VMS Help CC, Run-time functions, mbrtowc  *Conan The Librarian (sorry for the slow response - running on an old VAX)

  Determines the number of bytes needed to complete the next
  multibyte character, including shift sequences.

  Syntax:

        #include <wchar.h>

        size_t mbrtowc (wchar_t *pwc,
                        const char *s,
                        size_t n,
                        mbstate_t *ps);