Re: Terminating subtasks reliably
Re: Terminating subtasks reliably
- Subject: Re: Terminating subtasks reliably
- From: Julian Hsiao <email@hidden>
- Date: Sat, 3 Apr 2010 04:20:21 -0700
Why not have the parent call setsid() / setpgid() at the beginning? This should prevent the PID from getting recycled until every process in the process group exits.
Julian Hsiao
email@hidden
On Apr 2, 2010, at 8:10 PM, Michael Ash wrote:
> On Thu, Apr 1, 2010 at 7:19 PM, Dave Keck <email@hidden> wrote:
>>>> Look like a race condition. The man page says getppid() will not fail, but
>>>> not what happens after the parent dies. I will test this.
>>>
>>> If the parent dies, the process becomes the child of launchd. For a quick
>>> fix I checked for the parent PID greater than 1.
>>>
>>> A better fix, the parent process passed its PID to the child process as an
>>> argument. In the child process, if getppid() does not return the PID from
>>> the parent process, exit.
>>
>> Another problem is if the parent dies after the getppid(), and another
>> process starts with the same PID as the old parent, all before the
>> call to kevent(). In this scenario you'll now be waiting for an
>> unrelated process to die. This situation is likely impossible
>> currently with incrementing PIDs, but there's been mention of PID
>> randomization on darwin-dev:
>>
>> http://lists.apple.com/archives/darwin-dev/2009/Oct/msg00056.html
>>
>> I only mention this because I use this kevent method to detect a
>> parent's death, and have been looking for a better technique that
>> also doesn't require the parent's cooperation...
>
> Call getppid, set up the kevent listener, then before you actually
> block in kevent, call getppid *again*. If the answer matches the first
> call, then you can go ahead and call kevent. If the answer doesn't
> match, you've hit the race condition you outline here, your parent is
> dead, and you can exit immediately. If the parent quits after the
> second getppid but before the call to kevent, you should still get the
> exit notification because you registered it before the parent quit.
>
> Mike
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