Question

Because the lack of condition variable in windows(though it is introduced since vista, it’s not supported in windows XP and 2003), it is not very easy to implement a thread-safe queue in c++. Strategies for Implementing POSIX Condition Variables on Win32. What I required is to just use CriticalSection or Mutex and Event without using semaphore and condition variable.

I also tried to find an exact implementation that just using win32 native API, but no luck. So I finished one by myself. The problem is I am not 100% sure the code is thread-safe. Who can tell me it is OK or not?

class CEventSyncQueue
{
public:
    CEventSyncQueue(int nCapacity = -1);
    virtual ~CEventSyncQueue();
    virtual void Put(void* ptr);
    virtual void* Get();
protected:
    int m_nCapacity;
    CPtrList m_list;

    CRITICAL_SECTION m_lock;    
    HANDLE m_hGetEvent;
    HANDLE m_hPutEvent;
};

CEventSyncQueue::CEventSyncQueue(int nCapacity)
{
    m_nCapacity = nCapacity;

    ::InitializeCriticalSection(&m_lock);
    m_hPutEvent = ::CreateEvent(NULL, FALSE, FALSE, NULL);
    m_hGetEvent = ::CreateEvent(NULL, FALSE, FALSE, NULL);
}

CEventSyncQueue::~CEventSyncQueue()
{
    m_list.RemoveAll();

    ::CloseHandle(m_hGetEvent);
    ::CloseHandle(m_hPutEvent);

    ::DeleteCriticalSection(&m_lock);
}

void CEventSyncQueue::Put(void* ptr)
{
    ::EnterCriticalSection(&m_lock);

    while(m_nCapacity > 0 && m_list.GetCount() >= m_nCapacity)
    {
        ::LeaveCriticalSection(&m_lock);

        //wait
        if(::WaitForSingleObject(m_hPutEvent, INFINITE) != WAIT_OBJECT_0)
        {
            ASSERT(FALSE);
        }

        ::EnterCriticalSection(&m_lock);
    }
    if(m_nCapacity > 0)
    {
        ASSERT(m_list.GetCount() < m_nCapacity);
    }
    m_list.AddTail(ptr);

    ::SetEvent(m_hGetEvent);    //notifyAll
    ::LeaveCriticalSection(&m_lock);
}
void* CEventSyncQueue::Get()
{
    ::EnterCriticalSection(&m_lock);

    while(m_list.IsEmpty())
    {
        ::LeaveCriticalSection(&m_lock);

        //wait
        if(::WaitForSingleObject(m_hGetEvent, INFINITE) != WAIT_OBJECT_0)
        {
            ASSERT(FALSE);
        }

        ::EnterCriticalSection(&m_lock);
    }
    ASSERT(!m_list.IsEmpty());
    void* ptr = m_list.RemoveHead();

    ::SetEvent(m_hPutEvent);    //notifyAll
    ::LeaveCriticalSection(&m_lock);

    return ptr;
}

Answer 1

On second thoughts, it’s hardly necessary to explicitly implement a semaphore. Instead, just think about how you would implement a semaphore using events, and approach your the problem that way. My first attempt used manual-reset events, which was inefficient but manifestly correct, and then I optimized.

Please note that I haven’t debugged (or even compiled!) either of these code fragments, but they should give you the right idea. Here’s the manual-reset version:

class CEventSyncQueue
{
public:
    CEventSyncQueue(int nCapacity = -1);
    virtual ~CEventSyncQueue();
    virtual void Put(void* ptr);
    virtual void* Get();
protected:
    int m_nCapacity;
    CPtrList m_list;

    CRITICAL_SECTION m_lock;    
    HANDLE m_queue_not_empty;
    HANDLE m_queue_not_full;
};

CEventSyncQueue::CEventSyncQueue(int nCapacity)
{
    m_nCapacity = nCapacity;
    ::InitializeCriticalSection(&m_lock);
    m_queue_not_empty = ::CreateEvent(NULL, TRUE, FALSE, NULL);
    m_queue_not_full = ::CreateEvent(NULL, TRUE, TRUE, NULL);
}

CEventSyncQueue::~CEventSyncQueue()
{
    m_list.RemoveAll();
    ::CloseHandle(m_queue_not_empty);
    ::CloseHandle(m_queue_not_full);
    ::DeleteCriticalSection(&m_lock);
}

void CEventSyncQueue::Put(void* ptr)
{
    bool done = false;
    while (!done)
    {
        // If the queue is full, we must wait until it isn't.
        if(::WaitForSingleObject(m_queue_not_full, INFINITE) != WAIT_OBJECT_0)
        {
            ASSERT(FALSE);
        }

        // However, we might not be the first to respond to the event,
        // so we still need to check whether the queue is full and loop
        // if it is.
        ::EnterCriticalSection(&m_lock);
        if (m_nCapacity <= 0 || m_list.GetCount() < m_nCapacity)
        {
            m_list.AddTail(ptr);
            done = true;
            // The queue is definitely not empty.
            SetEvent(m_queue_not_empty);
            // Check whether the queue is now full.
            if (m_nCapacity > 0 && m_list.GetCount() >= m_nCapacity)
            {
                ResetEvent(m_queue_not_full);
            }
        }
        ::LeaveCriticalSection(&m_lock);
    }
}

void* CEventSyncQueue::Get()
{
    void *result = nullptr;
    while (result == nullptr)
    {
        // If the queue is empty, we must wait until it isn't.
        if(::WaitForSingleObject(m_queue_not_empty, INFINITE) != WAIT_OBJECT_0)
        {
            ASSERT(FALSE);
        }

        // However, we might not be the first to respond to the event,
        // so we still need to check whether the queue is empty and loop
        // if it is.
        ::EnterCriticalSection(&m_lock);
        if (!m_list.IsEmpty())
        {
            result = m_list.RemoveHead();
            ASSERT(result != nullptr);
            // The queue shouldn't be full at this point!
            ASSERT(m_nCapacity <= 0 || m_list.GetCount() < m_nCapacity);
            SetEvent(m_queue_not_full);
            // Check whether the queue is now empty.
            if (m_list.IsEmpty())
            {
                ResetEvent(m_queue_not_empty);
            }
        }
        ::LeaveCriticalSection(&m_lock);
    }
}

And here’s the more efficient, auto-reset events version:

class CEventSyncQueue
{
public:
    CEventSyncQueue(int nCapacity = -1);
    virtual ~CEventSyncQueue();
    virtual void Put(void* ptr);
    virtual void* Get();
protected:
    int m_nCapacity;
    CPtrList m_list;

    CRITICAL_SECTION m_lock;    
    HANDLE m_queue_not_empty;
    HANDLE m_queue_not_full;
};

CEventSyncQueue::CEventSyncQueue(int nCapacity)
{
    m_nCapacity = nCapacity;
    ::InitializeCriticalSection(&m_lock);
    m_queue_not_empty = ::CreateEvent(NULL, FALSE, FALSE, NULL);
    m_queue_not_full = ::CreateEvent(NULL, FALSE, TRUE, NULL);
}

CEventSyncQueue::~CEventSyncQueue()
{
    m_list.RemoveAll();
    ::CloseHandle(m_queue_not_empty);
    ::CloseHandle(m_queue_not_full);
    ::DeleteCriticalSection(&m_lock);
}

void CEventSyncQueue::Put(void* ptr)
{
    if (m_nCapacity <= 0)
    {
        ::EnterCriticalSection(&m_lock);
        m_list.AddTail(ptr);
        SetEvent(m_queue_not_empty);
        ::LeaveCriticalSection(&m_lock);
        return;
    }

    bool done = false;
    while (!done)
    {
        // If the queue is full, we must wait until it isn't.
        if(::WaitForSingleObject(m_queue_not_full, INFINITE) != WAIT_OBJECT_0)
        {
            ASSERT(FALSE);
        }

        // However, under some (rare) conditions we'll get here and find
        // the queue is already full again, so be prepared to loop.
        ::EnterCriticalSection(&m_lock);
        if (m_list.GetCount() < m_nCapacity)
        {
            m_list.AddTail(ptr);
            done = true;
            SetEvent(m_queue_not_empty);
            if (m_list.GetCount() < m_nCapacity)
            {
                SetEvent(m_queue_not_full);
            }
        }
        ::LeaveCriticalSection(&m_lock);
    }
}

void* CEventSyncQueue::Get()
{
    void *result = nullptr;
    while (result == nullptr)
    {
        // If the queue is empty, we must wait until it isn't.
        if(::WaitForSingleObject(m_queue_not_empty, INFINITE) != WAIT_OBJECT_0)
        {
            ASSERT(FALSE);
        }

        // However, under some (rare) conditions we'll get here and find
        // the queue is already empty again, so be prepared to loop.
        ::EnterCriticalSection(&m_lock);
        if (!m_list.IsEmpty())
        {
            result = m_list.RemoveHead();
            ASSERT(result != nullptr);
            // The queue shouldn't be full at this point!
            if (m_nCapacity <= 0) ASSERT(m_list.GetCount() < m_nCapacity);
            SetEvent(m_queue_not_full);
            if (!m_list.IsEmpty())
            {
                SetEvent(m_queue_not_empty);
            }
        }
        ::LeaveCriticalSection(&m_lock);
    }
}