I was using XML::Simple to parse a document and was told "junk after document element at line 77, column 0, byte 2910. So I wrote the following program to help me take a look at the data in the file around that byte position.

Does anyone have any input into whether I am using the right method of doing byte-level analysis of a file for bad data? Program output *precedes* the source code.

Program Output Starts on Next Line

< 47    /
< 99    c
< 100   d
< 62    >
* 10

> 60    <
> 99    c
> 100   d
> 62    >

[download]

Program

#!/usr/bin/perl

use strict;

my $f = shift || die "must supply filename";

open F, $f    or die "couldnt open $f: $!";

my $bytepos = shift || die "must supply bytepos";

my $offset  = 4;

my @range   = ($bytepos-$offset .. $bytepos+$offset);
my $range   =  @range;
my $text    =   join '', <F>;

my @substr  = ($range[0]-1, $range);
warn "substr @substr";
#my $chunk   = substr $text, @substr;
my $chunk   = substr $text, $range[0]-1, $range;
warn substr $chunk, 0, 20;

my $format  = "C" . $range;
my @unpack  = unpack $format, $chunk;

my $normal  = $range[0];
my @chunk   = split //, $chunk;
for (@range) {

  my $arydex = $_-$normal;

  if ($_ < $bytepos) {
    print '< ';
  }
  if ($_ > $bytepos) {
    print '> ';
  }
  if ($_ == $bytepos) {
    print '* ';
  }

  print unpack 'C', $chunk[$arydex];

  print "\t";

  print             $chunk[$arydex];

  print "\n";

}

[download]

In reply to Byte-level file inspection? by princepawn

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