diff --git a/src/structs.h b/src/structs.h
index 958a5d2..5f148a0 100644
--- a/src/structs.h
+++ b/src/structs.h
@@ -1940,6 +1940,10 @@
 # define CRYPT_M_BF	1
 # define CRYPT_M_BF2	2
 # define CRYPT_M_COUNT	3 /* number of crypt methods */
+
+// Currently all crypt methods work inplace.  If one is added that isn't then
+// define this.
+//  # define CRYPT_NOT_INPLACE 1
 #endif