execute UDF, ERROR 1305 (42000): FUNCTION does not exist

[hewei...@gmail.com]

I downloaded the library lib_mysqludf_sys 's source code, compiled,
and installed successfully.
ex:
SELECT * FROM mysql.func;
+----------+-----+---------------------+----------+
| name | ret | dl | type |
+----------+-----+---------------------+----------+
| sys_exec | 2 | lib_mysqludf_sys.so | function |
+----------+-----+---------------------+----------+
1 row in set (0.00 sec)

but when I exec it:
select sys_exec(commandline); or
select dbname.sys_exec(commandline);
or use upper case function name. but all the same error:
ERROR 1305 (42000): FUNCTION does not exist.

so why?