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9f7b84bb07
* Makefile.am configure.in include/libxml/xmlwin32version.h: preparing for a 2.4.3 release even if it may not be ready yet * catalog.c parser.c xmlIO.c include/libxml/catalog.h: redirected all file parsing lookup to go through the entity resolver, add to add an API to bypass it (needed to load catalogs themselves), some cleanup on the catalog code too. * nanoftp.c: small cleanup * doc/catalog.html: small update Daniel
394 lines
18 KiB
HTML
394 lines
18 KiB
HTML
<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"
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"http://www.w3.org/TR/html4/loose.dtd">
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<html>
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<head>
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<title>Libxml Catalog support</title>
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<meta name="GENERATOR" content="amaya V5.0">
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<meta http-equiv="Content-Type" content="text/html">
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</head>
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<body bgcolor="#ffffff">
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<h1 align="center">Libxml Catalog support</h1>
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<p>Location: <a
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href="http://xmlsoft.org/catalog.html">http://xmlsoft.org/catalog.html</a></p>
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<p>Libxml home page: <a href="http://xmlsoft.org/">http://xmlsoft.org/</a></p>
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<p>Mailing-list archive: <a
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href="http://mail.gnome.org/archives/xml/">http://mail.gnome.org/archives/xml/</a></p>
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<p>Version: $Revision: 1.2 $</p>
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<p>Table of Content:</p>
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<ol>
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<li><a href="#General">General overview</a></li>
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<li><a href="#definition">The definition</a></li>
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<li><a href="#Simple">Using catalogs</a></li>
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<li><a href="#Some">Some examples</a></li>
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<li><a href="#reference">How to tune catalog usage</a></li>
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<li><a href="#validate">How to debug catalog processing</a></li>
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<li><a href="#Declaring">How to create and maintain catalogs</a></li>
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<li><a href="#implemento">The implementor corner quick review of the
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API</a></li>
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<li><a href="#Other">Other resources</a></li>
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</ol>
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<h2><a name="General">General overview</a></h2>
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<p>What is a catalog ? Basically it's a lookup mechanism which is used when
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an entity (a file or a remote resource) reference another entity. The catalog
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lookup is inserted between the moment the reference is recognized by the
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software (XML parser, stylesheet processing, or even images referenced for
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inclusion in a rendering) and the time where loading that resource is
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actually started.</p>
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<p>It is basically used for 3 things:</p>
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<ul>
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<li>mapping from "logical" names, the public identifiers and a more
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concrete name usable for download (and URI). For example it can associate
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the logical name
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<p>"-//OASIS//DTD DocBook XML V4.1.2//EN"</p>
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<p>of the DocBook 4.1.2 XML DTD with the actual URL where it can be
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downloaded</p>
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<p>http://www.oasis-open.org/docbook/xml/4.1.2/docbookx.dtd</p>
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</li>
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<li>remapping from a given URL to another one, like an HTTP indirection
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saying that
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<p>"http://www.oasis-open.org/committes/tr.xsl"</p>
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<p>should really be looked at</p>
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<p>"http://www.oasis-open.org/committes/entity/stylesheets/base/tr.xsl"</p>
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</li>
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<li>providing a local cache mechanism allowing to load the entities
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associated to public identifiers or remote resources, this is a really
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important feature for any significant deployment of XML or SGML since it
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allows to avoid the aleas and delays associated to fetching remore
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resources.</li>
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</ul>
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<h2><a name="definition">The definitions</a></h2>
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<p>Libxml, as of 2.4.3 implements 2 kind of catalogs:</p>
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<ul>
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<li>the older SGML catalogs, the official spec is SGML Open Technical
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Resolution TR9401:1997, but is better understood by reading <a
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href="http://www.jclark.com/sp/catalog.htm">the SP Catalog page</a> from
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James Clark. This is relatively old and not the preferred mode of
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operation of libxml.</li>
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<li><a href="http://www.oasis-open.org/committees/entity/spec.html">XML
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Catalogs</a>
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is far more flexible, more recent, uses an XML syntax and should scale
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quite better. This is the default option of libxml.</li>
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</ul>
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<p></p>
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<h2><a name="Simple">Using catalog</a></h2>
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<p>In a normal environment libxml will by default check the presence of a
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catalog in /etc/xml/catalog, and assuming it has been correctly populated,
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the processing is completely transparent to the document user. To take a
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concrete example, suppose you are authoring a DocBook document, this one
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starts with the following DOCTYPE definition:</p>
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<pre><?xml version='1.0'?>
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<!DOCTYPE book PUBLIC "-//Norman Walsh//DTD DocBk XML V3.1.4//EN"
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"http://nwalsh.com/docbook/xml/3.1.4/db3xml.dtd"></pre>
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<p>When validating the document with libxml, the catalog will be
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automatically consulted to lookup the public identifier "-//Norman Walsh//DTD
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DocBk XML V3.1.4//EN" and the system identifier
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"http://nwalsh.com/docbook/xml/3.1.4/db3xml.dtd", and if these entities have
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been installed on your system and the catalogs actually point to them, libxml
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will fetch them from the local disk.</p>
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<p style="font-size: 10pt"><strong>Note</strong>: Really don't use this
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DOCTYPE example it's a really old version, but is fine as an example.</p>
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<p>Libxml will check the catalog each time that it is requested to load an
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entity, this include DTD, external parsed entities, stylesheets, etc ... If
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your system is correctly configured all the authoring phase and processing
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should use only local files, even if your document stay portable because it
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uses the canonical public and system ID, referencing the remote document.</p>
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<h2><a name="Some">Some examples:</a></h2>
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<p>Here is a couple of fragments from XML Catalogs used in libxml early
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regression tests in <code>test/catalogs</code> :</p>
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<pre><?xml version="1.0"?>
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<!DOCTYPE catalog PUBLIC "-//OASIS//DTD Entity Resolution XML Catalog V1.0//EN"
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"http://www.oasis-open.org/committees/entity/release/1.0/catalog.dtd">
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<catalog xmlns="urn:oasis:names:tc:entity:xmlns:xml:catalog">
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<public publicId="-//OASIS//DTD DocBook XML V4.1.2//EN"
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uri="http://www.oasis-open.org/docbook/xml/4.1.2/docbookx.dtd"/>
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...</pre>
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<p>This is the beginning of a catalog for DocBook 4.1.2, XML Catalogs are
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written in XML, there is a specific namespace for catalog elements
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"urn:oasis:names:tc:entity:xmlns:xml:catalog". The first entry in this
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catalog is a <code>public</code> mapping it allows to associate a Public
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Identifier with an URI.</p>
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<pre>...
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<rewriteSystem systemIdStartString="http://www.oasis-open.org/docbook/"
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rewritePrefix="file:///usr/share/xml/docbook/"/>
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...</pre>
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<p>A <code>rewriteSystem</code> is a very powerful instruction, it says that
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any URI starting with a given prefix should be looked at another URI
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constructed by replacing the prefix with an new one. In effect this acts like
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a cache system for a full area of the Web. In practice it is extremely useful
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with a file prefix if you have installed a copy of those resources on your
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local system.</p>
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<pre>...
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<delegatePublic publicIdStartString="-//OASIS//DTD XML Catalog //"
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catalog="file:///usr/share/xml/docbook.xml"/>
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<delegatePublic publicIdStartString="-//OASIS//ENTITIES DocBook XML"
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catalog="file:///usr/share/xml/docbook.xml"/>
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<delegatePublic publicIdStartString="-//OASIS//DTD DocBook XML"
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catalog="file:///usr/share/xml/docbook.xml"/>
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<delegateSystem systemIdStartString="http://www.oasis-open.org/docbook/"
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catalog="file:///usr/share/xml/docbook.xml"/>
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<delegateURI uriStartString="http://www.oasis-open.org/docbook/"
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catalog="file:///usr/share/xml/docbook.xml"/>
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...</pre>
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<p>Delegation is the core features which allows to build a tree of catalogs,
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easier to maintain than a single catalog, based on Public Identifier, System
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Identifier or URI prefixes it instruct the catalog software to lookup entries
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in another resource. This feature allow to build hierarchies of catalogs, the
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set of entries presented should be sufficient to redirect the resolution of
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all DocBook references to the specific catalog in
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<code>/usr/share/xml/docbook.xml</code> this one in turn could delegate all
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references for DocBook 4.2.1 to a specific catalog installed at the same time
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as the DocBook resources on the local machine.</p>
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<h2><a name="reference">How to tune catalog usage:</a></h2>
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<p>The user can change the default catalog behaviour by redirecting queries
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to its own set of catalogs, this can be done by setting the
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<code>XML_CATALOG_FILES</code> environment variable to a list of catalogs, an
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empty one should desactivate loading the default
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<code>/etc/xml/catalog</code> default catalog.</p>
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<p>@@More options are likely to be provided in the future@@</p>
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<h2><a name="validate">How to debug catalog processing:</a></h2>
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<p>Setting up the <code>XML_DEBUG_CATALOG</code> environment variable will
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make libxml output debugging informations for each catalog operations, for
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example:</p>
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<pre>orchis:~/XML -> xmllint --memory --noout test/ent2
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warning: failed to load external entity "title.xml"
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orchis:~/XML -> export XML_DEBUG_CATALOG=
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orchis:~/XML -> xmllint --memory --noout test/ent2
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Failed to parse catalog /etc/xml/catalog
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Failed to parse catalog /etc/xml/catalog
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warning: failed to load external entity "title.xml"
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Catalogs cleanup
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orchis:~/XML -> </pre>
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<p>The test/ent2 references an entity, running the parser from memory makes
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the base URI unavailable and the the "title.xml" entity cannot be loaded.
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Setting up the debug environment variable allows to detect that an attempt is
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made to load the <code>/etc/xml/catalog</code> but since it's not present the
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resolution fails.</p>
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<p>But the most advanced way to debug XML catalog processing is to use the
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<strong>xmlcatalog</strong> command shipped with libxml2, it allows to load
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catalogs and make resolution queries to see what is going on. This is also
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used for the regression tests:</p>
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<pre>orchis:~/XML -> ./xmlcatalog test/catalogs/docbook.xml "-//OASIS//DTD DocBook XML V4.1.2//EN"
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http://www.oasis-open.org/docbook/xml/4.1.2/docbookx.dtd
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orchis:~/XML -> </pre>
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<p>For debugging what is going on, adding one -v flags increase the verbosity
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level to indicate the processing done (adding a second flag also indicate
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what elements are recognized at parsing):</p>
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<pre>orchis:~/XML -> ./xmlcatalog -v test/catalogs/docbook.xml "-//OASIS//DTD DocBook XML V4.1.2//EN"
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Parsing catalog test/catalogs/docbook.xml's content
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Found public match -//OASIS//DTD DocBook XML V4.1.2//EN
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http://www.oasis-open.org/docbook/xml/4.1.2/docbookx.dtd
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Catalogs cleanup
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orchis:~/XML -> </pre>
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<p>A shell interface is also available to debug and process multiple queries
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(and for regression tests):</p>
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<pre>orchis:~/XML -> ./xmlcatalog -shell test/catalogs/docbook.xml "-//OASIS//DTD DocBook XML V4.1.2//EN"
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> help
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Commands available:
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public PublicID: make a PUBLIC identifier lookup
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system SystemID: make a SYSTEM identifier lookup
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resolve PublicID SystemID: do a full resolver lookup
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add 'type' 'orig' 'replace' : add an entry
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del 'values' : remove values
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dump: print the current catalog state
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debug: increase the verbosity level
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quiet: decrease the verbosity level
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exit: quit the shell
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> public "-//OASIS//DTD DocBook XML V4.1.2//EN"
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http://www.oasis-open.org/docbook/xml/4.1.2/docbookx.dtd
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> quit
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orchis:~/XML -> </pre>
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<p>This should be sufficient for most debugging purpose, this was actually
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used heavilly to debug the XML Catalog implementation itself.</p>
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<h2><a name="Declaring">How to create and maintain</a> catalogs:</h2>
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<p>Basically XML Catalogs are XML files, you can either use XML tools to
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manage them or use <strong>xmlcatalog</strong> for this. The basic step is
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to create a catalog the -create option provide this facility:</p>
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<pre>orchis:~/XML -> ./xmlcatalog --create tst.xml
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<?xml version="1.0"?>
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<!DOCTYPE catalog PUBLIC "-//OASIS//DTD Entity Resolution XML Catalog V1.0//EN"
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"http://www.oasis-open.org/committees/entity/release/1.0/catalog.dtd">
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<catalog xmlns="urn:oasis:names:tc:entity:xmlns:xml:catalog"/>
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orchis:~/XML -> </pre>
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<p>By default xmlcatalog does not overwrite the original catalog and save the
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result on the standard output, this can be overrident using the -noout
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option. The <code>-add</code> command allows to add entries in the
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catalog:</p>
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<pre>orchis:~/XML -> ./xmlcatalog --noout --create --add "public" "-//OASIS//DTD DocBook XML V4.1.2//EN" http://www.oasis-open.org/docbook/xml/4.1.2/docbookx.dtd tst.xml
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orchis:~/XML -> cat tst.xml
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<?xml version="1.0"?>
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<!DOCTYPE catalog PUBLIC "-//OASIS//DTD Entity Resolution XML Catalog V1.0//EN" "http://www.oasis-open.org/committees/entity/release/1.0/catalog.dtd">
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<catalog xmlns="urn:oasis:names:tc:entity:xmlns:xml:catalog">
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<public publicId="-//OASIS//DTD DocBook XML V4.1.2//EN"
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uri="http://www.oasis-open.org/docbook/xml/4.1.2/docbookx.dtd"/>
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</catalog>
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orchis:~/XML -> </pre>
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<p>The <code>-add</code> option will always take 3 parameters even if some of
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the XML Catalog constructs (like nextCatalog) will have only a single
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argument, just pass a third empty string, it will be ignored.</p>
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<p>Similary the <code>-del</code> option remove matching entries from the
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catalog:</p>
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<pre>orchis:~/XML -> ./xmlcatalog --del "http://www.oasis-open.org/docbook/xml/4.1.2/docbookx.dtd" tst.xml
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<?xml version="1.0"?>
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<!DOCTYPE catalog PUBLIC "-//OASIS//DTD Entity Resolution XML Catalog V1.0//EN" "http://www.oasis-open.org/committees/entity/release/1.0/catalog.dtd">
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<catalog xmlns="urn:oasis:names:tc:entity:xmlns:xml:catalog"/>
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orchis:~/XML -> </pre>
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<p>The catalog is now empty. Note that the maching of <code>-del</code> is
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exact and would have worked in a similar fashion with the Public ID
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string.</p>
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<p>This is rudimentary but should be sufficient to manage a not too complex
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catalog tree of resources.</p>
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<h2><a name="implemento">The implementor corner quick review of the
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API:</a></h2>
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<p>First an like for every other module of libxml, there is an automatically
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generated <a href="html/libxml-catalog.html">API page for catalog
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support</a>.</p>
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<p>The header for the catalog interfaces should be included as:</p>
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<pre>#include <libxml/catalog.h></pre>
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<p>The API is voluntarily kept very simple. First it is not obvious that
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applications really need access to it since it is the default behaviour of
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libxml (Note: it is possible to completely override libxml default catalog by
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using <a href="html/libxml-parser.html">xmlSetExternalEntityLoader</a> to
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plug an application specific resolver).</p>
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<p>Basically libxml support 2 catalog lists:</p>
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<ul>
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<li>the default one, global shared by all the application</li>
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<li>a per-document catalog, this one is built if the document uses the
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<code>oasis-xml-catalog</code> PIs to specify its own catalog list, it is
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associated to the parser context and destroyed when the parsing context
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is destroyed.</li>
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</ul>
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<p>the document one will be used first if it exists.</p>
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<h3>Initialization routines:</h3>
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<p>xmlInitializeCatalog(), xmlLoadCatalog() and xmlLoadCatalogs() should be
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used at startup to initialize the catalog, if the catalog should be
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initialized with specific values xmlLoadCatalog() or xmlLoadCatalogs()
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should be called before xmlInitializeCatalog() which would otherwise do a
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default initialization first.</p>
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<p>The xmlCatalogAddLocal() call is used by the parser to grow the document
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own catalog list if needed.</p>
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<h3>Preferences setup:</h3>
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<p>The XML Catalog spec requires the possibility to select default
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preferences between public and system delegation,
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xmlCatalogSetDefaultPrefer() allows this, xmlCatalogSetDefaults() and
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xmlCatalogGetDefaults() allow to control if XML Catalogs resolution should
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be forbidden, allowed for global catalog, for document catalog or both, the
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default is to allow both.</p>
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<p>And of course xmlCatalogSetDebug() allows to generate debug messages
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(through the xmlGenericError() mechanism).</p>
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<h3>Querying routines:</h3>
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<p>xmlCatalogResolve(), xmlCatalogResolveSystem(), xmlCatalogResolvePublic()
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and xmlCatalogResolveURI() are relatively explicit if you read the XML
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Catalog specification they correspond to section 7 algorithms, they should
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also work if you have loaded an SGML catalog with a simplified semantic.</p>
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<p>xmlCatalogLocalResolve() and xmlCatalogLocalResolveURI() are the same but
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operate on the document catalog list</p>
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<h3>Cleanup and Miscellaneous:</h3>
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<p>xmlCatalogCleanup() free-up the global catalog, xmlCatalogFreeLocal() is
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the per-document equivalent.</p>
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<p>xmlCatalogAdd() and xmlCatalogRemove() are used to dynamically modify the
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first catalog in the global list, and xmlCatalogDump() allows to dump a
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catalog state, those routines are primarily designed for xmlcatalog, I'm not
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sure that exposing more complex interfaces (like navigation ones) would be
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really useful.</p>
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<p>The xmlParseCatalogFile() is a function used to load XML Catalog files,
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it's similar as xmlParseFile() except it bypass all catalog lookups, it's
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provided because this functionality may be useful for client tools.</p>
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<h3>threaded environments:</h3>
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<p>Since the catalog tree is built progressively, some care has been taken to
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try to avoid troubles in multithreaded environments but without a
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test-and-set routine accessible from C this can't be fully garanteed, so the
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best is to use xmlGetExternalEntityLoader and set the entity loader routines
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to one of your code doing the synchronization.</p>
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<p></p>
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<h2><a name="Other">Other resources</a></h2>
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<p>The XML Catalog specification is relatively recent so there isn't much
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litterature to point at:</p>
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<ul>
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<li>You can find an good rant from Norm Walsh about <a
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href="http://www.arbortext.com/Think_Tank/XML_Resources/Issue_Three/issue_three.html">the
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need for catalogs</a>, it provides a lot of context informations even if
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I don't agree with everything presented.</li>
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<li>An <a href="http://home.ccil.org/~cowan/XML/XCatalog.html">old XML
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catalog proposal</a> from John Cowan</li>
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<li>The <a href="http://www.rddl.org/">Resource Directory Description
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Language</a> (RDDL) another catalog system but more oriented toward
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providing metadata for XML namespaces.</li>
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<li>the page from the OASIS Technical <a
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href="http://www.oasis-open.org/committees/entity/">Committee on Entity
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Resolution</a> who maintains XML Catalog, you will find pointers to the
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specification update, some background and pointers to others tools
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providing XML Catalog support</li>
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</ul>
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<p>If you have suggestions for corrections or additions, simply contact
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me:</p>
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<p><a href="mailto:daniel@veillard.com">Daniel Veillard</a></p>
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<p>$Id: catalog.html,v 1.2 2001/08/23 00:52:23 veillard Exp $</p>
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</body>
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</html>
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