Comment 43 for bug 1094278

There are two issues here:

cu randomly identifies the serial connection as dialin (nortscts) ot dialout (rtscts). I don't care about that distinction being broken. I fix that with the patch.

The other problem is 'line in use' which is caused by running cu as root. The permission issues are discussed above. You need to add yourself to whatever group cu is owned by and run as non-root user and/or change the permissions to make the device nodes owned by whatever is cu suid to.