4.2.7.5 Reference Tracking Mechanics

Objects tracking references to themselves maintain a list of references, a list in which each item corresponds a tracked reference. The list items identify the objects holding the references by the address of one of their attribute. The latter is typed as struct x1f4_nodelink_type.

See struct x1f4_nodelink_type.

Objects in the libaime type system (those that may hold references and be themselves referred) embed thus a struct x1f4_nodelink_type record. The address at which this record is stored is offered to the referred objects for up tracing purposes.

See Detecting Cycles.

The maintained references list is a list of pointers to struct x1f4_nodelink_type records.

The embedded struct x1f4_nodelink_type record needs to be set up before it is published.

Its slip field is the flag space required for tracking the already visited references when running the cycle detection algorithm and needs be NULL outside the cycle detection algorithm.

The other struct x1f4_nodelink_type record field, miss, is the address of the routine determining whether the object (which embeds the struct x1f4_nodelink_type record) is variable referred. The method is expected to return a non zero value if the object is variable referred, zero otherwise.

When called, the routine indicated by the miss field will be supplied with

• - the execution context
• - the address of some struct x1f4_nodelink_type pointer to construct the linked list of the visited references
• - the address of some struct x1f4_nodelink_type pointer to construct the linked list of the to be visited references
• - the address of the struct x1f4_nodelink_type record from which the routine address was picked up

as arguments, in this order.

What such routine would do is:

• check whether the object has been visited, by examinging the slip field of its last argument. If that’s the case it would return zero without further processing (for else the the cycle detection algorithm might run endlessly).
• check the reference count for non zero value. If the count is zero the object is in the process of destruction and the routine would return zero.
• compare the reference count with the size of the list of references hold by other objects. For that, the address at which the object is stored may have to be computed from the value of the same last argument. If the reference count is greater the object is variable referred and the routine is to return non zero. A zero list size would prompt a non zero return as well.
• flag the object as visited / set up the linked list of visited objects. The routine would do that by setting the slip field of its last argument with the value of the struct x1f4_nodelink_type pointer stored at the address indicated by its second argument (the previously visited object) and store its last struct x1f4_nodelink_type pointer argument (the address by which the object is to be known) at the address indicated by its second argument.
• add each reference in the list of references hold by other objects (to the currently examined object) to the list of the to be visited references. The method is to return zero when getting this far.