Re: MSDN volatile sample

From:
"Doug Harrison [MVP]" <dsh@mvps.org>
Newsgroups:
microsoft.public.vc.language
Date:
Sun, 30 Dec 2007 12:08:04 -0600
Message-ID:
<tomfn3hk2ga514q1p7dpkr2bnq001uanma@4ax.com>
On Sun, 30 Dec 2007 07:33:08 -0800, "Alexander Grigoriev"
<alegr@earthlink.net> wrote:

For your concern, I begin to suspect all of the code I wrote before about
multi-threaded parts. Maybe I need to add volatile to all shared data.


Adding 'volatile' would be wrong approach. Using proper synchronization
should do the right thing.


Correct. As is commonly said, "Volatile is neither necessary nor sufficient
for multithreaded programming." If you use synchronization such as mutexes,
you don't need volatile; conversely, if you use volatile instead of proper
synchronization, there's a good chance your code is wrong, and if it
appears to work, you're just winning the race conditions, which like all
gambling, can change at any time. To further appreciate the folly of trying
to solve multithreading problems with volatile, the OP should do what he
said and declare std::string and other objects of class type volatile and
see how many member functions he can call. The answer will almost certainly
be zero. Not only that, volatile does not penetrate deeply enough to be
very useful; for example:

struct X
{
   int* p;
};

volatile X x;

void f()
{
   int& p = *x.p; // Fine
}

Surely, it was desired that *x.p be considered volatile, but there's no way
to express this except in the class definition itself.

--
Doug Harrison
Visual C++ MVP

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