diff --git a/src/tools/pkcs15-tool.c b/src/tools/pkcs15-tool.c
index 0bb25c14..4c7be8d2 100644
--- a/src/tools/pkcs15-tool.c
+++ b/src/tools/pkcs15-tool.c
@@ -20,7 +20,11 @@
  */
 #include "config.h"
 
+#ifdef __APPLE__
+#define _XOPEN_SOURCE 600
+#else
 #define _XOPEN_SOURCE 500
+#endif
 #include <assert.h>
 #include <ctype.h>
 #ifdef _WIN32