diff --git a/su.c b/su.c
index d24157a..0d639e1 100644
--- a/su.c
+++ b/su.c
@@ -1,4 +1,5 @@
 /* See LICENSE file for copyright and license details. */
+#define _XOPEN_SOURCE
 #include <sys/types.h>
 #include <unistd.h>
 #include <errno.h>