January 2016 | Page 2 of 3 | The Old New Thing

A puzzle involving dynamic programming, or maybe it doesn’t, episode 2

Raymond ChenJanuary 18, 2016Jan 18, 201601/18/16

Here's a programming challenge: Given an array of numbers, count the number of ways the array can be partitioned into three contiguous parts such that each part has the same sum. For example, given the array { 1, 1, -1, 0, -1, 2, 2, 0, -2, 1 }, the answer is seven: 1 = 1 = (−1) + 0 + (−1) + 2 + 2 + 0 + (−2) + 1 1 = 1 + (&...

What does this crash in TppRaiseHandleStatus mean?

Raymond ChenJanuary 15, 2016Jan 15, 201601/15/16

A customer found that their program was crashing with this stack: "Since none of our code is on the stack, it is unclear what may have gone wrong here." A naming convention followed by many teams is to come up with a short prefix for a component and put that prefix on every function in that component. For example, all of the I/O manager ...

When you start talking about numbers as small as 2⁻¹²², you have to start looking more closely at the things you thought were zero

Raymond ChenJanuary 14, 2016Jan 14, 201601/14/16

GUID generation algorithm 4 fills the GUID with 122 random bits. The odds of two GUIDs colliding are therefore one in 2¹²², which is a phenomenally small number. When you are dealing with rates this low, you have to adjust your frame of reference. Some people don't like algorithm 4 because the algorithm is not designed to ...

My program for faking debugging demos

Raymond ChenJanuary 13, 2016Jan 13, 201601/13/16

When I present a debugging session at TechReady, there are a lot of things that can go wrong. A network hiccup might cause my debugger session to hang trying to load symbols over the network. An errant typo may cause the debugger to run off into the weeds. The machine I'm given to present on may be running an old version of Windows that the ...

Getting one’s paycheck from some mysterious pile of money

Raymond ChenJanuary 12, 2016Jan 12, 201601/12/16

My colleague who seems to get involved in all sorts of money-related shenanigans once found himself wrapped up in a money mystery not of his own devising. Back in the days before direct deposit, you received your pay in the form of a paper check. The group administrator would go through the halls with a handful of checks and personally hand ...

Creating a shell extension that applies only to files with a very specific name

Raymond ChenJanuary 11, 2016Jan 11, 201601/11/16

Today's Little Program isn't even a program. It demonstrates how to register a shell context menu command that applies only to files with a specific name. Let's say that you want a special context menu command, let's call it Party!, on any file called party.txt. If invoked, it runs CharMap because everybody knows that CharMap is a total party...

The FILE_FLAG_DELETE_ON_CLOSE flag applies to the handle, also known as the file object, which is not the same as the file

Raymond ChenJanuary 8, 2016Jan 8, 201601/8/16

A customer was having trouble with the FILE_FLAG_DELETE_ON_CLOSE flag. "We create a file that we want to be deleted automatically when we're finished with it. Whenever we open the file, we ask for GENERIC_READ | GENERIC_WRITE, and we allow all sharing (including FILE_SHARE_DELETE), and we pass FILE_FLAG_DELETE_ON_CLOSE. We can open the file ...

It rather involved being on the other side of this airtight hatchway: Attacking the system clock

Raymond ChenJanuary 7, 2016Jan 7, 201601/7/16

A security vulnerability report came in that went something like this: An attacker can trigger a buffer overflow in the XYZ component by setting the system time beyond the year 9999. This can be used as a precursor step for a further attack. Thanks for letting us know. We'll look at it. But is it a security vulnerability? In order to ...

Why does the mouse cursor jump a few pixels when you right-click on the Start button?

Raymond ChenJanuary 6, 2016Jan 6, 201601/6/16

Some time ago, I explained why the mouse cursor jumps a few pixels if you click on the very bottom row of pixels on the taskbar. Commenter ender noted that a similar jump occurs if you right-click the Start button. What's up with that? This is not an issue with Fitts's Law, because you already hit the target successfully. The purpose of the ...

Why did Windows NT decline to support an undocumented flag to TrackPopupMenuEx?

Raymond ChenJanuary 5, 2016Jan 5, 201601/5/16

Commenter kero points out that Windows 95 supported a flag called TPM_SYSMENU, but Windows NT doesn't. Why not? The TPM_SYSMENU was never documented. It was used internally by Windows 95 for something-or-other, but Windows NT didn't need to support it, so it didn't. Since it was an internal flag that was never documented, the ...

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