From: "Chris Günther" <cg_@...>
Date: 2005-03-24T01:49:48+09:00
Subject: Re: Reading and writing a binary file

Pit Capitain wrote:
> Chris G�nther schrieb:
> 
>> ... I am trying to patch binary data in a file.
>> Unfortunately the data I am patching is not the data I get when 
>> reading it back from the patched binary file.
>> Any ideas??
>> ...
>> File.open(FNAME,"ab+") {|fd|
>>     fd.pos=0
>> ...
> 
> 
> Hi Chris,
> 
> it seems that the mode "ab+" is your problem. According to [1], "a+" 
> means open the file for reading and writing, starting at the end of the 
> file. This also means (at least on windows), that the end of file is 
> position 0. So what you are doing above is writing the patched data 
> after the end of the original file.
> 
> For patching a binary file I'd change your code to:
> 
>   FNAME, FSIZE, OFFSET, VALUE = 'foooo.dat', 1024, 32,
>   [?c,?a,?f,?e].pack('cccc')
> 
>   # "wb+" for initial data: truncate existing content
>   File.open(FNAME,"wb+") {|fd|
>       bin_data = [0].pack('c')*FSIZE
>       fd.write(bin_data)
>   }
> 
>   # "rb+" for the patch: open at the beginning
>   File.open(FNAME,"rb+") {|fd|
>       fd.pos=OFFSET
>       fd.write(VALUE)
>       p "written modified piece of data = '#{VALUE}'"
>   }
> 
>   # "rb" for reading the data
>   File.open(FNAME,"rb") {|fd|
>       fd.pos=OFFSET
>       patched=fd.read(VALUE.length)
>       p "read modified piece of data = '#{patched}'"
>       puts "Oooops !!!" unless VALUE==patched
>   }
> 
> Regards,
> Pit
> 
> [1] http://www.ruby-doc.org/core/classes/IO.html
> 
> 
Hi Pit,
thanks for your contribution.
That's exactly the solution I already have. What I dont like about this 
solution is that you have to open and close file descriptors 3 times 
just for patching one little file (For sure a working solution but quite 
some overhead). Just for academical reason it seeems I have to fall back 
reading the C-sources :-( to find out what is going wrong under the hood.

Chris