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* AUTHORS: added William and Bjorn * include/libxml/*.h *.c README doc/*.html etc.: changed old email to daniel@veillard.com hopefully I won't have to do this again * doc/Makefile.am doc/html/*.html: cleanup makefile, checked that docs can be rebuilt cleanly now * include/libxml/xml*version.h*: removed include/libxml/xmlversion.h from CVs it's generated, added include/libxml/xmlwin32version.h also generated but which should change far less frequently. * catalog.c nanoftp.c: made sure to include libxml.h not libxml/xmlversion.h directly * include/libxml/*.h: include xmlwin32version.h instead of xmlversion.h when compiling on WIN32 and MSC Daniel
202 lines
7.4 KiB
HTML
202 lines
7.4 KiB
HTML
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN"
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"http://www.w3.org/TR/REC-html40/loose.dtd">
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<html>
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<head>
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<title>Libxml Input/Output handling</title>
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<meta name="GENERATOR" content="amaya V3.2.1">
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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 Input/Output handling</h1>
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<p>Location: <a
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href="http://xmlsoft.org/xmlio.html">http://xmlsoft.org/xmlio.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://xmlsoft.org/messages/">http://xmlsoft.org/messages/</a></p>
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<p>Version: $Revision: 1.4 $</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="#basic">The basic buffer type</a></li>
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<li><a href="#Input">Input I/O handlers</a></li>
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<li><a href="#Output">Output I/O handlers</a></li>
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<li><a href="#entities">The entities loader</a></li>
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<li><a href="#Example">Example of customized I/O</a></li>
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</ol>
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<h2><a name="General">General overview</a></h2>
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<p>The module <code><a
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href="http://xmlsoft.org/html/libxml-xmlio.html">xmlIO.h</a></code>
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provides the interfaces to the libxml I/O system. This consists of 4 main
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parts:</p>
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<ul>
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<li>Entities loader, this is a routine which tries to fetch the entities
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(files) based on their PUBLIC and SYSTEM identifiers. The default loader
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don't look at the public identifier since libxml do not maintain a
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catalog. You can redefine you own entity loader by using
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<code>xmlGetExternalEntityLoader()</code> and
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<code>xmlSetExternalEntityLoader()</code>. <a href="#entities">Check the
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example</a>.</li>
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<li>Input I/O buffers which are a commodity structure used by the parser(s)
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input layer to handle fetching the informations to feed the parser. This
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provides buffering and is also a placeholder where the encoding convertors
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to UTF8 are piggy-backed.</li>
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<li>Output I/O buffers are similar to the Input ones and fulfill similar
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task but when generating a serialization from a tree.</li>
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<li>A mechanism to register sets of I/O callbacks and associate them with
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specific naming schemes like the protocol part of the URIs.
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<p>This affect the default I/O operations and allows to use specific I/O
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handlers for certain names.</p>
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</li>
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</ul>
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<p>The general mechanism used when loading http://rpmfind.net/xml.html for
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example in the HTML parser is the following:</p>
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<ol>
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<li>The default entity loader calls <code>xmlNewInputFromFile()</code> with
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the parsing context and the URI string.</li>
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<li>the URI string is checked against the existing registered handlers using
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their match() callback function, if the HTTP module was compiled in, it is
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registered and its match() function will succeeds</li>
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<li>the open() function of the handler is called and if successful will
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return an I/O Input buffer</li>
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<li>the parser will the start reading from this buffer and progressively
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fetch information from the resource, calling the read() function of the
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handler until the resource is exhausted</li>
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<li>if an encoding change is detected it will be installed on the input
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buffer, providing buffering and efficient use of the conversion
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routines</li>
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<li>once the parser has finished, the close() function of the handler is
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called once and the Input buffer and associed resources are
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deallocated.</li>
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</ol>
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<p>The user defined callbacks are checked first to allow overriding of the
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default libxml I/O routines.</p>
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<h2><a name="basic">The basic buffer type</a></h2>
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<p>All the buffer manipulation handling is done using the
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<code>xmlBuffer</code> type define in <code><a
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href="http://xmlsoft.org/html/libxml-tree.html">tree.h</a> </code>which is
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a resizable memory buffer. The buffer allocation strategy can be selected to
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be either best-fit or use an exponential doubling one (CPU vs. memory use
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tradeoff). The values are <code>XML_BUFFER_ALLOC_EXACT</code> and
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<code>XML_BUFFER_ALLOC_DOUBLEIT</code>, and can be set individually or on a
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system wide basis using <code>xmlBufferSetAllocationScheme()</code>. A number
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of functions allows to manipulate buffers with names starting with the
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<code>xmlBuffer...</code> prefix.</p>
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<h2><a name="Input">Input I/O handlers</a></h2>
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<p>An Input I/O handler is a simple structure
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<code>xmlParserInputBuffer</code> containing a context associated to the
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resource (file descriptor, or pointer to a protocol handler), the read() and
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close() callbacks to use and an xmlBuffer. And extra xmlBuffer and a charset
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encoding handler are also present to support charset conversion when
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needed.</p>
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<h2><a name="Output">Output I/O handlers</a></h2>
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<p>An Output handler <code>xmlOutputBuffer</code> is completely similar to an
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Input one except the callbacks are write() and close().</p>
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<h2><a name="entities">The entities loader</a></h2>
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<p>The entity loader resolves requests for new entities and create inputs for
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the parser. Creating an input from a filename or an URI string is done through
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the xmlNewInputFromFile() routine. The default entity loader do not handle
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the PUBLIC identifier associated with an entity (if any). So it just calls
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xmlNewInputFromFile() with the SYSTEM identifier (which is mandatory in
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XML).</p>
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<p>If you want to hook up a catalog mechanism then you simply need to override
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the default entity loader, here is an example:</p>
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<pre>#include <libxml/xmlIO.h>
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xmlExternalEntityLoader defaultLoader = NULL;
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xmlParserInputPtr
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xmlMyExternalEntityLoader(const char *URL, const char *ID,
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xmlParserCtxtPtr ctxt) {
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xmlParserInputPtr ret;
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const char *fileID = NULL;
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/* lookup for the fileID depending on ID */
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ret = xmlNewInputFromFile(ctxt, fileID);
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if (ret != NULL)
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return(ret);
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if (defaultLoader != NULL)
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ret = defaultLoader(URL, ID, ctxt);
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return(ret);
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}
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int main(..) {
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...
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/*
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* Install our own entity loader
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*/
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defaultLoader = xmlGetExternalEntityLoader();
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xmlSetExternalEntityLoader(xmlMyExternalEntityLoader);
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...
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}</pre>
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<h2><a name="Example">Example of customized I/O</a></h2>
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<p>This example come from <a href="http://xmlsoft.org/messages/0708.html">a
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real use case</a>, xmlDocDump() closes the FILE * passed by the application
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and this was a problem. The <a
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href="http://xmlsoft.org/messages/0711.html">solution</a> was to redefine a
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new output handler with the closing call deactivated:</p>
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<ol>
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<li>First define a new I/O ouput allocator where the output don't close the
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file:
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<pre>xmlOutputBufferPtr
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xmlOutputBufferCreateOwn(FILE *file, xmlCharEncodingHandlerPtr encoder) {
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<EFBFBD><EFBFBD><EFBFBD><EFBFBD>xmlOutputBufferPtr ret;
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<EFBFBD><EFBFBD><EFBFBD><EFBFBD>
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<EFBFBD><EFBFBD><EFBFBD><EFBFBD>if (xmlOutputCallbackInitialized == 0)
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<EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD>xmlRegisterDefaultOutputCallbacks();
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<EFBFBD><EFBFBD><EFBFBD><EFBFBD>if (file == NULL) return(NULL);
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<EFBFBD><EFBFBD><EFBFBD><EFBFBD>ret = xmlAllocOutputBuffer(encoder);
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<EFBFBD><EFBFBD><EFBFBD><EFBFBD>if (ret != NULL) {
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<EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD>ret->context = file;
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<EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD>ret->writecallback = xmlFileWrite;
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<EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD>ret->closecallback = NULL; /* No close callback */
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<EFBFBD><EFBFBD><EFBFBD><EFBFBD>}
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<EFBFBD><EFBFBD><EFBFBD><EFBFBD>return(ret); <br>
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} </pre>
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</li>
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<li>And then use it to save the document:
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<pre>FILE *f;
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xmlOutputBufferPtr output;
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xmlDocPtr doc;
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int res;
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f = ...
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doc = ....
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output = xmlOutputBufferCreateOwn(f, NULL);
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res = xmlSaveFileTo(output, doc, NULL);
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</pre>
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</li>
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</ol>
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<p><a href="mailto:daniel@veillard.com">Daniel Veillard</a></p>
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<p>$Id: xmlio.html,v 1.4 2001/01/29 08:22:12 veillard Exp $</p>
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</body>
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</html>
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