Michael and Russ, thanks for your replies.

Failure of my application will not cause any life threatening situations but I also don't want a corrupted config file to work with.

I'm not concerned about failures cause by power or program glitches when writing the file. The application runs on a laptop which communicates with a midi device.

There are no secrets in the file but I also don't want to spend several evenings (it's a hobby project) making sure that allunwanted user action is checked and corrected.
So given the fact that the lines (midi messages) in the config file already contain a checksum I'm going to use it.
I'm going to save the config file twice. That's a good tip.
If one fails I will load the second one. If that one fails I'll load defaults.

Peter