Date: Fri, 3 Mar 2017 14:23:37 -0800 From: "Ngie Cooper (yaneurabeya)" <yaneurabeya@gmail.com> To: Dirk-Willem van Gulik <dirkx@webweaving.org> Cc: freebsd-hackers@freebsd.org Subject: Re: kill -0 <pid> --- side effect or supported Message-ID: <11A4B6AB-E51D-4754-8E80-4503687E0F84@gmail.com> In-Reply-To: <23F3BAC3-0D8B-4290-8DC2-818D67A0B6A9@webweaving.org> References: <23F3BAC3-0D8B-4290-8DC2-818D67A0B6A9@webweaving.org>
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> On Mar 3, 2017, at 14:12, Dirk-Willem van Gulik <dirkx@webweaving.org> wrote:
>
> I regularly use 'kill -0 <pid>' on FreeBSD as a way to test if a certain process is still running (but without actually sending the signal). And I think it has worked reliably since the mid 80's.
>
> Is it actually a properly supported use - as I recently happened to notice that it does not seem to be all that documented in kill(
It better work. I have code that relies on it :)…
It does work as you noted, according to truss:
# sudo truss -ff kill -0 1 2>&1
...
79940: kill(1,0) = 0 (0x0)
…
#
As noted in kill(2), this is one of the valid values:
a group of processes. The sig argument may be one of the signals
specified in sigaction(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.
So, the manpage for kill(1) is just lacking in the sense that -0 is supported.
Cheers!
-Ngie
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