Return_Values

   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).