A primitive that calls xdr_string(xdrs, sp, MAXUNSIGNED), where
    MAXUNSIGNED is the maximum value of an unsigned integer. This
    routine is useful because the ONC RPC client and server routines
    pass the XDR stream handle and a single pointer as parameters
    to any referenced XDR routines. The xdr_string routine, one
    of the most frequently used ONC RPC primitives, requires three
    parameters.

    While decoding, if *sp is NULL, the necessary storage is
    allocated to hold the NULL-terminated string and *sp is set to
    point to it.