in reply to Filtering my own stderr

This seems like such a dumb idea, but maybe it'll work... 
$|=1;

attach_stderr_filter();
while (<>) { 
    print        "To STDOUT: $_"; 
    print STDERR "To STDERR: $_"; 
}

sub attach_stderr_filter {
    my($pid);
    # This is the top secret part :)
    $pid = open(STDERR, '|-');
    defined $pid or die "Cannot fork: $!\n";
    if($pid) {
        return;
    }

    # child, now prints to STDERR.
    while (<STDIN>) { s/foo/bar/; print STDERR "$_"; }
}

[download]
Woot, what do you know, it DOES work. Cool!
--
Mike

Replies are listed 'Best First'.
Re: Re: Filtering my own stderr
by Anonymous Monk on Nov 02, 2002 at 13:38 UTC
    Mike

    Many thanks for your reply and sorry for taking a while to respond...

    <confusion>
    Er, yes, that does work. Ok, then. Given that open(STDERR, '|-'); was actually the first thing I tried and it did not work for me, I obviously did something stupid. It turns out that I did two stupid things. The first was that I did not try all the alternatives against a simple test case. The second was that I did not read the source of P::RD carefully enough.

    ...Time passes while Kevin reads P::RD properly this time...

    The output I am concerned about is actually written to a format, not directly to STDERR. Why does this matter? Because the format is opened by dup(2)ing STDERR, which gives a fresh file descriptor. So, I can sit there in the child waiting for output on STDERR until I am blue in the face and it will not get me anywhere.
    </confusion>

    So, what now? It does not seem as though I can open an output filter, since I do not know to which file descriptor I should listen. Perhaps I could spin around all of them, but that sounds pretty obscure.

    How about tie'ing the format to a suitable class? Well, this partially works. As it happens, P::RD both prints and writes to the format. The prints come through fine but the writes do not. It seems as though writes to a format cannot be tied. See the example code below: 

    use strict;
use warnings;

package Log::OPG::Info;
use Tie::Handle;
use base qw(Tie::StdHandle);
sub WRITE { print "Good\n"; }


package main;

format STDERR =
Bad
.

tie *STDERR, 'Log::OPG::Info';
print STDERR '';
write STDERR;

    [download]

    Whereas I hope for the output 

    Good
Good

    [download]
    I actually get 
    Good
Bad

    [download]

    So, either I am missing something obvious, or the follow-up question is:

    "How does one tie writes to a format?" 

    Cheers
Kevin

    P.S. I faintly remember that it is possible to name individual slots in hash table entries, along the lines of $::slot{SCALAR} and $::slot{FORMAT}. Perhaps that might form the basis for a solution, but the details escape me.

[reply]
[d/l]
[select]
      Rather than capturing your own STDERR, why not try wrapping your program with a short one that uses IPC::Open3 to launch the original program with STDERR captured.

      You can also do this kind of redirecting in the command-line invocation with any decent shell.

[reply]

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