1. if you mean with GETFIRSTFILE msg_getfirstfile than I don’t understand what this has to do with a scan preview.
for a scan preview I would suggest you do a normal scan just with a very low resolution and display it on the screen.
2. be a bit more specific in terms of what you are using for your app: any predefined commands, twack-commands, …..
what kind of error messages are you thinking of? errors occurring during setting the caps, the transfer,… or wrong input from the user?