Not all of these constants are available on every platform. When I encountered this sort of problem in the past, I looked up each item individually in the libc repository to see which platforms support that item. (See #3840, for example.)

I realized back then (and I still maintain) that this process would prove to be tedious and error-prone. Perhaps I should look into using the autocfg crate instead, but that's ultimately beyond the scope of this PR.

I tried with Window locale.h this afternoon and found out that Window API uses different naming convention (prefix with W, if I remember). Hence, if we would like to fix it, I'm afraid that we need to implement our own locale interaction. I will do it in another PR once this module takes shape.

Right now, I put this module limit to unix system only!

this is not always successful

>>> _locale.setlocale(0, '\0')
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ValueError: embedded null character

because CString implements std::ops::Deref<Target=CStr>, this is redundant.

libc::setlocale(args.category, lstr.as_ptr())

works exactly same way.