From: Rob Lee <robl@...>
Date: 2006-10-23T11:41:58+09:00
Subject: Binary file modification

Hi

I've been modifying a binary file that contains various data including 
some audio.  I'm trying to add my own audio (instead of the audio in the 
file) which I believe I have done but I need to modify the 
content-length header in the file which indicates the length of the 
audio sample.  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
f.close
=> an integer

This basically opens the file as a binary file, skips to the audio 
content header data position and then unpacks 3 bytes into a string 
(formatted as hex) which is then converted to an integer value.

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 = SOMEHEADERDATA
mydata += ["1024"].pack("someformat")
mydata += AUDIODATA
f=open(config[:file],"wb")
f.write(mydata)
f.close

However I'm a bit stuck on how to pack the data (if this is the correct 
solution).  If someone could point me in the correct direction it would 
be much appreciated.

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