Using boost::lock_guard for simple shared data locking

Question

I am a newcomer to the Boost library, and am trying to implement a simple producer and consumer threads that operate on a shared queue. My example implementation looks like this:

#include <iostream>
#include <deque>
#include <boost/thread.hpp>

boost::mutex mutex;
std::deque<std::string> queue;

void producer() 
{
    while (true) {
        boost::lock_guard<boost::mutex> lock(mutex);

        std::cout << "producer() pushing string onto queue" << std::endl;

        queue.push_back(std::string("test"));
    }
}

void consumer()
{
    while (true) {
        boost::lock_guard<boost::mutex> lock(mutex);

        if (!queue.empty()) {
            std::cout << "consumer() popped string " << queue.front() << " from queue" << std::endl;

            queue.pop_front();
        }
    }
}

int main()
{
    boost::thread producer_thread(producer);
    boost::thread consumer_thread(consumer);

    sleep(5);

    producer_thread.detach();
    consumer_thread.detach();

    return 0;
}

This code runs as I expect, but when main exits, I get

/usr/include/boost/thread/pthread/mutex.hpp:45:    
    boost::mutex::~mutex(): Assertion `!pthread_mutex_destroy(&m)' failed.
consumer() popped string test from queue
Aborted

(I'm not sure if the output from consumer is relevant in that position, but I've left it in.)

Am I doing something wrong in my usage of Boost?

Answer 1

A bit off-topic but relevant imo (...waits for flames in comments).

The consumer model here is very greedy, looping and checking for data on the queue continually. It will be more efficient (waste less CPU cycles) if you have your consumer threads awakened determistically when data is available, using inter-thread signalling rather than this lock-and-peek loop. Think about it this way: while the queue is empty, this is essentially a tight loop only broken by the need to acquire the lock. Not ideal?

void consumer()
{
    while (true) {
        boost::lock_guard<boost::mutex> lock(mutex);

        if (!queue.empty()) {
            std::cout << "consumer() popped string " << queue.front() << " from queue" << std::endl;

            queue.pop_front();
        }
    }
}

I understand that you are learning but I would not advise use of this in 'real' code. For learning the library though, it's fine. To your credit, this is a more complex example than necessary to understand how to use the lock_guard, so you are aiming high!

Eventually you will most likely build (or better if available, reuse) code for a queue that signals workers when they are required to do work, and you will then use the lock_guard inside your worker threads to mediate accesses to shared data.