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Date: Tue, 9 Oct 2001 15:37:02 +0200
From: Jan Hudec <bulb@ucw.cz>
To: "Kernel, Linux" <linux-kernel@vger.kernel.org>
Subject: Re: Desperately missing a working "pselect()" or similar...
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> 
> > > The select system call doesn't return EINTR when the signal is caught
> > > prior to entry into select.
> > 
> > Your friend there is siglongjmp/sigsetjmp - the same problem was true
> > with old versions of alarm that did
> > 
> >         alarm(num)
> >         pause()
> > 
> > on a heavily loaded box.
> > 
> > Using siglongjmp cures that
> 
> Hmmm... would you say the "siglongjmp" method is better than the "self-pipe"
> method for a select on both file descriptors and signals too?
> 
> As far as I can see the trade-off is (in the non-race-condition case)
> between having to call read() on the pipe (to empty it after receiving the
> signal) for the "self-pipe" method and having to call sigsetjump() every time
> before one enters select/poll.
> 
> My assumption would be that the "self-pipe" method is cheaper... right?

Well, but you don't have to call sigsetjmp before every select; just when you
enter the loop. Than just enable volatile flag, that the handler should now
use the siglongjmp... well, you have to care about 2 signals quickly following
one another and similar nasty cases anyway, so the pipe aproach is less
error-prone. When signal arives, select either returns EINTR, or says the pipe
is ready for writing, so you can save yourself the additional select call by
checking for both conditions.

--------------------------------------------------------------------------------
                  				- Jan Hudec `Bulb' <bulb@ucw.cz>
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