Question

I need help constructing a monitor that implements a semaphore, and simple C example will do.

This is to demonstrate that a monitor can be used any place a semaphore can be used.

Answer 1

If you say mutex/condvars are allowed, then check this:

#include <pthread.h>

typedef struct
{
  unsigned int count;
  pthread_mutex_t lock;
  pthread_cond_t cond;
} semaph_t;

int
semaph_init (semaph_t *s, unsigned int n)
{
  s->count = n;
  pthread_mutex_init (&s->lock, 0);
  pthread_cond_init (&s->cond, 0);
  return 0;
}

int
semaph_post (semaph_t *s)
{
  pthread_mutex_lock (&s->lock); // enter monitor
  if (s->count == 0)
    pthread_cond_signal (&s->cond); // signal condition
  ++s->count;
  pthread_mutex_unlock (&s->lock); // exit monitor
  return 0;
}

int
semaph_wait (semaph_t *s)
{
  pthread_mutex_lock (&s->lock); // enter monitor
  while (s->count == 0)
    pthread_cond_wait (&s->cond, &s->lock); // wait for condition
  --s->count;
  pthread_mutex_unlock (&s->lock); // exit monitor
  return 0;
}