1: Semaphores pseudocode
    2: ==============================
    3: 
    4:        int sem_wait(sem_t * sem);
    5:        int sem_trywait(sem_t * sem);
    6:        int sem_post(sem_t * sem);
    7:        int sem_getvalue(sem_t * sem, int * sval);
    8: 
    9: struct sem_t {
   10: 
   11:    unsigned int count;
   12:          - current semaphore count, also used as a futex
   13: }
   14: 
   15: sem_wait(sem_t *sem)
   16: {
   17:   for (;;) {
   18: 
   19:     if (atomic_decrement_if_positive(sem->count))
   20:       break;
   21: 
   22:     futex_wait(&sem->count, 0)
   23:   }
   24: }
   25: 
   26: sem_post(sem_t *sem)
   27: {
   28:   n = atomic_increment(sem->count);
   29:   // Pass the new value of sem->count
   30:   futex_wake(&sem->count, n + 1);
   31: }
   32: 
   33: sem_trywait(sem_t *sem)
   34: {
   35:   if (atomic_decrement_if_positive(sem->count)) {
   36:     return 0;
   37:   } else {
   38:     return EAGAIN;
   39:   }
   40: }
   41: 
   42: sem_getvalue(sem_t *sem, int *sval)
   43: {
   44:   *sval = sem->count;
   45:   read_barrier();
   46: }