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Re: Detect whether a writeable filehandle has closed?

by bliako (Prior)
on Jan 14, 2021 at 12:36 UTC ( #11126904=note: print w/replies, xml ) Need Help??

in reply to Detect whether a writeable filehandle has closed?

As others suggested the first thing to do would be to turn buffering off.

Then simply check the pid is alive before every write.

Which is essentially what you have suggested: to check if filehandle is open, but at a lower level, that of the process.

A tiny caveat would be if your user does not have permission to check whether a pid is alive. I think that's unlikely in unix. For Windows you can always install one of the thousand viruses out there to hijack the system and bypass whatever permissions. Kids don't do this - someone else will do it for you (that's a joke)

my $other_program = 'cat'; my $pid = open my $pipe, " | $other_program "; print $pipe "blahblah" if alive($pid);

I thought something as simple as this: sub alive { return kill 0, $_[0] } would work but it doesn't in my Fedora-linux-latest. It always return true!

If you are on a unix box, then it would be straightforward to check the pid using various methods (e.g. ps -o pid | grep -w $pid or in linux: via perl: -d "/proc/$pid". Additionally, relevant modules on CPAN do exist. It's unclear to me if they work for non-unix systems so I wont mention them here.

bw, bliako

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Re^2: Detect whether a writeable filehandle has closed?
by jdporter (Canon) on Jan 14, 2021 at 14:51 UTC

    Thank you. In fact, I had already tried the kill 0 technique, and it didn't work, just as you described.

    I'm going to try the /proc/$pid method. But I'm afraid that that might also yield a false negative if I haven't reaped the child yet. (ps shows "defunct".) (I suspect that that would also explain why kill 0 doesn't do what we expect either.)

      (ps shows "defunct".)

      You need waitpid to reap the zombie.

      perl -le'print map{pack c,($-++?1:13)+ord}split//,ESEL'

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