I already tried that. In fact, in the declare statement the "@" sign needs to be after the datatype. But when I call the function I tried both with the @ sign and with out it. Did not make any difference.

DECLARE INTEGER v_o_d IN "RLPOLK.DLL" STRING, STRING @
x = v_o_d(outvin, @options)
?options
x = v_o_d(outvin, options)
?options

Also in which documentation did you find that in order to get a value back you have to pass it by reference.