The code for this producer-consumer example was derived from a homework
from an earlier year.
The rest of this note describes how this code was constructed.

[ NOTE:  If you use McMini with this, please replace 'sem_' by 'mysem_' in
  partial_solution.c and producer-consumer.c.  Otherwise, McMini will
  think that 'sem_' is the real thing, and debug on that.  This code
  has already replaced 'sem_' with 'mysem_'.
    I've found that 'mcmini-gdb -m 30 ./producer-consumer'
  suffices to show the bug. ]

    Take the code from hw6 (producer-consumer.c and partial-solution.c),
    and in partial-solution.c, replace the original 'block()' function by
    the variation below:

void block(sem_t *sem, int blocking_count) {
  while (1) {
    pthread_mutex_lock( &(sem->mutex) );
    // if someone just posted and the count is back to what it was
    //   before I called sem_wait and blocked:
    if (sem->count == blocking_count + 1) {
      pthread_mutex_unlock( &(sem->mutex) );
      return;
    }
    pthread_mutex_unlock( &(sem->mutex) );
    sleep(1); // sleep for a second, and check later if count >= 0.
  }
}

Finally, you need to set the blocking_count somewhere in your code.
We will add it to sem_wait.  In the original partial_solution.c:sem_wait()
WE HAD:
  if (sem->count < 0) {
    // We must release lock, or no one can ever call sem_post() for us.
    pthread_mutex_unlock( &(sem->mutex) );
    block(sem);

YOU MUST CHANGE IT TO:
  if (sem->count < 0) {
    // We must release lock, or no one can ever call sem_post() for us.
    int blocking_count = sem->count;  // This is why we will block.
    pthread_mutex_unlock( &(sem->mutex) );
    block(sem, blocking_count); 

ALSO CHANGE:
void block(sem_t *sem); // Used by sem_wait()
TO:
void block(sem_t *sem, int blocking_count); // Used by sem_wait()