diff --git a/swaylock/main.c b/swaylock/main.c
index 50f145e36..cdd430297 100644
--- a/swaylock/main.c
+++ b/swaylock/main.c
@@ -1,3 +1,4 @@
+#define _XOPEN_SOURCE 700
 #define _POSIX_C_SOURCE 200809L
 #include <assert.h>
 #include <ctype.h>