Parsing a binary file in Perl and returning a value at a specified offset

Question

I have a binary file that I need to be able to parse through. I'm looking to specify an offset and then have the program return the byte value at that location.

I'm not sure about how to go about this. I've got the file open part, but I don't know how to get the program to jump to the location.

Answer 1

Use seek and the handy constants from the Fcntl module as in

#! /usr/bin/env perl

use bytes;
use strict;
use warnings;

use Fcntl ':seek';

open my $fh, "<", $0 or die "$0: open: $!";

seek $fh, 0, SEEK_END or die "$0: seek: $!";

my $last = tell $fh;
die "$0: tell: $!" if $last < 0;

for (1 .. 20) {
    my $offset = int rand($last + 1);
    seek $fh, $offset, SEEK_SET or die "$0: seek: $!";
    defined read $fh, my $byte, 1 or die "$0: read: $!";
    $byte = "\\$byte" if $byte eq "'" || $byte eq "\\";
    printf "offset %*d: \\x%02x%s\n",
      length $last, $offset,
      unpack("C", $byte),
      $byte =~ /[[:print:]]/a ? " '$byte'" : "";
}
__DATA__
? ??  ????  ???  ??  ???????? ?
???? :  ???????? ?:   ? ????????  ???????????????
?      ? ?   ? ? ? ? ?   ? ? ? ? ? ?    ?
?          ? ? ? ? ? ?   ? ? ? ? ? ? ? ? ? ?           ?
? ¡?dl?? ???? ?do? pu? ???p ???u ? ???? ???nl poo? ?

Sample output:

offset   47: \x65 'e'
offset  392: \x20 ' '
offset  704: \xf0
offset  427: \x5c '\''
offset  524: \x61 'a'
offset 1088: \x75 'u'
offset  413: \x20 ' '
offset 1093: \xbb
offset 1112: \xc9
offset  377: \x24 '$'
offset   64: \x46 'F'
offset  361: \x62 'b'
offset  898: \xf0
offset  566: \x5d ']'
offset  843: \xf0
offset 1075: \xc9
offset  280: \x20 ' '
offset    3: \x2f '/'
offset  673: \x8a
offset  153: \x20 ' '

The contents of the __DATA__ section were borrowed from Tom’s excellent suggestions for dealing with UTF-8 in Perl programs.