diff --git a/pintos-env/pintos/threads/thread.c b/pintos-env/pintos/threads/thread.c
index e73f893..361a083 100644
--- a/pintos-env/pintos/threads/thread.c
+++ b/pintos-env/pintos/threads/thread.c
@@ -436,9 +436,11 @@ thread_exit (void)
      when it calls thread_schedule_tail(). */
   intr_disable ();
   struct thread* t = thread_current();
+  //printf("Exit status: %d\n",t->exit_status);
   list_remove (&t->allelem);
-  thread_current ()->status = THREAD_DYING;
   sema_up(&t->waiting_sem);
+  sema_down(&t->parent->parent_sem);
+  t->status = THREAD_DYING;
   list_remove(&t->child_elem);
   schedule ();
   NOT_REACHED ();
@@ -585,6 +587,7 @@ init_thread (struct thread *t, const char *name, int priority)
   strlcpy (t->name, name, sizeof t->name);
   t->stack = (uint8_t *) t + PGSIZE;
   sema_init(&t->waiting_sem, 0);
+  sema_init(&t->parent_sem, 0);
   t->waited_on_before = 0;
   list_init(&t->children);
 
diff --git a/pintos-env/pintos/threads/thread.h b/pintos-env/pintos/threads/thread.h
index d6bdde2..95e2666 100755
--- a/pintos-env/pintos/threads/thread.h
+++ b/pintos-env/pintos/threads/thread.h
@@ -110,6 +110,8 @@ struct thread
     struct list_elem child_elem;
     struct list children;
 
+    struct semaphore parent_sem;
+
 #ifdef USERPROG
     /* Owned by userprog/process.c. */
     uint32_t *pagedir;                  /* Page directory. */
diff --git a/pintos-env/pintos/userprog/process.c b/pintos-env/pintos/userprog/process.c
index d6e9412..dd5194e 100755
--- a/pintos-env/pintos/userprog/process.c
+++ b/pintos-env/pintos/userprog/process.c
@@ -182,8 +182,6 @@ process_wait (tid_t child_tid)
 {
   //printf("process wait for: %s on: %ul\n", thread_current()->name, child_tid);
   
-  // TODO: check if tid is child
-  
   struct thread* child = thread_get_by_tid(child_tid);
   if (child == NULL || child->waited_on_before) {
     //printf("child not found\n");
@@ -193,8 +191,11 @@ process_wait (tid_t child_tid)
   //printf("child is: %s\n", child->name);
 
   sema_down(&child->waiting_sem);
+  int status = child->exit_status;
+  //printf("Wait exit: %s %d\n",child->name, status);
   child->waited_on_before = 1;
-  return child->exit_status;
+  sema_up(&(thread_current()->parent_sem));
+  return status;
 }
 
 /* Free the current process's resources. */
diff --git a/pintos-env/pintos/userprog/syscall.c b/pintos-env/pintos/userprog/syscall.c
index a0deea0..cfd5b0b 100755
--- a/pintos-env/pintos/userprog/syscall.c
+++ b/pintos-env/pintos/userprog/syscall.c
@@ -23,7 +23,6 @@ void
 syscall_init (void) 
 {
   intr_register_int (0x30, 3, INTR_ON, syscall_handler, "syscall");
-printf("syscall\n");
   memset(syscall_vec, (int)&syscall_nop, 128);
   syscall_vec[SYS_EXIT] = (handler)sys_exit;
   syscall_vec[SYS_WAIT] = (handler)sys_wait;
@@ -66,7 +65,7 @@ sys_exit (int status)
   struct thread* t = thread_current();
   printf("%s: exit(%d)\n", t->name, status);
   t->exit_status = status;
-  thread_exit ();
+  thread_exit();
   return -1;
 }
 
@@ -74,7 +73,9 @@ int
 sys_wait (tid_t tid)
 {
   printf("system call wait\n");
-  return process_wait(tid);
+  int status = process_wait(tid);
+  //printf("wait has returned status(%d)\n", status);
+  return status;
 }
 
 tid_t