hello @Tim Peters, thanks for all the hints,
Let me answer bottom first to give something back:
dtoa.c from David Gay is somewhat outdated, see:
some other discussions about similar attempts from Grisu / Dragon,
Schubfach, swift, Dragonbox, Drachennest …
sorry, the last one is long, in the end Morten managed to integrate
ryu into goffice which might serve as a template.
Reg. my original question: my ‘use case’ is not to ‘do my work’,
but constructing improvements to achieve more ‘decimal like’
results with doubles and long doubles. That works to some
degree, but could like testing. As I have seen python handling
some points - e.g. rounding - different to others it’s a good
inspiring cross check.
Perhaps sometimes someone will work in that area and can
care for this option, up then I’ll try to use strings and other
workarounds. I just did not want to ‘work around’ while evtl. a
simple solution is available and I just didn’t find.