libpthread: Implement sem_trywait() and sem_getvalue()

src/untrusted/pthread/nc_semaphore.c

 /*
  * Copyright (c) 2011 The Native Client Authors. All rights reserved.
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 /*
 * Native Client semaphore API
 */
 
@@ skipping 72 matching lines @@
     return 0;
 
   __sync_fetch_and_add(&sem->nwaiters, 1);
   do {
     __nc_irt_futex.futex_wait_abs(&sem->count, 0, NULL);
   } while (!decrement_if_positive(&sem->count));
   __sync_fetch_and_sub(&sem->nwaiters, 1);
   return 0;
 }
 
+int sem_trywait(sem_t *sem) {
+  if (decrement_if_positive(&sem->count))
+    return 0;
+
+  errno = EAGAIN;
+  return -1;
+}
+
 int sem_post(sem_t *sem) {
   /* Increment sem->count, checking for overflow. */
   int old_value;
   do {
     old_value = sem->count;
     if (old_value == SEM_VALUE_MAX) {
       errno = EOVERFLOW;
       return -1;
     }
   } while (!__sync_bool_compare_and_swap(&sem->count, old_value,
                                          old_value + 1));
 
   /*
    * We only need to call futex_wake() if there are waiters.  Note
    * that this might do an unnecessary call to futex_wake() if the
    * last waiter has already been woken using futex_wake() but has not
    * yet decremented sem->nwaiters.
    */
   if (sem->nwaiters != 0) {
     int woken_count;
     __nc_irt_futex.futex_wake(&sem->count, 1, &woken_count);
   }
   return 0;
 }
+
+int sem_getvalue(sem_t *sem, int *value) {
+  *value = sem->count;
+  return 0;
+}