RPC parameters

HI,Genode is new to me . I just start playing with it. I was looking at the Hello program and the RPC mechanism. as i understood RPC does not support passing pointer as parameter how i can pass a parameter by reference where i need to change its value in the server side. I found in the rpc.h that the parameter could be input and output in the same time. How i can use this ? are there any implemented examples . should i defined an my output structure if the remote procedure return multiple outputs. best.