March, 2011

  • The Old New Thing

    How do I create a topmost window that is never covered by other topmost windows?

    • 74 Comments

    We already know that you can't create a window that is always on top, even in the presence of other windows marked always-on-top. An application of the What if two programs did this? rule demonstrates that it's not possible, because whatever trick you use to be on-top-of-always-on-top, another program can use the same trick, and now you have two on-top-of-always-on-top windows, and what happens?

    A customer who failed to understand this principle asked for a way to establish their window as "super-awesome topmost". They even discovered the answer to the "and what happens?" rhetorical question posed above.

    We have overridden the OnLostFocus and OnPaint methods to re-assert the TopLevel and TopMost window properties, as well as calling BringToFront and Activate. The result is that our application and other applications end up fighting back and forth because both applications are applying similar logic. We tried installing a global hook and swallowing paint and focus events for all applications aside from our own (thereby preventing the other applications from having the opportunity to take TopMost ahead of us), but we found that this causes the other applications to crash. We're thinking of setting a timer and re-asserting TopMost when the timer fires. Is there a better way?

    This is like saying, "Sometimes I'm in a hurry, and I want to make sure I am the next person to get served at the deli counter. To do this, I find whoever has the lowest number, knock them unconscious, and steal their ticket. But sometimes somebody else comes in who's also in a hurry. That person knocks me unconscious and steals my ticket. My plan is to set my watch alarm to wake me up periodically, and each time it wakes me up, I find the person with the lowest number, knock them unconscious, and steal their ticket. Is there a better way?"

    The better way is to stop knocking each other unconscious and stealing each other's tickets.

    The customer (via the customer liaison) provided context for their question.

    This is not a general-purpose application. This application will be run on dedicated machines which operate giant monitors in retail stores. There are already other applications running on the computer which rotate through advertisements and other display information.

    The customer is writing another application which will also run on the machine. Most of the time, the application does nothing, but every so often, their application needs to come to the front and display its message, regardless of whatever the other applications are displaying. (For example, there might be a limited-time in-store promotion that needs to appear on top of the regular advertisements.)

    Unfortunately, all of these different programs were written by different vendors, and there is no coordination among them for who gets control of the screen. We were hoping that there was some way we could mark our window as "super topmost" so that when it came into conflict with another application running on the machine, it would win and the other application would lose.

    I'm thinking of recommending that the vendors all come up with some way of coordinating access to the screen so they can negotiate among themselves and not get into focus fights. (Easier said than done, since all the different applications running on the machine come from different vendors...)

    Since there is no coordination among the various applications, you're basically stuck playing a game of walls and ladders, hoping that your ladder is taller than everybody else's wall. The customer has pretty much found the tallest ladder which the window manager provides. There is no "super topmost" flag.

    Sure, you can try moving to another level of the system, like say creating a custom desktop, but all that does is give you a taller ladder. And then one of the other applications is going to say, "I need to display a store-wide page (manager to the deli please, manager to the deli), overriding all other messages, even if it's a limited-time in-store promotion." And they'll try something nastier, like enumerating all the windows in the system and calling ShowWindow(SW_HIDE).

    And then another application will say, "I need to display an important store-wide security announcement (Will the owner of a white Honda Civic, license plate 037-MSN, please return to your vehicle), overriding all other messages, even if it's a store-wide page." And it'll try something nastier, like setting their program as the screen saver, disabling the mouse and keyboard devices, and then invoking the screen saver on the secure desktop.

    And then another application will say, "I need to display a critical store-wide announcement (Fire in the automotive department. Everybody evacuate the building immediately), overriding all other messages, even if it's an important store-wide security announcement." And it'll try something nastier, like enumerating all the processes on the system, attaching to each one with debug privilege, and suspending all the threads.

    Stop the madness. The only sane way out is to have the programs coöperate to determine who is in control of the screen at any particular time.

    In response to my hypothetical game of walls and ladders, one of my colleagues wrote, "Note to self: Do not get into a walls-and-ladders contest with Raymond."

  • The Old New Thing

    Why did Win32 define BOOL as a signed int instead of an unsigned int?

    • 48 Comments

    Igor Levicki wants somebody from Microsoft to explain why BOOL was defined as a signed int instead of an unsigned int.

    You don't need to work for Microsoft to figure this out. All the information you need is publically available.

    Quoting from K&R Classic, which was the operative C standards document at the time Windows was being developed:

    7.6 Relational Operators

    The [relational operators] all yield 0 if the specified relation is false and 1 if it is true. The type of the result is int.

    Win32 defined BOOL as synonymous with int because Brian and Dennis said so. If you want to know why Brian and Dennis decided to have the result of relational operators be signed instead of unsigned, you'll have to ask them.

  • The Old New Thing

    Why can't Explorer decide what size a file is?

    • 42 Comments

    If you open Explorer and highlight a file whose size is a few kilobytes, you can find some file sizes where the Explorer Size column shows a size different from the value shown in the Details pane. What's the deal? Why can't Explorer decide what size a file is?

    The two displays use different algorithms.

    The values in the Size column are always given in kilobytes, regardless of the actual file size. File is 15 bytes? Show it in kilobytes. File is 2 gigabytes? Show it in kilobytes.

    The value shown in the Size column is rounded up to the nearest kilobyte. Your 15-byte file shows up as 1KB. This has been the behavior since Explorer was first introduced back in Windows 95, Why? I don't know; the reasons may have been lost to the mists of time. Though I suspect one of the reasons is that you don't want a file to show up as 0KB unless it really is an empty file.

    On the other hand, the value shown in the Details pane uses adaptive units: For a tiny file, it'll show bytes, but for a large file, it'll show megabytes or gigabytes or whatever. And the value is shown to three significant digits.

    The result is that a file which is, say, 19465 bytes in size (19.0088 kilobytes) shows up in the Size column as 20KB, since the Size column rounds up. On the other hand, the Details pane shows 19.0KB since it displays the value to three significant digits.

    It looks like Explorer can't make up its mind, and perhaps it can't, but the reason is that the two places on the screen which show the size round in different ways.

  • The Old New Thing

    Why is there the message '!Do not use this registry key' in the registry?

    • 40 Comments

    Under Software\Microsoft\Windows\Current­Version\Explorer\Shell Folders, there is a message to registry snoopers: The first value is called "!Do not use this registry key" and the associated data is the message "Use the SH­Get­Folder­Path or SH­Get­Known­Folder­Path function instead."

    I added that message.

    The long and sad story of the Shell Folders key explains that the registry key exists only to retain backward compatibility with four programs written in 1994. There's also a TechNet version of the article which is just as sad but not as long.

    One customer saw this message and complained, "That registry key and that TechNet article explain how to obtain the current locations of those special folders, but they don't explain how to change them. This type of woefully inadequate documentation only makes the problem worse."

    Hey, wow, a little message in a registry key and a magazine article are now "documentation"! The TechNet article is historical background. And the registry key is just a gentle nudge. Neither is documentation. It's not like I'm going to put a complete copy of the documentation into a registry key. Documentation lives in places like MSDN.

    But it seems that some people need more than a nudge; they need a shove. Let's see, we're told that the functions for obtaining the locations of known folders are SH­Get­Folder­Path and its more modern counterpart SH­Get­Known­Folder­Path. I wonder what the names of the functions for modifying those locations might be?

    Man that's a tough one. I'll let you puzzle that out for a while.

    Okay, here, I'll tell you: The corresponding functions go by the completely unobvious names SH­Set­Folder­Path and SH­Set­Known­Folder­Path.

    Sorry you had to use your brain. I'll let you get back to programming now.

  • The Old New Thing

    Function requirements are cumulative: If you fail to meet any of them, then all bets are off

    • 33 Comments

    A customer was having problems with the Wait­For­Multiple­Objects function:

    We are looking for a clarification of the behavior of Wait­For­Multiple­Objects. We have a thread that waits on two handles (call them Handle1 and Handle2) with Wait­For­Multiple­Objects, bWaitAll = FALSE. Under certain conditions, we signal Handle1 and close Handle2 from another thread while the wait is in progress. This results in WAIT_FAILED being returned from the wait call. MSDN is not clear on what the expected behavior is here. On the one hand, it says

    1. When bWait is FALSE, this function checks the handles in the array in order starting with index 0, until one of the objects is signaled. If multiple objects become signaled, the function returns the index of the first handle in the array whose object was signaled.

    This description implies that the wait should never fail, because the function should have noticed that Handle1 is signaled before it got around to noticing that Handle2 was invalid.

    On the other hand, the documentation also says

    1. If one of these handle is closed while the wait is still pending, the function's behavior is undefined.

    What behavior is guaranteed here?

    Once you violate a constraint (in this case, the constraint that the handles remain valid for the duration of the wait operation), you have failed to meet the prerequisites and the behavior is undefined. You can't say, "Well, sure you say I can't do X, but if you follow exactly the steps given in this description of how signaled objects are selected, then the function wouldn't even have noticed X before coming to a decision, so the fact that I broke one of the rules is irrelevant!"

    The description of the behavior of the Wait­For­Multiple­Objects function when bWait is FALSE is not an implementation specification. It's a description of how to interpret the behavior of the function. The algorithmic way the function behavior is described is to assist in understanding the behavior; it doesn't mean that the function actually follows the algorithm step-by-step. (In reality, there is no polling loop, as the algorithmic description implies. All the handles are waited on simultaneously. It's like Lebesgue integration: You integrate over the entire domain at once.)

    An algorithm-free description of the behavior when bWait is false might go like this:

    1. When bWait is FALSE, the function does not return until one of the handles in the array becomes signaled. The return value is the smallest index corresponding to a signaled handle.

    This description is purely declarative but gives you no mental model.

    It's like saying that "Water seeks its own level." Water doesn't have a will that compels it to behave in a certain way, but describing the behavior in that way makes reasoning about water easier.

  • The Old New Thing

    Why does my TIME_ZONE_INFORMATION have the wrong DST cutover date?

    • 30 Comments

    Public Service Announcement: Daylight Saving Time begins in most parts of the United States this weekend. Other parts of the world may change on a different day from the United States.

    A customer reported that they were getting incorrect values from the GetTimeZoneInformationForYear function.

    I have a program that calls GetTimeZoneInformationForYear, and it looks like it's returning incorrect DST transition dates. For example, GetTimeZoneInformationForYear(2010, NULL, &tzi) is returning March 2nd as the tzi.DaylightDate value, instead of the Expected March 14th date. The current time zone is Pacific Time.

    The value returned by GetTimeZoneInformationForYear (and GetTimeZoneInformation) is correct; you're just reading it wrong.

    As called out in the documentation for the TIME_ZONE_INFORMATION structure, the wDay field in the StandardDate and DaylightDate changes meaning depending on whether the wYear is zero or nonzero.

    If the wYear is nonzero, then the wDay has its usual meaning.

    But if the wYear is zero (and it is for most time zones), then the wDay encodes the week number of the cutover rather than the day number.

    In other words, that 2 does not mean "March 2nd". It means "the second week in March".

  • The Old New Thing

    No, not that M, the other M, the one called Max

    • 29 Comments

    Code names are rampant at Microsoft. One of the purposes of a code name is to impress upon the people who work with the project that the name is only temporary, and that the final name will come from the marketing folks (who sometimes pull through with a catchy name like Zune, and who sometimes drop the ball with a dud like Bob and who sometimes cough up monstrosities like Microsoft WinFX Software Development Kit for Microsoft® Pre-Release Windows Operating System Code-Named "Longhorn", Beta 1 Web Setup).

    What I find amusing are the project which change their code names. I mean, the code name is already a placeholder; why replace a placeholder with another placeholder?

    One such example is the experimental project released under the code name Max. The project founders originally named it M. Just the letter M. Not to be confused with this thing code named M or this other thing code named M.

    In response to a complaint from upper management about single-letter code names, the name was changed to Milkshake, and the team members even made a cute little mascot figure, with a straw coming out the top of his head like a milkshake.

    I'm not sure why the name changed a second time. Perhaps those upper level managers didn't think Milkshake was a dignified-enough name. For whatever reason, the name changed yet again, this time to Max. (Wikipedia claims that the project was named after the pet dog of one of the team members; I have been unable to confirm this. Because I haven't bothered trying.)

    There's no real punch line here, sorry. Just one example of the naming history of a project that went by many names.

    Bonus chatter: Apparently the upper management folks who complained about the single-letter code name M were asleep when another product was code-named Q (now known as Windows Home Server).

  • The Old New Thing

    Raymond's highly scientific predictions for the 2011 NCAA men's basketball tournament

    • 28 Comments

    Once again, it's time for Raymond to come up with an absurd, arbitrary criterion for filling out his NCAA bracket.

    This year, I look at the strength of the school's football team, on the theory that a school with a strong football team and a strong basketball team has clearly invested a lot in its athletics program. My ranking of football teams is about as scientific as my ranking of basketball teams:

    • If the school ended last season with a BCS ranking, I used that.
    • If a school wasn't ranked but received votes in the AP ranking, then I gave it a rank of 30 (and if two such schools faced each other, I looked at who got more votes).
    • If a school still isn't ranked, then I looked to see if it had been ranked at any time earlier in the season; if so, then I gave it a rank of 40.
    • If a school still isn't ranked, but it appeared on the equally-scientific ESPN Fan Rankings, then I gave it a rank of 50.
    • If a school still isn't ranked, but it has a Division I FBS football team, then I gave it a rank of 80. If two such schools faced each other, then I gave what appeared to be the weaker school a rank of 90.
    • If a school still isn't ranked, but it has a Division I FCS football team, then I gave it a rank of 100. If two such schools faced each other, then I gave what appeared to be the weaker schools a rank of 101. (Why 101 instead of 110? Who cares!)
    • If a school still isn't ranked, but it has a football team in some other division, then I gave it a rank of 150.
    • If a school still isn't ranked because its football team is new, then I gave it a rank of 200.
    • If a school still isn't ranked because it doesn't have a football team, but it had one in the past, then I gave it a rank of 300.
    • If a school still isn't ranked because it never had a football team, then I gave it a rank of 400.

    (As a special case, USC received its rank of 22 from two years ago, because it was forced to sit out the 2010 season as part of its punishment for "several major rules violations." Now that's what I call dedication to athletics!)

    I made up all these rules on the fly, which is why the spacing is so uneven and why they were not necessarily applied fairly across the board, but that's what makes it highly scientific.

    As before, once the field has been narrowed to eight teams, the results are determined by a coin flip.

    Update:

    • Correct predictions are in green.
    • Incorrect predictions are in red.
    • (!) marks upsets correctly predicted.
    • (*) marks upsets predicted but did not take place.
    • (x) marks actual upsets not predicted.

    Opening Round Games

    Texas-San Antonio(200) Alabama State
    (80)
    Alabama State(80)
    UAB(90) Clemson
    (80)
    Clemson(80)
    UNC-Asheville(400) Arkansas-Little Rock
    (300)
    Arkansas-Little Rock(300)
    USC(22*) USC
    (22*)
    VCU(400)

    East bracket

    1Ohio State(6) Ohio State
    (6)
    Ohio State
    (6)
    Ohio State Ohio State
    16Alabama State(80)
    8George Mason(400) Villanova
    (100) (*)
    9Villanova(100)
    5Kentucky(80) Kentucky
    (80)
    West Virginia
    (30) (*)
    12Princeton(100)
    4West Virginia(30) West Virginia
    (30)
    13Clemson(80)
    6Syracuse(80) Syracuse
    (80)
    Syracuse
    (80) (x)
    Washington
    11Indiana State(90)
    3Xavier(300) Xavier
    (300) (x)
    14Marquette(310)
    7Washington(30) Washington
    (30)
    Washington
    (30)
    10Georgia(50)
    2North Carolina(50) North Carolina
    (50)
    15Long Island(400)

    West bracket

    1Duke(90) Duke
    (90)
    Michigan
    (80) (x)
    Arizona Arizona
    16Hampton(100)
    8Michigan(80) Michigan
    (80)
    9Tennessee(90)
    5Texas(40) Texas
    (40)
    Arizona
    (40)
    12Oakland(400)
    4Arizona(40) Arizona
    (40)
    13Memphis(80)
    6Connecticut(30) Connecticut
    (30)
    Missouri
    (12) (*)
    Missouri
    11Bucknell(100)
    3Cincinnati(80) Missouri
    (12) (*)
    14Missouri(12)
    7Temple(80) Penn State
    (40) (*)
    San Diego State
    (30)
    10Penn State(40)
    2San Diego State(30) San Diego State
    (30)
    15Northern Colorado(200)

    Southeast bracket

    1Pittsburgh(80) Pittsburgh
    (80)
    Pittsburgh
    (80) (x)
    Wisconsin Michigan State
    16Arkansas-Little Rock(300)
    8Butler(100) Butler
    (100)
    9Old Dominion(101)
    5Wisconsin(4) Wisconsin
    (4)
    Wisconsin
    (4)
    12Belmont(150)
    4Kansas State(40) Kansas State
    (40)
    13Utah State(80)
    6BYU(39) BYU
    (39)
    BYU
    (39)
    Michigan State
    11Wofford(100)
    3St. John's(200) St. John's
    (200) (x)
    14Gonzaga(300)
    7UCLA(80) Michigan State
    (7) (*)
    Michigan State
    (7)
    10Michigan State(7)
    2Florida(30) Florida
    (30)
    15UCSB(400)

    Southwest bracket

    1Kansas(90) Kansas
    (90)
    Illinois
    (80) (*)
    Illinois Texas A&M
    16Boston University(300)
    8UNLV(90) Illinois
    (80) (!)
    9Illinois(80)
    5Louisville(82) Louisville
    (82) (x)
    Louisville
    (82)
    12Morehead State(100)
    4Vanderbilt(90) Vanderbilt
    (90) (x)
    13Richmond(91)
    6Purdue(90) Purdue
    (90)
    USC
    (22*)
    Texas A&M
    11Saint Peter's(300)
    3Georgetown(100) USC
    (22*)
    14USC(22*)
    7Texas A&M(18) Texas A&M
    (18) (x)
    Texas A&M
    (18)
    10Florida State(23)
    2Notre Dame(30) Notre Dame
    (30)
    15Akron(80)

    Finals

    Ohio State Ohio State Michigan State
    Arizona
    Michigan State Michigan State
    Texas A&M
  • The Old New Thing

    What's up with the mysterious inc bp in function prologues of 16-bit code?

    • 24 Comments

    A little while ago, we learned about the EBP chain. The EBP chain in 32-bit code is pretty straightforward because there is only one type of function call. But in 16-bit code there are two types of function calls, the near call and the far call.

    A near call pushes a 16-bit return address on the stack before branching to the function entry point, which must reside in the same code segment as the caller. The function then uses a ret instruction (a near return) when it wants to return to the caller, indicating that the CPU should resume execution at the specified address within the same code segment.

    By comparison, a far call pushes both the segment (or selector if in protected mode) and the offset of the return address on the stack (two 16-bit values), and the function being called is expected to use a retf instruction (a far return) to indicate that the CPU should pop two 16-bit values off the stack to determine where execution should resume.

    When Windows was first introduced, it ran on an 8086 with 384KB of RAM. This posed a challenge because the 8086 processor had no memory manager, had no CPU privilege levels, and had no concept of task switching. And in order to squeeze into 384KB of RAM, Windows needed to be able to load code from disk on demand and discard it when memory pressure required it.

    One of the really tricky parts of the real-mode memory manager was fixing up all the function pointers when code was loaded and unloaded. When you unloaded a function, you had to make sure that any existing code in memory that called that function didn't actually call it, because the function wasn't there. If you had a memory manager, you could mark the segment or page not present, but there is no such luxury on the 8086.

    There are multiple parts to the solution, but the part that leads to the answer to the title question is the way the memory manager patched up all the stacks in the system. After all, if you discarded a function, you had to make sure that any reference to that function as a return address on somebody's stack got fixed up before the code tried to execute that retf instruction and found itself returning to a function that didn't exist.

    And that's where the mysterious inc bp came from.

    The first rule of stack frames in real-mode Windows is that you must have a bp-based stack frame. FPO was not permitted. (Fortunately, FPO was also not very tempting because the 16-bit instruction set made it cumbersome to access stack memory by means other than the bp register, so the easiest way to do something was also the right way.) In other words, the first rule required that every stack have a valid bp chain at all times.

    The second rule of stack frames in real-mode Windows is that if you are going to return with a retf, then you must increment the bp register before you push it (and must therefore perform the corresponding decrement after you pop it). This second rule means that code which walks the bp chain can find the next function up the stack. If bp is even, then the function will use a near return, so it looks at the 16-bit value stored on the stack after the bp and doesn't change the cs register. On the other hand, if the bp is odd, then it knows to look at both the 16-bit offset and the 16-bit segment that were pushed on the stack.

    Okay, so let's put it all together: When code got discarded, the kernel walked all the stacks in the system (which it could now do due to these two rules), and if it saw that a return address corresponded to a function that got discarded, it patched the return address to point to a chunk of code which called back into the memory manager to reload the function, re-patch all the return addresses so they now point to the new address where the function got loaded (probably different from where the function was when it was discarded), and then jumped back to the original code as if nothing had happened.

    I continue to be amazed at how much Windows 1.0 managed to accomplish given that it had so little to work with. It even used an LRU algorithm to choose which functions to discard by implementing a software version of the "accessed bit", something that modern CPUs manage in hardware.

  • The Old New Thing

    News flash: Companies change their product to appeal to their customers

    • 24 Comments

    There was some apparent uproar because there was an industry which "changed the flavoring of their product depending on which market segment they were trying to appeal to."

    Well duh, don't all industries do this?

    The reason why this even remotely qualified as news didn't appear until the last five words of the article!

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