More mac and linux -- parallelization

My company just bought a bunch of 8-core workstations -- it is high time to work on programs that take advantage of a modern computing environment.

I had written a renderer that used pthread and semaphores to partition and then manage the job, so I thought I'd see if I could do the same thing on my mac.  Unfortunately, I had used unnamed semaphores (sem_init(...)) on my Linux boxes, and the program failed (all threads appeared to be finished immediately) on the Leopard box.

Turns out that when you examine the error return, sem_init() has an entry point and prototype in the library and header file respectively, but it's actually "Not Implemented".  So, I switched over to named semaphores, and it all works perfectly.

In particular, on my 8-core box, I was getting some 6x speedup.

// start the threads
for(thread_id = 0; thread_id < n_proc; thread_id++) {
        char    sem_name[64];
        printf("Starting thread %d\n", thread_id);
        sprintf(sem_name, "sem_%d\n", thread_id);
        if((semaphore[thread_id] = sem_open(sem_name, O_CREAT, 0777, 0)) == SEM_FAILED) {
                printf("sem_open failed\n");
                perror("sem_open error: ");
                return -1;
        }
        ret = pthread_create(&thread[thread_id], NULL, (void *)median_scanlines, (void *)thread_id);
}

// wait for all threads to finish -- when thread is done, it calls sem_post(), then sem_wait below will unblock
 for(thread_id = 0; thread_id < n_proc; thread_id++) {
        printf("Waiting for thread %d to finish\n", thread_id);
        sem_wait(semaphore[thread_id]);
}