[Python-Dev] Problem with signals in a single threaded application (original) (raw)

Ulisses Furquim ulissesf at gmail.com
Tue Jan 23 20:37:09 CET 2007

Hi,

I'm aware of the problems with signals in a multithreaded application, but I was using signals in a single-threaded application and noticed something that seemed wrong. Some signals were apparently being lost, but when another signal came in the python handler for that "lost" signal was being called.

The problem seems to be inside the signal module. The global variable is_tripped is incremented every time a signal arrives. Then, inside PyErr_CheckSignals() (the pending call that calls all python handlers for signals that arrived) we can return immediately if is_tripped is zero. If is_tripped is different than zero, we loop through all signals calling the registered python handlers and after that we zero is_tripped. This seems to be ok, but what happens if a signal arrives after we've returned from its handler (or even after we've checked if that signal arrived) and before we zero is_tripped? I guess we can have a situation where is_tripped is zero but some Handlers[i].tripped are not. In fact, I've inserted some debugging output and could see that this actually happens and then I've written the following test program to reproduce the problem.

#!/usr/bin/env python2.5

import sys import os import time import signal

def alarm_handler(*args): sys.stderr.write('alarmmmmmmmmmmm!\n')

def sigio_handler(*args): sys.stderr.write('Entering SIGIO handler\n') os.kill(os.getpid(), signal.SIGALRM) sys.stderr.write('Leaving SIGIO handler\n')

signal.signal(signal.SIGIO, sigio_handler) signal.signal(signal.SIGALRM, alarm_handler)

os.kill(os.getpid(), signal.SIGIO)

ini = time.time() while True : if time.time() - ini > 3.0: sys.stderr.write('Loop!\n') ini = time.time()

When we run this program, the handler for the SIGALRM isn't called after we return from the SIGIO handler. We return to our main loop and print 'Loop!' every 3 seconds aprox. and the SIGALRM handler is called only when another signal arrives (like when we hit Ctrl-C).

I've read some threads about signals in the archives and I was under the impression signals should work reliably on single-threaded applications. Am I right? I've thought about a way to fix this, but I don't know what is the current plan for signals support in python, so what can be done?

Best regards,

-- Ulisses

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