Today, WG14 voted N 2358 into C2X. This makes mblen() thread-safe by
disallowing the use of internal state in that function.

Since this could make existing implementations non-compliant (though
only one C implementation is known to be affected that way), we thought
that it would be good to notify you, so any C++ implementations would
know as early as possible, and could, if necessary, fix their mblen().

The C++ standard contains mblen(), and leaves the semantics of this
function to be specified by the C standard.

Philipp