From: ara.t.howard@...
Date: 2006-01-17T03:11:27+09:00
Subject: Re: DANGER ! Ruby-Newbie ahead: How to access binary files

On Tue, 17 Jan 2006, Meino Christian Cramer wrote:

> There is a file on my HD, which was written by a C program. The C program
> wrotes the contents of an array of structures (each array element was made
> from the same structure) to that file.
>
> Since accessing that file looks like a very low level and "procedure-based"
> thing to me I would be very interested how this job can be done in a most
> ruby-like, objectoriented way.

this is a perfect use case to abstract the error prone method of
reading/seeking/writing that one would typically do with binary data.  i use
mmap alot for these types of tasks at work, here is a little (silly) example:


first we build a c program to output an array of struct.  note that we output
the sizeof(struct) as the first part of the file - this is because we can't
know how the compiler will pad structs so we make sure the correct size is
encoded into the file:

     harp:~ > cat a.c
     #include <stdlib.h>
     #include <stdio.h>

     struct foobar { int foo; float bar; };

     main ()
     {
       struct foobar a[] = { {40, 40.0}, {2, 2.0} };
       int size = sizeof(struct foobar);
       fwrite (&size, sizeof(int), 1, stdout);
       fwrite (&a, sizeof(a), 1, stdout);
     }

     harp:~ > gcc a.c

     harp:~ > a.out > a


next we write a ruby class to access the data.  the access will be via mmap, so
any changes we make to the data can be tranparently written to disk with no
explicit io on our part - we simply use the objects as normal:

     harp:~ > cat a.rb
     #! /usr/bin/env ruby
     require "mmap"  # ftp://moulon.inra.fr/pub/ruby/

     class Integer
       SIZEOF = [42].pack("i").size
     end
     class Float
       SIZEOF = [42.0].pack("f").size
     end
     module Foobar
       class Struct
         def initialize mmap, offset
           @mmap, @offset = mmap, offset
         end
         def foo
           @mmap[@offset, Integer::SIZEOF].unpack("i").first
         end
         def foo= i
           @mmap[@offset, Integer::SIZEOF] = [Integer(i)].pack("i")
         end
         def bar
           @mmap[@offset + Integer::SIZEOF, Float::SIZEOF].unpack("f").first
         end
         def bar= f
           @mmap[@offset + Integer::SIZEOF, Float::SIZEOF] = [Float(f)].pack("f")
         end
         def inspect
           { "foo" => foo, "bar" => bar }.inspect
         end
       end
       class List < ::Array
         def initialize mmap
           @mmap = mmap
           @sizeof = mmap[0, Integer::SIZEOF].unpack("i").first
           offset = Integer::SIZEOF
           while((offset + @sizeof) <= mmap.size)
             struct = Struct::new @mmap, offset
             self << struct
             offset += @sizeof
           end
         end
       end
       class File
         attr "path"
         attr "list"
         attr "mmap"
         def initialize path
           @path = path
           open(@path, "r+"){|f| @mmap = Mmap::new f, "rw", Mmap::MAP_SHARED}
           @list = List::new @mmap
         end
         def self::new *a, &b
           ff = super
           mmap = ff.mmap
           ::ObjectSpace::define_finalizer(ff){ mmap.msync; mmap.munmap }
           ff
         end
       end
     end

     ff = Foobar::File::new ARGV.shift
     fl = ff.list

     p fl

     fl.each{|f| f.foo = 42 and f.bar = 42.0}  # automatically written!



the first time we run the progam we see the data the c program wrote:

     harp:~ > a.rb a
     [{"foo"=>40, "bar"=>40.0}, {"foo"=>2, "bar"=>2.0}]


but next time we see the data automatically written by the ruby program:


     harp:~ > a.rb a
     [{"foo"=>42, "bar"=>42.0}, {"foo"=>42, "bar"=>42.0}]


this is just a silly example, but it shows how objectification of something
like this might be done in a way that really makes working with the actual data
easier.

kind regards.

-a
-- 
strong and healthy, who thinks of sickness until it strikes like lightning?
preoccupied with the world, who thinks of death, until it arrives like
thunder?  -- milarepa