In Perl, why can't you use __DATA__ as a seekable filehandle?

Reading from DATA via the typical slurp works. Trying to use DATA as a filehandle on which I can do a seek does not work. Is anyone able to point me the to the obvious mistake I must be making?

Code:

#!/usr/bin/env perl

use strict;
use warnings;

if ($ARGV[0] eq 'seek' )
my $log_fh = *DATA;
$log_fh->seek(64,0);
print "n-- 64 --n",join ("", <$log_fh> );
else
while (<DATA>)
print $_;



exit;

__DATA__
01234567890123456789
1234567890123456789
1234567890123456789
12
X <- That X is the 64th char in
this file.
Y <- That Y is the 106th char in this file.
junk
more junk.
bye!

$ perl file_from_data.pl slurp
01234567890123456789
1234567890123456789
1234567890123456789
12
X <- That X is the 64th char in
this file.
Y <- That Y is the 106th char in this file.
junk
more junk.
bye!


Running the while() loop:

$ perl file_from_data.pl slurp
01234567890123456789
1234567890123456789
1234567890123456789
12
X <- That X is the 64th char in
this file.
Y <- That Y is the 106th char in this file.
junk
more junk.
bye!


Running the seek(), it appears to not start at DATA but the start of the script:

$ perl file_from_data.pl seek

-- 64 --
'seek' )
my $log_fh = *DATA;
$log_fh->seek(64,0);
print "n-- 64 --n",join ("", <$log_fh> );
else
while (<DATA>)
print $_;



exit;

__DATA__
01234567890123456789
1234567890123456789
1234567890123456789
12
X <- That X is the 64th char in
this file.
Y <- That Y is the 106th char in this file.
junk
more junk.
bye!


This is an old Perl:

$ perl -v

This is perl 5, version 16, subversion 3 (v5.16.3) built for x86_64-linux-
thread-multi


DATA

tell(DATA)

DATA

1 Answer
1

Running the seek(), it appears to not start at DATA but the start of the script

I don't think you're making any mistake at all. That's exactly what happens. DATA is a filehandle that is open on your source file. Before your first read() from that filehandle, the file pointer is positioned immediately after the __DATA__ token in the file. But you can use seek() to move the file pointer to any position at all in the file.

DATA

read()

__DATA__

seek()

I guess it would be harder to implement a "special case" filehandle that wasn't able to move back before its initial position.

my $log_fh = *main::DATA; my $whence = $log_fh->tell(); $log_fh->seek($whence + 64,0); print "n-- 64 --n",join ("", <$log_fh> );

__DATA__

SEEK_CUR

tell

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