Toplevels may be closed by several means, including
with the X button on top or
Alt-F4. Here's example code how to react to this event and have it
confirmed. Code for saving files, status, etc. could also go in there
(before the exit, of course ;-)
    wm protocol . WM_DELETE_WINDOW {
        if {[tk_messageBox \
                    -icon    question \
                    -type    yesno \
                    -default no \
                    -message "Do you want to go?" \
                    -title   "Quit Application?"] == "yes"} {
            exit
        }
    } ;# DKF - brought here by RS.

Jeffrey Hobbs writes: However, you can do the usual:
    bind . <Destroy> { puts "away I go" }

The advantage with the first ([wm] protocol .. [http://tcl.activestate.com/man/tcl8.4/TkCmd/wm.htm#M25]) is just that you get it
before the window goes away (in fact, you have to explicitly have that
destroy or it will stay), which means it still exists and you can save
info that might still be in the widgets. With the latter, you are
already on the way down, but it still works. In the latter you also
have to ensure you check on the %W, because <Destroy> will otherwise
trigger for all the widgets in .top (bindtags).

[DKF] - This illustrates quite nicely the difference between trapping a
request (what [[wm protocol]] handles) and a notification (what
<Destroy> notices.) By the time the <Destroy> arrives it is too late
to veto it - the window in question has already gone the way of the
dodo - whereas WM_DELETE_WINDOW is really the window manager saying
"Would you mind deleting this window, please?" and as such you can do
whatever you want with it. Making it pop up 30 new windows in response
is probably not a good move though... :^)

[NEM] - One point to remember with WM_DELETE_WINDOW though, is to make
certain that your code for the handler is completely bug free (or you
catch all errors). Otherwise, you can be left with a window that is
impossible to close, without forcibly killing the process.













----
Donal explain that, in such usages as
      wm protocol <window> WM_DELETE_WINDOW {#}
"An empty string is taken as asking for the
default behaviour (to destroy the appropriate toplevel.) Workaround is
to set the protocol handler to a non-empty do-nothing script (comments
are a favourite...)"






----
The syntax is
      wm protocol window ?name? ?command?
'''?name?''' here names "an atom corresponding to a window manager
protocol ..."  These atoms therefore are dependent on the governing
window managers.  Among those generally available are
   * WM_SAVE_YOURSELF
   * WM_DELETE_WINDOW
   * WM_TAKE_FOCUS
   * ...














The first three have a historical association with [Motif], or [ICCCM] more specifically.
There apparently are not recognized WM_ICONIFY and such; instead,
management of iconification is done with the <Unmap> event:
     [example here]