Re^2: Unpacking variable length bit fields

Oops, I spoke to soon. The field length specifies the number of bits (multiple of 8) but I want to extract the fields themselves as bytes not as bit strings of 1/0's.

My problems is that using something like: unpack('C/C'...) would unpack C bytes for each filed rather than C bits for each field which is what I want. So I really would like to divide by 8 but I'm not sure how to do that. I imagine I could unpack as a bit string of 1/0's then split, then repack but that seems klugey.

It's a screwy format, but I don't have control over it....

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Re^3: Unpacking variable length bit fields
by BrowserUk (Patriarch) on Jun 19, 2012 at 05:30 UTC

The the easiest way is to convert the ascii-ised binary back to real numbers:

$bitstream = "\x18\xf0\xff\x0f\x10\xaa\x55\x20\xde\xad\xbe\xef";;

( $f1, $f2, $f3 ) = map unpack( 'L', pack( 'b32', $_ )), unpack 'C/b C
+/b C/b', $bitstream;;

print for $f1, $f2, $f3;;
1048560
21930
4022250974
[download]

This assumes maximum 32-bit values. You could use Q if you are on a 64-bit platform and Perl and need that.

By converting them all to the maximum sized integer the platform can handle you save having to make decisions and they'd end up that way anyway.

You might need to adjust the L (or the 'b32' to 'B32' or 'B64') for the endianness of the originating platform.

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