Re^3: IP Header

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.

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Re: Re^3: IP Header by Anonymous Monk on Nov 02, 2002 at 23:39 UTC
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]
B versus b by Thelonius (Priest) on Nov 03, 2002 at 14:27 UTC
In addition to the problems Aristotle outlined above, there is one more problem. The functions unpack("b") and vec() unpack the bits in the wrong order within bytes. If you unpack("B", $foo), you will get all the bits (as a string) in the right order. Here is one way to get your final answer, although I'm not sure that `eval "0b$_"` is the best solution: `($ver, $hlen, $tos, $len) = map { eval "0b$_" } unpack "A4A4A8A16", un +pack("B", $a);` [download]	[reply] [d/l] [select]
Re: B versus b by Aristotle (Chancellor) on Nov 03, 2002 at 14:36 UTC
++ for pointing out the `B` vs `b` concern, but you're wrong about vec. And that code is an eval $REDFLAG;, you should do it thusly: `my ($ver, $hlen, $tos, $len) = map pack "B", unpack "A4A4A8A16", unpack "B", $foo;` However, that will not honour the network byte ordering in `$len`. Makeshifts last the longest.	[reply] [d/l]
Re^5: IP Header by Aristotle (Chancellor) on Nov 02, 2002 at 23:49 UTC
Argh. This ain't my day. My parameters were in the wrong order, it's `my ($ver, $hlen) = (vec($ver_hlen, 0, 4), vec($ver_hlen, 1, 4));` or also `my ($ver, $hlen) = map(vec($ver_hlen, $_, 4), (0 .. 1));` The `4` means I want to treat this as an "array" of 4-bit-elements, and the offset specifies the "index" in that "array". I hope I haven't left any errors still to correct this time. grumble Makeshifts last the longest.	[reply] [d/l] [select]