"Chris Guenther" <chris_guenther / freenet.de> schrieb im Newsbeitrag
news:d1mhu3$bi6$1 / news.mch.sbs.de...
> Hi,
>
> I am having trouble reading values from an existing (or maybe not yet
> exisiting) binary file:
> My target is to open a binary file for reading and writing (if it does
not
> exisit it should be created).
> If the file was not yes existing I'd like to fill it with binary data to
a
> certain predefined size.
> Later on I'd like to position my file position, and read and write
whatever
> I like.
>
> The following piece does not work as intended:
>
>   require 'pp'
>   UNDER_UNIX = CONFIG['target_os'].to_s.index('linux')!=nil
>   filename='foo.bin'
>   size=1024
>   dont_write=FileTest.exists?(filename) &&
File.stat(filename).size==size
>   begin
>        fd=File.open(filename,"ab+")
>        fd.binmode unless UNDER_UNIX          # assuming !UNIX=WINDOWS
>   rescue => e
>        pp e
>        fd=File.open(filename,"wb+")
>        puts "opened #{filename} with 'wib+'"
>   end
>
>   fd.sync=true
>   fd.seek(0)
>   unless dont_write
>       bin_data = [0].pack('c')*(size)
>       fd.write(bin_data)
>       # OK now I really have a file of size 1024 with a binay 0 contents
>   end
>
>    ###.... then sometime later using the same file descriptor (fd) :
>     requestes_stream_position = 900
>     fd.pos=(requestes_stream_position)
>     p "tell=#{fd.tell}"
>     # Now please read 30 binary 0s into a binary string
>     bin_value=fd.read(30)
>
>     #### I ASSUMED THAT bin_value now is a "binary string of size 30",
but
> unfortunately it is nil !!!
>
>
>
> Thanks in adcance,
> C.

I don't know what you're doing wrong.  Your script works fine for me.
Btw, you don't close the fd.

Here's how I'd do it:

filename='foo.bin'
size=1024

File.open(filename, "ab+") do |fd|
  fd.seek(0, IO::SEEK_END)

  if fd.tell < size
    fd.seek 0
    fd.write( "\000" * size );
  elsif fd.tell > size
    # if you need the file to be exact this size
    fd.truncate size
  end

  requestes_stream_position = 900
  fd.seek requestes_stream_position

  bin_value = fd.read(30)
  p bin_value
end

Notes:

 - don't change binmode depending on platform, always use "b" for binary
files.

 - use File.open with block for proper close / cleanup

Kind regards

    robert