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

Dear Monks

I was just exploring the vec function but I already encountered a problem, I was not able to unpack it. Here is the code I used
#! /usr/bin/perl 

use strict ;
use warnings ;


my $bitstring = "" ;

my $offset = 0 ;

for my $i (0..20) {
    vec( $bitstring, $offset++, 4 ) = $i ;
}

my $bits = unpack("A4*", $bitstring);

print "$bits\n" ;

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From the result I realized that I do not fully understand vec and maybe also pack/unpack
so any help is appreciated!!

Thnx

LuCa

Replies are listed 'Best First'.
Re: HowTo unpack a vec (bitstring)
by Joost (Canon) on Jun 19, 2006 at 10:00 UTC
    Just to clarify: I think your main confusion is about how the C, a and A templates for (un)pack work. Observe: 
    #! /usr/bin/perl 

use strict ;
use warnings ;

my $bindata = pack("CaA", 65, "B", "C");

print "$bindata\n";

print join(", ",unpack("H2H2H2",$bindata)),"\n";

print join(", ",unpack("C*",$bindata)),"\n";

print join(", ",unpack("A1A1A1",$bindata)),"\n";

print join(", ",unpack("a1a1a1",$bindata)),"\n";

    [download]
    "What should it profit a man, if he should win a flame war, yet lose his cool?"
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Re: HowTo unpack a vec (bitstring)
by Joost (Canon) on Jun 19, 2006 at 09:13 UTC
    Erm.. 
    for my $i (0..20) {
    vec( $bitstring, $offset++, 4 ) = $i ;
}

    [download]
    That sets consecutive nybbles (half-bytes) to 0,1,2,3 .. etc. 

    my $bits = unpack("A4*", $bitstring);

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    This "unpacks" an ascii string of 4 bytes (the * at the end doesn't do anything since you didn't give a format letter). In other words, it just returns the first 4 bytes of $bitstring without converting them in any way.

    I don't think there's an unpack template for nybbles. If you want to see what you put in $bitstring, you might want to use the bit string template: "b*": 

    print unpack("b*",$bitstring);

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    output (broken up for readability): 
    0000100001001100001010
1001101110000110010101
1101001110110111111100
0010000100110000100000

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    or hex: 
    print unpack("h*",$bitstring);

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    output: 
    0123456789abcdef012340

    [download]
    "What should it profit a man, if he should win a flame war, yet lose his cool?"
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[d/l]
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      Ok, I made the following changes:
      vec ($bitstring, $offset++,8) = $_ ;

      [download]
      So now I can do
      my $bits = unpack("x1 C1", $bitstring);

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      However I cannot do
      my $bits = unpack("x1 A1", $bitstring);

      [download]
      So why is it not possible to extract an integer as an ASCII character ?

      LuCa
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