A simple Common Lisp finalizer for foreign objects when using CFFI and TRIVIAL-GARBAGE.

simple-finalizer

-*- mode: org -*-

* INTRODUCTION

This is the README for SIMPLE-FINALIZER, a package that wraps the 
finalizer functionality provided by TRIVIAL-GARBAGE and CFFI.

SIMPLE-FINALIZER depends on TRIVIAL-GARBAGE and CFFI.
 
ASDF packaging is provided; 

(asdf:oos 'asdf:load-op :simple-finalizer)

After loading the system, try the following example;
 
(make-instance 'simfin:foreign-object :fp (cffi:foreign-alloc :char :count 20))

(simfin:gc-p *)
(simfin:free **)

* DESCRIPTION

Simple-finalizer manages the deletion of a foreign object by implementing a wrapper object 
that contains both the foreign-object and the function to free the foreign object.
The foreign object may be deleted explictly by calling FREE, or automatically when the 
object is finalized (at time of garbage collection).

Use MAKE-INSTANCE to create a new FOREIGN-OBJECT, as show below.

(make-instance 'simfin:foreign-object :fp <foreign-object> :free <free-function> :gc <enable-finalization>)

FOREIGN-OBJECT wraps the foreign object stored in FP. The foreign object is finalized 
when the wrapper is garbage collected using the function in :FREE and :GC is T.

FP returns a reference to the foreign object or NIL if the object has already been 
deleted using FREE.

(simfin:FP *obj*)

The object can be explicitely freed using FREE. Once freed, the reference to the foreign 
object as well as the deletion function are set to NIL. Attempting to enable finalization
on a previously deleted foreign object will result in an error.

(simfin:FREE *obj*)

GC-P will return T if the object will be deleted at finalization/garbage collection.
Returns NIL if the foreign object was previously deleted or if the foreign object must 
be freed manually using FREE.

(simfin:GC-P *obj*)

FREE-P returns T if the foreign object was previously freed

(simfin:FREE-P *obj*)

Object deletion at finalization is enabled or disabled by setting GC-P to T or NIL, as follows;

(SETF (GC-P *obj*) T)   ; Enable finalization
(SETF (GC-P *obj*) NIL) ; Disable finalization

THIS-FP a reference to the foreign object. Unlike FP which can be overridden 
by a subclass, THIS-FP must never be overridden and must always point to the foreign object.

An important point to note is that the function passed in :FREE does not act as an object 
'destructor' in the C++ or Java sense because there are no destructors in Common Lisp. 
:FREE is not called when FOREIGN-OBJECT goes out of scope, but rather when the LISP
object is being garbage collected. Another point to note is that by the time the function 
in :FREE is called, the CLOS slots for the object will already have been garbage-collected.
  
Comments, criticisms, additions, and optimizations are welcome at the
email address below.
 
Luke Crook
luke@balooga.com


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