The Old New Thing

Why you should never suspend a thread

December 9, 2003 Dec 9, 2003 12/9/03

Raymond Chen

It's almost as bad as terminating a thread. Instead of just answering a question, I'm going to ask you the questions and see if you can come up with the answers. Consider the following program, in (gasp) C#: When you run this program and hit Enter to suspend, the program hangs. But if you change the worker function to just "for...

Which access rights bits belong to whom?

December 4, 2003 Dec 4, 2003 12/4/03

Raymond Chen

Each ACE in a security descriptor contains a 32-bit access mask. Which bits belong to whom? The access rights mask is a 32-bit value. The upper 16 bits are defined by the operating system and the lower 16 bits are defined by the object being secured. For example, consider the value 0x00060002 for the access rights mask. This breaks down as ...

Which window style bits belong to whom?

December 3, 2003 Dec 3, 2003 12/3/03

Raymond Chen

There are 64 bits of styles in the parameters to CreateWindowEx. Which ones belong to whom? Windows defines the meanings of the high word of the dwStyle parameter and all of the bits in the dwExStyle parameter. The low 16 bits of the dwStyle parameter are defined by the implementor of the window class (by the person who calls RegisterClass...

Which message numbers belong to whom?

December 2, 2003 Dec 2, 2003 12/2/03

Raymond Chen

Valid window messages break down into four categories. 0 .. 0x3FF (WM_USER-1): System-defined messages. The meanings of these messages are defined by the operating system and cannot be changed. Do not invent new messages here. Since the meanings are defined by Windows, the operating system understands how to parse the WPARAM and LPARAM ...

Other tricks with WM_GETDLGCODE

November 26, 2003 Nov 26, 2003 11/26/03

Raymond Chen

The WM_GETDLCODE message lets you influence the behavior of the dialog manager. A previous entry on using WM_GETDLGCODE described the DLGC_HASSETSEL flag which controls whether edit control content is auto-selected when focus changes. I was going to write a bit about the other flags, but it turns out that Knowledge Base Article 83302 ...

Make sure the buttons match the question

November 18, 2003 Nov 18, 2003 11/18/03

Raymond Chen

When your program displays a dialog box with buttons, please make the buttons match the text. Consider this dialog, which appears after you install patches from Windows Update: It asks a yes/no question, but the options are "OK" and "Cancel". Either the buttons should...

Preventing edit control text from being autoselected in a dialog box

November 14, 2003 Nov 14, 2003 11/14/03

Raymond Chen

By default, when the user TABs to an edit control in a dialog box, the entire contents of the edit control are autoselected. This occurs because the edit control responds with the DLGC_HASSETSEL flag in response to the WM_GETDLGCODE message. To prevent it from happening, remove that flag. All this subclass procedure does is remove the ...

Another different type of dialog procedure

November 13, 2003 Nov 13, 2003 11/13/03

Raymond Chen

The other method of using a window-procedure-like dialog box is to change the rules of the game. Normally, the window procedure for a dialog box is the DefDlgProc function, which calls the dialog procedure and then takes action if the dialog procedure indicated that it desired the default action to take place. The dialog procedure is ...

Answer to previous exercise about m_fRecursing

November 13, 2003 Nov 13, 2003 11/13/03

Raymond Chen

Answer to previous exercise: The m_fRecursing flag does not need to be per-instance. It only needs to be valid long enough that the recursive call that comes immediately afterwards can be detected. However, a global variable would not work because two threads might be inside the recursive DefDlgProc call simultaneously. But a thread-local ...