X11 -- how do you prevent window from closing if user closes it

I trying to use X11 and let's say I create a text editor. Let's say the user is using the text editor, makes some changes and then clicks on the [X] to close the window before saving changes.I want to popup a "are you sure" before the window is closed but X11 doesn't allow this. How does

Howto process X11 events async'ly

ource but can't follow it. Thanks in advance. I've asked this question many times @ linuxforum.com but no one there understands the problem. Peter Quiring