daemon/fork: defunct parent not going away nicely

Cagao has asked for the wisdom of the Perl Monks concerning the following question:

I'm trying to create a simple daemon process, ie, fork, exit the parent, and have the child inherited by the system to do its stuff.

I've tried this with the following code but running under cron doesn't seem to behave - I want it under cron to make make sure it restarts every hour in case it ever falls over.

my $pid = fork;

if ( ! defined $pid )
{
    # fork failed
    
    throw_invalid "Couldn't fork";
}
elsif ( $pid > 0 )
{
    # parent

    exit;
}
else
{
    while ( 1 ) { sleep 1; # and do other snazzy stuff forever }
}
[download]

$ ps -ef 

rbrown   29884  3162  0 11:58 ?        00:00:00 crond
rbrown   29885 29884  2 11:58 ?        00:00:03 solr-job-queue- <defunct>
rbrown   29886     1  2 11:58 ?        00:00:02 /usr/bin/perl -w /home/me/solr-job-queue-process.pl

Is it all because the parent can't die until it's child has, and that's why cron isn't clearing it from the process table?

Surely there's a way round this?

Any guidance greatly appreciated.

Comment on daemon/fork: defunct parent not going away nicely Download Code

Replies are listed 'Best First'.
Re: daemon/fork: defunct parent not going away nicely by dave_the_m (Monsignor) on Oct 14, 2011 at 11:28 UTC
It'll be defunct because crond hasn't yet reaped the exited process. As to why it hasn't, I don't know; but I would speculate that crond is waiting for one for more of the STDIN/STDOUT/STDRR it passed to it's child to reach EOF; and maybe your child (crond's grandchild) is inheriting those file descriptors? Dave.	[reply]
Re^2: daemon/fork: defunct parent not going away nicely by Eliya (Vicar) on Oct 14, 2011 at 11:55 UTC
I would speculate that crond is waiting for one for more of the STDIN/STDOUT/STDRR... That's right. crond is generally attempting to read output from the jobs it started, which it would then typically try to mail somewhere. (This is kind of like if you were to run your script using backticks from another Perl script (or the shell, unless it auto-reaps) — which would produce the exact same behavior of leaving a zombie behind.) As the (grand)child process inherits the standard file handles, they're not being closed when the parent part of your script exits, i.e. the child's duplicates remain open. The solution is to close those handles, or more properly, do as described in perlipc.	[reply]
Re^3: daemon/fork: defunct parent not going away nicely by Cagao (Monk) on Oct 14, 2011 at 13:26 UTC
thanks for the background and link. I remember this same issue years ago with a cgi-script used to start a daemon process, but kept the browser progress waiting, even tho the request had "finished". I just closed STDIN/OUT/ERR in the child before it does anything.	[reply]
Re^2: daemon/fork: defunct parent not going away nicely by Cagao (Monk) on Oct 14, 2011 at 11:39 UTC
Cheers Dave, That sounds very familiar now, needing to close then re-open filehandles, specifically stdin/out/err.	[reply]
Re: daemon/fork: defunct parent not going away nicely by Your Mother (Archbishop) on Oct 14, 2011 at 13:30 UTC
One more: Proc::Daemon.	[reply]
Re: daemon/fork: defunct parent not going away nicely by Anonymous Monk on Oct 14, 2011 at 11:13 UTC
Proc::Background, Proc::Pidfile	[reply]
Re^2: daemon/fork: defunct parent not going away nicely by Cagao (Monk) on Oct 14, 2011 at 11:15 UTC
Thanks, tho I'd rather understand why/what's going on with this one.	[reply]