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From: Sean McNeil <sean@mcneil.com>
To: Daniel Eischen <eischen@vigrid.com>
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cc: freebsd-threads@freebsd.org
Subject: Re: kill(pid,0) sends a signal or not?
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On Mon, 2004-06-21 at 07:36, Daniel Eischen wrote:
> On Sun, 20 Jun 2004, Sean McNeil wrote:
> 
> > I'm trying to trace down an issue with kse threads and firefox.  There
> > is an odd "trick" I haven't seen before:
> > 
> >  // kill(pid,0) is a neat trick to check if a
> >  // process exists
> >  if (kill(pid, 0) == 0 || errno != ESRCH)
> > 
> > Does this really work?  It is kind of odd that it I appear to get a
> > signal (if the traceback is accurate) with the signal set to 0:
> > 
> > #10 0x0000000202bc7a80 in thr_resume_wrapper (sig=0, siginfo=0x4,
> >     ucp=0x7fffffffd4c0) at /usr/src/lib/libpthread/thread/thr_kern.c:1112
> > 
> > This later causes a sig 11 and the program core dumps.
> > 
> > Any info on how threads are suppose to behave when a process does a
> > kill(pid,0) would be greatly appreciated.
> 
> kill(pid, 0) shouldn't result in a signal.  libpthread doesn't do
> anything with kill() and the kernel shouldn't cause a signal for 0
> either.  What does ktrace show?

It wasn't generating a sig 0.  What I was seeing was the inner workings
of the threads where a "sig" variable was set to 0, but the actual
signal was 11.  Everything is working as designed as far as I can tell.