diff --git a/util/shm.c b/util/shm.c
index 5eb87bb89..455cfe865 100644
--- a/util/shm.c
+++ b/util/shm.c
@@ -1,3 +1,4 @@
+#define _XOPEN_SOURCE 700
 #include <errno.h>
 #include <fcntl.h>
 #include <string.h>