Re: MT Design Question

"Chris M. Thomasson" <cristom@charter.net> wrote in message
news:6oweo.76824$1F6.66279@newsfe01.iad...


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Don't forget, Windows has kernel semaphore objects. We can use the
interlocked API and a single kernel semaphore to build a "fast-pathed"
semaphore:

<pseudo-code sketch>
______________________________________________________________
struct fsem
{
    LONG m_count; // = 0
    HANDLE m_waitset; // = Windows semaphore; value of zero

    void post()
    {
        if (InterlockedIncrement(&m_count) < 1)
        {
            ReleaseSemaphore(m_waitset, 1, NULL);
        }
    }

    void wait()
    {
        if (InterlockedDecrement(&m_count) < 0)
        {
            WaitForSingleObject(m_waitset, INFINITE);
        }
    }
};
______________________________________________________________

Now we can build the simple queue using the synchronization primitive above:

<pseudo-code sketch>
______________________________________________________________
struct node
{
    node* m_next;
};

struct queue
{
    node* m_head; // = NULL;
    node* m_tail;
    fsem m_waitset;
    CRITICAL_SECTION m_lock;

    void push(node* n)
    {
        n->m_next = NULL;

        EnterCriticalSection(&m_lock);

        if (! m_head)
        {
            m_head = n;
        }

        else
        {
            m_tail->m_next = n;
        }

        m_tail = n;

        LeaveCriticalSection(&m_lock);

        m_waitset.post();
    }

    node* pop()
    {
        m_waitset.wait();

        EnterCriticalSection(&m_lock);

        node* n = m_head;

        assert(n);

        m_head = n->m_next;

        LeaveCriticalSection(&m_lock);

        return n;
    }
};
______________________________________________________________

IMHO, the Windows synchronization primitives are fairly flexible indeed. The
standardized interlocked API is _very_ nice as well.

IMVHO, one of the coolest features Windows provides is the ability to issue
a process-wide memory barrier. That is it makes every CPU in the processes
affinity mask execute a memory barrier by broadcasting an IPI:

http://msdn.microsoft.com/en-us/library/ms683148(VS.85).aspx

To damn bad it's only for Vista and higher.