From: Robert Klemme <bob.news@...>
Date: 2004-05-08T07:58:56+09:00
Subject: Re: signaling another thread


"Edgardo Hames" <ehames@vates.com> schrieb im Newsbeitrag
news:409BD97D.8060805@vates.com...
> Robert Klemme wrote:
>
> > That code is not thread safe.  At least @i is accessed unsychronized.
> > While it *may* work it is unsafe.
>
> I know it is unsafe. I just wanted to write an example on how a thread
> could be working and stop when a certain event was notified,

Your thread does neither stop nor is it interrupted.  WarnLow#m just does an
endless loop and increments @i without taking any notice of #update calls.
It is the same instance but a different thread context.  You can view that
easily by including the current thread in your printout like

class WarnHigh < Warner
   def update(time, price)       # callback for observer
     if price > @limit
       print "#{Thread.current.inspect}: +++ #{time}: Price above #@limit:
#{price}\n"
     end
   end
end

You'll notice that it is the same as the main program, which happens to
execute Ticker#run

> which you
> cannot do if you where using a Condition Variable.

That's right: with a condition variable the thread is suspended and does
nothing.

> A Condition Variable
> could be used to guarantee exclusive access to @i.

For exclusive access a mutex is sufficient.  The condition variable buys you
the added convenience that a thread is suspended while waiting for a
condition change (technically for a signal to occur).

    robert


> Ed
>
> >>----8<--------8<--------8<--------8<----
> >>require "observer"
> >>
> >>
> >>
> >>
> >>class Ticker
> >>   include Observable
> >>   def run
> >>     i = 0
> >>     while (i < 130)
> >>       changed(i > 70)
> >>       notify_observers(Time.now, i)
> >>       i = i + 1
> >>     end
> >>   end
> >>end
> >>
> >>class Warner
> >>   def initialize(ticker, limit)
> >>     @limit = limit
> >>     ticker.add_observer(self)   # all warners are observers
> >>   end
> >>end
> >>
> >>
> >>class WarnLow < Warner
> >>   def m
> >>     @i = 0
> >>     loop do
> >>       @i = @i + 1
> >>     end
> >>   end
> >>   def update(time, price)       # callback for observer
> >>     if price < @limit
> >>       print "#@i #{time.to_s}: Price below #@limit: #{price}\n"
> >>     end
> >>   end
> >>end
> >>
> >>
> >>
> >>
> >>class WarnHigh < Warner
> >>   def update(time, price)       # callback for observer
> >>     if price > @limit
> >>       print "+++ #{time.to_s}: Price above #@limit: #{price}\n"
> >>     end
> >>   end
> >>end
> >>
> >>
> >>
> >>
> >>ticker = Ticker.new()
> >>Thread.new {
> >>         wl = WarnLow.new(ticker, 80)
> >>         wl.m
> >>}
> >>WarnHigh.new(ticker, 120)
> >>ticker.run
> >>
>
>
> -- 
> Edgardo Hames
> Consultor Inform�tico
> Vates S.A. Ingenier�a de Software
>
> ehames@vates.com
> Tel: +54 +351 +4240133 int. 121
> 9 de Julio 228 - 6� piso. C�rdoba
>
>
>