The mbsrtowcs function converts a sequence of multibyte
characters, beginning in the conversion state described by the
object pointed to by ps, from the array indirectly pointed to by
src, into a sequence of corresponding wide characters.
If dst is not a NULL pointer, the converted characters are stored
into the array pointed to by dst. Conversion continues up to and
including a terminating null character, which is also stored.
Conversion stops earlier for one of the following reasons:
o A sequence of bytes is encountered that does not form a valid
multibyte character.
o If dst is not a NULL pointer, when len codes have been stored
into the array pointed to by dst.
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 of reaching a terminating null wide character), or the
address just beyond the last multibyte character converted (if
any). If conversion stopped because of reaching a terminating
null wide character, the resulting state described is the initial
conversion state.