The CFLN has a page for callbacks.   These are useful in certain rare cases but there is very little information available about how to use them, MgErr and InstanceDataPointer are defined in extcode.h and, it seems, InstanceDataPointer is a pointer passed by LabVIEW but there the information ends. Does LabVIEW free the InstanceDataPointer? - it is readable in the Abort, Reserve and Unreserve. Are we limited to a pointer sized variable? It's a (void *) so can we resize t