Most prospective buyers would be turned off by a dead battery, but that didn't bother me one bit.

    The replacement laptop arrived a few days ago, and it runs great. I wiped the drive and reinstalled Windows XP from scratch. (Challenging because the laptop doesn't come with a bootable CD-ROM drive. I had to use floppies!) I may install a handful of programs but that's all. I don't like installing software on my computer. The more programs you install, the more likely there's going to be a conflict somewhere.

    The old laptop has already started being scavenged for parts. A friend of mine needed a replacement laptop hard drive, so I gave him my old one. The battery and power brick can of course be used by the new laptop. The memory from the old Vaio is no use, since the Vaio has only one memory expansion slot. The other parts of the old laptop aren't much use for anything aside from spares. Perhaps I should put the old laptop on concrete blocks on my front lawn.

    Next time (if there is a next time), the story of the dead AlphaServer.

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