- /* Filled in by nl_sock_transact_batch(). */
- struct ofpbuf *reply; /* Reply (NULL if reply was an error code). */
+ /* The client must initialize 'reply' to one of:
+ *
+ * - NULL, if it does not care to examine the reply.
+ *
+ * - Otherwise, to an ofpbuf with a memory allocation of at least
+ * NLMSG_HDRLEN bytes.
+ */
+ struct ofpbuf *reply; /* Reply (empty if reply was an error code). */