Sophie: sbcl-1.0.31-1mdv2010.0 i586

sbcl-1.0.31-1mdv2010.0.i586.rpm

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<h3 class="section">8.1 Introduction to the Foreign Function Interface</h3>

<!-- AKA "Introduction to Aliens" in the CMU CL manual -->
<p>Because of Lisp's emphasis on dynamic memory allocation and garbage
collection, Lisp implementations use non-C-like memory representations
for objects.  This representation mismatch creates friction when a Lisp
program must share objects with programs which expect C data.  There
are three common approaches to establishing communication:

     <ul>
<li>The burden can be placed on the foreign program (and programmer) by
requiring the knowledge and use of the representations used internally
by the Lisp implementation.  This can require a considerable amount of
&ldquo;glue&rdquo; code on the C side, and that code tends to be sensitively
dependent on the internal implementation details of the Lisp system.

     <li>The Lisp system can automatically convert objects back and forth
between the Lisp and foreign representations.  This is convenient, but
translation becomes prohibitively slow when large or complex data
structures must be shared. This approach is supported by the SBCL
<acronym>FFI</acronym>, and used automatically by the when passing integers and
strings.

     <li>The Lisp program can directly manipulate foreign objects through the
use of extensions to the Lisp language.

   </ul>

   <p>SBCL, like CMUCL before it, relies primarily on the automatic
conversion and direct manipulation approaches. The <code>SB-ALIEN</code>
package provides a facility wherein foreign values of simple scalar
types are automatically converted and complex types are directly
manipulated in their foreign representation.  Additionally the
lower-level System Area Pointers (or <acronym>SAP</acronym>s) can be used where
necessary to provide untyped access to foreign memory.

   <p>Any foreign objects that can't automatically be converted into Lisp
values are represented by objects of type <code>alien-value</code>.  Since
Lisp is a dynamically typed language, even foreign objects must have a
run-time type; this type information is provided by encapsulating the
raw pointer to the foreign data within an <code>alien-value</code> object.

   <p>The type language and operations on foreign types are
intentionally similar to those of the C language.

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