CODEWITHSUNDEEP
cmultithreadingpthreadsmutexrace-condition
Yves
Yves

Reputation: 12371

Why doesn't the pthread condition variable work

I'm coding to simulate a selling-ticket situation:
Two threads are selling some tickets one by one and one thread is to show sold out when the number of the tickets is 0.
So I try to use a condition variable. Here is the code:

int a = 10;
pthread_mutex_t mtx;
pthread_cond_t cond;


void * funca(void *p)
{
    while(1)
    {
        pthread_mutex_lock(&mtx);
        if (a == 0)
        {
            break;
        }
        a--;
        pthread_mutex_unlock(&mtx);
        sleep(1);
    }
    pthread_mutex_unlock(&mtx);
    pthread_cond_broadcast(&cond);
    return NULL;
}

void * funcb(void *p)
{
    while(1)
    {
        pthread_mutex_lock(&mtx);
        if (a == 0)
        {
            break;
        }
        a--;
        pthread_mutex_unlock(&mtx);
        sleep(1);
    }
    pthread_mutex_unlock(&mtx);
    pthread_cond_broadcast(&cond);
    return NULL;
}

void * funcc(void *p)
{
    pthread_mutex_lock(&mtx);
    while(a != 0)
    {
            pthread_cond_wait(&cond, &mtx);
            printf("I'm nothing");
    }
    pthread_mutex_unlock(&mtx);
    printf("sold out\n");
    return NULL;
}

However, when I execute the code above, I get a infinite loop with many I'm nothing printed.

Upvotes: 0

Views: 90

Answers (1)

caf
caf

Reputation: 239011

You haven't initialised your mutex and condition variable. When you declare the variables, use:

pthread_mutex_t mtx = PTHREAD_MUTEX_INITIALIZER;
pthread_cond_t cond = PTHREAD_COND_INITIALIZER;

There's also no point having two identical functions for funca() and funcb(), you can just have two different threads execute the same function.

Upvotes: 3

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