diff --git a/dvhlib.c b/dvhlib.c
--- a/dvhlib.c
+++ b/dvhlib.c
@@ -1,6 +1,7 @@
 /*
  * Disk Volume Header Library
  */
+#define _XOPEN_SOURCE 500
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>