Thank you for your encouragement :)

I read a bit more about the topic, and it seems that the issue with my approach is that the decimal representation might end up exactly halfway between two floats, and then the result of parsing it depends on the rounding mode that the parser uses. (By default scanf should use round-to-even, but I'm not sure all implementations do so)

In the PostgreSQL docs I found a curious fact: They use an algorithm that makes sure the printed decimal is never exactly half way between two representable floats, so the result of scanning the decimal representation doesn't depend on the rounding mode.