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

[Ngie Cooper (yaneurabeya)]

> 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
     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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