synchronization error in producer consumer using pthreads and semaphore

Question

I have written a simple producer consumer using pthreads and semaphores. I am getting out of order(consumer consuming before producer has produced) output sometimes. Please help me in finding the problem . I have verified the logic using various sources and tutorials but still getting undesired results.

    #include <iostream>
    #include <pthread.h>
    #include <semaphore.h>
    #include <string>
    #include <sstream>
    #include <unistd.h>
    #include "pthread_barrier.hpp"

    sem_t empty;
    sem_t full;
    sem_t lock;
    pthread_mutex_t wlock;

    pthread_barrier_t pbarrier;
    pthread_barrier_t cbarrier;

    pthread_attr_t tattr;

    #define BUFF_SIZE 100

    volatile bool buffer[BUFF_SIZE];
    int prodIterator = 0;
    int consIterator = 0;

    void *Producer(void *args)
    {   
        pthread_barrier_wait(&pbarrier);
        while(1) {
            sem_wait(&empty);
            sem_wait(&lock);
            buffer[prodIterator] = true;
            pthread_mutex_lock(&wlock);
            std::stringstream str;
            std::cout<<"producer produced = "<<prodIterator<<"\n";
            pthread_mutex_unlock(&wlock);
            prodIterator = (++prodIterator)% BUFF_SIZE;
            sem_post(&lock);
            sem_post(&full);
            sleep(1);
        }
    }

    void *Consumer(void *args)
    {
        pthread_barrier_wait(&cbarrier);
        while(1) {
            sem_wait(&full);
            sem_wait(&lock);
            buffer[consIterator] = false;
            pthread_mutex_lock(&wlock);
            std::cout<<"Consumer consumed = "<<consIterator<<"\n";
            pthread_mutex_unlock(&wlock);
            consIterator = (++consIterator)% BUFF_SIZE;
            sem_post(&lock);
            sem_post(&empty);
            sleep(1);
        }   
    }


    int main()
    {
        sem_init(&empty, 0, BUFF_SIZE);
        sem_init(&full, 0, 0);
        sem_init(&lock, 0, 1);

        pthread_mutex_init(&wlock, NULL);

        pthread_t prod[10];
        pthread_t cons[10];

        unsigned pcount = 5;
        unsigned ccount = 2;
        pthread_barrier_init(&pbarrier, NULL, pcount);
        pthread_barrier_init(&cbarrier, NULL, ccount);

        pthread_attr_init(&tattr);
        pthread_attr_setdetachstate(&tattr, PTHREAD_CREATE_DETACHED);

        for(int i=0;i<5;i++) {
            pthread_create(&prod[i], &tattr, Producer, NULL);
        }

        for(int i=0;i<2;i++) {
            pthread_create(&cons[i], &tattr, Consumer, NULL);
        }

        pthread_exit(NULL);
    }

Answer 1

I have found out the problem. I am using mac osx and mac osx does not support unnmaed emaphores. It only supports named semaphores. So in place of using unnamed semaphores in my code, i needed to replace it with named semaphores to make the code work. OSX has very poor support for pthreads. It also does not support pthread barriers. So i needed to replace the semaphore init lines with following:

           full = sem_open("/semaphore1", O_CREAT, 0777, 1);
           empty = sem_open("/semaphore2", O_CREAT, 0777, 1);
           lock = sem_open("/semaphore3", O_CREAT, 0777, 1);

           and the declaration of semaphore variables needed be replace by :

           sem_t *empty;
           sem_t *full;
           sem_t *lock;

           very poor support indeed for pthreads in mac osx