• The Old New Thing

    The walls of my friend's house sometimes randomly got corrupted

    • 16 Comments

    One evening, I had a series of three dreams. In each one, I visited an unusual home.

    In the third dream, I visited the home of a friend of mine. He lived in a white stucco split-level, a stereotypical suburban home. What made the house interesting was that if you did things just right, dark dots would appear on the wall and slowly consume it.

    My friend explained, "This house is running a very old build of DirectX, and sometimes it just does that."

    We set up a repro and calculated that when the dots appeared, stack usage was exactly 5124 bytes. This was a 16-bit house, and the stack overflow into the heap caused a return address to be changed to point into the DirectDraw flood fill function.

    When I told my friend about this strange dream, he quipped, "I've since then upgraded to a 64-bit house."

  • The Old New Thing

    How does the C runtime know whether to use the static-linking or dynamic-linking version of the header file?

    • 14 Comments

    In response to a description of what happens when you get dll­import wrong, nksingh asks, "This seems like a problem for the CRT. As far as I know, VC gives you the option of statically or dynamically linking the CRT. But it seems like the headers will have to make a choice to support one thing better than the other. Conditional compilation would work, but then people would have to remember to include a #define somewhere. Is this dllimport vs. static linking thing something the compiler could figure out on its own if you're doing Link-time codegen?"

    Let's start from the beginning.

    Yes, this would be a problem for the CRT since it wouldn't know whether to declare the functions as normal static functions or as dllimport-style functions, and the headers have to make a choice which way to go.

    And if you look at the headers, you can see that it is indeed done via conditional compilation.

    ...
    _CRTIMP int __cdecl fflush(FILE * _File);
    ...
    

    This magic _CRTIMP symbol is defined in crtdefs.h like so:

    /* Define _CRTIMP */
    #ifndef _CRTIMP
    #ifdef _DLL
    #define _CRTIMP __declspec(dllimport)
    #else  /* _DLL */
    #define _CRTIMP
    #endif  /* _DLL */
    #endif  /* _CRTIMP */
    

    Conditional compilation decides whether _CRTIMP expands to __declspec(dllimport) or to nothing at all, depending on whether the _DLL symbol is defined.

    And yet nobody bothers writing #define _DLL before they #include <stdio.h>. There must be something else going on.

    In fact, we can run some experiments to see what's going on.

    #ifdef _DLL
    #error "_DLL is defined"
    #else
    #error "_DLL is not defined"
    #endif
    

    Save this as dummy.c and run a few tests.

    C:\tests> cl /MT dummy.c
    dummy.c
    dummy.c(4) : fatal error C1189: #error :  "_DLL is not defined"
    
    C:\tests> cl /MD dummy.c
    dummy.c
    dummy.c(2) : fatal error C1189: #error :  "_DLL is defined"
    

    Well how's about that. The compiler uses the /MT and /MD flag to decide whether or not to define the preprocessor symbol _DLL, which is the secret signal it passes to the crtdef.h header file to control the conditional compilation.

    The compiler has to use this technique instead of deferring the decision to link-time code generation because it cannot assume that everybody has enabled link-time code generation. (Indeed, we explicitly did not in our sample command lines.)

    If link-time code generation were enabled, then is this something that could be deferred until that point?

    In principle yes, because link-time code generation in theory could just make the .obj file a copy of the source file (and all the header files) and do all the actual compiling at link time. This is a sort of extreme way of doing it, but I guess it could've been done that way.

    On the other hand, it also means that the compiler folks would have to come up with a new nonstandard extension that means "This function might be a normal static function or it might be a dll­import function. I haven't decided yet; I'll tell you later."

    Seeing as how the CRT already has to solve the problem in the case where there is no link-time code generation, it doesn't seem worth the effort to add a feature to link-time-code generation that you don't actually need. It would be a feature for which the only client is the C runtime library itself, for which the C runtime library already requires a separate solution when link-time code generation is disabled, and for which that separate solution still works when link-time code generation is enabled.

    No engineering purpose is served by writing code just for the sake of writing code.

  • The Old New Thing

    Things I've written that have amused other people: Episode 6

    • 4 Comments

    On an internal discussion group, somebody decided to quote one of my blog entries.

    Jim Medding was amused by my response: Bloggers are just idiots with a Web site.

    (My blog was dug up in order to refute a claim, even though the blog entry applied to a situation different from the one under discussion.)

  • The Old New Thing

    You gotta fight for your right to parry

    • 17 Comments

    (The headline was stolen from one of my cleverer friends.)

    I'm taking time out of my busy vacation to draw your attention (if it hasn't been already) to the story of South Korean fencer Shin A Lam, who got screwed out of her trip to the finals of the women's individual epeé.¹ The story is heart-wrenching, and the photojournalism is moving in its simplicity. (Assisted in large part by the rule that fencers must not leave the piste while the appeal is in progress.)

    The FIE, fencing's international governing body, later chose to award Shin a special medal "For aspiration to win and respect of the rules." Apparently, the FIE's timekeeping equipment was not up to the task because "They never expected this kind of thing to happen in the last second, three attacks. Their timekeeping machine is only in seconds, not points of a second."

    Um, right. Because nobody expected fencing to have sudden bursts of intense, frantic activity which require split-second accuracy.

    ¹ To avoid matches which last indefinitely, fencing has a concept known as priority: Before the final overtime round, one of the fencers is chosen at random to have priority. If no winner is determined by the final round, then the fencer with priority is declared the winner. Shin had priority, so if the ruling had been no-touch, she would have been the one to advance.

  • The Old New Thing

    Microspeak: Engagement

    • 23 Comments

    Meetings are so passé. You no longer have a meeting with a customer; you have an engagement:

    I have a customer engagement tomorrow and they have a question surrounding Feature X.

    Note that this use of the phrase customer engagement is different from the process known as customer engagement. The process is an ongoing interaction, a long-term activity to build customer loyalty.

    The author of the above sentence is not using it in the process sense (because you don't have "a" customer engagement; rather, a meeting is one component of the overall process of customer engagement). Nope, the author is just trying to sound cool.

  • The Old New Thing

    The new dietary restriction landscape

    • 25 Comments

    Non appetit, the modern quandary of preparing a dinner for people who all have different types of dietary restrictions. Depending on whom I invite to dinner, I may have to put together a meal that conforms to one or more of the following restrictions: low-fat, pescetarian, vegetarian, nondairy, non-pork, non-beef. (Yes, many of these categories overlap.) To the best of my knowledge, none of my dinner guests have had nut allergies or been gluten intolerant.

    I remember telling a story once and mentioning that the subject of the story was a vegetarian. The person I was telling the story to (who is from an older generation) asked, "Are they so poor that they can't afford meat?" Because in an older era, people were vegetarians because they had no choice.

  • The Old New Thing

    Neuroscience can be used for good or for evil; this one might fall in the evil bucket

    • 5 Comments

    Marketplace radio interviews Martin Lindstrom, author of Buyology: Truth and Lies About Why We Buy (another book in the series Short catchy title: Long boring subtitle) about how stores get people to buy more stuff by taking advantage of how our brains are wired.

    (Unfortunately, at the time I checked, the Smell and Beer bonus tracks were broken. You can try to console yourself with Paddy Hirsch's explanation of margin calls in terms of Girl Scout Cookies.)

    Update: The Smell and Beer links work now.

  • The Old New Thing

    Microspeak: Dialogue

    • 24 Comments

    Why have a conversation when you can dialogue?

    I think this is minimal work, but do others care? If they don't, then this is one for the ideas that failed bin. If they do, well let's dialogue...

    No need to talk when you can dialogue.

  • The Old New Thing

    In the search for the subtle source of the problem, you sometimes overlook the obvious one

    • 36 Comments

    A customer was encountering a problem with lots of duplicate GUIDs. How is that possible? The whole point of the GUID generation algorithm is to work hard to avoid duplication. Was one of the fundamental assumptions of the algorithm broken? Maybe there was a duplicate MAC? Was the clock regressing?

    One of my colleagues pointed out that in the search for the subtle source of the problem, you sometimes overlook the obvious one. In fact, this is the most common source of problems with so-called duplicate GUIDs. As he so tersely puts it: "A GUID can easily be duplicated by simply copying it."

    In other words, you have a duplicate GUID because you duplicated the GUID. This can happen, for example, if you have a Clone method on an object which creates an exact duplicate of the object. Since the GUID is a property of the object, it too gets cloned. Then you add the clone to the same database as the original.

    Boom, instant duplicate.

  • The Old New Thing

    2011 Q1 link clearance: Microsoft blogger edition

    • 11 Comments

    It's that time again: Linking to other Microsoft bloggers.

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