MySQL 5.7 Reference Manual
        MySQL 5.7 Release Notes
        
        If you want to add support for a new character set named
        MYSET that includes multibyte
        characters, you must use multibyte character functions in the
        ctype-
        source file in the MYSET.cstrings directory.
      
        The existing character sets provide the best documentation and
        examples to show how these functions are implemented. Look at
        the ctype-*.c files in the
        strings directory, such as the files for
        the euc_kr, gb2312,
        gbk, sjis, and
        ujis character sets. Take a look at the
        MY_CHARSET_HANDLER structures to see how they
        are used. See also the CHARSET_INFO.txt
        file in the strings directory for
        additional information.