diff --git a/usr.sbin/ctm/mkCTM/mkctm.c b/usr.sbin/ctm/mkCTM/mkctm.c
index 80db47f66a0c..196a51039752 100644
--- a/usr.sbin/ctm/mkCTM/mkctm.c
+++ b/usr.sbin/ctm/mkCTM/mkctm.c
@@ -546,6 +546,7 @@ main(int argc, char **argv)
 			logf = fopen(optarg, "w");
 			if (!logf)
 				err(1, "%s", optarg);
+			setlinebuf(logf);
 			break;
 		case 'q':
 			verbose--;