123 lines
5.8 KiB
HTML
123 lines
5.8 KiB
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<!DOCTYPE html>
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<html>
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<!-- This manual documents Guile version 3.0.10.
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Copyright (C) 1996-1997, 2000-2005, 2009-2023 Free Software Foundation,
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Inc.
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Copyright (C) 2021 Maxime Devos
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Copyright (C) 2024 Tomas Volf
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Permission is granted to copy, distribute and/or modify this document
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under the terms of the GNU Free Documentation License, Version 1.3 or
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any later version published by the Free Software Foundation; with no
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Invariant Sections, no Front-Cover Texts, and no Back-Cover Texts. A
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copy of the license is included in the section entitled "GNU Free
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Documentation License." -->
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<title>Guardians (Guile Reference Manual)</title>
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<meta name="description" content="Guardians (Guile Reference Manual)">
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<link href="index.html" rel="start" title="Top">
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<link href="Concept-Index.html" rel="index" title="Concept Index">
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<link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">
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<link href="Memory-Management.html" rel="up" title="Memory Management">
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<link href="Weak-References.html" rel="prev" title="Weak References">
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</head>
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<body lang="en">
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<div class="subsection-level-extent" id="Guardians">
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<div class="nav-panel">
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<p>
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Previous: <a href="Weak-References.html" accesskey="p" rel="prev">Weak References</a>, Up: <a href="Memory-Management.html" accesskey="u" rel="up">Memory Management and Garbage Collection</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Concept-Index.html" title="Index" rel="index">Index</a>]</p>
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</div>
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<hr>
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<h4 class="subsection" id="Guardians-1"><span>6.17.4 Guardians<a class="copiable-link" href="#Guardians-1"> ¶</a></span></h4>
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<p>Guardians provide a way to be notified about objects that would
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otherwise be collected as garbage. Guarding them prevents the objects
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from being collected and cleanup actions can be performed on them, for
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example.
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</p>
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<p>See R. Kent Dybvig, Carl Bruggeman, and David Eby (1993) "Guardians in
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a Generation-Based Garbage Collector". ACM SIGPLAN Conference on
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Programming Language Design and Implementation, June 1993.
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</p>
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<dl class="first-deffn">
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<dt class="deffn" id="index-make_002dguardian"><span class="category-def">Scheme Procedure: </span><span><strong class="def-name">make-guardian</strong><a class="copiable-link" href="#index-make_002dguardian"> ¶</a></span></dt>
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<dt class="deffnx def-cmd-deffn" id="index-scm_005fmake_005fguardian"><span class="category-def">C Function: </span><span><strong class="def-name">scm_make_guardian</strong> <var class="def-var-arguments">()</var><a class="copiable-link" href="#index-scm_005fmake_005fguardian"> ¶</a></span></dt>
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<dd><p>Create a new guardian. A guardian protects a set of objects from
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garbage collection, allowing a program to apply cleanup or other
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actions.
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</p>
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<p><code class="code">make-guardian</code> returns a procedure representing the guardian.
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Calling the guardian procedure with an argument adds the argument to
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the guardian’s set of protected objects. Calling the guardian
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procedure without an argument returns one of the protected objects
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which are ready for garbage collection, or <code class="code">#f</code> if no such object
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is available. Objects which are returned in this way are removed from
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the guardian.
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</p>
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<p>You can put a single object into a guardian more than once and you can
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put a single object into more than one guardian. The object will then
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be returned multiple times by the guardian procedures.
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</p>
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<p>An object is eligible to be returned from a guardian when it is no
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longer referenced from outside any guardian.
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</p>
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<p>There is no guarantee about the order in which objects are returned
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from a guardian. If you want to impose an order on finalization
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actions, for example, you can do that by keeping objects alive in some
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global data structure until they are no longer needed for finalizing
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other objects.
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</p>
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<p>Being an element in a weak vector, a key in a hash table with weak
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keys, or a value in a hash table with weak values does not prevent an
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object from being returned by a guardian. But as long as an object
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can be returned from a guardian it will not be removed from such a
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weak vector or hash table. In other words, a weak link does not
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prevent an object from being considered collectible, but being inside
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a guardian prevents a weak link from being broken.
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</p>
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<p>A key in a weak key hash table can be thought of as having a strong
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reference to its associated value as long as the key is accessible.
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Consequently, when the key is only accessible from within a guardian,
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the reference from the key to the value is also considered to be
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coming from within a guardian. Thus, if there is no other reference
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to the value, it is eligible to be returned from a guardian.
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</p></dd></dl>
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</div>
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<hr>
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<div class="nav-panel">
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<p>
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Previous: <a href="Weak-References.html">Weak References</a>, Up: <a href="Memory-Management.html">Memory Management and Garbage Collection</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Concept-Index.html" title="Index" rel="index">Index</a>]</p>
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</div>
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