From: Joel VanderWerf <vjoel@...>
Date: 2006-07-30T00:43:09+09:00
Subject: Re: How can I parse binary files?


Hi, all. Sorry to respond so late to this thread. Thanks to Daniel for 
his excellent and thorough responses!

Just one comment below on Daniel's solution...

Daniel Martin wrote:
> require 'bit-struct'
> class MRKHeader < BitStruct
>     unsigned :version,      32, "Version",     :endian => :native
>     unsigned :uid_Validity, 32, "UIDValidity", :endian => :native
>     unsigned :uid_next,     32, "UIDNext",     :endian => :native
>     unsigned :last_write_counter, 32, "LastWriteCounter", :endian => :native
>     rest     :unused,           "Unused"
>     # Override so that it gets padded properly
>     def MRKHeader.round_byte_length
>       super
>       36
>     end
> end

I'd suggest defining a fixed-length "unused" field instead of a "rest" 
field. The rest construct is better for variable length data, such as a 
payload at the end of a packet.

So instead of

       rest     :unused,           "Unused"

you can consume the bytes with

       char     :unused,         (36*8-4*32), "Unused"

Then you don't have to worry about whether overriding #round_byte_length 
is the right thing to do or not.

The disadvantage to doing it this way is that inspect will print out 
stuff you don't care about. Solving this problem gets to the suggestions 
  Daniel made in another post, so I'll respond separately.

-- 
       vjoel : Joel VanderWerf : path berkeley edu : 510 665 3407