diff --git a/swaylock/render.c b/swaylock/render.c
index 79609e96..cb3ed276 100644
--- a/swaylock/render.c
+++ b/swaylock/render.c
@@ -1,5 +1,7 @@
-#include <wayland-client.h>
+#define _POSIX_C_SOURCE 199506L
 #include <math.h>
+#include <stdlib.h>
+#include <wayland-client.h>
 #include "cairo.h"
 #include "background-image.h"
 #include "swaylock/swaylock.h"