The wcsrtombs function converts a sequence of wide characters
    from the array indirectly pointed to by src into a sequence of
    corresponding multibyte characters, beginning in the conversion
    state described by the object pointed to by ps.

    If dst is a not a NULL pointer, the converted characters are then
    stored into the array pointed to by dst. Conversion continues up
    to and including a terminating null wide character, which is also
    stored.

    Conversion stops earlier in two cases:

    o  When a code is reached that does not correspond to a valid
       multibyte character

    o  If dst is not a NULL pointer, when the next multibyte
       character would exceed the limit of len total bytes to be
       stored into the array pointed to by dst

    Each conversion takes place as if by a call to the wcrtomb
    function.

    If dst is not a NULL pointer, the pointer object pointed to by
    src is assigned either a NULL pointer (if the conversion stopped
    because it reached a terminating null wide character) or the
    address just beyond the last wide character converted (if any).
    If conversion stopped because it reached a terminating null
    wide character, the resulting state described is the initial
    conversion state.

    If the wcsrtombs function is called as a counting function,
    which means that dst is a NULL pointer, the value of the internal
    mbstate_t object will remain unchanged.

    See also wcrtomb.