CODEWITHSUNDEEP
c++cmultithreadingpthreads
user2847598
user2847598

Reputation: 1299

wait for all threads finishing one job then doing another

Suppose there are NUM_TREADS threads and they have to complete job1() before doing job2(). How to guarantee this to happen, like this:

void thread_func(void *arg)
{
    while(1) {
        job1();
        some_kind_of_waiting();
        job2();
    }
    return NULL;
}

Will a semaphore like following work or are there any other/better solutions?

{
    static int done;
    static sem_t semaphore;

    if(__sync_fetch_and_add(&done, 1) == THREAD_NUMS-1) {
        done = 0;
        for(i = 0; i < THREAD_NUMS-1; i++)
            sem_post(&semaphore);
    } else
        sem_wait(&semaphore);
}

Thanks.

Upvotes: 0

Views: 160

Answers (2)

caf
caf

Reputation: 239011

This precisely the problem that pthreads barriers are intended to solve. Initialise the barrier with NUM_THREADS (in your main function, prior to spawning the threads):

pthread_barrier_t barrier;

pthread_barrier_init(&barrier, NULL, NUM_THREADS);

and use pthread_barrier_wait() to synchronise:

void *thread_func(void *arg)
{
    while(1) {
        job1();
        pthread_barrier_wait(&barrier);
        job2();
    }
    return NULL;
}

If you also need the threads to wait until all other threads have compelted job2() before any of them can start on job1() again, you can add a second wait on the barrier:

void *thread_func(void *arg)
{
    while(1) {
        job1();
        pthread_barrier_wait(&barrier);
        job2();
        pthread_barrier_wait(&barrier);
    }
    return NULL;
}

Upvotes: 3

eerorika
eerorika

Reputation: 238311

You describe a high level concurrency control structure typically called by the name barrier. The C++ standard library has no implementation of barrier although it has been proposed and will hopefully be part of C++ standard library in the future.

Until the standard library provides a solution, you can of course implement the barrier yourself (you can use a condition variable), use a platform specific threading API or a wrapper library.

Upvotes: 1

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