Difficulty producing correct checksum

Anonymous Monk has asked for the wisdom of the Perl Monks concerning the following question:

Hello everyone again, I recently posted here with some help from ikegami, but I am still unable to produce the correct checksum (I am trying to match it to the checksum provided by my coworker). I would like to raed each 32 bit chunk of binary, and then turn it to decimal value, and add it to the checksum (which starts at 0). Does my ($n = read FILE, $buffer, 4) work? Is that reading 32 bits? Also, does my restraining work at the end of the script? Thanks for any and all help.

#!/usr/bin/perl -w
  
  use strict;
  use FileHandle;
  use warnings;

  my $data = "../file.bin";
  my $buffer;
  my $string;
  my $n = 0;
  my $checksum = 0;
  my $num = 0;
  my $str = unpack("B32", pack("N", shift));
  my $bits = 4;
  my $decimal;


open FILE, $data or die $!;
binmode FILE;  

    while (($n = read FILE, $buffer, 4))
 {
    
    $decimal = unpack("N", pack("B32", $buffer));
    
    {$checksum = $checksum + $decimal;
    #$num = $num + $bits;    
    
    #print "1. $num\n";
    print "2. $decimal\n";
    #print "3. $n bytes read\n";
    print "4. $checksum \n";
    }
    
    $checksum %= 2**32;
    
    }
[download]

Whenever i run this script, the number is never correct. Are there any obvious errors here?

Comment on Difficulty producing correct checksum Download Code

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Re: Difficulty producing correct checksum by spazm (Monk) on Jul 16, 2009 at 16:18 UTC
your read statement should be correct, since you've used binmode on the filehandle and don't have a :utf8 pragma. you're reading 4 x 8bit bytes. The final read is not guaranteed to be 32 bits, you should check $n to see how many bytes were read from the stream. I'm not sure of the behavior of unpack if you don't feed it enough data. Since you're using Big-Endian, it may see your final read as much larger than expected. Your final line $checksum %= 2^32, is going to constrain your checksum to be mod 2^32. If that's what you want, then, yes, it should work. You're doing your bit match in Big-Endian. Is there any chance that the code you're trying to match does it little-endian? Your B in unpack and N in pack match endianness, so that's good. Suggestion: make a 4 byte test file and run the other checksum program on it. Use this as the basis for your first test of your checksum program. Then progress to a 5 yte test file, to test the question about the truncated final read. perldoc read: Note the characters: depending on the status of the filehandle, either (8-bit) bytes or characters are read. By default all filehandles operate on bytes, but for example if the filehandle has been opened with the :utf8 I/O layer (see "open", and the open pragma, open), the I/O will operate on UTF-8 encoded Unicode characters, not bytes.	[reply]
Re^2: Difficulty producing correct checksum by Anonymous Monk on Jul 16, 2009 at 18:07 UTC
Hey! thanks for all of your help guys! it finally works!	[reply]
Re^3: Difficulty producing correct checksum by spazm (Monk) on Jul 17, 2009 at 16:15 UTC
What were the issues and how did you fix them? Let's see the final code!	[reply]
Re: Difficulty producing correct checksum by jrsimmon (Hermit) on Jul 16, 2009 at 16:01 UTC
Have you considered simply using the checksum feature of unpack itself? In addition to fields allowed in pack(), you may prefix a field with a %<number> to indicate that you want a <number>-bit checksum of the items instead of the items themselves. Default is a 16-bit checksum. Checksum is calculated by summing numeric values of expanded values (for string fields the sum of "ord($char)" is taken, for bit fields the sum of zeroes and ones). Or, if that isn't what you're trying to accomplish, what about Digest::MD5 or Digest::Adler32?	[reply]
Re: Difficulty producing correct checksum by Anonymous Monk on Jul 16, 2009 at 16:06 UTC
Does my ($n = read FILE, $buffer, 4) work? Is that reading 32 bits? yes, it's reading 4 characters(if 1 character=1 byte)=> 32bits `$decimal = unpack("N", pack("B32", $buffer));` what are you trying to do here ? you said "turn it to decimal value", you mean ascii code ? if so consider using ord	[reply] [d/l]