diff --git a/flock.c b/flock.c
index 5d36bdf..9d025e2 100644
--- a/flock.c
+++ b/flock.c
@@ -3,6 +3,7 @@
 #include <sys/wait.h>
 
 #include <errno.h>
+#include <fcntl.h>
 #include <stdio.h>
 #include <unistd.h>