Thread Safe queue in C++

Question

Is this the correct way to make a Thread Safe Queue in C++ which can handle unsigned char* arrays of binary data?

Notice that in the data is produced from the main thread and not a created pthread, which makes me question if the pthread_mutex_t will actually work correctly on the push and pop.

Thread Safe Queue

#include <queue>
#include <pthread.h>

class ts_queue
{

private:

    std::queue<unsigned char*> _queue_;
    pthread_mutex_t mutex;
    pthread_cond_t cond;

public:

    ts_queue()
    {
        pthread_mutex_init(&mutex, NULL);
        pthread_cond_init(&cond, NULL);
    }

    void push(unsigned char* data)
    {
        pthread_mutex_lock(&mutex);

        _queue_.push(data);

        pthread_cond_signal(&cond);
        pthread_mutex_unlock(&mutex);
    }

    void pop(unsigned char** popped_data)
    {
        pthread_mutex_lock(&mutex);

        while (_queue_.empty() == true)
        {
            pthread_cond_wait(&cond, &mutex);
        }

        *popped_data = _queue_.front();
        _queue_.pop();

        pthread_mutex_unlock(&mutex);
    }
};

CONSUMER TEST:

void *consumer_thread(void *arguments)
{
    ts_queue *tsq = static_cast<ts_queue*>(arguments);

    while (true)
    {
        unsigned char* data = NULL;

        tsq->pop(&data);

        if (data != NULL)
        {
            // Eureka! Received from the other thread!!!
            // Delete it so memory keeps free.
            // NOTE: In the real scenario for which I need
            // this class, the data received are bitmap pixels
            // and at this point it would be processed
            delete[] data;
        }
    }

    return 0;
}

PRODUCER TEST:

void main()
{
    ts_queue tsq;

    // Create the consumer
    pthread_t consumer;
    pthread_create(&consumer, NULL, consumer_thread, &tsq));

    // Start producing
    while(true)
    {
        // Push data. 
        // Expected behaviour: memory should never run out, as the
        // consumer should receive the data and delete it.
        // NOTE: test_data in the real purpose scenario for which I 
        // need this class would hold bitmap pixels, so it's meant to 
        // hold binary data and not a string

        unsigned char* test_data = new unsigned char [8192];
        tsq.push(test_data);
    }

    return 0;
}

Answer 1

How do you know the consumer never gets the data? When I try your program out, I get a segmentation fault, and GDB tells me the consumer did get a pointer, but it's an invalid one.

I believe your problem is that you have a data race on the _queue_ member. push() calls _queue_.push(data) (a write on _queue_ ) while holding push_mutex and pop() calls _queue_.front() (a read on _queue_ ) and _queue_.pop() (another write on _queue_ ) while holding pop_mutex , but push() and pop() can occur at the same time, causing both threads to be writing (and reading) _queue_ at the same time, a classical data-race.