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

OK, I admit it... pack and unpack confuse me... each time I think I have them figured out, I realize I really don't. Are there any resources (other then perldoc -f unpack) with some more advanced examples?

The particular problem I'm having at the moment is unpacking a C struct like this: 

  struct {
    unsigned long var1:16, var2:16;
    unsigned long var3:24, var4:8;
    unsigned long var5:6,  var6:26;
  };

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The first 2, I got (I think), the second 2 I THOUGHT I got, but was wrong.. the last 2... well, frankly I'm just not too sure. Here's what I tried: 

  $fh->read($buf, 12);
  @var = unpack("s s C3 C b6 b26", $buf);

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Now, $var[0] and $var[1] end up as 2 byte shorts (as I expected), $var[2] ends up as 1 byte, not the 3 bytes I was expecting... it looks like the 2nd and 3rd bytes are put into $var[3] and $var[4]... which of course throws the rest off track.

How do I grab 3 bytes and put that into a single var, not split into 3 separate vars?

With regards to the "b6" and "b26"... well... I don't really want the value returned as binary data... I just want a 6 bit integer and a 26 bit integer respectively. How do I do that?

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Re: Unpacking struct
by Zaxo (Archbishop) on Apr 01, 2005 at 03:26 UTC

    Bitwise operations may be your best choice. There are no 24-, 6-, or 26-bit types in unpack's formats, and vec demands its chunks be a power of two bits wide and aligned. Here is one way to do it, unpacking to three ints and doing the bit operations. 

    $fh->read(my $buf, 12);
my ($var1, $var2, @raw) = unpack 's s L2' $buf;
my ($var3,$var4) = map { ($_ >> 8, $_ & 0xff) } $raw[0];
my ($var5,$var6) = map { ($_ >> 26, $_ & 0x3fffffff) } $raw[1];

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    That's not quite as unportable as it looks, but it's still on the edge.

    Update: Differs from Stevie-O's by bit order of the C struct. I'm not sure which is correct, so I'd bet on his. Adjust to what works :-)

    After Compline,
    Zaxo

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Re: Unpacking struct
by Stevie-O (Friar) on Apr 01, 2005 at 03:09 UTC
    pack and unpack can handle very many things, but bitfields are not one of them. You'll have to do some bit-level manipulation to extract the values you want; I would recommend you do this: 
    $fh->read($buf, 12);
# note that your original code used 's', which is signed,
# but your struct declaration says unsigned!
@var = unpack('S S L L', $buf);
my ($var1, $var2) = ($var[0], $var[1]);
my $var3 = $var[2] & 0xFFFFFF;
my $var4 = $var[2] >> 24;
my $var5 = $var[3] & 0x3F;
my $var6 = $var[3] >> 6;

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    --Stevie-O 
    $"=$,,$_=q>|\p4<6 8p<M/_|<('=>
.q>.<4-KI<l|2$<6%s!<qn#F<>;$,
.=pack'N*',"@{[unpack'C*',$_]
}"for split/</;$_=$,,y[A-Z a-z]
         {}cd;print lc

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Re: Unpacking struct
by Random_Walk (Prior) on Apr 01, 2005 at 10:04 UTC
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Re: Unpacking struct
by ikegami (Patriarch) on Apr 01, 2005 at 02:46 UTC
    Are those bit fields? Have you looked into vec instead of unpack?
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