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From: cudcv@warwick.ac.uk (Rob McMahon)
Newsgroups: comp.mail.elm,comp.unix.wizards
Subject: Re: newmail also displays on console (bug)
Message-ID: <123@titania.warwick.ac.uk>
Date: 17 Mar 89 23:39:58 GMT
References: <316@wubios.wustl.edu> <890001@hpausla.HP.COM> <5142@b-tech.ann-arbor.mi.us> <4774@pbhyf.PacBell.COM> <2918@epimass.EPI.COM> <117@titania.warwick.ac.uk> <330@wubios.wustl.edu> <91@dsinc.UUCP>
Reply-To: cudcv@warwick.ac.uk (Rob McMahon)
Followup-To: comp.unix.wizards
Organization: Computing Services, Warwick University, UK
Lines: 32

I'm cross-posting this to comp.unix.wizards, and sending follow-ups there,
since this has stopped being an elm question.  I'll report back here if no-one
else does.

In article <91@dsinc.UUCP> syd@dsinc.UUCP (Syd Weinstein) writes:
>In article <330@wubios.wustl.edu> david@wubios.UUCP (David J. Camp) writes:
>:I have studied the code in newmail.c, and I do not see how the process put
>:in the background is automatically killed upon logout.  Perhaps someone
>:in-the-know can explain to me how this is done.
>
>It's really very simple.  We do nothing, and when the init process sends a
>SIGHUP, we take the default action, to die.

[But processes not in the process group of the terminal on systems with job
control never see the SIGHUP]

This is quite an interesting problem really.  If the background process does
not fork, it can check getppid() occasionally, when it returns 1 it knows it
has been orphaned and can exit.  If it does fork things gets more tricky.  It
can try to kill(shell_pid, 0), but a) not all systems allow this, and b)
there's a chance of pid reuse.  The BSD `leave' program uses getlogin() and
compares it against a stashed copy.  At least this means that the only person
it's going to annoy is yourself.

Anyone got any better ideas on how a process which forks and carries on
running in the background can detect when the original shell has gone away ?

Rob
-- 
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Rob McMahon, Computing Services, Warwick University, Coventry CV4 7AL, England