diff --git a/client/pool-buffer.c b/client/pool-buffer.c
index 6555a3ae8..588bd06cf 100644
--- a/client/pool-buffer.c
+++ b/client/pool-buffer.c
@@ -1,4 +1,4 @@
-#define _XOPEN_SOURCE 600
+#define _POSIX_C_SOURCE 200809
 #include <assert.h>
 #include <cairo/cairo.h>
 #include <fcntl.h>
diff --git a/sway/commands.c b/sway/commands.c
index 9e0fda552..e72b89167 100644
--- a/sway/commands.c
+++ b/sway/commands.c
@@ -1,4 +1,4 @@
-#define _XOPEN_SOURCE 600
+#define _POSIX_C_SOURCE 200809
 #include <ctype.h>
 #include <stdarg.h>
 #include <stdlib.h>
diff --git a/sway/commands/bar.c b/sway/commands/bar.c
index 9d62dc837..f760888e4 100644
--- a/sway/commands/bar.c
+++ b/sway/commands/bar.c
@@ -1,4 +1,4 @@
-#define _XOPEN_SOURCE 600
+#define _POSIX_C_SOURCE 200809
 #include <string.h>
 #include <strings.h>
 #include <wlr/util/log.h>
diff --git a/sway/commands/output/background.c b/sway/commands/output/background.c
index 900a5e753..ddad679df 100644
--- a/sway/commands/output/background.c
+++ b/sway/commands/output/background.c
@@ -1,4 +1,4 @@
-#define _XOPEN_SOURCE 600
+#define _POSIX_C_SOURCE 200809
 #include <libgen.h>
 #include <strings.h>
 #include <unistd.h>
diff --git a/sway/tree/workspace.c b/sway/tree/workspace.c
index e7c1e36b8..1957d94fc 100644
--- a/sway/tree/workspace.c
+++ b/sway/tree/workspace.c
@@ -1,4 +1,4 @@
-#define _XOPEN_SOURCE 600
+#define _POSIX_C_SOURCE 200809
 #include <ctype.h>
 #include <limits.h>
 #include <stdbool.h>
diff --git a/swaybar/ipc.c b/swaybar/ipc.c
index 972869c6f..0e60c10c4 100644
--- a/swaybar/ipc.c
+++ b/swaybar/ipc.c
@@ -1,4 +1,4 @@
-#define _XOPEN_SOURCE 600
+#define _POSIX_C_SOURCE 200809
 #include <limits.h>
 #include <string.h>
 #include <strings.h>