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<html><head><title>XLISP *gc-hook*</title>
<link rel="stylesheet" type="text/css" href="reference.css">
</head>
<body>
<a href="../start.htm">Nyquist / XLISP 2.0</a> -
<a href="../manual/contents.htm">Contents</a> |
<a href="../tutorials/tutorials.htm">Tutorials</a> |
<a href="../examples/examples.htm">Examples</a> |
<a href="reference-index.htm">Reference</a>
<hr>
<h1>*gc-hook*</h1>
<hr>
<p><table cellpadding="0" cellspacing="0" style="margin-left:10px"><tbody>
<tr valign="top">
<td><nobr>Type:</nobr></td>
<td><nobr> - </nobr></td>
<td width="100%"><nobr>system variable</nobr></td>
</tr>
<tr valign="top">
<td><nobr>Source:</nobr></td>
<td><nobr> - </nobr></td>
<td width="100%"><nobr>xldmem.c</nobr></td>
</tr>
</tbody></table></p>
<h2>Syntax</h2>
<dl>
<dt> *gc-hook*</dt>
</dl>
<h2>Description</h2>
<p>*gc-hook* is a system variable that allows a user function to be
performed everytime garbage is collected [either explicitly with
<a href="gc.htm">gc</a> or automatically]. The default value for
*gc-hook* is <a href="nil.htm">NIL</a>. When *gc-hook* is set to
a non-<a href="nil.htm">NIL</a> symbol, it is enabled to execute
the specified user routine. The user routine can be a quoted symbol or a
closure. There are two parameters to the user routine, the total number of
nodes and current free nodes after the garbage collection.</p>
<h2>Examples</h2>
<pre class="example">
*gc-hook* <font color="#008844">; returns NIL</font>
(gc) <font color="#008844">; returns NIL</font>
(defun mygchook (&rest stuff) <font color="#008844">; define the hook</font>
(print stuff)
(print "my hook"))
(setq *gc-hook* 'mygchook) <font color="#008844">; set up *GC-HOOK*</font>
(gc) <font color="#008844">; prints (2640 232)</font>
<font color="#008844">; "my hook"</font>
<font color="#008844">; returns NIL</font>
(setq *gc-flag* T) <font color="#008844">; turn on the system GC message</font>
(gc) <font color="#008844">; prints</font>
<font color="#008844">; [ gc: total 2640, (2640 241)</font>
<font color="#008844">; "my hook"</font>
<font color="#008844">; 236 free ]</font>
<font color="#008844">; returns NIL</font>
(setq *gc-flag* NIL) <font color="#008844">; turn off GC message</font>
(setq *gc-hook* (lambda (x y) <font color="#008844">; enable user routine</font>
(princ "\007"))) <font color="#008844">; that beeps at every GC</font>
(gc) <font color="#008844">; beeps</font>
(defun expand-on-gc (total free) <font color="#008844">; define EXPAND-ON-GC</font>
(if (< (/ free 1.0 total) .1) <font color="#008844">; IF free/total < .10</font>
(progn (expand 2) <font color="#008844">; THEN expand memory</font>
(princ "\007")))) <font color="#008844">; and beep</font>
<font color="#008844">; NOTE: XLISP already gets more nodes</font>
<font color="#008844">; automatically, this is just an example.</font>
(setq *gc-hook* 'expand-on-gc) <font color="#008844">; enable EXPAND-ON-GC</font>
(gc) <font color="#008844">; beeps when low on nodes</font>
</pre>
<p><b>Note:</b> The *gc-hook* and <a href="global-gc-flag.htm">*gc-flag*</a>
facilities can interact. If you do printing in the *gc-hook* user form and
enable <nobr><a href="global-gc-flag.htm">*gc-flag*</a> ,</nobr> the
*gc-hook* printing will come out in the middle of the
<a href="global-gc-flag.htm">*gc-flag*</a> message.</p>
<p><b>Note:</b> The *gc-hook* user form is evaluated after the execution of
the actual garbage collection code. This means that if the user form causes
an error, it does not prevent a garbage collection.</p>
<p><b>Note:</b> Since *gc-hook* is set to a symbol, the user defined form
can be changed by doing another <a href="defun.htm">defun</a> [or
whatever] to the symbol in *gc-hook*. Note also that you should define the
symbol first and then set *gc-hook* to the symbol. If you don't, an
automatic garbage collection might occur before you set *gc-hook*,
generating an error and stopping your program.</p>
<p>See the
<a href="../manual/xlisp-man-011.htm#gc-hook">*gc-hook*</a>
system variable in the <nobr>XLISP 2.0</nobr> manual.</p>
<p><nobr> <a href="#top">Back to Top</nobr></a></p>
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