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Memory management

This article will explain the memory management in AngelScript in detail. It's probably more detail than most developers need to know, but some may want to know exactly how it works in order to evaluate AngelScript better.

# Overview of the memory management

AngelScript uses a hybrid method with both reference counting and garbage collection, where reference counting is the main method and the garbage collection is only for backup when circular references needs to be resolved.

This design was chosen because reference counting is the easiest way of passing objects between script engine and application while still keeping track of live objects. If pure garbage collection was used, the script engine would have to know about the entire memory space of the application where script objects could be stored, otherwise it wouldn't be able to know if the objects are still alive or not.

The garbage collector is only used for those object types that have a possibility of forming a circular reference with itself or other objects, e.g. objects that have a member variable as a handle to the same type as itself. Whenever such an object is created, the garbage collector is informed so that it can keep track of the object.

The garbage collector can be executed automatically by the engine, but the application may also choose to do it manually to have better control over when it is done. The garbage collector is incremental, so it can be executed in small steps throughout the application execution without halting the rest of the processing for longer periods.

See Also
  Garbage collection

# Reference counting algorithm

All built-in script objects are automatically reference counted, and the application just need to make sure to call the AddRef and Release methods appropriately if it is storing any references to the objects outside the script engine.

Application registered objects may or may not be reference counted. Those that should be reference counted must implement the ADDREF and RELEASE behaviours so that AngelScript can notify the objects of the changes to the reference count. The application needs to keep track of the reference counter itself and free the memory when there are no more references to the object. This is usually done by having an integer member variable in the object structure itself to hold the reference count. It can also be implemented with a separate structure for holding the reference counter, but it adds extra overhead as the reference counter must be searched for with each change.

See Also
  Registering an object type

# Garbage collector algorithm

The garbage collector, used to handle the scenarios where reference counting isn't enough, uses the following algorithm.

1. Destroy garbage: Free all objects with only one reference. This reference is held by the GC itself so therefore it is known that nobody else is referencing the object.

2. Clear counters: Clear the GC counter for the remaining objects. This counter will be used to count the references to the object reachable from the GC. At the same time the objects are also flagged, so that we can detect if the references change while the garbage collector is running. This flag is automatically cleared when incrementing or decrementing the reference counter.

3. Count GC references: For each of the objects in the GC, tell it to increment the GC counter for all objects it holds references to. This is only done for objects still flagged, because if the flag is no longer set we know it has been referenced by the application, thus it is considered alive along with all objects it holds references to.

4. Mark live objects: Build a list of all objects in the GC that are not flagged or that doesn't have the GC count equal to the reference counter. These are objects that are considered alive. For each of the objects in the list, add all of the references in the object to the list as well, unless they are already in the list. As the list is traversed it will grow with new objects, these objects will also have their references added to the list. This way we're following the chain of references so we're gathering all objects that are alive.

5. Verify unmarked objects: Make one more pass over the list of objects in the GC to see if any of the objects that were not marked as alive has had the flag cleared, if it has then the run the 'mark live objects' step again.

6. Break circular references: All objects that have not been marked as alive by this time are involved in unreachable circular references and must be destroyed. The objects are destroyed by telling the objects to release the references they hold, thus breaking the circular references. When all circular references have been broken, the entire GC routine is restarted which will free the objects. If there were no dead objects, then the GC is finished.

All of the steps are incremental, i.e. they can be interrupted to allow the application and scripts to execute before continuing the garbage collection. Step 1 can also be executed individually at any time during the cycle, this permits to free up memory for objects that are not involved in cyclic memory without having to wait for the detection cycle to complete.

The GC also has a notion of new and old generations. All new objects are placed in the new generation. In this generation, only trivial garbage collection is done, i.e. the 'destroy garbage' step above. Once the objects in the new generation have survived a few iterations without being destroyed, they are moved to the old generation. In the old generation the full garbage collection algorithm is executed.

This division of new and old generations help reduce the work done by the GC to detect circular references, and thus improve the performance of the application.

See Also
  Garbage collected objects

# Memory heap

By default AngelScript uses the memory heap accessed through the standard malloc and free global functions when allocating memory. Usually this means that AngelScript shares the memory heap with the application, though if the AngelScript library is compiled as a DLL or a shared object the heap may actually be separate from the application heap.

Some applications need extra control over the memory heap that the standard C/C++ library doesn't provide. This is common with video games for consoles, that have limited memory and cannot afford loosing space due to fragmentation or many small allocations. AngelScript aids these applications as well as it is possible to register custom memory allocation routines with the library, giving the application exact control over the memory used by AngelScript.

See Also
  asSetGlobalMemoryFunctions