Using signal.alarm to terminate a thread

Adrian Casey adrian.casey at internode.on.net
Wed Nov 15 12:32:53 CET 2006


Nick Craig-Wood wrote:

> Adrian Casey <adrian.casey at internode.on.net> wrote:
>> > Adrian Casey <adrian.casey at internode.on.net> wrote:
>> > import os, pexpect, threading
>> > 
>> > def runyes():
>> >         print "Running yes command..."
>> >         pexpect.run('yes', timeout=5)
>> > 
>> > t = threading.Thread(target=runyes)
>> > t.start()
>> > t.join()
>> > 
>>  The timeout parameter will not work in this case.  If you run the sample
>>  code above, it will run forever.
> 
> The above runs just fine for me, stopping after 5 seconds.  Did you
> try it?
> 
>>  The 'yes' command presents a class of command which can not be
>>  easily be handled by pexpect.
> 
> Worked for me under Debian/testing.
> 
>> As far as I know, mixing threads and signals is OK provided the
>> parent creates the alarm.
> 
> There are so many pitfalls here that I advise you not to try.  From
> the linuxthreads FAQ
> 
>   J.3: How shall I go about mixing signals and threads in my program?
> 
>   The less you mix them, the better. Notice that all pthread_*
>   functions are not async-signal safe, meaning that you should not
>   call them from signal handlers. This recommendation is not to be
>   taken lightly: your program can deadlock if you call a pthread_*
>   function from a signal handler!
> 
>   The only sensible things you can do from a signal handler is set a
>   global flag, or call sem_post on a semaphore, to record the delivery
>   of the signal. The remainder of the program can then either poll the
>   global flag, or use sem_wait() and sem_trywait() on the semaphore.
> 
>   Another option is to do nothing in the signal handler, and dedicate
>   one thread (preferably the initial thread) to wait synchronously for
>   signals, using sigwait(), and send messages to the other threads
>   accordingly.
> 
> Note also that the signal can be delivered to any thread which
> complicates things.
> 
I'm running Kubuntu 06-06 with python 2.4.3 and the above code runs forever
at 100% cpu utilization.  I shall look into semaphores. However, that means
another thread whose sole purpose is to watch the semaphore.

Thanks for you help.

Cheers.
Adrian.



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