Using Condition Variables

The following code implements a producer/consumer queue. The queue is represented as a bounded circular buffer, and is protected by a critical section. The code uses two condition variables: one used by producers (BufferNotFull) and one used by consumers (BufferNotEmpty).

The code calls the InitializeConditionVariable function to create the condition variables. The consumer threads call the SleepConditionVariableCS function to wait for items to be added to the queue and the WakeConditionVariable function to signal the producer that it is ready for more items. The producer threads call SleepConditionVariableCS to wait for the consumer to remove items from the queue and WakeConditionVariable to signal the consumer that there are more items in the queue.

Windows Server 2003 and Windows XP:  Condition variables are not supported.


#include <windows.h>
#include <stdlib.h>
#include <stdio.h>

#define BUFFER_SIZE 10
#define PRODUCER_SLEEP_TIME_MS 500
#define CONSUMER_SLEEP_TIME_MS 2000

LONG Buffer[BUFFER_SIZE];
LONG LastItemProduced;
ULONG QueueSize;
ULONG QueueStartOffset;

ULONG TotalItemsProduced;
ULONG TotalItemsConsumed;

CONDITION_VARIABLE BufferNotEmpty;
CONDITION_VARIABLE BufferNotFull;
CRITICAL_SECTION   BufferLock;

BOOL StopRequested;

DWORD WINAPI ProducerThreadProc (PVOID p)
{
    ULONG ProducerId = (ULONG)(ULONG_PTR)p;

    while (true)
    {
        // Produce a new item.

        Sleep (rand() % PRODUCER_SLEEP_TIME_MS);

        ULONG Item = InterlockedIncrement (&LastItemProduced);

        EnterCriticalSection (&BufferLock);

        while (QueueSize == BUFFER_SIZE && StopRequested == FALSE)
        {
            // Buffer is full - sleep so consumers can get items.
            SleepConditionVariableCS (&BufferNotFull, &BufferLock, INFINITE);
        }

        if (StopRequested == TRUE)
        {
            LeaveCriticalSection (&BufferLock);
            break;
        }

        // Insert the item at the end of the queue and increment size.

        Buffer[(QueueStartOffset + QueueSize) % BUFFER_SIZE] = Item;
        QueueSize++;
        TotalItemsProduced++;

        printf ("Producer %u: item %2d, queue size %2u\r\n", ProducerId, Item, QueueSize);

        LeaveCriticalSection (&BufferLock);

        // If a consumer is waiting, wake it.

        WakeConditionVariable (&BufferNotEmpty);
    }

    printf ("Producer %u exiting\r\n", ProducerId);
    return 0;
}

DWORD WINAPI ConsumerThreadProc (PVOID p)
{
    ULONG ConsumerId = (ULONG)(ULONG_PTR)p;

    while (true)
    {
        EnterCriticalSection (&BufferLock);

        while (QueueSize == 0 && StopRequested == FALSE)
        {
            // Buffer is empty - sleep so producers can create items.
            SleepConditionVariableCS (&BufferNotEmpty, &BufferLock, INFINITE);
        }

        if (StopRequested == TRUE && QueueSize == 0)
        {
            LeaveCriticalSection (&BufferLock);
            break;
        }

        // Consume the first available item.

        LONG Item = Buffer[QueueStartOffset];

        QueueSize--;
        QueueStartOffset++;
        TotalItemsConsumed++;

        if (QueueStartOffset == BUFFER_SIZE)
        {
            QueueStartOffset = 0;
        }

        printf ("Consumer %u: item %2d, queue size %2u\r\n", 
            ConsumerId, Item, QueueSize);

        LeaveCriticalSection (&BufferLock);

        // If a producer is waiting, wake it.

        WakeConditionVariable (&BufferNotFull);

        // Simulate processing of the item.

        Sleep (rand() % CONSUMER_SLEEP_TIME_MS);
    }

    printf ("Consumer %u exiting\r\n", ConsumerId);
    return 0;
}

int main ( void )
{
    InitializeConditionVariable (&BufferNotEmpty);
    InitializeConditionVariable (&BufferNotFull);

    InitializeCriticalSection (&BufferLock);

    DWORD id;
    HANDLE hProducer1 = CreateThread (NULL, 0, ProducerThreadProc, (PVOID)1, 0, &id);
    HANDLE hConsumer1 = CreateThread (NULL, 0, ConsumerThreadProc, (PVOID)1, 0, &id);
    HANDLE hConsumer2 = CreateThread (NULL, 0, ConsumerThreadProc, (PVOID)2, 0, &id);

    puts ("Press enter to stop...");
    getchar();

    EnterCriticalSection (&BufferLock);
    StopRequested = TRUE;
    LeaveCriticalSection (&BufferLock);

    WakeAllConditionVariable (&BufferNotFull);
    WakeAllConditionVariable (&BufferNotEmpty);

    WaitForSingleObject (hProducer1, INFINITE);
    WaitForSingleObject (hConsumer1, INFINITE);
    WaitForSingleObject (hConsumer2, INFINITE);

    printf ("TotalItemsProduced: %u, TotalItemsConsumed: %u\r\n", 
        TotalItemsProduced, TotalItemsConsumed);
    return 0;
}


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