#include <semaphore.h>
#include "pthread_impl.h"

int sem_trywait(sem_t* sem) {
  int val;
  while ((val = sem->__val[0]) > 0) {
    int new = val - 1 - (val == 1 && sem->__val[1]);
    if (a_cas(sem->__val, val, new) == val)
      return 0;
  }
  errno = EAGAIN;
  return -1;
}