in reply to IP Header

Your problem is that each unpack starts over at bit 0 of the binary data. But unpack is perfectly capable of returning any number of fields in one fell swoop, so this is what you really wanted: my ($ver, $hlen, $tos, $len) = unpack "b4b4b8n", $foo;

Makeshifts last the longest.

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Re: Re: IP Header
by Anonymous Monk on Nov 03, 2002 at 00:10 UTC
    Just to throw a wrench in the works ;-) The binary data is packed in Network bit order, won't pulling it out using C2 not respect that? So i tried this: 
    my ($data) = unpack "N", $foo;
my ($ver, $hlen) = (vec($data,0,4), vec($data,1,4));
my $tos = vec($data, 1, 8);
my $len = vec($data, 1, 16);

    [download]
    Which returned completely un-expected results. Is it not possible to use vec on data pulled using "N"? (yes, i know it can be more compact, split up for readability.)
[reply]
[d/l]
      You misunderstand the purpose of that format. The network ordering is important for multibyte fields. The order of bytes inside such a field may differ from what the host platform expects. The order of fields however does not change, regardless of platform. Two byte-sized fields are always extracted using C2; a single 16-bit field is extracted using n.

      Makeshifts last the longest.

[reply]
Re: Re: IP Header
by Anonymous Monk on Nov 02, 2002 at 22:10 UTC
    My understanding of the way IP headers are formed is that $ver should equal 4 when all is said and done. So, while grabbing it all in one swoop solves the repeating data I am still not obtaining the expected results.

    If you have 4 bits that represent, say, an integer how do you unpack to a usable value?
[reply]
      Oh. Silly me. You have a problem there: unpack consumes its input in byte chunks. Even if you specify b4, it will consume a full byte. (If you specificy b10, it will eat two full bytes.) Note also that b8 will return something of the form 00011111, not 31 - for that purpose you need C (unsigned character). For in-byte bit fiddling, you need vec: 
      my ($ver_hlen, $tos, $len) = unpack "C2n", $foo;
my ($ver, $hlen) = (vec($ver_hlen, 4, 0), vec($ver_hlen, 4, 1));

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      or more compactly my ($ver, $hlen) = map( vec($ver_hlen, 4, $_), (0 .. 1) );       See perldoc -f vec for details.

      Makeshifts last the longest.

[reply]
[d/l]
[select]
        Thank you for your help thus far. I am now getting the expected results for $tos and $len, however, $ver and $hlen are still not right.

        first, your "vec($ver_hlen, 4, 0)" brings up "illegal number of bits", due to 0 but being a power of 2 between 1 and 32 (from perldoc -f vec).

        I am having trouble understanding exactly what the bits field is. Is that the number of bits that you want to read in? if so, wouldn't: 
        my ($ver, $hlen) = (vec($ver_hlen, 0, 4), vec($ver_hlen, 4, 4));

        [download]
        be better? Using the the changes i made above, I get unexpected results for the values for $ver and $hlen, which leads me to believe I am wrong.
[reply]
[d/l]

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