diff --git a/c/README b/c/README
index 0314012..785e6de 100644
--- a/c/README
+++ b/c/README
@@ -1,6 +1,8 @@
-./prog <mode> <IFNAME>
+./prog [<mode> <IFNAME>]
 
 mode:
+0 is default
+
 0: rainbow
 1: red
 2: green
@@ -10,3 +12,4 @@ mode:
 
 IFNAME:
 Interface name like "eth0" or "enp5s0"
+enp5s0 is default