<html><head><meta http-equiv="Content-Type" content="text/html; charset=ANSI_X3.4-1968"><title>5.2. Boehm-Weiser garbage collector</title><link rel="stylesheet" href="ecl.css" type="text/css"><meta name="generator" content="DocBook XSL Stylesheets V1.75.2"><link rel="home" href="index.html" title="The ECL manual"><link rel="up" href="ch20.html" title="Chapter 5. Memory Management"><link rel="prev" href="ch20.html" title="Chapter 5. Memory Management"><link rel="next" href="ch20s03.html" title="5.3. Memory limits"></head><body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF"><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="3" align="center">5.2. Boehm-Weiser garbage collector</th></tr><tr><td width="20%" align="left"><a accesskey="p" href="ch20.html">Prev</a> </td><th width="60%" align="center">Chapter 5. Memory Management</th><td width="20%" align="right"> <a accesskey="n" href="ch20s03.html">Next</a></td></tr></table><hr></div><div class="section" title="5.2. Boehm-Weiser garbage collector"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a name="ext.memory.boehm"></a>5.2. Boehm-Weiser garbage collector</h2></div></div></div><p>First of all, the garbage collector must be able to determine which
objects are alive and which are not. In other word, the collector must able
to find all references to an object. One possiblity would be to know where
all variables of a program reside, and where is the stack of the program and
its size, and parse all data there, discriminating references to lisp
objects. To do this precisely one would need a very precise control of the
data and stack segments, as well as how objects are laid out by the C
compiler. This is beyond <span class="application">ECL</span>'s scope and wishes and it can make
coexistence with other libraries (C++, Fortran, etc) difficult.</p><p>The Boehm-Weiser garbage collector, on the other hand, is a
conservative garbage collector. When scanning memory looking for references
to live data, it guesses, conservatively, whether a word is a pointer or
not. In case of doubt it will consider it to be a pointer and add it to the
list of live objects. This may cause certain objects to be retained longer
than what an user might expect but, in our experience, this is the best of
both worlds and <span class="application">ECL</span> uses certain strategies to minimize the amount of
misinterpreted data.</p><p>More precisely, <span class="application">ECL</span> uses the garbage collector with the following
settings:</p><div class="itemizedlist"><ul class="itemizedlist" type="disc"><li class="listitem"><p>The collector will not scan the data sectors. If you embed
<span class="application">ECL</span> in another program, or link libraries with <span class="application">ECL</span>, you will have to
notify <span class="application">ECL</span> which variables point to lisp objects.</p></li><li class="listitem"><p>The collector is configured to ignore pointers that point
to the middle of allocated objects. This minimizes the risk of
misinterpreting integers as pointers to life obejcts.</p></li><li class="listitem"><p>It is possible to register finalizers that are invoked when
an object is destroyed, but for that you should use <span class="application">ECL</span>'s API and
understand the restriction described later in <a class="xref" href="ch20s05.html" title="5.5. Finalization">Section 5.5</a></p></li></ul></div><p>Except for finalization, which is a questionable feature, the previous
settings are not very relevant for Common Lisp programmers, but are crucial
for people interested in embedding in or cooperating with other C, C++ or
Fortran libraries. Care should be taken when manipulating directly the GC
library to avoid interfering with <span class="application">ECL</span>'s expectations.</p></div><div class="navfooter"><hr><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="ch20.html">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="ch20.html">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="ch20s03.html">Next</a></td></tr><tr><td width="40%" align="left" valign="top">Chapter 5. Memory Management </td><td width="20%" align="center"><a accesskey="h" href="index.html">Home</a></td><td width="40%" align="right" valign="top"> 5.3. Memory limits</td></tr></table></div></body></html>
