`yes`: use strcat.

src/yes.c

@@ -78,7 +78,7 @@ int yes(int argc, char **argv) {
   int n = 0;
   int splen = strlen(spammy);
   do {
-    sprintf(buf, "%s%s", buf, spammy);
+    strcat(buf, spammy);
     n += splen;
   } while (n+splen < len);
   do {