Sophie: sbcl-1.0.31-1mdv2010.0 i586

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

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<h4 class="subsection">8.4.2 External Foreign Variables</h4>

<p>External foreign names are strings, and Lisp names are symbols. When
an external foreign value is represented using a Lisp variable, there
must be a way to convert from one name syntax into the other. The
macros <code>extern-alien</code>, <code>define-alien-variable</code> and
<code>define-alien-routine</code> use this conversion heuristic:

     <ul>
<li>Alien names are converted to Lisp names by uppercasing and replacing
underscores with hyphens.

     <li>Conversely, Lisp names are converted to alien names by lowercasing and
replacing hyphens with underscores.

     <li>Both the Lisp symbol and alien string names may be separately
specified by using a list of the form

     <pre class="lisp">          (alien-string lisp-symbol)
</pre>
     </ul>

<div class="defun">
&mdash; Macro: <b>sb-alien:define-alien-variable</b><var> name type<a name="index-sb_002dalien_003adefine_002dalien_002dvariable-292"></a></var><br>
<blockquote><p><a name="index-define_002dalien_002dvariable-293"></a>
The <code>define-alien-variable</code> macro defines <var>name</var> as an
external foreign variable of the specified foreign <code>type</code>. 
<var>name</var> and <code>type</code> are not evaluated.  The Lisp name of the
variable (see above) becomes a global alien variable.  Global alien
variables are effectively &ldquo;global symbol macros&rdquo;; a reference to the
variable fetches the contents of the external variable.  Similarly,
setting the variable stores new contents &ndash; the new contents must be
of the declared <code>type</code>. Someday, they may well be implemented
using the <acronym>ANSI</acronym> <code>define-symbol-macro</code> mechanism, but as
of SBCL 0.7.5, they are still implemented using an older more-or-less
parallel mechanism inherited from CMUCL.

        <p>For example, to access a C-level counter <var>foo</var>, one could write

     <pre class="lisp">          (define-alien-variable "foo" int)
          ;; Now it is possible to get the value of the C variable foo simply by
          ;; referencing that Lisp variable:
          (print foo)
          (setf foo 14)
          (incf foo)
</pre>
        </blockquote></div>

<div class="defun">
&mdash; Function: <b>sb-alien:get-errno</b><var><a name="index-sb_002dalien_003aget_002derrno-294"></a></var><br>
<blockquote><p><a name="index-get_002derrno-295"></a>
Since in modern C libraries, the <code>errno</code> &ldquo;variable&rdquo; is typically
no longer a variable, but some bizarre artificial construct
which behaves superficially like a variable within a given thread,
it can no longer reliably be accessed through the ordinary
<code>define-alien-variable</code> mechanism. Instead, SBCL provides
the operator <code>sb-alien:get-errno</code> to allow Lisp code to read it. 
</p></blockquote></div>

<div class="defun">
&mdash; Macro: <b>sb-alien:extern-alien</b><var> name type<a name="index-sb_002dalien_003aextern_002dalien-296"></a></var><br>
<blockquote><p><a name="index-extern_002dalien-297"></a>
The <code>extern-alien</code> macro returns an alien with the specified
<var>type</var> which points to an externally defined value.  <var>name</var> is
not evaluated, and may be either a string or a symbol.  <var>type</var> is
an unevaluated alien type specifier. 
</p></blockquote></div>

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