diff --git a/libmango/include-user/mango/msg.h b/libmango/include-user/mango/msg.h
index 870a8ef..aaf3e96 100644
--- a/libmango/include-user/mango/msg.h
+++ b/libmango/include-user/mango/msg.h
@@ -38,7 +38,8 @@ extern kern_status_t msg_read(
 	msgid_t id,
 	size_t offset,
 	struct iovec *out,
-	size_t nr_out);
+	size_t out_count,
+	size_t *nr_read);
 extern kern_status_t msg_read_handles(
 	kern_handle_t channel,
 	msgid_t id,