From: Sean Russell <ser@...>
Date: 2001-07-05T09:59:57+09:00
Subject: [ruby-talk:17260] Re: ruby community

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On Tuesday 03 July 2001 20:40, you wrote:
> Well, Guy totally came to the rescue, he dissected a bad function and
> explained to me that when something is said to not be "thread safe" it
> means that you have made a system call that could block, and you need
> to deal with that situation.  The way to deal with such a situation is
> something like this (if I understood correctly):

I'd hate to confuse the issue, but thread-safedness affects more than just 
system calls.  It affects any shared object accessed by two or more threads, 
whereby operations are not atomic and therefore prone to functional errors.  
The common, trivial, example is:

   class SomeClass
      attr_reader _items
      def initialize
         _items = []
      end
      def do_something
         if _items.length > 0      # Danger, Will Robinson!
            item = _items.pop
            # Do something with item, expecting it to be non-nil
         end
      end
   end

If you use this class in a threaded environment, it is possible that two 
threads could call do_something on the same object at (nearly) the same time, 
and that the scheduler could interrupt the code right after the length 
comparison.  The second thread would execute, pop the stack, and when the 
first thread resumes, it would be in trouble.

In any case, it isn't just system calls that have to be checked for 
thread-safedness, but any object which is being used by multiple threads.

- --- SER   Deutsch|Esperanto|Francais|Linux|Java|Dirigibles|GPG|Pegwit
"Did you ever notice that while MS stands for Microsoft, it also stands
for the dread disease Multiple Sclerosis? Well, why not? They're both
slow, crippling and ultimately deadly."   -- Monika McDole
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