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Message-ID: <3b667ff0-7778-4b48-866f-1dd112317c84@SystematicSW.ab.ca>
Date: Tue, 1 Jul 2025 11:17:08 -0600
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Subject: Re: setpgid non-compliance with POSIX?
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From: Brian Inglis via Cygwin <cygwin AT cygwin DOT com>
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On 2025-06-30 13:07, Jeremy Drake via Cygwin wrote:
> On Mon, 30 Jun 2025, Corinna Vinschen via Cygwin wrote:
> 
>> On Jun 28 12:32, Jeremy Drake via Cygwin wrote:
>>> I was looking at setpgid yesterday, and it seems deficient with respect to
>>> the errors required to be returned by the POSIX docs in
>>> https://pubs.opengroup.org/onlinepubs/9799919799/functions/setpgid.html
>>>
>>> Basically, it seems that setpgid allows any arbitrary positive integer to
>>> be set as pgid, rather than restricting to setting it to either the
>>> process's pid or to an existing pgid in the same session (sid).
>>
>> Yeah, error checking in setpgid is a bit on the lacking side.  With only
>> minor changes, the function is basically 25 years old :}
> 
> The POSIX docs metioned that an old BSD allowed setting arbitrary pgid
> 
>> Off the top of my head I don't know how to check the pgid already exists.
>> Do you?
> 
> I think it'd have to iterate all processes and check pgid and sid members
> of pinfo until it found a match.

Shouldn't need to - pgid is the pid of a process spawned by init, or pid of a 
login shell, and sid is that pgid or that of the parent process, or pgid is that 
of a job shell or of the parent process.

So each process sid also pgid is either its pid or that of its parent process: 
they are only set either to the process pid or inherited.

Try head /proc/self/*id & and head /proc/$$/*id as demos.

Does any of that help? ;^>

-- 
Take care. Thanks, Brian Inglis              Calgary, Alberta, Canada

La perfection est atteinte                   Perfection is achieved
non pas lorsqu'il n'y a plus rien à ajouter  not when there is no more to add
mais lorsqu'il n'y a plus rien à retrancher  but when there is no more to cut
                                 -- Antoine de Saint-Exupéry

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