Question

Following code is only to show how to use condition variable to synchronize threads(one producer and many consumers) as exercising.
See the line for code ‘usleep(100);’.When I comment this line,two consumer threads seem got the mutex locked at the same time after the producer thread exit,and then wait on ‘cond’,this raise race condition.
But if I uncomment it.everything goes well(seems).

My question is how can two consumer threads lock one mutex at the same time?This demo should also work even after I don’t call usleep()?Thanks for your time in advance.

This is the output after usleep removed from loops in producer.Please note the last two ‘lock’ in the output.

$ ./pthread_cond 
Producer 3062414192 beginning...
Producer locked mutex self:3062414192
Producer is creating work:1
Producer finished creating work:1
Producer unlock self:3062414192
Producer locked mutex self:3062414192
Producer is creating work:2
Producer finished creating work:2
Producer unlock self:3062414192
Producer locked mutex self:3062414192
Producer is creating work:3
Producer finished creating work:3
Producer unlock self:3062414192
Producer locked mutex self:3062414192
Producer is creating work:4
Producer finished creating work:4
Producer unlock self:3062414192
Producer 3062414192 exit after creating 4 works...
produce joined,but 4 work remained
Consumer 3070806896 beginning...
Consumer locked mutex self:3070806896
to wait on cond self:3070806896
Consumer 3079199600 beginning...
Consumer locked mutex self:3079199600
to wait on cond self:3079199600

Implemented:

#include <pthread.h>
#include <stdio.h>
#include <stdlib.h>
#include <assert.h>

#define MAX_COUSUMER 2

#define TOTAL_WORK 4

int g_work_counter=0;

pthread_mutex_t mut = PTHREAD_MUTEX_INITIALIZER;
pthread_cond_t cond=PTHREAD_COND_INITIALIZER;

void *producer_thread(void* arg)
{
    int i;

    printf("Producer %lu beginning...\n",pthread_self());
    for(i=0;i<TOTAL_WORK;i++)
    {
        assert(pthread_mutex_lock(&mut)==0);
        printf("Producer locked mutex self:%lu\n",pthread_self());
        printf("Producer is creating work:%d\n",g_work_counter+1);
        g_work_counter++;
        printf("Producer finished creating work:%d\n",g_work_counter);
        pthread_cond_broadcast(&cond);
        assert(pthread_mutex_unlock(&mut)==0);
        printf("Producer unlock self:%lu\n",pthread_self());

        //usleep(100);
    }

    printf("Producer self:%lu exit after creating %d works...\n",pthread_self(),i);//counter starts from 0
    pthread_exit(NULL);
}

void *consumer_thread(void *arg)
{
    printf("Consumer %lu beginning...\n",pthread_self());
    //use pthread_cancel in main
    pthread_detach(pthread_self());

    while(1)
    {
        assert(pthread_mutex_lock(&mut)==0);
        printf("Consumer locked mutex self:%lu\n",pthread_self());
        printf("to wait on cond self:%lu\n",pthread_self());
        assert(pthread_cond_wait(&cond,&mut)==0);
        if(g_work_counter)
        {
            printf("Consumer %lu is performing work:%d\n",pthread_self(),g_work_counter);
            g_work_counter--;
            printf("Consumer %lu finished performing work:%d\n",pthread_self(),g_work_counter+1);
        }
        assert(pthread_mutex_unlock(&mut)==0);
        printf("Consumer unlock self:%lu\n",pthread_self());
    }

    //no output (pthread_cancel is called)
    printf("Consumer %lu exit...\n",pthread_self());
    pthread_exit(NULL);
}

int main(int argc,char* argv[])
{
    pthread_t producer;
    pthread_t consumers[MAX_COUSUMER];
    int i;

    for(i=0;i<MAX_COUSUMER;i++)
    {
        if(pthread_create(&consumers[i],NULL,consumer_thread,NULL)!=0)
        {
            printf("pthread_create failed for consumer_thread %d\n",i);
        }
    }

    pthread_create(&producer,NULL,producer_thread,NULL);

    if(pthread_join(producer,NULL)!=0)
    {
        printf("pthread_join failed for producer_thread %lu\n",consumers[i]);
    }
    printf("producer joined,but %d work remained\n",g_work_counter);

    //wait for the consumers
    while(g_work_counter>0)
        ;

    //cancel the consumer,for they are detached
    for(i=0;i<MAX_COUSUMER;i++)
    {
        if(pthread_cancel(consumers[i])!=0)
        {
            printf("pthread_cancel failed for consumer_thread %d\n",i);
        }
    }

    pthread_mutex_destroy(&mut);
    pthread_cond_destroy(&cond);
    return 0;
}

Answer 1

When a thread waits on a condition, it releases the lock. When it is woken up, it reacquires it. In this code, a consumer should only wait if the buffer is empty.

Another problem lies in main, actually, with this line: while(g_work_counter>0). You don’t have the lock at that point, so it’s not safe to check g_work_counter. I’m also not too sure about pthread_detach(pthread_self());. Shouldn’t that be called by main on its own child?

As a general note, if you want to check for deadlocks and otherwise debug your pthreads code, you should use the pthread_mutexattr_foo functions to set up an error-checking mutex, and check the return value with more than just an assert(==0).