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I need help with "pack" and "unpack"

bobt_1234
New User

Feb 12, 2014, 12:38 PM

Post #1 of 3 (2645 views)

I need help with "pack" and "unpack"	Can't Post

Can someone shed some light on how to use the "pack" and "unpack" functions to encode and decode binary data?

The following code yields unexpected results. Basically, I'm trying to write a very simple binary to character to decimal converter using pack and unpack, with little success.

Code

my $b = "00000110"; 
print "B before pack = $b\n"; 
my $v = pack("B8", $b); 
print "v after pack = $v\n"; # <== what I expect. 
 
# I get expected results here, basically undoing what I did above. 
my $v1 = unpack("B8 ", $v); 
$v1 = $v1 || ""; 
print "V1 after unpack = $v1\n"; 
 
# Unpacking the packed value in 'i' (integer) format. 
my $n = unpack("i*", $v); 
$n = $n || ""; 
print "n = $n\n"; 
# Would expect to get 6 here, but don't (?????)

Laurent_R
Veteran / Moderator

Feb 12, 2014, 2:00 PM

Post #2 of 3 (2636 views)

Re: [bobt_1234] I need help with "pack" and "unpack" [In reply to]	Can't Post

Can you please:
- explain what results you are really expecting for your code;
- explain what your results are;
- specify how the results differ from your expectation.

(OK, I have run your code, I know what the results are, but I would like a synoptical comparison of what you get and what you expected, how and why.)

I am very far from being an expert on the pack/unpack functions (I really don't use them often), but I have used them a few times lately and thus have some (limited) idea on how they work, and, prima facie, I am not so surprised by the results. So, please explain why you expect something else.

Meanwhile, these links might help you it you don't know them already:

http://perldoc.perl.org/functions/pack.html
http://perldoc.perl.org/functions/unpack.html

Granted, this documentation is not among the best pieces of documentation I have seen recently, but, well, you and I (and others) have to live with that. The other link gives in my view a less detailed but better overview:

http://perldoc.perl.org/perlpacktut.html

bobt_1234
New User

Feb 13, 2014, 7:36 AM

Post #3 of 3 (2589 views)

Re: [Laurent_R] I need help with "pack" and "unpack" [In reply to]	Can't Post

I've solved the problem!

I had to use the correct descriptor to correctly unpack an 8-bit packed value.

Since "pack" and "unpack" are inverse functions, I expected to get back the original value after running something through pack and unpack. Unpacking using the "i" descriptor should return the value in decimal instead of binary format (in this case, 6 instead of 00001100).

It didn't work because $v is 8 bits (the result of pack using "B8"), and the "i" descriptor wants a 32-bit value. The "c" descriptor is needed to unpack an 8-bit integer.

Code

my $b = "00001100"; # Binary 6. 
my $v = pack("B8", $b); 
print "Packed value = $v\n"; # <-- a "spade" or the printable representation of Ascii-6. 
my $n = unpack("i*", $v); # <-- Should unpack the value as "6" instead of "0110". 
print "Unpacked value as decimal = $n\n"; # <-- but prints blank space instead. 
 
my $nn = unpack("c*", $v); # <-- Unpack, treating $v as 8-bits.  Alas this works. 
print "Unpacked value is $nn\n"; # <-- Ah, now it prints "6" as I expected ("0110"B ==> "6"D).

Thanks for your time and support.

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