152 lines
7.4 KiB
HTML
152 lines
7.4 KiB
HTML
<!DOCTYPE html>
|
|
<html>
|
|
<!-- Created by GNU Texinfo 7.1, https://www.gnu.org/software/texinfo/ -->
|
|
<head>
|
|
<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
|
|
<!-- This manual documents Guile version 3.0.10.
|
|
|
|
Copyright (C) 1996-1997, 2000-2005, 2009-2023 Free Software Foundation,
|
|
Inc.
|
|
|
|
Copyright (C) 2021 Maxime Devos
|
|
|
|
Copyright (C) 2024 Tomas Volf
|
|
|
|
|
|
Permission is granted to copy, distribute and/or modify this document
|
|
under the terms of the GNU Free Documentation License, Version 1.3 or
|
|
any later version published by the Free Software Foundation; with no
|
|
Invariant Sections, no Front-Cover Texts, and no Back-Cover Texts. A
|
|
copy of the license is included in the section entitled "GNU Free
|
|
Documentation License." -->
|
|
<title>Creating Foreign Objects (Guile Reference Manual)</title>
|
|
|
|
<meta name="description" content="Creating Foreign Objects (Guile Reference Manual)">
|
|
<meta name="keywords" content="Creating Foreign Objects (Guile Reference Manual)">
|
|
<meta name="resource-type" content="document">
|
|
<meta name="distribution" content="global">
|
|
<meta name="Generator" content=".texi2any-real">
|
|
<meta name="viewport" content="width=device-width,initial-scale=1">
|
|
|
|
<link href="index.html" rel="start" title="Top">
|
|
<link href="Concept-Index.html" rel="index" title="Concept Index">
|
|
<link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">
|
|
<link href="Defining-New-Foreign-Object-Types.html" rel="up" title="Defining New Foreign Object Types">
|
|
<link href="Type-Checking-of-Foreign-Objects.html" rel="next" title="Type Checking of Foreign Objects">
|
|
<link href="Defining-Foreign-Object-Types.html" rel="prev" title="Defining Foreign Object Types">
|
|
<style type="text/css">
|
|
<!--
|
|
a.copiable-link {visibility: hidden; text-decoration: none; line-height: 0em}
|
|
div.example {margin-left: 3.2em}
|
|
span:hover a.copiable-link {visibility: visible}
|
|
-->
|
|
</style>
|
|
<link rel="stylesheet" type="text/css" href="https://www.gnu.org/software/gnulib/manual.css">
|
|
|
|
|
|
</head>
|
|
|
|
<body lang="en">
|
|
<div class="subsection-level-extent" id="Creating-Foreign-Objects">
|
|
<div class="nav-panel">
|
|
<p>
|
|
Next: <a href="Type-Checking-of-Foreign-Objects.html" accesskey="n" rel="next">Type Checking of Foreign Objects</a>, Previous: <a href="Defining-Foreign-Object-Types.html" accesskey="p" rel="prev">Defining Foreign Object Types</a>, Up: <a href="Defining-New-Foreign-Object-Types.html" accesskey="u" rel="up">Defining New Foreign Object Types</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>
|
|
</div>
|
|
<hr>
|
|
<h4 class="subsection" id="Creating-Foreign-Objects-1"><span>5.5.2 Creating Foreign Objects<a class="copiable-link" href="#Creating-Foreign-Objects-1"> ¶</a></span></h4>
|
|
|
|
<p>Foreign objects contain zero or more “slots” of data. A slot can hold
|
|
a pointer, an integer that fits into a <code class="code">size_t</code> or <code class="code">ssize_t</code>,
|
|
or a <code class="code">SCM</code> value.
|
|
</p>
|
|
<p>All objects of a given foreign type have the same number of slots. In
|
|
the example from the previous section, the <code class="code">image</code> type has one
|
|
slot, because the slots list passed to
|
|
<code class="code">scm_make_foreign_object_type</code> is of length one. (The actual names
|
|
given to slots are unimportant for most users of the C interface, but
|
|
can be used on the Scheme side to introspect on the foreign object.)
|
|
</p>
|
|
<p>To construct a foreign object and initialize its first slot, call
|
|
<code class="code">scm_make_foreign_object_1 (<var class="var">type</var>, <var class="var">first_slot_value</var>)</code>.
|
|
There are similarly named constructors for initializing 0, 1, 2, or 3
|
|
slots, or initializing <var class="var">n</var> slots via an array. See <a class="xref" href="Foreign-Objects.html">Foreign Objects</a>, for full details. Any fields that are not explicitly
|
|
initialized are set to 0.
|
|
</p>
|
|
<p>To get or set the value of a slot by index, you can use the
|
|
<code class="code">scm_foreign_object_ref</code> and <code class="code">scm_foreign_object_set_x</code>
|
|
functions. These functions take and return values as <code class="code">void *</code>
|
|
pointers; there are corresponding convenience procedures like
|
|
<code class="code">_signed_ref</code>, <code class="code">_unsigned_set_x</code> and so on for dealing with
|
|
slots as signed or unsigned integers.
|
|
</p>
|
|
<p>Foreign objects fields that are pointers can be tricky to manage. If
|
|
possible, it is best that all memory that is referenced by a foreign
|
|
object be managed by the garbage collector. That way, the GC can
|
|
automatically ensure that memory is accessible when it is needed, and
|
|
freed when it becomes inaccessible. If this is not the case for your
|
|
program – for example, if you are exposing an object to Scheme that was
|
|
allocated by some other, Guile-unaware part of your program – then you
|
|
will probably need to implement a finalizer. See <a class="xref" href="Foreign-Object-Memory-Management.html">Foreign Object Memory Management</a>, for more.
|
|
</p>
|
|
<p>Continuing the example from the previous section, if the global variable
|
|
<code class="code">image_type</code> contains the type returned by
|
|
<code class="code">scm_make_foreign_object_type</code>, here is how we could construct a
|
|
foreign object whose “data” field contains a pointer to a freshly
|
|
allocated <code class="code">struct image</code>:
|
|
</p>
|
|
<div class="example">
|
|
<pre class="example-preformatted">SCM
|
|
make_image (SCM name, SCM s_width, SCM s_height)
|
|
{
|
|
struct image *image;
|
|
int width = scm_to_int (s_width);
|
|
int height = scm_to_int (s_height);
|
|
|
|
/* Allocate the `struct image'. Because we
|
|
use scm_gc_malloc, this memory block will
|
|
be automatically reclaimed when it becomes
|
|
inaccessible, and its members will be traced
|
|
by the garbage collector. */
|
|
image = (struct image *)
|
|
scm_gc_malloc (sizeof (struct image), "image");
|
|
|
|
image->width = width;
|
|
image->height = height;
|
|
|
|
/* Allocating the pixels with
|
|
scm_gc_malloc_pointerless means that the
|
|
pixels data is collectable by GC, but
|
|
that GC shouldn't spend time tracing its
|
|
contents for nested pointers because there
|
|
aren't any. */
|
|
image->pixels =
|
|
scm_gc_malloc_pointerless (width * height, "image pixels");
|
|
|
|
image->name = name;
|
|
image->update_func = SCM_BOOL_F;
|
|
|
|
/* Now wrap the struct image* in a new foreign
|
|
object, and return that object. */
|
|
return scm_make_foreign_object_1 (image_type, image);
|
|
}
|
|
</pre></div>
|
|
|
|
<p>We use <code class="code">scm_gc_malloc_pointerless</code> for the pixel buffer to tell the
|
|
garbage collector not to scan it for pointers. Calls to
|
|
<code class="code">scm_gc_malloc</code>, <code class="code">scm_make_foreign_object_1</code>, and
|
|
<code class="code">scm_gc_malloc_pointerless</code> raise an exception in out-of-memory
|
|
conditions; the garbage collector is able to reclaim previously
|
|
allocated memory if that happens.
|
|
</p>
|
|
|
|
</div>
|
|
<hr>
|
|
<div class="nav-panel">
|
|
<p>
|
|
Next: <a href="Type-Checking-of-Foreign-Objects.html">Type Checking of Foreign Objects</a>, Previous: <a href="Defining-Foreign-Object-Types.html">Defining Foreign Object Types</a>, Up: <a href="Defining-New-Foreign-Object-Types.html">Defining New Foreign Object Types</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>
|
|
</div>
|
|
|
|
|
|
|
|
</body>
|
|
</html>
|