Question

I intent to have two threads waiting for signals from a third thread.

These two threads do the same work but only one of them gets the signal at a time. Once a certain condition is meet(number of signal captured), they terminate themselves.

Then in the end the main thread cancel the third thread.

I got deadlock, but could not figure out where is the problem.

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

pthread_mutex_t mutex = PTHREAD_MUTEX_INITIALIZER;
pthread_cond_t cond = PTHREAD_COND_INITIALIZER;

int n = 0;
int count = 0;

void* func1(void *ptr)
{
    while(1)
    {
        pthread_mutex_lock(&mutex);

        // wait for func3 to signal
        pthread_cond_wait(&cond, &mutex);
        count++;

        if(count > 10)
        {
            printf("number %d terminate func1\n", n);
            return (NULL);
        }
        else
        {
            printf("func1 got number:%d\n", n);
        }
        pthread_mutex_unlock(&mutex);
    }
}

void* func2(void *ptr)
{
    while(1)
    {
        pthread_mutex_lock(&mutex);

        // wait for func3 to signal
        pthread_cond_wait(&cond, &mutex);
        count++;

        if(count > 10)
        {
            printf("number %d terminate func2\n", n);
            return (NULL);
        }
        else
        {
            printf("func2 got number:%d\n", n);
        }
        pthread_mutex_unlock(&mutex);
    }
}    

void* func3(void *ptr)
{
    while(1)
    {
        pthread_mutex_lock(&mutex);
        n++;
        pthread_cond_signal(&cond);
        pthread_mutex_unlock(&mutex);
    }
}


int main(int argc, char *argv[])
{
    pthread_t t1, t2, t3;

    pthread_create(&t1, NULL, func1, NULL);
    pthread_create(&t2, NULL, func2, NULL);
    pthread_create(&t3, NULL, func3, NULL);

    pthread_join(t1, NULL);
    pthread_join(t2, NULL);

    pthread_cancel(t3);

    return 0;
}

Answer 1

You’re not unlocking the mutex when func1 or func2 exits.