From: Rob Lee <robl@...>
Date: 2006-10-23T20:05:52+09:00
Subject: Re: Binary file modification

Ben Nagy wrote:
>> -----Original Message-----
>> From: list-bounce@example.com 
>> [mailto:list-bounce@example.com] On Behalf Of Rob Lee
> [...]
>> I've been reading the content-length data from 
>> the file 
>> using something similar to :
>> 
>> f=open(config[:file],"rb")
>> f.pos=AUDIO_CONTENT_HEADER_OFFSET
>> length=f.read(3).unpack("H2H2H2").hex.to_i
> 
> That should die because Array#hex doesn't exist, but I get the idea. If 
> you
> do this kind of thing a lot with different kinds of data then you should
> note that to_i and to_s both take optional base arguments, so
> foo.unpack('H*').first.to_i(16) should work

Thanks for pointing that out, it's strange as the code definately works 
(it returns correct content lengths repeatedly for different files) but 
as you say Array#hex doesn't exist - I'll look into this ...

> 
> [...]
>> I'd like to be able to reverse this process and take any an integer 
>> value (1024 in the case shown below) and write it to a binary 
>> file with 
>> some header and footer data - something like :
> [...]
>> mydata += ["1024"].pack("someformat")
> [...]
>> However I'm a bit stuck on how to pack the data (if this is 
>> the correct 
>> solution).
> 
> Here's the problem - if you need exactly three bytes then you are going 
> to
> have to apply your own padding, since the pack routines will only pack
> directly as a long or a short which will mostly be 4 and 2 bytes - both 
> of
> which could cause you problems. My hacks always involve <<'ing a single 
> byte
> integer onto a string. In your case, here is a horrible oneliner which 
> you
> should not use because it is gross.
> 
> num=1024
> num.to_s(16)[0..2].instance_eval {(self.reverse + '0' * (6 -
> self.length)).reverse}.scan(/../).inject('') {|s,byte| s << 
> byte.to_i(16)}
> 

I'm intrigued by this, I can see you are repacking the int value into 
three bytes but I'm still unclear as to how you would then add this to 
the file.  I guess it needs to be packed as 3 bytes - my guess was

num=1024
newpacket += [num].pack("3C")

However this doesn't seem to work using the reading scheme above.  Could 
you tell me where I'm going wrong ?

> You could also try googling BitStruct, which is a ruby library that 
> might
> help by defining these headers and footers as structure objects.
> 

I'm looking at using bitstruct for the next version of the project, but 
as I've almost got this working now I'd like to avoid a re-write at this 
stage if possible !

> Cheers,
> 
> ben

Thanks for all the help - you've saved me hours of work already !

-- 
Posted via http://www.ruby-forum.com/.