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Next: <a rel="next" accesskey="n" href="Coercing-Foreign-Values.html#Coercing-Foreign-Values">Coercing Foreign Values</a>,
Up: <a rel="up" accesskey="u" href="Operations-On-Foreign-Values.html#Operations-On-Foreign-Values">Operations On Foreign Values</a>
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<h4 class="subsection">8.3.1 Accessing Foreign Values</h4>
<div class="defun">
— Function: <b>sb-alien:deref</b><var> pointer-or-array &rest indices<a name="index-sb_002dalien_003aderef-268"></a></var><br>
<blockquote><p><a name="index-deref-269"></a>
The <code>sb-alien:deref</code> function returns the value pointed to by a
foreign pointer, or the value of a foreign array element. When
dereferencing a pointer, an optional single index can be specified to
give the equivalent of C pointer arithmetic; this index is scaled by
the size of the type pointed to. When dereferencing an array, the
number of indices must be the same as the number of dimensions in the
array type. <code>deref</code> can be set with <code>setf</code> to assign a new
value.
</p></blockquote></div>
<div class="defun">
— Function: <b>sb-alien:slot</b><var> struct-or-union slot-name<a name="index-sb_002dalien_003aslot-270"></a></var><br>
<blockquote><p><a name="index-slot-271"></a>
The <code>sb-alien:slot</code> function extracts the value of the slot named
<var>slot-name</var> from a foreign <code>struct</code> or <code>union</code>. If
<var>struct-or-union</var> is a pointer to a structure or union, then it is
automatically dereferenced. <code>sb-alien:slot</code> can be set with
<code>setf</code> to assign a new value. Note that <var>slot-name</var> is
evaluated, and need not be a compile-time constant (but only constant
slot accesses are efficiently compiled).
</p></blockquote></div>
<h5 class="subsubsection">8.3.1.1 Untyped memory</h5>
<p>As noted at the beginning of the chapter, the System Area Pointer
facilities allow untyped access to foreign memory. <acronym>SAP</acronym>s can
be converted to and from the usual typed foreign values using
<code>sap-alien</code> and <code>alien-sap</code> (described elsewhere), and also
to and from integers - raw machine addresses. They should thus be
used with caution; corrupting the Lisp heap or other memory with
<acronym>SAP</acronym>s is trivial.
<div class="defun">
— Function: <b>sb-sys:int-sap</b><var> machine-address<a name="index-sb_002dsys_003aint_002dsap-272"></a></var><br>
<blockquote><p><a name="index-int_002dsap-273"></a>
Creates a <acronym>SAP</acronym> pointing at the virtual address
<var>machine-address</var>.
</p></blockquote></div>
<div class="defun">
— Function: <b>sb-sys:sap-ref-32</b><var> sap offset<a name="index-sb_002dsys_003asap_002dref_002d32-274"></a></var><br>
<blockquote><p><a name="index-sap_002dref_002d32-275"></a>
Access the value of the memory location at <var>offset</var> bytes from
<var>sap</var>. This form may also be used with <code>setf</code> to alter the
memory at that location.
</p></blockquote></div>
<div class="defun">
— Function: <b>sb-sys:sap=</b><var> sap1 sap2<a name="index-sb_002dsys_003asap_003d-276"></a></var><br>
<blockquote><p><a name="index-sap_003d-277"></a>
Compare <var>sap1</var> and <var>sap2</var> for equality.
</p></blockquote></div>
<p>Similarly named functions exist for accessing other sizes of word,
other comparisons, and other conversions. The reader is invited to
use <code>apropos</code> and <code>describe</code> for more details
<pre class="lisp"> (apropos "sap" :sb-sys)
</pre>
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Mandriva
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2010.0
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