In many applications, C programmers attach double meaning to the values of a pointer. Typically the value NULL (or zero) means data is not needed, yet some application-specific interpretation applies. In essence, the C programmer is encoding a discriminated union efficiently by overloading the interpretation of the value of a pointer. For instance, a NULL pointer value for gnp could indicate that the person's assets and liabilities are unknown. That is, the pointer value encodes two things: whether the data is known and, if it is known, where it is located in memory. Linked lists are an extreme example of the use of application-specific pointer interpretation.
The primitive xdr_reference() cannot and does not attach any special meaning to a null-value pointer during serialization. That is, passing an address of a pointer that has a value of value of NULL to xdr_reference() when serializing data most likely causes a memory fault and, on the UNIX system, a core dump.