The threading aspect is one thing yes, but I think programmers forgetting or never realizing that these functions will magically behave differently for some of their users is a bigger problem.

I don't think having to pass a locale arguments would be that big of a problem - you could always have wrapper functions for the C locale, although they should be implemented directly for performance.

> What you want is for the locale stuff to be broken out into a complete separate library: a whole separate set of loc_* functions: loc_strtod, loc_printf, and so on.*

Yes, that would be ideal.