Clips multiple EnvEval queries invalidate previous

Question

I had a another strange problem that I solved already. But I'm not sure I just luckily fixed it or I really understand what's going on. So basically I have perform a query on my facts via:

DATA_OBJECT decay_tree_fact_list;
std::stringstream clips_query;
clips_query << "(find-all-facts ((?f DecayTree)) TRUE)";
EnvEval(clips_environment_, clips_query.str().c_str(), &decay_tree_fact_list);

Then I go through the list of facts and retrieve the needed information. There I also make another "subquery" for each of the found facts above in the following way

DATA_OBJECT spin_quantum_number_fact_list;
std::stringstream clips_query;
clips_query << "(find-fact ((?f SpinQuantumNumber)) (= ?f:unique_id "
  << spin_quantum_number_unique_id << "))";
EnvEval(clips_environment_, clips_query.str().c_str(),
  &spin_quantum_number_fact_list);

This all works fine for the first DecayTree fact, no matter at which position I start, but for the next one it crashes, because the fact address is bogus. I traced the problem down to the subquery I make. So what I did to solve the problem was to save all the DecayTree fact addresses in a vector and then process that. Since I could not find any information about my theory so far I wanted to ask here.

So my question is quite simple, and would be: If I perform two queries, after each other, does the retrieved information of the first query get invalidated as soon as I call the second query?

Answer 1

The EnvEval function should be marked in the documentation as triggering garbage collection, but it is not. CLIPS internally represents string, integers, floats, and other primitives similar to other languages (such as Java) which allow instances of classes such as String, Integer, and Float. As these values are dynamically created, they need to be subject to garbage collection when they are no longer used. Internally CLIPS uses reference counts to determine whether these values are referenced, but when these values are returned to a user's code it is not possible to know if they are referenced without some action from the user's code.

When you call EnvEval, the value it returns is exempt from garbage collection. It is not exempt the next time EnvEval is called. So if you immediately process the value returned or save it (i.e. allocate storage for a string and copy the value from CLIPS or save the fact addresses from a multifield in an array), then you don't need to worry about the value returned by CLIPS being garbage collected by a subsequent EnvEval call.

If you want to execute a series of EnvEval calls (or other CLIPS function which may trigger garbage collection) without having to worry about values being garbage collected, wrap the calls within EnvIncrementGCLocks/EnvDecrementGCLocks

EnvIncrementGCLocks(theEnv);
   ... Your Calls ...
EnvDecrementGCLocks(theEnv);  

Garbage collection for all the values returned to your code will be temporarily disabled while you make the calls and then when you finish by calling EnvDecrementGCLocks the values will be garbage collected.

There's some additional information on garbage collection in section 1.4 of the Advanced Programming Guide.