x                  The number of bytes comprising the multibyte
                       character.
    0                  The next n or fewer bytes complete the
                       multibyte character that corresponds to the
                       null wide character (which is the value stored
                       if pwc is not a NULL pointer). The wide-
                       character code corresponding to a null byte
                       is zero.
    -1                 Indicates an encoding error. 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).