Terminating program run from thread (not just the thread)

[Jeffrey Barish]

My Tkinter GUI starts a thread that runs ogg123 using popen3.  The
thread reads the stderr from ogg123 to determine when ogg123 is done
and, until then, to update a progress meter.  I need to be able to stop
ogg123 before it reaches the end of its task.  If I terminate the
thread that used popen3 to start ogg123, ogg123 does not terminate. 
popen3 does not return a pid, so I don't see any way to terminate the
program.  Suggestions?
-- 
Jeffrey Barish