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Re: busyloop in sigchld_handler


From: Andreas Schwab
Subject: Re: busyloop in sigchld_handler
Date: Mon, 12 Mar 2007 20:28:33 +0100
User-agent: Gnus/5.110006 (No Gnus v0.6) Emacs/22.0.91 (gnu/linux)

Sam Steingold <address@hidden> writes:

> I don't know the details, but the kernel obviously has to do SOMETHING
> when the child process terminates: it has to notice the state change so
> that wait3 in emacs will return "yes, the child is dead" instead of
> "nothing for you yet". The busyloop prevents the kernel from doing
> anything for some time.

But the busyloop does not happen when there is nothing to do, it only
happens when wait3 is interrupted.  A zero return will cause the handler
to return immediately.

> Now, it might be better to remove the WNOHANG option instead (except that
> signal handlers are not supposed to hang),

If you don't use WNOHANG you open up a race where several processes may
have their status changed, but only one signal is sent (non-realtime
signals are not queued).

Andreas.

-- 
Andreas Schwab, SuSE Labs, address@hidden
SuSE Linux Products GmbH, Maxfeldstraße 5, 90409 Nürnberg, Germany
PGP key fingerprint = 58CA 54C7 6D53 942B 1756  01D3 44D5 214B 8276 4ED5
"And now for something completely different."




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