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<h4 class="subsection">8.2.2 Foreign Types and Lisp Types</h4>
<p>The foreign types form a subsystem of the SBCL type system. An
<code>alien</code> type specifier provides a way to use any foreign type as a
Lisp type specifier. For example,
<pre class="lisp"> (typep <var>foo</var> '(alien (* int)))
</pre>
<p>can be used to determine whether <var>foo</var> is a pointer to a foreign
<code>int</code>. <code>alien</code> type specifiers can be used in the same ways
as ordinary Lisp type specifiers (like <code>string</code>.) Alien type
declarations are subject to the same precise type checking as any
other declaration. See <a href="Precise-Type-Checking.html#Precise-Type-Checking">Precise Type Checking</a>.
<p>Note that the type identifiers used in the foreign type system overlap
with native Lisp type specifiers in some cases. For example, the type
specifier <code>(alien single-float)</code> is identical to
<code>single-float</code>, since foreign floats are automatically converted
to Lisp floats. When <code>type-of</code> is called on an alien value that
is not automatically converted to a Lisp value, then it will return an
<code>alien</code> type specifier.
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