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.
use Fcntl qw(:seek);
my($fh, $filename, $byte_position, $byte_value);
$filename = "/some/file/name/goes/here";
$byte_position = 42;
open($fh, "<", $filename)
|| die "can't open $filename: $!";
binmode($fh)
|| die "can't binmode $filename";
sysseek($fh, $byte_position, SEEK_CUR) # NB: 0-based
|| die "couldn't see to byte $byte_position in $filename: $!";
sysread($fh, $byte_value, 1) == 1
|| die "couldn't read byte from $filename: $!";
printf "read byte with ordinal value %#02x at position %d\n",
ord($byte_value), $byte_position;
use autodie v2.12 every time, which I don’t really trust people to do, nor have. Remember that this has to run on a basic vendor Perl installation. I’m also worried about folks snipping pieces of it without that, in which case they have really big trouble. It’s a cute feature for quickies, but you have to be very careful with such advice.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__
🐫 𝕲𝖔 𝕿𝖍𝖔𝖚 𝖆𝖓𝖉 𝕯𝖔 𝕷𝖎𝖐𝖊𝖜𝖎𝖘𝖊 🐫
𝓔𝓭𝓲𝓽 : 𝙎𝙞𝙢𝙥𝙡𝙚𝙨𝙩 ℞: 𝟕 𝘿𝙞𝙨𝙘𝙧𝙚𝙩𝙚 𝙍𝙚𝙘𝙤𝙢𝙢𝙚𝙣𝙙𝙖𝙩𝙞𝙤𝙣𝙨
🎅 𝕹 𝖔 𝕸 𝖆 𝖌 𝖎 𝖈 𝕭 𝖚 𝖑 𝖑 𝖊 𝖙 🎅
💩 𝔸 𝕤 𝕤 𝕦 𝕞 𝕖 𝔹 𝕣 𝕠 𝕜 𝕖 𝕟 𝕟 𝕖 𝕤 𝕤 💩
😈 ¡ƨdləɥ ƨᴉɥʇ ədoɥ puɐ ʻλɐp əɔᴉu ɐ əʌɐɥ ʻʞɔnl 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.
perldoc -f seek