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

You give your threads (producer & consumer) the mutex object and then detach them. They are supposed to run forever. Then you exit from your program and the mutex object is no longer valid. Nevertheless your threads still try to use it, they don’t know that it is no longer valid. If you had used the NDEBUG define you would have got a coredump.

Are you trying to write a daemon application and this is the reason for detaching threads?