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<h4 class="subsection">8.3.3 Foreign Dynamic Allocation</h4>
<p>Lisp code can call the C standard library functions <code>malloc</code> and
<code>free</code> to dynamically allocate and deallocate foreign
variables. The Lisp code shares the same allocator with foreign C
code, so it's OK for foreign code to call <code>free</code> on the result of
Lisp <code>sb-alien:make-alien</code>, or for Lisp code to call
<code>sb-alien:free-alien</code> on foreign objects allocated by C
code.
<div class="defun">
— Macro: <b>sb-alien:make-alien</b><var> type size<a name="index-sb_002dalien_003amake_002dalien-286"></a></var><br>
<blockquote><p><a name="index-make_002dalien-287"></a>
The <code>sb-alien:make-alien</code> macro
returns a dynamically allocated foreign value of the specified
<var>type</var> (which is not evaluated.) The allocated memory is not
initialized, and may contain arbitrary junk. If supplied,
<var>size</var> is an expression to evaluate to compute the size of the
allocated object. There are two major cases:
<ul>
<li>When <var>type</var> is a foreign array type, an array of that type is
allocated and a pointer to it is returned. Note that you must use
<code>deref</code> to change the result to an array before you can use
<code>deref</code> to read or write elements:
<pre class="lisp"> (cl:in-package "CL-USER") ; which USEs package "SB-ALIEN"
(defvar *foo* (make-alien (array char 10)))
(type-of *foo*) ⇒ (alien (* (array (signed 8) 10)))
(setf (deref (deref foo) 0) 10) ⇒ 10
</pre>
<p>If supplied, <var>size</var> is used as the first dimension for the
array.
<li>When <var>type</var> is any other foreign type, then an object for that
type is allocated, and a pointer to it is returned. So
<code>(make-alien int)</code> returns a <code>(* int)</code>. If <var>size</var> is
specified, then a block of that many objects is allocated, with the
result pointing to the first one.
</ul>
</blockquote></div>
<div class="defun">
— Function: <b>sb-alien:free-alien</b><var> foreign-value<a name="index-sb_002dalien_003afree_002dalien-288"></a></var><br>
<blockquote><p><a name="index-free_002dalien-289"></a>
The <code>sb-alien:free-alien</code> function
frees the storage for <var>foreign-value</var>,
which must have been allocated with Lisp <code>make-alien</code>
or C <code>malloc</code>.
<p>See also the <code>sb-alien:with-alien</code> macro, which allocates foreign
values on the stack.
</p></blockquote></div>
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