From: Robert Klemme <bob.news@...>
Date: 2003-12-05T19:32:06+09:00
Subject: Re: how to navigate in a file ?


"Mark J. Reed" <markjreed@mail.com> schrieb im Newsbeitrag
news:20031205025328.GA10409@mulan.thereeds.org...
> On Fri, Dec 05, 2003 at 01:58:22AM +0100, Boris BXS Schulz wrote:
> > Hi,
> >
> > I would like to open a binary file and read its contents.
> > The opening is no problem, but if I wanted to jump to a specific byte,
> > how would I do that ? Also, how would I read this specific byte ? I
have
> > some code reading the byte, but it quite ugly. Is there something like
> > DATA.readbyte(position) ?
>
> What you're looking for is File#seek(offset).  For instance:
>
> foo.rb(main):001:0> # Create a binary file where the byte at position N
= N
> foo.rb(main):002:0* File.open("binfile", "wb") do
> foo.rb(main):003:1*     |f|
> foo.rb(main):004:1*     256.times do
> foo.rb(main):005:2*     |b|
> foo.rb(main):006:2*     f.write(b.chr)
> foo.rb(main):007:2>     end
> foo.rb(main):008:1> end
> => 256
> foo.rb(main):009:0>
> foo.rb(main):010:0* # Seek to position 128 and read the byte there
> foo.rb(main):011:0* File.open("binfile", "rb") do
> foo.rb(main):012:1*     |f|
> foo.rb(main):013:1*     f.seek(128)
> foo.rb(main):014:1>     b = f.getc
> foo.rb(main):015:1>     puts b
> foo.rb(main):016:1> end
> 128
> => nil
> foo.rb(main):017:0>
>

Note that for reading and writing "r+b" is the appropriate open mode.

File.open("io-test.txt", "r+b") do |io|
  puts "read"
  io.seek(0)

  while ( b = io.read(1) )
    printf "%4d  0x%02x  %s\n", io.tell, b[0], b.inspect
  end

  puts "write"
  io.seek(0)

  io.write( "xxx" )

  puts "read again"
  io.seek(0)

  while ( b = io.read(1) )
    printf "%4d  0x%02x  %s\n", io.tell, b[0], b.inspect
  end
end

Regards

    robert