Right now, there's a bit of a discrepancy between these two, even though there shouldn't be. While the libc crate has very complicated logic to determine how the c_* types are defined, the standard library uses a much simpler logic.

I'm not sure how much these types are desired outside of libc; the only type that I've really seen used across the standard library is c_char, which honestly could just be replaced with an opaque struct with repr(u8) if it weren't already stabilised.