diff --git a/ChangeLog b/ChangeLog
index 8f76a10402..00ce4b7e0e 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,8 @@
+Wed Dec 23 02:59:26 2015  Nobuyoshi Nakada  <nobu@ruby-lang.org>
+
+	* configure.in (__STDC_WANT_LIB_EXT1__): necessary to use memset_s
+	  in strict C99 mode.
+
 Wed Dec 23 02:34:36 2015  Nobuyoshi Nakada  <nobu@ruby-lang.org>
 
 	* regexec.c (match_at): move direct threaded VM code to get rid of
diff --git a/configure.in b/configure.in
index cbefd7c1db..cdb1b95425 100644
--- a/configure.in
+++ b/configure.in
@@ -2317,6 +2317,8 @@ AC_CHECK_FUNCS(utimes)
 AC_CHECK_FUNCS(wait4)
 AC_CHECK_FUNCS(waitpid)
 
+AS_IF([test "$ac_cv_func_memset_s" = yes], [AC_DEFINE([__STDC_WANT_LIB_EXT1__], 1)])
+
 AS_IF([test "$ac_cv_func_getcwd" = yes], [
     AC_CACHE_CHECK(if getcwd allocates buffer if NULL is given, [rb_cv_getcwd_malloc],
 	[AC_TRY_RUN([