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<h3 class="section">7.1 Garbage Collection</h3>
<p>SBCL provides additional garbage collection functionality not
specified by ANSI. Weak pointers allow references to objects to be
maintained without keeping them from being garbage collected, and
“finalization” hooks are available to cause code to be executed when
an object has been garbage collected. Additionally users can specify
their own cleanup actions to be executed with garbage collection. See
also <code>make-hash-table</code> for information on weak hash tables.
<p><a name="Function-sb_002dext_003afinalize"></a>
<div class="defun">
— Function: <b>sb-ext:finalize</b><var> object function &key dont-save<a name="index-sb_002dext_003afinalize-147"></a></var><br>
<blockquote><p><a name="index-sb_002dext_003afinalize-148"></a>Arrange for the designated <code>function</code> to be called when there
are no more references to <code>object</code>, including references in
<code>function</code> itself.
<p>If <code>dont-save</code> is true, the finalizer will be cancelled when
<code>save-lisp-and-die</code> is called: this is useful for finalizers
deallocating system memory, which might otherwise be called
with addresses from the old image.
<p>In a multithreaded environment <code>function</code> may be called in any
thread. In both single and multithreaded environments <code>function</code>
may be called in any dynamic scope: consequences are unspecified
if <code>function</code> is not fully re-entrant.
<p>Errors from <code>function</code> are handled and cause a <code>warning</code> to be
signalled in whichever thread the <code>function</code> was called in.
<p>Examples:
<pre class="lisp"> ;;; good (assumes RELEASE-HANDLE is re-entrant)
(let* ((handle (get-handle))
(object (make-object handle)))
(finalize object (lambda () (release-handle handle)))
object)
</pre>
<pre class="lisp"> ;;; bad, finalizer refers to object being finalized, causing
;;; it to be retained indefinitely
(let* ((handle (get-handle))
(object (make-object handle)))
(finalize object (lambda () (release-handle (object-handle object)))))
</pre>
<pre class="lisp"> ;;; bad, not re-entrant
(defvar *rec* nil)
</pre>
<pre class="lisp"> (defun oops ()
(when *rec*
(error "recursive OOPS"))
(let ((*rec* t))
(gc))) ; or just cons enough to cause one
</pre>
<pre class="lisp"> (progn
(finalize "oops" #'oops)
(oops)) ; causes GC and re-entry to #'oops due to the finalizer
; -> ERROR, caught, WARNING signalled
</pre>
</blockquote></div>
<p><a name="Function-sb_002dext_003acancel_002dfinalization"></a>
<div class="defun">
— Function: <b>sb-ext:cancel-finalization</b><var> object<a name="index-sb_002dext_003acancel_002dfinalization-149"></a></var><br>
<blockquote><p><a name="index-sb_002dext_003acancel_002dfinalization-150"></a>Cancel any finalization for <code>object</code>.
</p></blockquote></div>
<p><a name="Function-sb_002dext_003amake_002dweak_002dpointer"></a>
<div class="defun">
— Function: <b>sb-ext:make-weak-pointer</b><var> object<a name="index-sb_002dext_003amake_002dweak_002dpointer-151"></a></var><br>
<blockquote><p><a name="index-sb_002dext_003amake_002dweak_002dpointer-152"></a>Allocate and return a weak pointer which points to <code>object</code>.
</p></blockquote></div>
<p><a name="Function-sb_002dext_003aweak_002dpointer_002dvalue"></a>
<div class="defun">
— Function: <b>sb-ext:weak-pointer-value</b><var> weak-pointer<a name="index-sb_002dext_003aweak_002dpointer_002dvalue-153"></a></var><br>
<blockquote><p><a name="index-sb_002dext_003aweak_002dpointer_002dvalue-154"></a>If <code>weak-pointer</code> is valid, return the value of <code>weak-pointer</code> and <code>t</code>.
If the referent of <code>weak-pointer</code> has been garbage collected,
returns the values <code>nil</code> and <code>nil</code>.
</p></blockquote></div>
<p><a name="Variable-sb_002dext_003a_002aafter_002dgc_002dhooks_002a"></a>
<div class="defun">
— Variable: <b>sb-ext:*after-gc-hooks*</b><var><a name="index-sb_002dext_003a_002aafter_002dgc_002dhooks_002a-155"></a></var><br>
<blockquote><p><a name="index-sb_002dext_003a_002aafter_002dgc_002dhooks_002a-156"></a>Called after each garbage collection, except for garbage collections
triggered during thread exits. In a multithreaded environment these hooks may
run in any thread.
</p></blockquote></div>
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Mandriva
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i586
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