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From: John Baldwin <jhb@freebsd.org>
To: Pawel Jakub Dawidek <pjd@freebsd.org>
Subject: Re: svn commit: r244154 - head/bin/ps
Date: Fri, 14 Dec 2012 11:52:15 -0500
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Cc: Konstantin Belousov <kostikbel@gmail.com>, svn-src-head@freebsd.org,
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On Thursday, December 13, 2012 6:12:44 am Pawel Jakub Dawidek wrote:
> On Wed, Dec 12, 2012 at 11:06:52PM +0200, Konstantin Belousov wrote:
> > On Wed, Dec 12, 2012 at 03:45:04PM +0000, Pawel Jakub Dawidek wrote:
> > > Author: pjd
> > > Date: Wed Dec 12 15:45:03 2012
> > > New Revision: 244154
> > > URL: http://svnweb.freebsd.org/changeset/base/244154
> > > 
> > > Log:
> > >   Use kern.max_pid sysctl to obtain maximum PID number instead of using local
> > >   define.
> > It is pid_max, not max_pid.
> > 
> > But the change is wrong. The kern.pid_max only limits newly allocated pids,
> > it does not magically moves existing pids, which are out of range, to the
> > limited region. See the corresponding commit log for the description.
> > It was added to make it easier to run FreeBSD 1.x binaries on the modern
> > kernels.
> 
> I saw CTLFLAG_TUN on the sysctl and assumed it is read-only...
> How about defining BSD_PID_MAX in sys/proc.h, which would be visible by
> userland as well and setting PID_MAX to BSD_PID_MAX?
> 
> This would also help bsnmpd.
> 
> 	http://people.freebsd.org/~pjd/patches/PID_MAX.patch

This doesn't help your actual use case though where you want to boot a kernel
with a different PID_MAX.  I would much rather our tools learn such constants
from the kernel via sysctl than have them compiled in.  So, I would add a new
sysctl which exports the true PID_MAX constant (and is read-only and never
changes) and use that in ps, etc.

-- 
John Baldwin