In our working producer and consumer solutions with a single buffer slot using condition variables, we used two condition variables. Why was the second condition variable necessary?

When using a semaphore to ensure the child thread runs before the parent executes further as in the code snippet below, what value should the semaphore be initialized too (i.e., what should X be)?

sem_t s;
void *child(void *arg) {
  printf("child");
  sem_post(&s); // signal here: child is done
  return NULL;
}
   
int main(int argc, char *argv[]) {
  sem_init(&s, 0, X); // what should X be?
  printf("parent: begin");
  pthread_t c;
  pthread_create(c, NULL, child, NULL);
  sem_wait(&s); // wait here for child
  printf("parent: end");
  return 0;
}

The code for our reader-writer lock from class/the textbook is shown below. What is the main disadvantage of this code?

typedef struct _rwlock_t {
  sem_t lock; 	// binary semaphore (basic lock)
  sem_t writelock; 	// used to allow ONE writer or MANY readers
  int readers; 	// count of readers reading in critical section
} rwlock_t;
void rwlock_init(rwlock_t *rw) {
  rw->readers = 0;
  sem_init(&rw->lock, 0, 1);
  sem_init(&rw->writelock, 0, 1);
}
void rwlock_acquire_readlock(rwlock_t *rw) {
  sem_wait(&rw->lock);
  rw->readers++;
  if (rw->readers == 1)
    sem_wait(&rw->writelock); // first reader acquires writelock
  sem_post(&rw->lock);
}
void rwlock_release_readlock(rwlock_t *rw) {
  sem_wait(&rw->lock);
  rw->readers--;
  if (rw->readers == 0)
    sem_post(&rw->writelock); // last reader releases writelock
  sem_post(&rw->lock);
}
void rwlock_acquire_writelock(rwlock_t *rw) {
  sem_wait(&rw->writelock);
}
void rwlock_release_writelock(rwlock_t *rw) {
  sem_post(&rw->writelock);
}

Which of the following is true about the advantages and disadvantages of using pthread_cond_broadcast() over pthread_cond_signal()?

1   sem_t m;
2   sem_init(&m, 0, X);
3   
4   sem_wait(&m);
5   //critical section here
6   sem_post(&m);

When the value of a semaphore is negative what does this number indicate?