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Date: Fri, 4 Jan 2019 22:29:10 +0200
From: Konstantin Belousov <kostikbel@gmail.com>
To: Fabian Freyer <fabian.freyer@physik.tu-berlin.de>
Cc: Christian Barthel <bch@online.de>, freebsd-jail@freebsd.org,
 stefan@gronke.net
Subject: Re: kqueue(2) kevents for jails
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On Fri, Jan 04, 2019 at 09:11:58PM +0100, Fabian Freyer wrote:
> On 1/4/19 5:14 PM, Konstantin Belousov wrote:
> > No, kevent(2) is not suitable mechanism to notify about jail state changes.
> > If anything in the existing system can be reused for such notifications,
> > it is devctl(4) notifications which are handled by devd(8).  Look at the
> > man pages and for existing notifications in kernel code, e.g.
> > sys/kern/kern_conf.c notify*() for how devfs does it.
> 
> Can any running binary subscribe to devd(8) events or does that require 
> a configuration change in /etc/devd.conf?

Only one reader is supported, effectively.  devctl(4) tries to limit opens
naively.  But then even if you have the file descriptor and fork or pass
it over unix domain socket, single event can be only read by one reader.