diff --git a/src/lxc/tools/lxc_monitor.c b/src/lxc/tools/lxc_monitor.c
index 72dca8e21..c60e14ff5 100644
--- a/src/lxc/tools/lxc_monitor.c
+++ b/src/lxc/tools/lxc_monitor.c
@@ -30,6 +30,7 @@
 #include <libgen.h>
 #include <poll.h>
 #include <regex.h>
+#include <stddef.h>
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>