From: Robert Klemme <bob.news@...>
Date: 2005-03-22T22:59:53+09:00
Subject: Re: Reading and writing a binary file


"Chris G�nther" <cg_@gmx.de> schrieb im Newsbeitrag
news:42400070.2050609@gmx.de...
> Thanks Robert,
>
> you are absolutely right with your remarks using a block. The code I
> posted was just a small copied fraction of the real code, in which I
> cannor use a block because the same fd will be stored in a hash for
> later usage by another method.
> Anyway, meanwhile I found the cause of this problem (again you are
> right, it was not in the code I posted) , but now I have another related
> problem that tricks my mind. 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??
>
>
>
> Thanks again in advance,
> Chris
>
>
>
>
>
> #!/usr/bin/env ruby
>
> FNAME, FSIZE, OFFSET, VALUE = 'foooo.dat', 1024, 32,
> [?c,?a,?f,?e].pack('cccc')
>
> File.open(FNAME,"ab+") {|fd|
>      fd.pos=0
>      bin_data = [0].pack('c')*FSIZE
>      fd.write(bin_data)
> }
>
> File.truncate(FNAME,FSIZE)
>
> File.open(FNAME,"ab+") {|fd|
>      fd.pos=0
>      data=fd.read(FSIZE)
>      data[OFFSET,VALUE.length]=VALUE
>      fd.pos=0
>      fd.write(data)
>      p "written modified piece of data = '#{data[OFFSET,VALUE.length]}'"
> }

You truncate too often:
> File.truncate(FNAME,FSIZE)

> File.open(FNAME,"ab+") {|fd|
>      fd.pos=OFFSET
>      patched=fd.read(VALUE.length)
>      p "read modified piece of data = '#{patched}'"
>      puts "Oooops !!!" unless VALUE==patched
> }

Careful when you're copying and pasting.

Regards

    robert