diff --git a/ChangeLog b/ChangeLog
index 643ae69700..6fc912903a 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,8 @@
+Tue Apr 10 16:53:21 2012  Nobuyoshi Nakada  <nobu@ruby-lang.org>
+
+	* thread_pthread.c: add prototype declarations for older Mac OS X.
+	  [ruby-core:43376][Bug #6170]
+
 Tue Apr 10 15:35:21 2012  Koichi Sasada  <ko1@atdot.net>
 
 	* compile.c (iseq_set_sequence): show a hint if there are duplicated
diff --git a/thread_pthread.c b/thread_pthread.c
index 666d007f05..de8325e087 100644
--- a/thread_pthread.c
+++ b/thread_pthread.c
@@ -465,6 +465,8 @@ static rb_thread_t *register_cached_thread_and_wait(void);
 #define STACKADDR_AVAILABLE 1
 #elif defined HAVE_PTHREAD_GET_STACKADDR_NP && defined HAVE_PTHREAD_GET_STACKSIZE_NP
 #define STACKADDR_AVAILABLE 1
+void *pthread_get_stackaddr_np(pthread_t);
+size_t pthread_get_stacksize_np(pthread_t);
 #elif defined HAVE_THR_STKSEGMENT || defined HAVE_PTHREAD_STACKSEG_NP
 #define STACKADDR_AVAILABLE 1
 #elif defined HAVE_PTHREAD_GETTHRDS_NP