Re^4: CRC of a string

Thank you very much for your great answers. So I could learn a lot.

Here the final solution:

#!/usr/bin/perl

use strict;
use warnings;

my $str = "/etc/test2.sh"; # checksum32: 990644297
my $checksum32 = &calcChecksum32($str);

print $checksum32 . "\n";

sub calcChecksum32()
{
    my $str = $_[0];
    my $nb_pad_bytes = 4 - (length($str) % 4);

    $str .= "\0" x $nb_pad_bytes;
    unpack('%32N*', $str);
}
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Comment on Re^4: CRC of a string Download Code

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Re^5: CRC of a string by AnomalousMonk (Archbishop) on Nov 20, 2009 at 08:57 UTC
Note that the calculation `4 - (length($str) % 4)` will cause four null bytes to be appended to a string that is an exactly even number of `'N'` fields long. As this effectively adds a zero to an arithmetic checksum, there should be no effect. However, there may be other, similar situations in which there is a significant effect! The current unpack-ing behavior of the `N` field specifier and others of its ilk is to ignore any bytes at the end of a string that do not constitute a complete field. If this is true, it is sufficient to simply append a constant string of three null bytes (in the case of the `N` specifier) to the end of every* string. Unfortunately, this behavior does not seem not be explicitly specified in the current documentation. Also note that you define the `calcChecksum32` function with a `()` prototype for an empty argument list, then invoke the function (with an argument!) with the `&` sigil, which causes any prototype to be ignored. A good rule is to avoid prototypes unless you understand exactly how they work; most programmers, especially those from a C/C++ background, simply don't. It is also good practice to avoid using the `&` sigil when invoking functions, using, e.g., `calcChecksum32($string)` instead. Among other things, this will allow prototypes, if you ever do use them, to actually have some effect! See perlsub for info on prototypes, subroutine invocation, etc.	[reply] [d/l] [select]
Re^6: CRC of a string by Dirk80 (Pilgrim) on Nov 27, 2009 at 10:36 UTC
Hi, Thank you very much for your answer. Now I read the whole tutorial perlpacktut. And I did many experiments with pack/unpack. And now I know the behaviour of "N". If you use "N" with unpack and in memory is less than 4 bytes then nothing is unpacked. "N" with unpack is only unpacking something if there are at least 4 bytes in memory left. So you were totally right. Adding "\0\0\0" to the string will always work. Because the pack tutorial was so interesting I still did not read the perlsub tutorial. But this will come next. Because you are right. At the moment I don't know the difference between calling a function with & or without, ... Greetings, Dirk	[reply]