Jim Meyering wrote:
> Conclusion: we'll have to make btowc work properly in the C locale, too.

Perhaps something like the attached (untested) patch? The basic idea is to have 
btowc and mbtowc use a fixed mbrtowc if the latter has the C-locale problem in 
question. While we're at it, btowc should invoke mbrtowc not mbtowc, as btowc is 
thread-safe but mbtowc is not.