volatile int val = 0;

/* cmpxchg
   atomic_inc */

/* retorno valor anterior */
int atomic_inc_max(int *val) {
  int r;
  do {
    r = *val;
    if (r == MAX)
      return MAX;
  } while (cmpxchg(&val, r, r+1) != r);
  return r;
}

/* retorno valor anterior */
int atomic_inc_max(int *val) {
  int r = atomic_inc(val);
  if (r < MAX)
    return r;
  else {
    *val = MAX;
    return MAX;
  }
}

ant = atomic_dec(&val);