From: Scott Peacock <nyaminche@...>
Date: 2011-01-14T09:45:37+09:00
Subject: Re: Thread Variable race condition

Hi Karan,

Your problem seems to be that you are sharing state between all
the threads you are creating.

When you create a thread, all variables that are in scope at the point
it is created are accessible to the thread.

So the variable 'i' is visible to all the threads you create (which is
why
you can print) but it is visible to *all* the threads you create (which
is why you can print duplicates).

As Robert says, there's no guarantee about scheduling. So at the point
you increment 'i' you could have up to 4 threads which have not yet
printed. (It can't be five because you're blocking the thread that makes
threads every fifth thread). In practice you're only seeing 2 at a time
but it could be 3 or 4. Three or four are just really unlikely.

Then when you increment 'i' they all might print but because they all
access the same variable 'i' (it is in scope to them) they all print the
same thing.

I think you want to pass in 'i' as a variable to the thread. This will
create a thread local variable that will not increment when you
increment 'i' and so every thread will print the value of 'i' at the
point the thread was created.

i = 1
while(i < 100)

   t = Thread.new(i) do |i_copy|
      echo(i_copy)
   end
   t.join if (i % 5 == 0)
   i = i + 1
end

Is this the problem you were looking to solve?

Thanks,
Scott

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