Question

I’m having a little trouble with pthreads. Basically, I want to catch a SIGINT and have all threads cleanup and exit. What I have (skeleton code):

main.c:

sig_atomic_t running;

void handler(int signal_number)
{
    running = 0;
}

int main(void)
{
    queue job_queue = new_job_queue();

    running = 1;

    struct sigaction sa;
    memset(&sa, 0, sizeof(sa));
    sa.sa_handler = &handler;
    sigaction(SIGINT, &sa, NULL);

    /* create a bunch of threads */
    init_threads(&job_queue);

    while(running) {
        /* do stuff */
    }

    cleanup();

    return (0);
}

threads.c

extern sig_atomic_t running;
pthread_mutex_t queue_mutex = PTHREAD_MUTEX_INITIALIZER;
sem_t queue_count;

void init_threads(queue *q)
{
    int numthreads = 12; /* say */

    sem_init (&queue_count, 0, 0);

    pthread_t worker_threads[numthreads];

    int i;
    for(i=0;i<numthreads;i++)
        pthread_create(&worker_threads[i], NULL, &thread_function, q);
}

void * thread_function(void *args)
{
    pthread_detatch(pthread_self());

    queue *q = (queue *)args;

    while(running) {

        job *j = NULL;

        sem_wait(&queue_count);

        pthread_mutex_lock(&queue_mutex);
        j = first_job_in_queue(q);
        pthread_mutex_unlock(&queue_mutex);

        if(j) {
            /*do something*/
        }

    }

    return (NULL);
}

I am having little luck with this. Since you’re not guarenteed which thread gets the signal I thought this was a good way to go. But I am having a problem where sem_wait() in threads.c is hanging, which is expected but not desired. The while(running) loop in threads.c seems redundant. Should I maybe do a pthread_kill() to all the threads from main? Any obvious problems with the above skeleton code? Is there a better/easier way to go about doing this?

Thanks.

Answer 1

What you can do is to call sem_post() from the handler until all threads are unlocked. In the thread function, immediately after sem_wait() you should check the value of the running variable and if it’s zero break breom the while.

The code in the handler could be something like the following:

int sval;    
sem_getvalue(&queue_count, &sval);
while (sval < 0) {
    sem_post(&queue_count);
    sem_getvalue(&queue_count, &sval);
}

Of course return values should be verified for errors