diff --git a/src/start_kdeinit/start_kdeinit.c b/src/start_kdeinit/start_kdeinit.c
--- a/src/start_kdeinit/start_kdeinit.c
+++ b/src/start_kdeinit/start_kdeinit.c
@@ -19,6 +19,8 @@
 
 #include <config-kdeinit.h>
 
+#define _POSIX_SOURCE /* kill(2) */
+
 #include <errno.h>
 #include <fcntl.h>
 #include <signal.h>