c++ - How to synchronize threads (consumer/producer) -

i have 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 , wait signal 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 example works fine when consumer thread takes mutex first. when producer thread acquires mutex @ first, have 1 integer in queue consumer can't pop.

how can let consumer thread, acquire mutex before producer.

note: looking better way launching 1 thread before other.

thanks,

one problem see consumer doesn't check work do, blindly pops queue.

the second problem see increment count in 1 , decrement in other, how ever reach termination condition?

take ninja "count--" out of consumer , should work. still, might want following inside consumer:

// wait producer thing , tell there work do. while ( myqueue.empty() ) {     pthread_cond_wait(&condition_var, &count_mutex); } // we've been told there's work queue, // , know there's on queue. // consume entire queue. while ( !myqueue.empty() ) {   myqueue.pop(); }  // treat count protected mutex, hoist test 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 , wait signal relase mutex     pthread_mutex_lock( &count_mutex );      // wait producer thing , tell there work do.     while ( myqueue.empty() )     pthread_cond_wait( &condition_var, &count_mutex );      // we've been told there's work queue,     // , know there's on queue.     // consume entire queue.     while ( myqueue.empty() == false ) {         myqueue.pop();     }      // count protected lock check if we're done before unlock.     workdone = (count >= count_done);     pthread_mutex_unlock( &count_mutex );  } return null;