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<title>avram - a virtual machine code interpreter: 3.1.6 Deconstruction Functions</title>

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<a name="Deconstruction-Functions"></a>
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<a name="Deconstruction-Functions-1"></a>
<h3 class="subsection">3.1.6 Deconstruction Functions</h3>

<p>A fast native implementation of the deconstruction operation is provided
<a name="index-deconstruction-3"></a>
by the functions declared in &lsquo;<tt>decons.h</tt>&rsquo;.
</p>
<dl>
<dt><a name="index-avm_005finitialize_005fdecons"></a><u>Function:</u> void <b>avm_initialize_decons</b><i> ()</i></dt>
<dd><p>This should be called prior to the first call to
<code>avm_deconstruction</code>, so as to initialize some necessary internal
data structures. Results will be undefined if it is not.
</p></dd></dl>
<dl>
<dt><a name="index-avm_005fcount_005fdecons"></a><u>Function:</u> void <b>avm_count_decons</b><i> ()</i></dt>
<dd><p>For ecologically sound memory management, this function should be called
at the end of a run to verify that there have been no leaks due to the
deconstruction functions, which there won&rsquo;t be unless the code in
&lsquo;<tt>decons.c</tt>&rsquo; has been ineptly modified. An error message to the
effect of unreclaimed &ldquo;points&rdquo; could be the result otherwise.
</p></dd></dl>
<dl>
<dt><a name="index-avm_005fdeconstruction"></a><u>Function:</u> list <b>avm_deconstruction</b><i> (list <var>pointer</var>, list <var>operand</var>, int *<var>fault</var>)</i></dt>
<dd><p>Deconstructions are performed by this function, as described in
<a href="Field.html#Field">Field</a>. In the <code>silly</code> program notation (<a href="A-Simple-Lisp-Like-Language.html#A-Simple-Lisp-Like-Language">A Simple Lisp Like Language</a>), this function computes the value
of ([[<code>field</code>]] <code><var>pointer</var></code>) <code><var>operand</var></code>.
</p>
<p>For example, using the fixed list <code>avm_join(NULL,NULL)</code> as the
<code><var>pointer</var></code> parameter will cause a copy of the operand itself
to be returned as the result. A <code><var>pointer</var></code> equal to
<code>avm_join(NULL,avm_join(NULL,NULL))</code> will cause a copy of
<code>operand-&gt;tail</code> to be returned, and so on. A <code>NULL</code>
<code><var>pointer</var></code> causes an internal error.
</p>
<p>If the deconstruction is invalid, as in the case of the tail of an empty
list, the invalid deconstruction error message is returned as the
result, and the <code>*<var>fault</var></code> parameter is set to a non-zero value.
The <code>*<var>fault</var></code> parameter is also set to a non-zero value in the event
of a memory overflow, and the memory overflow message is returned.
</p></dd></dl>

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