Re: Am I or Alexandrescu wrong about singletons?

From:
James Kanze <james.kanze@gmail.com>
Newsgroups:
comp.lang.c++.moderated
Date:
Wed, 24 Mar 2010 05:34:25 CST
Message-ID:
<146a4d24-1f1a-46b7-8201-0f19574a6324@q16g2000yqq.googlegroups.com>
On Mar 23, 2:05 pm, "Leigh Johnston" <le...@i42.co.uk> wrote:

"Joshua Maurice" <joshuamaur...@gmail.com> wrote in message

news:b897c547-0237-4d21-8a54-7c0cc80cd39a@u15g2000prd.googlegroups.com...


      [...]

Sometimes you have to use common sense:


Modern memory models don't respect common sense very much.

thread A:
finished = false;
spawn_thread_B();
while(!finished)
{
  /* do work */
}

thread B:
/* do work */
finished = true;

If finished is not volatile and compiler optimizations are
enabled thread A may loop forever.


And making finished volatile doesn't change anything in this
regard. At least not with Sun CC or g++ under Solaris, g++
under Linux on PC, and VC++8.0 under Windows on a 64 bit PC.

The behaviour of optimizing compilers in the real world can
make volatile necessary to get correct behaviour in
multi-threaded designs.


As has been pointed out: volatile is never sufficient, and when
you use whatever is sufficient, volatile ceases to be necessary.

You don't always have to use a memory barriers or a mutexes
when performing an atomic read of some state shared by more
than one thread.


Only if you want it to work.

--
James Kanze

--
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