Question

I’m creating n threads & then starting then execution after a barrier breakdown.

In global data space:

int bkdown = 0;

In main():

pthread_barrier_init(&bar,NULL,n);

for(i=0;i<n;i++)
{
pthread_create(&threadIdArray[i],NULL,runner,NULL);
if(i==n-2)printf("breakdown imminent!\n");
if(i==n-1)printf("breakdown already occurred!\n");
}

In thread runner function:

void *runner(void *param)
{
 pthread_barrier_wait(&bar);

 if(bkdown==0){bkdown=1;printf("barrier broken down!\n");}

        ...

 pthread_exit(NULL);
}

Expected order:

breakdown imminent!
barrier broken down!
breakdown already occurred!

Actual order: (tested repeatedly)

breakdown imminent!
breakdown already occurred!
barrier broken down!!

Could someone explain why the I am not getting the "broken down" message before the "already occurred" message?

Answer 1

for(i=0;i<n;i++)
{
pthread_create(&threadIdArray[i],NULL,runner,NULL);
if(i==n-2)printf("breakdown imminent!\n");
if(i==n-1)printf("breakdown already occurred!\n");
}

Nothing stops this loop from executing until i == n-1 . pthread_create() just fires off a thread to be run. It doesn’t wait for it to start or end. Thus you’re at the mercy of the scheduler, which might decide to continue executing your loop, or switch to one of the newly created threads (or do both, on a SMP system).

You’re also initalizing the barrier to n, so in any case none of the threads will get past the barrier until you’ve created all of them.