diff --git a/pintos-env/pintos/threads/thread.c b/pintos-env/pintos/threads/thread.c
index f8ba0f2..62424df 100644
--- a/pintos-env/pintos/threads/thread.c
+++ b/pintos-env/pintos/threads/thread.c
@@ -426,8 +426,10 @@ thread_exit (void)
      and schedule another process.  That process will destroy us
      when it calls thread_schedule_tail(). */
   intr_disable ();
-  list_remove (&thread_current()->allelem);
+  struct thread* t = thread_current();
+  list_remove (&t->allelem);
   thread_current ()->status = THREAD_DYING;
+  sema_up(&t->waiting_sem);
   schedule ();
   NOT_REACHED ();
 }
@@ -572,6 +574,7 @@ init_thread (struct thread *t, const char *name, int priority)
   t->status = THREAD_BLOCKED;
   strlcpy (t->name, name, sizeof t->name);
   t->stack = (uint8_t *) t + PGSIZE;
+  sema_init(&t->waiting_sem, 0);
 
   if (thread_mlfqs) {
     t->nice_init = priority > 20 || priority < 20 ? 0 : priority;
@@ -710,6 +713,21 @@ schedule (void)
   thread_schedule_tail (prev);
 }
 
+struct thread *
+thread_get_by_tid (tid_t tid)
+{
+  struct list_elem *e;
+  for (e = list_begin (&all_list);
+       e != list_end (&all_list);
+       e = list_next (e))
+    {
+      struct thread *t = list_entry (e, struct thread, allelem);
+      if (t->tid == tid)
+        return t;
+    }
+  return NULL;
+}
+
 /* Returns a tid to use for a new thread. */
 static tid_t
 allocate_tid (void)
diff --git a/pintos-env/pintos/threads/thread.h b/pintos-env/pintos/threads/thread.h
index a37b265..dcebaaf 100755
--- a/pintos-env/pintos/threads/thread.h
+++ b/pintos-env/pintos/threads/thread.h
@@ -8,6 +8,7 @@
 #include <list.h>
 #include <stdint.h>
 #include "threads/fpr_arith.h"
+#include "threads/synch.h"
 
 /* States in a thread's life cycle. */
 enum thread_status
@@ -99,6 +100,7 @@ struct thread
     int nice;
     int nice_init;
     FPReal recent_cpu;
+    struct semaphore waiting_sem;
 
 #ifdef USERPROG
     /* Owned by userprog/process.c. */
@@ -147,5 +149,5 @@ int thread_get_nice (void);
 void thread_set_nice (int);
 int thread_get_recent_cpu (void);
 int thread_get_load_avg (void);
-
-#endif /* threads/thread.h */
+struct thread* thread_get_by_tid(tid_t);
+#endif /* threads/thread.h */
\ No newline at end of file
diff --git a/pintos-env/pintos/userprog/process.c b/pintos-env/pintos/userprog/process.c
index c0e5215..92990d1 100755
--- a/pintos-env/pintos/userprog/process.c
+++ b/pintos-env/pintos/userprog/process.c
@@ -86,9 +86,15 @@ start_process (void *file_name_)
    This function will be implemented in problem 2-2.  For now, it
    does nothing. */
 int
-process_wait (tid_t child_tid UNUSED) 
+process_wait (tid_t child_tid) 
 {
-  return -1;
+  struct thread* child = thread_get_by_tid(child_tid);
+  if (child == NULL) {
+    return -1;
+  }
+
+  sema_down(&child->waiting_sem);
+  return 0;
 }
 
 /* Free the current process's resources. */