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# 8. Finalization

## 8.1. Overview

Finalization is implemented internally using the Guardian Pool Class (design.mps.poolmrg). Objects can be registered for finalization using an interface function (called `mps_finalize()`). Notification of finalization is given to the client via the messaging interface. `PoolClassMRG` (design.mps.poolmrg) implements a Message Class which implements the finalization messages.

## 8.2. History

.hist.0: Incomplete design. David Jones, 1997-02-14.

.hist.1: Converted from MMInfo database design document. Richard Brooksby, 2002-06-07.

.hist.2: Converted to reStructuredText. Gareth Rees, 2013-04-13.

## 8.3. Requirements

.req: Historically only Dylan had requirements for finalization, see req.dylan.fun.final. Now (2003-02-19) Configura have requirements for finalization. Happily they are very similar.

## 8.4. Architecture

### 8.4.1. External interface

mps_res_t `mps_finalize`(mps_arena_t arena, mps_addr_t obj)

.if.register: Increases the number of times that the object located at `obj` has been registered for finalization by one. The object must have been allocated from the arena (space). Any finalization messages that are created for this object will appear on the arena’s message queue. The MPS will attempt to finalize the object that number of times.

void `mps_definalize`(mps_arena_t arena, mps_addr_t obj)

.if.deregister: `mps_definalize()` reduces the number of times that the object located at `obj` has been registered for finalization by one. It is an error to definalize an object that has not been registered for finalization.

.if.deregister.not: At the moment (1997-08-20) `mps_definalize()` is not implemented.

void `mps_message_finalization_ref`(mps_addr_t *mps_addr_return, mps_arena_t mps_arena, mps_message_t mps_message)

.if.get-ref: `mps_message_finalization_ref()` returns the reference to the finalized object stored in the finalization message.

## 8.5. Implementation

.int.over: Registering an object for finalization corresponds to allocating a reference of rank FINAL to that object. This reference is allocated in a guardian object in a pool of `PoolClassMRG` (see design.mps.poolmrg).

.int.arena.struct: The MRG pool used for managing final references is kept in the Arena (Space), referred to as the “final pool”.

.int.arena.lazy: The pool is lazily created. It will not be created until the first object is registered for finalization.

.int.arena.flag: There is a flag in the Arena that indicates whether the final pool has been created yet or not.

Res `ArenaFinalize`(Arena arena, Ref addr)

.int.finalize.create: Creates the final pool if it has not been created yet.

.int.finalize.alloc: Allocates a guardian in the final pool.

.int.finalize.write: Writes a reference to the object into the guardian object.

.int.finalize.all: That’s all.

.int.finalize.error: If either the creation of the pool or the allocation of the object fails then the error will be reported back to the caller.

.int.finalize.error.no-unwind: This function does not need to do any unwinding in the error cases because the creation of the pool is not something that needs to be undone.

.int.arena-destroy.empty: `ArenaDestroy()` empties the message queue by calling `MessageEmpty()`.

.int.arena-destroy.final-pool: If the final pool has been created then `ArenaDestroy()` destroys the final pool.

.access: `mps_message_finalization_ref()` needs to access the finalization message to retrieve the reference and then write it to where the client asks. This must be done carefully, in order to avoid breaking the invariants or creating a hidden root.

.access.invariants: We protect the invariants by using special routines `ArenaRead()` and `ArenaPoke()` to read and write the reference. This works as long as there’s no write-barrier collection. [Instead of `ArenaPoke()`, we could put in an `ArenaWrite()` that would be identical to `ArenaPoke()`, except for AVERring the invariant (or it can just AVER there are no busy traces unflipped). When we get write-barrier collection, we could change it to do the real thing, but in the absence of a write-barrier, it’s functionally identical to `ArenaPoke()`. Pekka 1997-12-09]

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