117 lines
6.9 KiB
HTML
117 lines
6.9 KiB
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<!DOCTYPE html>
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<html>
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<!-- Created by GNU Texinfo 7.1, https://www.gnu.org/software/texinfo/ -->
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<head>
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<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
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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>Non-Immediate Objects (Guile Reference Manual)</title>
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<meta name="description" content="Non-Immediate Objects (Guile Reference Manual)">
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<meta name="keywords" content="Non-Immediate Objects (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="The-SCM-Type-in-Guile.html" rel="up" title="The SCM Type in Guile">
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<link href="Allocating-Heap-Objects.html" rel="next" title="Allocating Heap Objects">
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<link href="Immediate-Objects.html" rel="prev" title="Immediate Objects">
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<link rel="stylesheet" type="text/css" href="https://www.gnu.org/software/gnulib/manual.css">
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</head>
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<body lang="en">
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<div class="subsubsection-level-extent" id="Non_002dImmediate-Objects">
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<div class="nav-panel">
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<p>
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Next: <a href="Allocating-Heap-Objects.html" accesskey="n" rel="next">Allocating Heap Objects</a>, Previous: <a href="Immediate-Objects.html" accesskey="p" rel="prev">Immediate Objects</a>, Up: <a href="The-SCM-Type-in-Guile.html" accesskey="u" rel="up">The SCM Type in Guile</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="subsubsection" id="Non_002dImmediate-Objects-1"><span>9.2.5.3 Non-Immediate Objects<a class="copiable-link" href="#Non_002dImmediate-Objects-1"> ¶</a></span></h4>
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<p>A Scheme object of type <code class="code">SCM</code> that does not fulfill the
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<code class="code">SCM_IMP</code> predicate holds an encoded reference to a heap object.
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This reference can be decoded to a C pointer to a heap object using the
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<code class="code">SCM_UNPACK_POINTER</code> macro. The encoding of a pointer to a heap
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object into a <code class="code">SCM</code> value is done using the <code class="code">SCM_PACK_POINTER</code>
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macro.
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</p>
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<a class="index-entry-id" id="index-cells_002c-deprecated-concept"></a>
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<p>Before Guile 2.0, Guile had a custom garbage collector that allocated
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heap objects in units of 2-word <em class="dfn">cells</em>. With the move to the
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BDW-GC collector in Guile 2.0, Guile can allocate heap objects of any
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size, and the concept of a cell is now obsolete. Still, we mention
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it here as the name still appears in various low-level interfaces.
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</p>
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<dl class="first-deftypefn">
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<dt class="deftypefn" id="index-SCM_005fUNPACK_005fPOINTER"><span class="category-def">Macro: </span><span><code class="def-type">scm_t_bits *</code> <strong class="def-name">SCM_UNPACK_POINTER</strong> <code class="def-code-arguments">(SCM <var class="var">x</var>)</code><a class="copiable-link" href="#index-SCM_005fUNPACK_005fPOINTER"> ¶</a></span></dt>
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<dt class="deftypefnx def-cmd-deftypefn" id="index-SCM2PTR"><span class="category-def">Macro: </span><span><code class="def-type">scm_t_cell *</code> <strong class="def-name">SCM2PTR</strong> <code class="def-code-arguments">(SCM <var class="var">x</var>)</code><a class="copiable-link" href="#index-SCM2PTR"> ¶</a></span></dt>
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<dd><p>Extract and return the heap object pointer from a non-immediate
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<code class="code">SCM</code> object <var class="var">x</var>. The name <code class="code">SCM2PTR</code> is deprecated but
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still common.
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</p></dd></dl>
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<dl class="first-deftypefn">
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<dt class="deftypefn" id="index-_0028scm_005ft_005fbits"><span class="category-def">Macro: </span><span><code class="def-type">SCM_PACK_POINTER</code> <strong class="def-name">(scm_t_bits</strong> <code class="def-code-arguments">* <var class="var">x</var>)</code><a class="copiable-link" href="#index-_0028scm_005ft_005fbits"> ¶</a></span></dt>
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<dt class="deftypefnx def-cmd-deftypefn" id="index-PTR2SCM"><span class="category-def">Macro: </span><span><code class="def-type">SCM</code> <strong class="def-name">PTR2SCM</strong> <code class="def-code-arguments">(scm_t_cell * <var class="var">x</var>)</code><a class="copiable-link" href="#index-PTR2SCM"> ¶</a></span></dt>
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<dd><p>Return a <code class="code">SCM</code> value that encodes a reference to the heap object
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pointer <var class="var">x</var>. The name <code class="code">PTR2SCM</code> is deprecated but still
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common.
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</p></dd></dl>
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<p>Note that it is also possible to transform a non-immediate <code class="code">SCM</code>
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value by using <code class="code">SCM_UNPACK</code> into a <code class="code">scm_t_bits</code> variable.
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However, the result of <code class="code">SCM_UNPACK</code> may not be used as a pointer to
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a heap object: only <code class="code">SCM_UNPACK_POINTER</code> is guaranteed to transform
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a <code class="code">SCM</code> object into a valid pointer to a heap object. Also, it is
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not allowed to apply <code class="code">SCM_PACK_POINTER</code> to anything that is not a
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valid pointer to a heap object.
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</p>
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<p>Summary:
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</p><ul class="itemize mark-bullet">
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<li>Only use <code class="code">SCM_UNPACK_POINTER</code> on <code class="code">SCM</code> values for which
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<code class="code">SCM_IMP</code> is false!
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</li><li>Don’t use <code class="code">(scm_t_cell *) SCM_UNPACK (<var class="var">x</var>)</code>! Use
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<code class="code">SCM_UNPACK_POINTER (<var class="var">x</var>)</code> instead!
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</li><li>Don’t use <code class="code">SCM_PACK_POINTER</code> for anything but a heap object pointer!
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</li></ul>
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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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Next: <a href="Allocating-Heap-Objects.html">Allocating Heap Objects</a>, Previous: <a href="Immediate-Objects.html">Immediate Objects</a>, Up: <a href="The-SCM-Type-in-Guile.html">The SCM Type in Guile</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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</body>
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</html>
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