multithreading thread switching issue

Question

I have a producer and consumer thread that are being created from main. They perform correctly well, except for the cout statement

class myclass{
int x;
// stuff
}

void foo1(myclass* ob){
setX(ob->x);
// stuff
}

void foo2(myclass* ob){

cout << ob->x << endl; // ONLY THIS DOESN'T EXECUTE

ob->getX();
// stuff
}

int main(){
myclass* ob = new myclass();
boost::thread producer_thread(boost::bind(foo1, ob));
boost::thread consumer_thread(boost::bind(foo2, ob));
// stuff

producer_thread.join();
consumer_thread.join();

}

Everything works fine (including showX that displays x, except for the cout. What's wrong?

Answer 1

Your threads are sharing the object without actually any lock on it. producer is not exiting before consumer started accessing the object. Using producer_thread.join() before boost::thread consumer_thread(boost::bind(foo2, ob)) should resolve this, which is not the best fix. Using mutex locks will be ideal.

Answer 2

Adding to the previous answer, you can also use a state variable or condition variable to ensure that your object is not getting written upon / processed by a thread when some other thread is working on it. In other words, you can have a state variable whose value is changed by each function to a unique number upon completion and each function will start operating when the state variable is assumes the value that the previous function is supposed to set it to.