> I still don't understand.  Can't there be a bidirectional mapping
> between symbols and numbers *on each machine*?  The mapping doesn't
> change from run to run on the same machine, at least not between reboots.

Yes, that was the idea. That's what is done by all the other "errno"
mappers listed in the prior art section.

If some API gives you an errno integer on a given machine, you can use
the codeset to translate into a symbol on that machine.