Date: Sun, 5 Mar 2017 22:08:01 +0100 From: Dirk-Willem van Gulik <dirkx@webweaving.org> To: "Ngie Cooper (yaneurabeya)" <yaneurabeya@gmail.com> Cc: "Rodney W. Grimes" <freebsd-rwg@pdx.rh.CN85.dnsmgr.net>, freebsd-hackers@freebsd.org Subject: Re: kill -0 <pid> --- side effect or supported Message-ID: <FBCA1111-5C6A-4E89-B6FE-94606EA21B9C@webweaving.org> In-Reply-To: <35C6F2AA-309E-4D58-8191-AB99F0195BEC@gmail.com> References: <201703032230.v23MUg5b072955@pdx.rh.CN85.dnsmgr.net> <35C6F2AA-309E-4D58-8191-AB99F0195BEC@gmail.com>
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> On 3 Mar 2017, at 23:42, Ngie Cooper (yaneurabeya) = <yaneurabeya@gmail.com> wrote: >=20 >>=20 >> On Mar 3, 2017, at 14:30, Rodney W. Grimes = <freebsd-rwg@pdx.rh.CN85.dnsmgr.net> wrote: >>=20 >> -- Start of PGP signed section. >> [ Charset UTF-8 unsupported, converting... ] >>>=20 >>>> On Mar 3, 2017, at 14:12, Dirk-Willem van Gulik = <dirkx@webweaving.org> wrote: >>>>=20 >>>> 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. >>>>=20 >>>> 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( >>>=20 >>> It better work. I have code that relies on it :)? >>>=20 >>> It does work as you noted, according to truss: >>>=20 >>> # sudo truss -ff kill -0 1 2>&1 >>> ... >>> 79940: kill(1,0) =3D 0 (0x0) >>> ? >>> # >>>=20 >>> As noted in kill(2), this is one of the valid values: >>>=20 >>> 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 >> ^^^^^^^^^^^^^^^^ >>=20 >> That bit of information should be promoted from kill(2) to kill(1) by >> adding 0 to the list as ?. >=20 > Actually=E2=80=A6 it is mentioned in kill(1) =E2=80=94 you just have = to read between the lines: >=20 > -signal_number > A non-negative decimal integer, specifying the signal to = be sent > instead of the default TERM. >=20 > 0 is technically a non-negative real number. >=20 > It might be a good idea to clarify this point/behavior by pointing to = kill(2) for the signal behavior/description noted above. Posix @ Opengroup says: = http://pubs.opengroup.org/onlinepubs/009695399/functions/kill.html The kill() function shall send a signal to a process or a group = of processes specified by pid. The signal to be sent is specified by sig = and is either one from the list given in <signal.h> or 0. If sig is 0 = (the null signal), error checking is performed but no signal is actually = sent. The null signal can be used to check the validity of pid. I guess this could be added - with perhaps the note/warning on the = 'zombie' processes - as this is subtly different between unixes. Dw
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