How to synchronize threads (consumer/producer)

Question

I have the following code:

#include <stdio.h> #include <stdlib.h> #include <pthread.h> #include <queue> using namespace std; queue<int> myqueue; pthread_mutex_t count_mutex = PTHREAD_MUTEX_INITIALIZER; pthread_cond_t condition_var = PTHREAD_COND_INITIALIZER; void *consumer(void*); void *producer(void*); #define COUNT_DONE 10 int count = 0; main() { pthread_t thread1, thread2; pthread_create( &thread2, NULL, &consumer, NULL); pthread_create( &thread1, NULL, &producer, NULL); pthread_join( thread1, NULL); pthread_join( thread2, NULL); printf("Final count: %d\n",count); system("PAUSE"); return EXIT_SUCCESS; } void *consumer(void*) { for(;;) { // Lock mutex and then wait for signal to relase mutex printf("consumer mutex lock \n"); pthread_mutex_lock( &count_mutex ); printf("consumer mutex locked\n"); // Wait while functionCount2() operates on count // mutex unlocked if condition varialbe in functionCount2() signaled. printf("consumer wait\n"); pthread_cond_wait( &condition_var, &count_mutex ); printf("consumer condition woke up\n"); myqueue.pop();count--; printf("Counter value consumer: %d\n",count); printf("consumer mutex unlock\n"); pthread_mutex_unlock( &count_mutex ); if(count >= COUNT_DONE) return(NULL); } } void * producer(void*) { for(;;) { printf("producer mutex lock\n"); pthread_mutex_lock( &count_mutex ); printf("producer mutex locked\n"); if( count < COUNT_DONE) { myqueue.push(1); count++; printf("Counter value producer: %d\n",count); printf("producer signal\n"); pthread_cond_signal( &condition_var ); } printf("producer mutex unlock\n"); pthread_mutex_unlock( &count_mutex ); if(count >= COUNT_DONE) return(NULL); Sleep(5000); } }

This is example works fine when consumer thread takes the mutex first. But then when the producer thread acquires the mutex initially at first, I will always have 1 integer in the queue that the consumer can't pop.

How can I let the consumer thread, initially acquire the mutex before the producer.

Note: I am looking for a better way than launching one thread before the other.

Thanks,

kfsone · Accepted Answer · 2013-05-20 01:24:16Z

One problem I see is that your consumer doesn't actually check for work to do, it just blindly pops from the queue.

The second problem I see is that you increment count in one and decrement it in the other, so how do you ever reach the termination condition?

Take the ninja "count--" out of consumer and it should work. Still, you might want to do the following inside consumer:

// Wait for producer to do its thing and tell us there is work to do. while ( myqueue.empty() ) { pthread_cond_wait(&condition_var, &count_mutex); } // we've been told there's work to do with the queue, // and we know there's something ON the queue. // consume the entire queue. while ( !myqueue.empty() ) { myqueue.pop(); } // treat count as protected by the mutex, so hoist this test into the lock. bool workDone = (count >= COUNT_DONE); pthread_mutex_unlock(&count_mutex); if(workDone) return break;

edit: preferred version of consumer:

bool workDone = false; while(workDone == false) { // Lock mutex and then wait for signal to relase mutex pthread_mutex_lock( &count_mutex ); // Wait for producer to do its thing and tell us there is work to do. while ( myqueue.empty() ) pthread_cond_wait( &condition_var, &count_mutex ); // we've been told there's work to do with the queue, // and we know there's something ON the queue. // consume the entire queue. while ( myqueue.empty() == false ) { myqueue.pop(); } // count is protected by the lock so check if we're done before we unlock. workDone = (count >= COUNT_DONE); pthread_mutex_unlock( &count_mutex ); } return NULL;

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