diff --git a/libraries/libmdb/mtest.c b/libraries/libmdb/mtest.c
index 6b084f9658..1f43341d3a 100644
--- a/libraries/libmdb/mtest.c
+++ b/libraries/libmdb/mtest.c
@@ -1,3 +1,4 @@
+#define _XOPEN_SOURCE 500		/* srandom(), random() */
 #include <stdio.h>
 #include <stdlib.h>
 #include <time.h>