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

[Ngie Cooper (yaneurabeya)]

> On Mar 3, 2017, at 14:30, Rodney W. Grimes <freebsd-rwg at pdx.rh.CN85.dnsmgr.net> wrote:
> 
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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 ?.

Actually… it is mentioned in kill(1) — you just have to read between the lines:

     -signal_number
             A non-negative decimal integer, specifying the signal to be sent
             instead of the default TERM.

0 is technically a non-negative real number.

It might be a good idea to clarify this point/behavior by pointing to kill(2) for the signal behavior/description noted above.

Thanks,
-Ngie
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