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<TITLE>Finalization in the Boehm-Demers-Weiser collector </TITLE>
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<H1>Finalization</h1>
Many garbage collectors provide a facility for executing user code
just before an object is collected. This can be used to reclaim any
system resources or non-garbage-collected memory associated with the
object.
Experience has shown that this can be a useful facility.
It is indispensable in cases in which system resources are embedded
in complex data structures (<I>e.g.</i> file descriptors
in the <A type="text/plain" HREF="../include/cord.h">cord package</a>).
<P>
Our collector provides the necessary functionality through
<TT>GC_register_finalizer</tt> in
<A type="text/plain" HREF="../include/gc.h">gc.h</a>, or by
inheriting from <TT>gc_cleanup</tt>
in <A type="text/plain" HREF="../include/gc_cpp.h">gc_cpp.h</a>.
<P>
However, finalization should not be used in the same way as C++
destructors. In well-written programs there will typically be
very few uses of finalization. (Garbage collected programs that
interact with explicitly memory-managed libraries may be an exception.)
<P>
In general the following guidelines should be followed:
<UL>
<LI>
Actions that must be executed promptly do not belong in finalizers.
They should be handled by explicit calls in the code (or C++
destructors if you prefer). If you expect the action to occur at
a specific point, this is probably not hard.
<LI>
Finalizers are intended for resource reclamation.
<LI>
Scarce system resources should be managed explicitly whenever
convenient. Use finalizers only as a backup mechanism for the
cases that would be hard to handle explicitly.
<LI>
If scarce resources are managed with finalization, the allocation
routine for that resource (<I>e.g.</i> open for file handles) should force
a garbage collection (two if that doesn't suffice) if it finds itself
short of the resource.
<LI>
If extremely scarce resources are managed by finalization (<I>e.g.</i>
file descriptors on systems which have a limit of 20 open files),
it may be necessary to introduce a descriptor caching scheme to
hide the resource limit.
(<I>E.g.</i>, the program would keep real file descriptors
for the 20 most recently used logically open files.
Any other needed files would be closed after saving their state.
They would then be reopened on demand.
Finalization would logically close the file, closing the
real descriptor only if it happened to be cached.)
Note that most modern systems (<I>e.g.</i> Irix®) allow hundreds or
thousands of open files, and this is typically not an issue.
<LI>
Finalization code may
be run anyplace an allocation or other call to the collector
takes place.
In multi-threaded programs, finalizers have to obey the normal
locking conventions to ensure safety.
Code run directly from finalizers should not acquire locks that may
be held during allocation. This restriction can be easily circumvented
by registering a finalizer which enqueues the real action for execution
in a separate thread.
<P>
In single-threaded code, it is also often easiest to have finalizers
queue actions, which are then explicitly run during an
explicit call by the user's program.
</ul>
<H1>Topologically Ordered Finalization</h1>
Our <A HREF="overview.html">conservative garbage collector</a> supports
a form of finalization
(with <TT>GC_register_finalizer</tt>)
in which objects are finalized in topological
order. If <I>A</i> points to <I>B</i>, and both are registered for
finalization, it is guaranteed the <I>A</i> will be finalized first.
This usually guarantees that finalization procedures see only
unfinalized objects.
<P>
This decision is often questioned, particularly since it has an obvious
disadvantage. The current implementation finalizes long chains of
finalizable objects one per collection. This is hard to avoid, since
the first finalizer invoked may store a pointer to the rest of the chain
in a global variable, making it accessible again. Or it may mutate the
rest of the chain.
<P>
Cycles involving one or more finalizable objects are never finalized.
<H1>
Why topological ordering?
</h1>
It is important to keep in mind that the choice of finalization ordering
matters only in relatively rare cases. In spite of the fact that it has
received a lot of discussion, it is not one of the more important
decisions in designing a system. Many, especially smaller, applications
will never notice the difference. Nonetheless, we believe that topologically
ordered finalization is the right choice.
<P>
To understand the justification, observe that if <I>A</i>s
finalization procedure does not refer to <I>B</i>, we could fairly easily have
avoided the dependency. We could have split <I>A</i> into <I>A'</i>
and <I>A''</i> such that any references to <I>A</i> become references to
<I>A'</i>, <I>A'</i> points to <I>A''</i> but not vice-versa, only fields
needed for finalization are stored in <I>A''</i>, and <I>A''</i> is enabled
for finalization. (<TT>GC_register_disappearing_link</tt> provides an
alternative mechanism that does not require breaking up objects.)
<P>
Thus assume that <I>A</i> actually does need access to <I>B</i> during
finalization. To make things concrete, assume that <I>B</i> is
finalizable because it holds a pointer to a C object, which must be
explicitly deallocated. (This is likely to be one of the most common
uses of finalization.) If <I>B</i> happens to be finalized first,
<I>A</i> will see a dangling pointer during its finalization. But a
principal goal of garbage collection was to avoid dangling pointers.
<P>
Note that the client program could enforce topological ordering
even if the system didn't. A pointer to <I>B</i> could be stored in
some globally visible place, where it is cleared only by <I>A</i>s
finalizer. But this puts the burden to ensure safety back on the
programmer.
<P>
With topologically ordered finalization, the programmer
can fail to split an object, thus leaving an accidental cycle. This
results in a leak, which is arguably less dangerous than a dangling
pointer. More importantly, it is <I>much</i> easier to diagnose,
since the garbage collector would have to go out of its way not to
notice finalization cycles. It can trivially report them.
<P>
Furthermore unordered finalization does not really solve the problem
of cycles. Consider the above case in which <I>A</i>s
finalization procedure depends on <I>B</i>, and thus a pointer to <I>B</i>
is stored in a global data structure, to be cleared by <I>A</i>s finalizer.
If there is an accidental pointer from <I>B</i> back to <I>A</i>, and
thus a cycle, neither <I>B</i> nor <I>A</i> will become unreachable.
The leak is there, just as in the topologically ordered case, but it is
hidden from easy diagnosis.
<P>
A number of alternative finalization orderings have been proposed, e.g.
based on statically assigned priorities. In our opinion, these are much
more likely to require complex programming discipline to use in a large
modular system. (Some of them, e.g. Guardians proposed by Dybvig,
Bruggeman, and Eby, do avoid some problems which arise in combination
with certain other collection algorithms.)
<P>
Fundamentally, a garbage collector assumes that objects reachable
via pointer chains may be accessed, and thus should be preserved.
Topologically ordered finalization simply extends this to object finalization;
an finalizable object reachable from another finalizer via a pointer chain
is presumed to be accessible by the finalizer, and thus should not be
finalized.
<H1>Programming with topological finalization</h1>
Experience with Cedar has shown that cycles or long chains of finalizable
objects are typically not a problem.
Finalizable objects are typically rare.
There are several ways to reduce spurious dependencies between finalizable
objects. Splitting objects as discussed above is one technique.
The collector also provides <TT>GC_register_disappearing_link</tt>, which
explicitly nils a pointer before determining finalization ordering.
<P>
Some so-called "operating systems" fail to clean up some resources associated
with a process. These resources must be deallocated at all cost before
process exit whether or not they are still referenced. Probably the best
way to deal with those is by not relying exclusively on finalization.
They should be registered in a table of weak pointers (implemented as
disguised pointers cleared by the finalization procedure that deallocates
the resource). If any references are still left at process exit, they
can be explicitly deallocated then.
<H1>Getting around topological finalization ordering</h1>
There are certain situations in which cycles between finalizable objects are
genuinely unavoidable. Most notably, C++ compilers introduce self-cycles
to represent inheritance. <TT>GC_register_finalizer_ignore_self</tt> tells the
finalization part of the collector to ignore self cycles.
This is used by the C++ interface.
<P>
Finalize.c actually contains an intentionally undocumented mechanism
for registering a finalizable object with user-defined dependencies.
The problem is that this dependency information is also used for memory
reclamation, not just finalization ordering. Thus misuse can result in
dangling pointers even if finalization doesn't create any.
The risk of dangling pointers can be eliminated by building the collector
with -DJAVA_FINALIZATION. This forces objects reachable from finalizers
to be marked, even though this dependency is not considered for finalization
ordering.
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