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).