From: Justin Collins <justincollins@...>
Date: 2008-03-07T09:51:58+09:00
Subject: Re: Threading weirdness

Philip Scott wrote:
> Hi guys,
>
> ######################################## PROGRAM 2: DOESNT WORK, 
> TERMINATES
> require 'thread'
> require 'digest/md5'
>
> a = []
> b = []
>
> # Fill two identical arrays full of 10 random strings
> for i in (0..9)
>    a[i] = Digest::MD5.hexdigest(rand(1000).to_s).to_s
>    b[i] = a[i]
> end
>
> print "Starting threads\n"
> Thread.abort_on_exception = true
>
> # Start a hundred threads reading items from the two arrays and 
> comparing them
> for x in (0..100)
>     testThread = Thread.new do
>         while true
>             i = rand(9)
>             if (a[i] != b[i])
>                 raise Exception.new("Threading Broke")
>             end
>         end
>     end
> end
>
> print "And we're off"
>
> # Spin forever (or at least until an Exception gets thrown)
> while true do
> end
>

The variable 'i' is shared across all threads because it was created 
when you initialized the arrays and is still in scope when you create 
the threads.

-Justin