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<html><head><title>A garbage collector for C and C++</title></head>
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<table bgcolor="#f0f0ff" cellpadding="10%">
  <tbody><tr>
  <td><a href="http://www.hpl.hp.com/personal/Hans_Boehm/gc/gcinterface.html">Interface Overview</a></td>
  <td><a href="http://www.hpl.hp.com/personal/Hans_Boehm/gc/04tutorial.pdf">Tutorial Slides</a></td>
  <td><a href="http://www.hpl.hp.com/personal/Hans_Boehm/gc/faq.html">FAQ</a></td>
  <td><a href="http://www.hpl.hp.com/personal/Hans_Boehm/gc/simple_example.html">Example</a></td>
  <td><a href="http://www.hpl.hp.com/personal/Hans_Boehm/gc/gc_source">Download</a></td>
  <td><a href="http://www.hpl.hp.com/personal/Hans_Boehm/gc/license.txt">License</a></td>
  </tr>
</tbody></table>
<h1>A garbage collector for C and C++</h1>
<ul>
<li><a href="#platforms">Platforms</a>
</li><li><a href="#multiprocessors">Scalable multiprocessor versions</a>
</li><li><a href="#details">Some collector details</a>
</li><li><a href="#further">Further reading</a>
</li><li><a href="#users">Current users</a>
</li><li><a href="#collector">Local Links for this collector</a>
</li><li><a href="#background">Local Background Links</a>
</li><li><a href="#contacts">Contacts and Mailing List</a>
</li></ul>
[ This is an updated version of the page formerly at
<tt>http://reality.sgi.com/boehm/gc.html</tt>
and before that at
<a href="ftp://parcftp.xerox.com/pub/gc/gc.html">
<tt>ftp://parcftp.xerox.com/pub/gc/gc.html</tt></a>.]
<p>
The <a href="http://www.hpl.hp.com/personal/Hans_Boehm">Boehm</a>-<a href="http://www.cs.cornell.edu/annual_report/00-01/bios.htm#demers">Demers</a>-<a href="http://www-sul.stanford.edu/weiser/">Weiser</a>
conservative garbage collector can
be used as a garbage collecting
replacement for C <tt>malloc</tt> or C++ <tt>new</tt>.
It allows you to allocate memory basically as you normally would,
without explicitly deallocating memory that is no longer useful.
The collector automatically recycles memory when it determines
that it can no longer be otherwise accessed.
A simple example of such a use is given
<a href="http://www.hpl.hp.com/personal/Hans_Boehm/gc/simple_example.html">here</a>.
</p><p>
The collector is also used by a number of programming language
implementations that either use C as intermediate code, want
to facilitate easier interoperation with C libraries, or
just prefer the simple collector interface.
For a more detailed description of the interface, see
<a href="http://www.hpl.hp.com/personal/Hans_Boehm/gc/gcinterface.html">here</a>.
</p><p>
Alternatively, the garbage collector  may be used as
a <a href="http://www.hpl.hp.com/personal/Hans_Boehm/gc/leak.html">leak detector</a>
for C or C++ programs, though that is not its primary goal.
</p><p>
Typically several versions will be available.
Usually you should first try to use
<a href="http://www.hpl.hp.com/personal/Hans_Boehm/gc/gc_source/gc.tar.gz"><tt>gc_source/gc.tar.gz</tt></a>,
which is normally an older, more stable version.
</p><p>
If that fails, try the latest explicitly numbered version
in <a href="http://www.hpl.hp.com/personal/Hans_Boehm/gc/gc_source/">
<tt>gc_source/</tt></a>.
Later versions may contain additional features, platform support,
or bug fixes, but are likely to be less well tested.
Note that versions containing the letters <tt>alpha</tt> are even less
well tested than others, especially on non-HP platforms.
</p><p>
A slightly older version of the garbage collector is now also
included as part of the
<a href="http://gcc.gnu.org/">GNU compiler</a>
distribution.  The source
code for that version is available for browsing
<a href="http://gcc.gnu.org/cgi-bin/cvsweb.cgi/gcc/boehm-gc/">here</a>.
</p><p>
The arguments for and against conservative garbage collection
in C and C++ are briefly
discussed in
<a href="http://www.hpl.hp.com/personal/Hans_Boehm/gc/issues.html">issues.html</a>.  The beginnings of
a frequently-asked-questions list are <a href="http://www.hpl.hp.com/personal/Hans_Boehm/gc/faq.html">here</a>.
</p><p>
The garbage collector code is copyrighted by
<a href="http://www.hpl.hp.com/personal/Hans_Boehm">Hans-J. Boehm</a>,
Alan J. Demers,
<a href="http://www.xerox.com/">Xerox Corporation</a>,
<a href="http://www.sgi.com/">Silicon Graphics</a>,
and
<a href="http://www.hp.com/">Hewlett-Packard Company</a>.
It may be used and copied without payment of a fee under minimal restrictions.
See the README file in the distribution  or the
<a href="http://www.hpl.hp.com/personal/Hans_Boehm/gc/license.txt">license</a> for more details.
<b>IT IS PROVIDED AS IS,
WITH ABSOLUTELY NO WARRANTY EXPRESSED OR IMPLIED.  ANY USE IS AT YOUR OWN RISK</b>.
</p><p>
Empirically, this collector works with most unmodified C programs,
simply by replacing
<tt>malloc</tt> with <tt>GC_malloc</tt> calls,
replacing <tt>realloc</tt> with <tt>GC_realloc</tt> calls, and removing
free calls.  Exceptions are discussed
in <a href="http://www.hpl.hp.com/personal/Hans_Boehm/gc/issues.html">issues.html</a>. 
</p><h2><a name="platforms">Platforms</a></h2>
The collector is not completely portable, but the distribution
includes ports to most standard PC and UNIX/Linux platforms.
The collector should work on Linux, *BSD, recent Windows versions,
MacOS X, HP/UX, Solaris,
Tru64, Irix and a few other operating systems.
Some ports are more polished than others.
<p>
Irix pthreads, Linux threads, Win32 threads, Solaris threads
(old style and pthreads),
HP/UX 11 pthreads, Tru64 pthreads, and MacOS X threads are supported
in recent versions.
</p><h3>Separately distributed ports</h3>
For MacOS 9/Classic use, Patrick Beard's latest port is available from
<a href="http://homepage.mac.com/pcbeard/gc/">
<tt>http://homepage.mac.com/pcbeard/gc/</tt></a>.
(Unfortunately, that's now quite dated.
I'm not in a position to test under MacOS.  Although I try to
incorporate changes, it is impossible for
me to update the project file.)
<p>
Precompiled versions of the collector for NetBSD are available
<a href="ftp://ftp.netbsd.org/pub/NetBSD/packages/pkgsrc/devel/boehm-gc/README.html">here</a>
or
<a href="http://www.netbsd.org/packages/devel/boehm-gc/README.html">here</a>.
</p><p>
<a href="http://www.debian.org/">Debian Linux</a> includes prepackaged
versions of the collector.
</p><h2><a name="multiprocessors">Scalable multiprocessor versions</a></h2>
Kenjiro Taura, Toshio Endo, and Akinori Yonezawa have made available
a <a href="http://www.yl.is.s.u-tokyo.ac.jp/gc/">parallel collector</a>
based on this one.  Their collector takes advantage of multiple processors
during a collection.  Starting with collector version 6.0alpha1
we also do this, though with more modest processor scalability goals.
Our approach is discussed briefly in
<a href="http://www.hpl.hp.com/personal/Hans_Boehm/gc/scale.html"><tt>scale.html</tt></a>.
<h2><a name="details">Some Collector Details</a></h2>
The collector uses a <a href="http://www.hpl.hp.com/personal/Hans_Boehm/gc/complexity.html">mark-sweep</a> algorithm.
It provides incremental and generational
collection under operating systems which provide the right kind of
virtual memory support.  (Currently this includes SunOS[45], IRIX,
OSF/1, Linux, and Windows, with varying restrictions.)
It allows <a href="http://www.hpl.hp.com/personal/Hans_Boehm/gc/finalization.html"><i>finalization</i></a> code
to be invoked when an object is collected.
It can take advantage of type information to locate pointers if such
information is provided, but it is usually used without such information.
ee the README and
<tt>gc.h</tt> files in the distribution for more details.
<p>
For an overview of the implementation, see <a href="http://www.hpl.hp.com/personal/Hans_Boehm/gc/gcdescr.html">here</a>.
</p><p>
The garbage collector distribution includes a C string
(<a href="http://www.hpl.hp.com/personal/Hans_Boehm/gc/gc_source/cordh.txt"><i>cord</i></a>) package that provides
for fast concatenation and substring operations on long strings.
A simple curses- and win32-based editor that represents the entire file
as a cord is included as a
sample application.
</p><p>
Performance of the nonincremental collector is typically competitive
with malloc/free implementations.  Both space and time overhead are
likely to be only slightly higher
for programs written for malloc/free
(see Detlefs, Dosser and Zorn's
<a href="ftp://ftp.cs.colorado.edu/pub/techreports/zorn/CU-CS-665-93.ps.Z">Memory Allocation Costs in Large C and C++ Programs</a>.)
For programs allocating primarily very small objects, the collector
may be faster; for programs allocating primarily large objects it will
be slower.  If the collector is used in a multithreaded environment
and configured for thread-local allocation, it may in some cases
significantly outperform malloc/free allocation in time.
</p><p>
We also expect that in many cases any additional overhead
will be more than compensated for by decreased copying etc.
if programs are written
and tuned for garbage collection.
</p><h1><a name="further">Further Reading:</a></h1>
<b>The beginnings of a frequently asked questions list for this
collector are <a href="http://www.hpl.hp.com/personal/Hans_Boehm/gc/faq.html">here</a></b>.
<p>
<b>The following provide information on garbage collection in general</b>:
</p><p>
Paul Wilson's <a href="ftp://ftp.cs.utexas.edu/pub/garbage">garbage collection ftp archive</a> and <a href="ftp://ftp.cs.utexas.edu/pub/garbage/gcsurvey.ps">GC survey</a>.
</p><p>
The Ravenbrook <a href="http://www.memorymanagement.org/">
Memory Management Reference</a>.
</p><p>
David Chase's
<a href="http://www.iecc.com/gclist/GC-faq.html">GC FAQ</a>.
</p><p>
Richard Jones'
<a href="http://www.ukc.ac.uk/computer_science/Html/Jones/gc.html">
GC page</a> and
<a href="http://www.cs.kent.ac.uk/people/staff/rej/gcbook/gcbook.html">
his book</a>.
</p><p>
<b>The following papers describe the collector algorithms we use
and the underlying design decisions at
a higher level.</b>
</p><p>
(Some of the lower level details can be found
<a href="http://www.hpl.hp.com/personal/Hans_Boehm/gc/gcdescr.html">here</a>.)
</p><p>
The first one is not available
electronically due to copyright considerations.  Most of the others are
subject to ACM copyright.
</p><p>
Boehm, H., "Dynamic Memory Allocation and Garbage Collection", <i>Computers in Physics
9</i>, 3, May/June 1995, pp. 297-303.  This is directed at an otherwise sophisticated
audience unfamiliar with memory allocation issues.  The algorithmic details differ
from those in the implementation.  There is a related letter to the editor and a minor
correction in the next issue.
</p><p>
Boehm, H., and <a href="http://www.ubiq.com/hypertext/weiser/weiser.html">M. Weiser</a>,
<a href="http://www.hpl.hp.com/personal/Hans_Boehm/spe_gc_paper">"Garbage Collection in an Uncooperative Environment"</a>,
<i>Software Practice &amp; Experience</i>, September 1988, pp. 807-820.
</p><p>
Boehm, H., A. Demers, and S. Shenker, <a href="http://www.hpl.hp.com/personal/Hans_Boehm/gc/papers/pldi91.ps.Z">"Mostly Parallel Garbage Collection"</a>, Proceedings
of the ACM SIGPLAN '91 Conference on Programming Language Design and Implementation,
<i>SIGPLAN Notices 26</i>, 6 (June 1991), pp. 157-164.
</p><p>
Boehm, H., <a href="http://www.hpl.hp.com/personal/Hans_Boehm/gc/papers/pldi93.ps.Z">"Space Efficient Conservative Garbage Collection"</a>, Proceedings of the ACM
SIGPLAN '93 Conference on Programming Language Design and Implementation, <i>SIGPLAN
Notices 28</i>, 6 (June 1993), pp. 197-206.
</p><p>
Boehm, H., "Reducing Garbage Collector Cache Misses",
<i> Proceedings of the 2000 International Symposium on Memory Management </i>.
<a href="http://portal.acm.org/citation.cfm?doid=362422.362438">
Official version.</a>
<a href="http://www.hpl.hp.com/techreports/2000/HPL-2000-99.html">
Technical report version.</a>  Describes the prefetch strategy
incorporated into the collector for some platforms.  Explains why
the sweep phase of a "mark-sweep" collector should not really be
a distinct phase.
</p><p>
M. Serrano, H. Boehm,
"Understanding Memory Allocation of Scheme Programs",
<i>Proceedings of the Fifth ACM SIGPLAN International Conference on
Functional Programming</i>, 2000, Montreal, Canada, pp. 245-256.
<a href="http://www.acm.org/pubs/citations/proceedings/fp/351240/p245-serrano/">
Official version.</a>
<a href="http://www.hpl.hp.com/techreports/2000/HPL-2000-62.html">
Earlier Technical Report version.</a>  Includes some discussion of the
collector debugging facilities for identifying causes of memory retention.
</p><p>
Boehm, H.,
"Fast Multiprocessor Memory Allocation and Garbage Collection",
<a href="http://www.hpl.hp.com/techreports/2000/HPL-2000-165.html">
HP Labs Technical Report HPL 2000-165</a>.  Discusses the parallel
collection algorithms, and presents some performance results.
</p><p>
Boehm, H., "Bounding Space Usage of Conservative Garbage Collectors",
<i>Proceeedings of the 2002 ACM SIGPLAN-SIGACT Symposium on Principles of
Programming Languages</i>, Jan. 2002, pp. 93-100.
<a href="http://portal.acm.org/citation.cfm?doid=503272.503282">
Official version.</a>
<a href="http://www.hpl.hp.com/techreports/2001/HPL-2001-251.html">
Technical report version.</a>
Includes a discussion of a collector facility to much more reliably test for
the potential of unbounded heap growth.
</p><p>
<b>The following papers discuss language and compiler restrictions necessary to guaranteed
safety of conservative garbage collection.</b>
</p><p>
We thank John Levine and JCLT for allowing
us to make the second paper available electronically, and providing PostScript for the final
version.
</p><p>
Boehm, H., <a href="http://www.hpl.hp.com/personal/Hans_Boehm/gc/papers/pldi96.ps.gz">``Simple
Garbage-Collector-Safety''</a>, Proceedings
of the ACM SIGPLAN '96 Conference on Programming Language Design
and Implementation.
</p><p>
Boehm, H., and D. Chase,  <a href="http://www.hpl.hp.com/personal/Hans_Boehm/gc/papers/boecha.ps.gz">
``A Proposal for Garbage-Collector-Safe C Compilation''</a>,
<i>Journal of C  Language Translation 4</i>, 2 (Decemeber 1992), pp. 126-141.
</p><p>
<b>Other related information: </b>
</p><p>
The Detlefs, Dosser and Zorn's <a href="ftp://ftp.cs.colorado.edu/pub/techreports/zorn/CU-CS-665-93.ps.Z">Memory Allocation Costs in Large C and C++ Programs</a>.
 This is a performance comparison of the Boehm-Demers-Weiser collector to malloc/free,
using programs written for malloc/free.
</p><p>
Joel Bartlett's <a href="ftp://ftp.digital.com/pub/DEC/CCgc">mostly copying conservative garbage collector for C++</a>.
</p><p>
John Ellis and David Detlef's <a href="ftp://parcftp.xerox.com/pub/ellis/gc/gc.ps">Safe Efficient Garbage Collection for C++</a> proposal.
</p><p>
Henry Baker's <a href="http://home.pipeline.com/%7Ehbaker1/">paper collection</a>.
</p><p>
Slides for Hans Boehm's <a href="http://www.hpl.hp.com/personal/Hans_Boehm/gc/myths.ps">Allocation and GC Myths</a> talk.
</p><h1><a name="users">Current users:</a></h1>
Known current users of some variant of this collector include:
<p>
The runtime system for <a href="http://gcc.gnu.org/java">GCJ</a>,
the static GNU java compiler.
</p><p>
<a href="http://w3m.sourceforge.net/">W3m</a>, a text-based web browser.
</p><p>
Some versions of the Xerox DocuPrint printer software.
</p><p>
The <a href="http://www.mozilla.org/">Mozilla</a> project, as leak
detector.
</p><p>
The <a href="http://www.go-mono.com/">Mono</a> project,
an open source implementation of the .NET development framework.
</p><p>
The <a href="http://www.gnu.org/projects/dotgnu/">DotGNU Portable.NET
project</a>, another open source .NET implementation.
</p><p>
The <a href="http://irssi.org/">Irssi IRC client</a>.
</p><p>
<a href="http://titanium.cs.berkeley.edu/">The Berkeley Titanium project</a>.
</p><p>
<a href="http://www.nag.co.uk/nagware_fortran_compilers.asp">The NAGWare f90 Fortran 90 compiler</a>.
</p><p>
Elwood Corporation's <a href="http://www.elwood.com/eclipse-info/index.htm">
Eclipse</a> Common Lisp system, C library, and translator.
</p><p>
The <a href="http://www-sop.inria.fr/mimosa/fp/Bigloo/">Bigloo
Scheme</a>
and <a href="http://kaolin.unice.fr/%7Eserrano/camloo.html">Camloo ML
compilers</a>
written by Manuel Serrano and others.
</p><p>
Brent Benson's <a href="http://ftp.cs.indiana.edu/pub/scheme-repository/imp/">libscheme</a>.
</p><p>
The <a href="http://www.cs.rice.edu/CS/PLT/packages/mzscheme/index.html">MzScheme</a> scheme implementation.
</p><p>
The <a href="http://www.cs.washington.edu/research/projects/cecil/www/cecil-home.html">University of Washington Cecil Implementation</a>.
</p><p>
<a href="http://www.icsi.berkeley.edu/Sather/">The Berkeley Sather implementation</a>.
</p><p>
<a href="http://www.cs.berkeley.edu/%7Eharmonia/">The Berkeley Harmonia Project</a>.
</p><p>
The <a href="http://www.cs.arizona.edu/sumatra/toba/">Toba</a> Java Virtual
Machine to C translator.
</p><p>
The <a href="http://www.gwydiondylan.org/">Gwydion Dylan compiler</a>.
</p><p>
The <a href="http://gcc.gnu.org/onlinedocs/gcc/Objective-C.html">
GNU Objective C runtime</a>.
</p><p>
<a href="http://www.math.uiuc.edu/Macaulay2">Macaulay 2</a>, a system to support
research in algebraic geometry and commutative algebra.
</p><p>
The <a href="http://www.vestasys.org/">Vesta</a> configuration management
system.
</p><p>
<a href="http://www.visual-prolog.com/vip6">Visual Prolog 6</a>.
</p><p>
<a href="http://asymptote.sf.net/">Asymptote LaTeX-compatible
vector graphics language.</a>

</p><h1><a name="collector">More collector information at this site</a></h1>
<a href="http://www.hpl.hp.com/personal/Hans_Boehm/gc/simple_example.html">A simple illustration of how to build and
use the collector.</a>.
<p>
<a href="http://www.hpl.hp.com/personal/Hans_Boehm/gc/gcinterface.html">Description of alternate interfaces to the
garbage collector.</a>
</p><p>
<a href="http://www.hpl.hp.com/personal/Hans_Boehm/gc/04tutorial.pdf">Slides from an ISMM 2004  tutorial about the GC.</a>
</p><p>
<a href="http://www.hpl.hp.com/personal/Hans_Boehm/gc/faq.html">A FAQ (frequently asked questions) list.</a>
</p><p>
<a href="http://www.hpl.hp.com/personal/Hans_Boehm/gc/leak.html">How to use the garbage collector as a leak detector.</a>
</p><p>
<a href="http://www.hpl.hp.com/personal/Hans_Boehm/gc/debugging.html">Some hints on debugging garbage collected
applications.</a>
</p><p>
<a href="http://www.hpl.hp.com/personal/Hans_Boehm/gc/gcdescr.html">An overview of the implementation of the
garbage collector.</a>
</p><p>
<a href="http://www.hpl.hp.com/personal/Hans_Boehm/gc/tree.html">The data structure used for fast pointer lookups.</a>
</p><p>
<a href="http://www.hpl.hp.com/personal/Hans_Boehm/gc/scale.html">Scalability of the collector to multiprocessors.</a>
</p><p>
<a href="http://www.hpl.hp.com/personal/Hans_Boehm/gc/gc_source">Directory containing garbage collector source.</a>

</p><h1><a name="background">More background information at this site</a></h1>
<a href="http://www.hpl.hp.com/personal/Hans_Boehm/gc/bounds.html">An attempt to establish a bound on space usage of
conservative garbage collectors.</a>
<p>
<a href="http://www.hpl.hp.com/personal/Hans_Boehm/gc/complexity.html">Mark-sweep versus copying garbage collectors
and their complexity.</a>
</p><p>
<a href="http://www.hpl.hp.com/personal/Hans_Boehm/gc/conservative.html">Pros and cons of conservative garbage collectors,
in comparison to other collectors.
</a>
</p><p>
<a href="http://www.hpl.hp.com/personal/Hans_Boehm/gc/issues.html">Issues related to garbage collection vs.
manual memory management in C/C++.</a>
</p><p>
<a href="http://www.hpl.hp.com/personal/Hans_Boehm/gc/example.html">An example of a case in which garbage collection
results in a much faster implementation as a result of reduced
synchronization.</a>
</p><p>
<a href="http://www.hpl.hp.com/personal/Hans_Boehm/gc/nonmoving">Slide set discussing performance of nonmoving
garbage collectors.</a>
</p><p>
<a href="http://www.hpl.hp.com/personal/Hans_Boehm/popl03/web">
Slide set discussing <i>Destructors, Finalizers, and Synchronization</i>
(POPL 2003).</a>
</p><p>
<a href="http://portal.acm.org/citation.cfm?doid=604131.604153">
Paper corresponding to above slide set.</a>
(<a href="http://www.hpl.hp.com/techreports/2002/HPL-2002-335.html">
Technical Report version</a>.)
</p><p>
<a href="http://www.hpl.hp.com/personal/Hans_Boehm/gc/gc_bench.html">A Java/Scheme/C/C++ garbage collection benchmark.</a>
</p><p>
<a href="http://www.hpl.hp.com/personal/Hans_Boehm/gc/myths.ps">Slides for talk on memory allocation myths.</a>
</p><p>
<a href="http://www.hpl.hp.com/personal/Hans_Boehm/gc/gctalk.ps">Slides for OOPSLA 98 garbage collection talk.</a>
</p><p>
<a href="http://www.hpl.hp.com/personal/Hans_Boehm/gc/papers">Related papers.</a>
</p><h1><a name="contacts">Contacts and Mailing List</a><a></a></h1>
<a>We have recently set up two mailing list for collector announcements
and discussions:
</a><ul>
<li><a href="mailto:gc-announce@linux.hpl.hp.com">gc-announce@linux.hpl.hp.com</a>
is used for announcements of new versions.  Postings are restricted.
We expect this to always remain a very low volume list.
</li><li><a href="mailto:gc@linux.hpl.hp.com">gc@linux.hpl.hp.com</a> is used for
discussions, bug reports, and the like.  Subscribers may post.
On-topic posts by nonsubscribers will usually also be accepted, but
it may take some time to review them.
</li></ul>
To subscribe to these lists, send a mail message containing the
word "subscribe" to
<a href="mailto:gc-announce-request@linux.hpl.hp.com?subject=subscribe">gc-announce-request@linux.hpl.hp.com</a>
or to
<a href="mailto:gc-request@linux.hpl.hp.com?subject=subscribe">gc-request@linux.hpl.hp.com</a>.
(Please ignore the instructions about web-based subscription.
The listed web site is behind the HP firewall.)
<p>
The archives for these lists appear
<a href="http://www.hpl.hp.com/hosted/linux/mail-archives">here</a>.
The gc list archive may also be read at
<a href="http://dir.gmane.org/gmane.comp.programming.garbage-collection.boehmgc">gmane.org</a>.
</p><p>
Some prior discussion of the collector has taken place on the gcc
java mailing list, whose archives appear
<a href="http://gcc.gnu.org/ml/java/">here</a>, and also on
<a href="http://lists.tunes.org/mailman/listinfo/gclist">gclist@iecc.com</a>.
</p><p>
Comments and bug reports may also be sent to
(<a href="mailto:Hans_Boehm@hp.com">Hans.Boehm@hp.com</a>) or
(<a href="mailto:boehm@acm.org">boehm@acm.org</a>), but the gc
mailing list is usually preferred.
 
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