Re^6: Unexpected result using tell/seek within the __DATA_

Have a second look at my previous post. I did get 14 and 16 with the hex editor.

None of PerlIO, clib and the OS know anything of __DATA__, so they don't reporting the file positions different after the first __DATA__ is encountered.

Special Literals

Text after __DATA__ may be read via the filehandle PACKNAME::DATA , where PACKNAME is the package that was current when the __DATA__ token was encountered. The filehandle is left open pointing to the contents after __DATA__. It is the program's responsibility to close DATA when it is done reading from it.

Correct me if I'm wrong but the way I understand it is Perl has to compile and preprocess the program file. It reads the file until EOF or __END__ or __DATA__. According to the above section DATA is left open pointing to just after __DATA__. That means when you do a "tell DATA" like I did at the beginning of the script there is nothing for tell() to calculate: It just has to spit out the file location. Then, after reading from DATA if you do another tell DATA it has to do the math.

I just ran the script on Knoppix linux and this problem didn't show up. The seek went to the correct location and there were no partial lines printed.

I'm sorry that I couldn't explain about the offsets better. I know what I meant and it makes sense to me. I thought it was an interesting problem that others would be interested in or I wouldn't have posted it.

I've had a splitting headache all day and now that I've had some Tylenol and a snack I'm off to relax a little.

Comment on Re^6: Unexpected result using tell/seek within the __DATA__ file

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Re^7: Unexpected result using tell/seek within the __DATA__ file by ikegami (Patriarch) on Mar 12, 2011 at 06:33 UTC
I can confirm the bug (ActivePerl 5.12.1). It doesn't happen with PERLIO=:crlf on a linux machine. (5.12.2, default build config). Here's a good demonstration: `#!/usr/bin/perl use strict; use warnings; my @data_positions = tell(DATA); while (<DATA>){ if (/^__DATA__$/) { push @data_positions, tell(DATA); } } my @fh_positions; open(my $fh, '<', $0) or die; while (<$fh>){ if (/^__DATA__$/) { push @fh_positions, tell($fh); } } print("@data_positions\n"); print("@fh_positions\n"); __DATA__ ab __DATA__ ab __DATA__ ab __DATA__ lotsa junk nothing` [download] `389 401 414 426 389 403 419 433` [download]	[reply] [d/l] [select]

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Re^7: Unexpected result using tell/seek within the __DATA__ file
by ikegami (Patriarch) on Mar 12, 2011 at 06:33 UTC

I can confirm the bug (ActivePerl 5.12.1). It doesn't happen with PERLIO=:crlf on a linux machine. (5.12.2, default build config).

Here's a good demonstration:

#!/usr/bin/perl
use strict;
use warnings;

my @data_positions = tell(DATA);
while (<DATA>){
    if (/^__DATA__$/) {
        push @data_positions, tell(DATA);
    }
}

my @fh_positions;
open(my $fh, '<', $0) or die;
while (<$fh>){
    if (/^__DATA__$/) {
        push @fh_positions, tell($fh);
    }
}

print("@data_positions\n");
print("@fh_positions\n");

__DATA__
ab
__DATA__
ab

__DATA__
ab
__DATA__
lotsa junk
nothing
[download]

389 401 414 426
389 403 419 433
[download]

[reply]
[d/l]
[select]