diff --git a/tools/xenstore/include/xenstore_lib.h b/tools/xenstore/include/xenstore_lib.h
index 5a10c6c937..462b7b9d40 100644
--- a/tools/xenstore/include/xenstore_lib.h
+++ b/tools/xenstore/include/xenstore_lib.h
@@ -19,6 +19,7 @@
 #ifndef XENSTORE_LIB_H
 #define XENSTORE_LIB_H
 
+#include <stddef.h>
 #include <stdbool.h>
 #include <limits.h>
 #include <errno.h>