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Re: File descriptors and screen and you

From: Michael Schroeder
Subject: Re: File descriptors and screen and you
Date: Thu, 8 Feb 2007 20:58:36 +0100
User-agent: Mutt/

On 2/6/07, Alan Young <address@hidden> wrote:
> I have a bit of perl code that I'm trying to make work:
> sub DB::get_fork_TTY {
>    open my $SCREEN, qq{&3>1 screen -t 'Child $$' sh -c "tty &1>3 ; sleep
> 1000000" |}
>      or return '';
>    my $tty = <$SCREEN>;
>    return $tty;
> }
> I'm getting bad file descriptor errors, so I'm assuming screen is
> cleaning up what it hands to it's children.  Which makes sense.

The problem is actually something else (but you're right, screen
cleans up the filedescriptors): the 'screen' command in your
script just tells the screen "backend" process that it should create
a new window, so the new window is no child of your script and
cannot share the file descriptor.

> I want to avoid going the tmp file route if at all possible.  Is there a
> way I can echo tty's output so that I can grab it from a filehandle?

You need some interprocess communication way. Some ways could be
- a tmp file (but you don't like that)
- a named pipe (but that's close to a tmp file)
- some code using sockets (complicated)
- using /proc/<pid>/<fd> to access the file descriptors of your
  process. But that's not very portable. Something like:

    open my $SCREEN, qq{screen -t 'Child $$' sh -c "tty >/proc/\$\$/fd/1 ; 
sleep 1000000" ; sleep 10 |}

  (The sleep 10 is needed to keep the process alive long enough.)


Michael Schroeder           address@hidden

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