x The number of bytes comprising the multibyte
character.
0 Indicates that s is a NULL pointer or a
pointer to a null byte.
-1 Indicates an encoding error, in which case
the next n or fewer bytes do not contribute
to a complete and valid multibyte character.
errno is set to EILSEQ; the conversion state
is undefined.
-2 Indicates an incomplete but potentially valid
multibyte character (all n bytes have been
processed).