Greets,

I need to open many filehandles against the same file. They should all be independent, meaning that they should all be able to seek, read, etc without affecting each other. If I read perlopentut correctly, this should do the trick: 

open(my $orig, "<", "foofile";
open(my $dupe, "<&", $orig);

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That works as expected on Perl 5.8.6 on my OS X laptop. However, It doesn't work with either 5.8.6, 5.8.0, or bleadperl on a RedHat9 box. seeking/reading against the original messes up the dupe. The dupe SAYS that it's in the right spot, but it doesn't read the data that's there. Here's a test script... 
#!/usr/bin/perl
use strict;
use warnings;

open (my $junkfile, '>', "junkfile");
my $junk = '';

for my $first_byte (0 .. 255) {
    for my $second_byte (0 .. 255) {
        $junk .= pack('CC', $first_byte, $second_byte);
        $junk .= "\0" x 2;
    }
}
print $junkfile $junk;
close $junkfile;

open(my $orig, '<', "junkfile") or die $!; 
seek( $orig, 256, 0);
print_locations("Locations after seeking ORIG to 256: ");

my $correct;
read($orig, $correct, 8);
print_locations("Locations after reading 8 bytes from ORIG: ");

my @bytes = unpack('C*', $correct);
print "ORIG read these bytes: @bytes\n";

open(my $dupe, "<&", $orig) or die $!; 
print_locations("Locations immediately after duping: ");

seek( $dupe, 256, 0);
print_locations("Locations after seeking DUPE to 256: ");

seek($orig, 120, 0);
print_locations("Locations after seeking ORIG to 120: ");

my $useless;
read($orig, $useless, 20);
print_locations("Locations after reading 20 bytes from ORIG: ");

my $test;
read($dupe, $test, 8);
print_locations("Locations after reading 8 bytes from DUPE: ");

@bytes = unpack('C*', $test); 
print "DUPE read these bytes: @bytes\n";

sub print_locations {
    my $label = shift;
    print "$label\n    ";
    print "ORIG " . tell($orig) . "  "; 
    print "DUPE ";
    print defined $dupe ? tell($dupe) : "not valid";
    print "\n";
}

[download]

... and its output:

On RedHat9: 

$ /usr/local/blead/bin/perl5.9.3 many.plx 
Locations after seeking ORIG to 256: 
    ORIG 256  DUPE not valid
Locations after reading 8 bytes from ORIG: 
    ORIG 264  DUPE not valid
ORIG read these bytes: 0 64 0 0 0 65 0 0
Locations immediately after duping: 
    ORIG 264  DUPE 264
Locations after seeking DUPE to 256: 
    ORIG 264  DUPE 256
Locations after seeking ORIG to 120: 
    ORIG 120  DUPE 256
Locations after reading 20 bytes from ORIG: 
    ORIG 140  DUPE 256
Locations after reading 8 bytes from DUPE: 
    ORIG 140  DUPE 264
DUPE read these bytes: 4 30 0 0 4 31 0 0
$

[download]

On OS X: 

$ perl many.plx
Locations after seeking ORIG to 256: 
    ORIG 256  DUPE not valid
Locations after reading 8 bytes from ORIG: 
    ORIG 264  DUPE not valid
ORIG read these bytes: 0 64 0 0 0 65 0 0
Locations immediately after duping: 
    ORIG 264  DUPE 4096
Locations after seeking DUPE to 256: 
    ORIG 264  DUPE 256
Locations after seeking ORIG to 120: 
    ORIG 120  DUPE 256
Locations after reading 20 bytes from ORIG: 
    ORIG 140  DUPE 256
Locations after reading 8 bytes from DUPE: 
    ORIG 140  DUPE 264
DUPE read these bytes: 0 64 0 0 0 65 0 0
$

[download]

What's the right way to dupe a filehandle?

--
Marvin Humphrey
Rectangular Research ― http://www.rectangular.com

In reply to Duping filehandles by creamygoodness

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