os.system vs os.system inside thread -> why is there any difference?

[przemas_r at o2.pl]

Hello everybody.

I've found python's behaviour I can't understand. Namely the way 
os.system () works depending if it's being run in Thread's run function 
or run in 'normal' way.

 >>> os.system ('arping -w 1 -c 1 -I eth1 192.168.0.2')
ARPING 192.168.0.2 from 62.233.239.115 eth1
Sent 1 probes (1 broadcast(s))
Received 0 response(s)
256
 >>>

Here as you can see, arping command quits sending packets after 1 second 
and outputs failure (Received 0 response(s)) information.

Now the same thing, but running from thread:

 >>> class A (threading.Thread):
...     def __init__ (self):
...             threading.Thread.__init__ (self)
...     def run (self):
...             os.system ('arping -w 1 -c 1 -I eth1 192.168.0.2')
...
 >>> A ().start ()
 >>> ARPING 192.168.0.2 from 62.233.239.115 eth1
<lots of time and nothing happens>

Now, although the only difference is that command arping is being run in 
thread, command never quits, informing about it's failure (-w 1 switch 
means that it should quit in 1 second). Mistery.

Please help me if you know what's going on.

--
Przemysław Różycki