'''Q.''' Is there a way a user can press a "stop"
button, for example, to stop the running script without killing the main
application?

'''A1.''' by mailto:kris.klindworth@carle.com : 
You can get much of the effect you want by selectively
redefining key TCL commands.  You could, for instance, have the cancel
button set a "cancelled" global.  Then, you can define a cancelled function
which checks for the existence of this global and calls "error" if it is
exists.
Finally, you can redefine commands like while so that they will insert a
call
to the cancelled command at the top of each loop.  It might look
something like this...

 proc cancelled {} {
    global cancelled
    # give the user a chance to hit the cancel button
    update
     if [info exists cancelled] {
         error "Script Cancelled by User"
     }
 }

 rename while _while
 proc while {condition body} {
        uplevel [list _while $condition "cancelled;$body"]
 }

If you redefine the proc command and the loop commands then you will
get pretty good coverage.  You might also want use the after command to
set a time limit on the execution.   Something like...
 set timeLimit 1000
 set timerId [after $timeLimit {set timedOut 1}]
Then you would have the cancelled function check this variable as well.
Just be sure that you run "after cancel $timerId" and unset the cancelled
and timedOut variables when you return from the users scripts.

'''A2.''' by  Michael Barth mailto:m.barth@de.bosch.com :
The solution I have implemented is simple: create a new process with the
user script. Additionally you can redirect the standard output of the
"user process" to a text widget.
 # tcl code not tested:
 button .interrupt -command {close $FID}
 pack .interrupt

 set FID [open |userscript r]
 fileevent readable $FID writeToTextWidget
''(From posts in news:comp.lang.tcl, slightly adapted by RS. Note however that frequent updates make long runs last even longer ;-)''