>> It is because input parameter of your UDF is CSTRING.
>> Rewrite UDF to use one of Firebird's native types, CHAR or VARCHAR.
>
> Why do you think is a problem?
>
> I have plenty of UDFs which use CString for parameters. They work fine
> for me.

Do you mean input parameters, passed via xsqlda structure ?
Can you send some example command ?

Ivan