void MOUNTPROC3_NULL(void) = 0;
Procedure NULL does not do any work. It is made available to allow server response testing and timing.
It is important that this procedure do no work at all so that it can be used to measure the overhead of processing a service request. By convention, the NULL procedure should never require any authentication. A server may choose to ignore this convention, in a more secure implementation, where responding to the NULL procedure call acknowledges the existence of a resource to an unauthenticated client.
Since the NULL procedure takes no MOUNT protocol arguments and returns no MOUNT protocol response, it can not return a MOUNT protocol error. However, it is possible that some server implementations may return RPC errors based on security and authentication requirements.