For any data type T used in an RPC function, an XDR function bool_t xdr_T(XDR* xdrs, T*ptr) is generated when T is not one of the primitive types. This function takes a pointer to an object of type T as the second argument, and performs encode, decode or free operation based on the value of xdrs->x_op.
One possible use of the XDR functions is for serializing data to and from files.