kill -0 <pid> --- side effect or supported

[Rodney W. Grimes]

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> 
> > On Mar 3, 2017, at 14:12, Dirk-Willem van Gulik <dirkx at 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
                                ^^^^^^^^^^^^^^^^

That bit of information should be promoted from kill(2) to kill(1) by
adding 0 to the list as ?.

>      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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-- 
Rod Grimes                                                 rgrimes at freebsd.org