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Re: Parallelism a la make -j <n> / GNU parallel

From: John Kearney
Subject: Re: Parallelism a la make -j <n> / GNU parallel
Date: Sun, 06 May 2012 05:25:26 +0200
User-agent: Mozilla/5.0 (Windows NT 6.1; WOW64; rv:12.0) Gecko/20120428 Thunderbird/12.0.1

Am 05.05.2012 06:28, schrieb Mike Frysinger:
> On Friday 04 May 2012 16:17:02 Chet Ramey wrote:
>> On 5/4/12 2:53 PM, Mike Frysinger wrote:
>>> it might be a little racy (wrt checking cnt >= 10 and then doing a wait),
>>> but this is good enough for some things.  it does lose visibility into
>>> which pids are live vs reaped, and their exit status, but i more often
>>> don't care about that ...
>> What version of bash did you test this on?  Bash-4.0 is a little different
>> in how it treats the SIGCHLD trap.
> bash-4.2_p28.  wait returns 145 (which is SIGCHLD).
>> Would it be useful for bash to set a shell variable to the PID of the just-
>> reaped process that caused the SIGCHLD trap?  That way you could keep an
>> array of PIDs and, if you wanted, use that variable to keep track of live
>> and dead children.
> we've got associative arrays now ... we could have one which contains all the 
> relevant info:
>       declare -A BASH_CHILD_STATUS=(
>               ["pid"]=1234
>               ["status"]=1    # WEXITSTATUS()
>               ["signal"]=13   # WTERMSIG()
>       )
> makes it easy to add any other fields people might care about ...
> -mike
Is there actually a guarantee that there will be 1 SIGCHLD for every
exited process.
Isn't it actually a race condition?
what happens if 2 subprocesses exit simultaneously.
or if a process exits while already in the SIGCHLD trap.
I mean my normal interpretation of a interrupt/event/trap is just a
notification that I need to check what has happened. Or that there was
an event not the extent of the event?
I keep feeling that the following is bad practice

trap ': $(( --cnt ))' SIGCHLD

and would be better something like this

trap 'cnt=$(jobs -p | wc -w)' SIGCHLD

as such you would need something more like.
declare -a BASH_CHILD_STATUS=([1234]=1 [1235]=1 [1236]=1)

declare -a BASH_CHILD_STATUS_SIGNAL=([1234]=13 [1235]=13 [1236]=13)

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