diff --git a/cryptfs.h b/cryptfs.h
index ca3cc95..caf99a4 100644
--- a/cryptfs.h
+++ b/cryptfs.h
@@ -34,6 +34,8 @@
 
 /* definitions of flags in the structure below */
 #define CRYPT_MNT_KEY_UNENCRYPTED 0x1 /* The key for the partition is not encrypted. */
+#define CRYPT_ENCRYPTION_IN_PROGRESS 0x2 /* Set when starting encryption,
+                                          * clear when done before rebooting */
 
 #define CRYPT_MNT_MAGIC 0xD0B5B1C4