Re: InterlockedExchange64

From:

Leandro Oliveira <loliveira@gmail.com>

Newsgroups:

microsoft.public.vc.language

Date:

Mon, 13 Aug 2007 22:35:00 -0000

Message-ID:

<1187044500.989067.70570@l70g2000hse.googlegroups.com>

Igor,

Thanks a lot! It is working now:

//////////////////////////////////

volatile __int64 g;

unsigned __stdcall Check(void*)
{
    __int64 t;

    while (t = ::InterlockedExchangeAdd64(&g, 0),
          ((t == 0) ||
          (t == 0xffffffffffffffff))) ;

    ::DebugBreak();

    cout << t;

    return 0L;
}

int main(int argc, char **argv)
{
    ::_beginthreadex(NULL, 0, Check, NULL, 0, NULL);

    __int64 v[] = {0, 0xffffffffffffffff};

    int i = 0;

    while (TRUE)
    {
        ::InterlockedExchange64(&g, v[i %= 2, i++]);
    }

    return 0;
}

////////////////////////////////////////////////////

On Aug 13, 7:11 pm, "Igor Tandetnik" <itandet...@mvps.org> wrote:

Leandro Oliveira <lolive...@gmail.com> wrote:

I am studying the InterlockedExchange64 function behavior and i think
I did sth wrong. IMO, the line that contains teh call
to ::DebugBreak() shouldn=B4t neve be executed. Am I right?

//////////////////////////////////////////////////////////////////////

volatile __int64 g;

unsigned __stdcall Check(void*)
{
__int64 t;

InterlockedExchange64(&t, g);

t is allocated on the stack. It's impossible for two threads to assign
to the same variable concurrently, since each thread has its own stack
and its own copy of t. Why do you feel the need to use
InterlockedExchange64 here, and not just

t = g;

? If you hope that InterlockedExchange64 somehow reads g atomically, you
are mistaken: it only assigns to t atomically (which is quite pointless
here). To perform an atomic read, do

__int64 local_g = InterlockedExchangeAdd64(&g, 0);

--
With best wishes,
Igor Tandetnik

With sufficient thrust, pigs fly just fine. However, this is not
necessarily a good idea. It is hard to be sure where they are going to
land, and it could be dangerous sitting under them as they fly
overhead. -- RFC 1925