diff --git a/src/cmd/diff/diffreg.c b/src/cmd/diff/diffreg.c
index ed4d17b8..478c1b1b 100644
--- a/src/cmd/diff/diffreg.c
+++ b/src/cmd/diff/diffreg.c
@@ -66,6 +66,9 @@
 *	3*(number of k-candidates installed),  typically about
 *	6n words for files of length n.
 */
+
+#define class diffclass
+
 /* TIDY THIS UP */
 struct cand {
 	int x;