--- a/src/checkpid.c
+++ b/src/checkpid.c
@@ -20,6 +20,7 @@
 
 #include "rrdcollect.h"
 #include <fcntl.h>
+#include <sys/file.h> /* flock() */
 
 /* lockfile in rrdcollect.pid */
 /* five digits+"\n\0" */