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From: guy@auspex.auspex.com (Guy Harris)
Newsgroups: comp.mail.misc
Subject: Re: Checking for new mail (and killing bkgnd process on logout?)
Message-ID: <2403@auspex.auspex.com>
Date: 31 Aug 89 18:17:48 GMT
References: <1989Aug30.053137.2047@ctr.columbia.edu> <9834@multimax.Encore.COM>
Reply-To: guy@auspex.auspex.com (Guy Harris)
Organization: Auspex Systems, Santa Clara
Lines: 24

>This is an approach I'd tried earlier (I used SIGCHLD, actually) and
>it worked OK, but I wasn't comfortable with the possibility that the
>parent might try to do something with the signal instead of dropping
>it.

To quote from the 4.3-tahoe "kill" man page:

	"Kill" sends the signal "sig" to a process, specified by the
	process number "pid".  "Sig" may be one of the signals specified in
	"sigvec(2)", or it may be 0, in which case error checking is
	performed but no signal is actually sent.  This can be used to
	check the validity of "pid".

and from the SVID:

	The signal that is to be sent is specified by the argument "sig"
	and is either one from the list given in SIGNAL(BA_OS) or 0.  If
	"sig" is 0 (the null signal), error checking is performed but
	no signal is actually sent.  This can be used to check the
	validity of "pid".

In 4.[23]BSD and System V, at least, your fears are groundless; the code
does what the documentation claims it does - the parent doesn't even
*see* "signal 0".