The Old New Thing

An easy way to determine whether you have a particular file permission

Sometimes you might want to determine whether you can do something without actually doing it. For example, you might want to know whether you have a particular permission in a directory, say permission to delete files from it. One way is to retrieve the ACL and then check whether the current user has the desired permission. The AccessCheck ...

What does SHGFI_USEFILEATTRIBUTES mean?

One of the flags you can pass to the SHGetFileInfo function is SHGFI_USEFILEATTRIBUTES. What does this flag mean? It means, "Do not access the disk. Pretend that the file/directory exists, and that its file attributes are what I passed as the dwFileAttributes parameter. Do this regardless of whether it actually exists or not." You...

High-performance multithreading is very hard

Among other things, you need to understand weak memory models. Hereby incorporating by reference Brad Abrams' discussion of volatile and MemoryBarrier(). In particular, Vance Morrison's discussion of memory models is important reading. (Though I think Brad is being too pessimistic about volatile. Ensuring release semantics at the...

Extending the Internet Explorer context menu

In a comment, Darrell Norton asked for a "View in Mozilla" option for Internet Explorer. You can already do this. Internet Explorer's context menu extension mechanism has been in MSDN for years. Let me show you how you can create this extension yourself. First, create the following registry key: Of course, you need to change C:\...

Do you know when your destructors run? Part 2.

Continuing from yesterday, here's another case where you have to watch your destructors. Yesterday's theme was destructors that run at the wrong time. Today, we're going to see destructors that don't run at all! Assume there's an ObjectLock class which takes a lock in its constructor and releases it in its destructor. Pretty standard ...

Do you know when your destructors run? Part 1.

Larry Osterman discussed the importance of knowing when your global destructors run, but this problem is not exclusive to global objects. You need to take care even with local objects. Consider: Easy as pie. And there's a bug here. When does the destructor for that smart-pointer run? Answer: When the object goes out of scope, which...

String sorting is not done by ASCII code any more.

Just because you have the ASCII table memorized doesn't mean you know how sorting works. I remember a bug filed where somebody said that the "sort" command was sorting underscores incorrectly: this was claimed to be wrong "because underscore character follow uppercase letters and precedes lowercase letters". Well perhaps it does if ...

Links about COM threading models

There have been a few good articles about threading models, one from Eric Lippert and another from Larry Osterman. Go off and read them if you haven't already...

How do the FILE_SHARE_* bits interact with the desired access bits?

It's really not that complicated. If you permit, say, , then you're saying, "I'm okay with other people reading this file while I have it open." And if you leave off the flag, then you're saying, "I do not want other people reading this file while I have it open." Now all that's left to do is work out what that means. So suppose you ...