Question

I tried to write a program that can have mutiple-threaded reads and writes. It works fine with one read and one write, but it ended in deadlock when I use two reads and one write.

Can anyone help to check?

const int BUF_SIZE = 10;
const int ITERS = 100;

boost::mutex io_mutex;

class buffer
{
public:

    typedef boost::mutex::scoped_lock scoped_lock;

    buffer() : p(0), c(0), full(0)
    {}

    void put(int m)
    {
        scoped_lock lock(mutex);
        if (full == BUF_SIZE)
        {
            {
                boost::mutex::scoped_lock lock(io_mutex);
                std::cout << "Buffer is full. Waiting..." << std::endl;
            }
            while (full == BUF_SIZE)
                cond.wait(lock);
        }

        buf[p] = m;

        p = (p+1) % BUF_SIZE;

        ++full;

        cond.notify_all();
    }

    int get()
    {
        scoped_lock lk(mutex);

        if (full == 0)
        {
            {
                boost::mutex::scoped_lock lock(io_mutex);
                std::cout << "Buffer is empty. Waiting..." << std::endl;
            }

            while (full == 0)
                cond.wait(lk);
        }

        int i = buf[c];

        c = (c+1) % BUF_SIZE;

        --full;

        cond.notify_one();

        return i;
    }

private:
    boost::mutex mutex;

    boost::condition cond;

    unsigned int p, c, full;

    int buf[BUF_SIZE];
};

buffer buf;

void writer()
{
    for (int n = 0; n < ITERS; ++n)
    {
        {
            boost::mutex::scoped_lock lock(io_mutex);
            std::cout << "sending: " << n << std::endl;
        }
        buf.put(n);
    }
}

void reader()
{
    for (int x = 0; x < ITERS; ++x)
    {
        int n = buf.get();
        {
            boost::mutex::scoped_lock lock(io_mutex);
            std::cout << "reader1: received: " << n <<  std::endl;
        }
    }
}

int main(int argc, char* argv[])
{
    boost::thread thrd1(&reader);
    boost::thread thrd2(&writer);
    boost::thread thrd3(&reader);

    std::string str;

    thrd1.join();
    thrd2.join();
    thrd3.join();

    std::getline(std::cin,str);
}

Answer 1

You possibly lost the forest for the trees a bit here. Your code writes ITERS values but tries to read 2 * ITERS values since you have two readers. That can’t work.

You’ll at least have to write 2 * ITERS values to get the program to complete.