The Old New Thing

Why can't I create my dialog with DialogBox, DialogBoxParam, CreateDialog, CreateDialogParam, or the indirect versions of same?

November 29, 2013 Nov 29, 2013 11/29/13

Raymond Chen

One of the purposes of my dialog manager series was to help people diagnose problems with their dialog boxes. But since I embedded the tips inside the series body, it's hard for people to find them, and I still end up answering the same questions over and over. So here it is in a separate article that hopefully people can find. Why your ...

If you try to declare a variadic function with an incompatible calling convention, the compiler secretly converts it to cdecl

November 28, 2013 Nov 28, 2013 11/28/13

Raymond Chen

Consider the following function on an x86 system: The function declares itself as , which is a callee-clean convention. But a variadic function cannot be callee-clean since the callee does not know how many parameters were passed, so it doesn't know how many it should clean. The Microsoft Visual Studio C/C++ compiler resolves this ...

The case of the DLL that refuses to load

November 27, 2013 Nov 27, 2013 11/27/13

Raymond Chen

A customer reported that they had a problem that occurred only on some machines but not others. Their application called and the call succeeded on some machines, but failed on others with error ("The specified module could not be found"). The path was a fully-qualified path to a file that was confirmed to exist and be readable. If the...

Extracting GPS coordinates from a photo and plotting it on a map

November 25, 2013 Nov 25, 2013 11/25/13

Raymond Chen

Today's Little Program extracts GPS coordinates from a photo and plots it on a map. Remember, Little Programs do little to no error checking, because that's how they roll. We start with a simple function that takes a latitude and longitude and opens a Web page that highlights that coordinate. In a real program, you probably would do ...

How do I get the effect of CW_USEDEFAULT positioning on a window I've already created?

November 22, 2013 Nov 22, 2013 11/22/13

Raymond Chen

A customer wanted to know how to get the effect of positioning on a window that already exists. In particular, they wanted to be able to reposition a dialog box to get the cascade effect, but since you can't actually pass in a dialog template, the repositioning has to be done after the fact. (Presumably in the handler, which runs before ...

How do I get the path to the default user's profile?

November 21, 2013 Nov 21, 2013 11/21/13

Raymond Chen

A customer wanted to know how to get the path to the default user's profile. On older versions of Windows, the default location of the default user's profile was . Then it moved to . Now it's in . And the location may have been customized, so in principle it could be anywhere. The function to get the default user profile's directory is is ...

How can I launch an unelevated process from my elevated process and vice versa?

November 18, 2013 Nov 18, 2013 11/18/13

Raymond Chen

Going from an unelevated process to an elevated process is easy. You can run a process with elevation by passing the verb to or . Going the other way is trickier. For one thing, it's really hard to munge your token to remove the elevation nature properly. And for another thing, even if you could do it, it's not the right thing to do, ...

Why is my FormatMessage call crashing trying to read my insertion parameter?

November 14, 2013 Nov 14, 2013 11/14/13

Raymond Chen

A customer was looking for assistance in debugging a crash in their product. The stack trace looked like this: The string being formatted is , and the insertion is a long (but valid) string. A unit test which passes a similarly long object name to does not crash. What is the problem? There are clues in the stack trace. The natural ...

What's the point of the various …WhenCallbackReturns functions?

November 8, 2013 Nov 8, 2013 11/8/13

Raymond Chen

The thread pool provides a number of functions named . What's the point of all these functions? Why can't you just do the operation yourself immediately before returning? We saw last time. What's the point of the others? Basically, the same thing as . It's a way to release a resource after execution has left the function and the callback ...

Partially eliminating the need for SetThreadpoolCallbackLibrary and reducing the cost of FreeLibraryAndExitThread

November 7, 2013 Nov 7, 2013 11/7/13

Raymond Chen

Update: Daniel points out that there is still a race condition here, so this trick won't work. Rats. The documentation for the says This prevents a deadlock from occurring when one thread in DllMain is waiting for the callback to end, and another thread that is executing the callback attempts to acquire the loader lock. If the DLL ...