diff --git a/missing/explicit_bzero.c b/missing/explicit_bzero.c
index 1220e5f9ad..59417e158e 100644
--- a/missing/explicit_bzero.c
+++ b/missing/explicit_bzero.c
@@ -1,12 +1,9 @@
 #ifndef __STDC_WANT_LIB_EXT1__
-#define __STDC_WANT_LIB_EXT1__ 1
+#define __STDC_WANT_LIB_EXT1__ 1 /* for memset_s() */
 #endif
 
 #include "ruby/missing.h"
 #include <string.h>
-#ifdef HAVE_MEMSET_S
-# include <string.h>
-#endif
 
 #ifdef _WIN32
 #include <windows.h>
diff --git a/util.c b/util.c
index f39e17b8b7..3c08879ce5 100644
--- a/util.c
+++ b/util.c
@@ -14,7 +14,7 @@
 #endif
 
 #ifndef __STDC_WANT_LIB_EXT1__
-#define __STDC_WANT_LIB_EXT1__ 1
+#define __STDC_WANT_LIB_EXT1__ 1 /* for qsort_s() */
 #endif
 
 #include "ruby/internal/config.h"