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| 10 | </style><title>I/O Interfaces</title></head><body bgcolor="#8b7765" text="#000000" link="#a06060" vlink="#000000"><table border="0" width="100%" cellpadding="5" cellspacing="0" align="center"><tr><td width="120"><a href="http://swpat.ffii.org/"><img src="epatents.png" alt="Action against software patents" /></a></td><td width="180"><a href="http://www.gnome.org/"><img src="gnome2.png" alt="Gnome2 Logo" /></a><a href="http://www.w3.org/Status"><img src="w3c.png" alt="W3C Logo" /></a><a href="http://www.redhat.com/"><img src="redhat.gif" alt="Red Hat Logo" /></a><div align="left"><a href="http://xmlsoft.org/"><img src="Libxml2-Logo-180x168.gif" alt="Made with Libxml2 Logo" /></a></div></td><td><table border="0" width="90%" cellpadding="2" cellspacing="0" align="center" bgcolor="#000000"><tr><td><table width="100%" border="0" cellspacing="1" cellpadding="3" bgcolor="#fffacd"><tr><td align="center"><h1>The XML C parser and toolkit of Gnome</h1><h2>I/O Interfaces</h2></td></tr></table></td></tr></table></td></tr></table><table border="0" cellpadding="4" cellspacing="0" width="100%" align="center"><tr><td bgcolor="#8b7765"><table border="0" cellspacing="0" cellpadding="2" width="100%"><tr><td valign="top" width="200" bgcolor="#8b7765"><table border="0" cellspacing="0" cellpadding="1" width="100%" bgcolor="#000000"><tr><td><table width="100%" border="0" cellspacing="1" cellpadding="3"><tr><td colspan="1" bgcolor="#eecfa1" align="center"><center><b>Developer Menu</b></center></td></tr><tr><td bgcolor="#fffacd"><form action="search.php" enctype="application/x-www-form-urlencoded" method="get"><input name="query" type="text" size="20" value="" /><input name="submit" type="submit" value="Search ..." /></form><ul><li><a href="index.html" style="font-weight:bold">Main Menu</a></li><li><a href="html/index.html" style="font-weight:bold">Reference Manual</a></li><li><a href="examples/index.html" style="font-weight:bold">Code 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href="site.xsl">stylesheet</a></li></ul></td></tr></table><table width="100%" border="0" cellspacing="1" cellpadding="3"><tr><td colspan="1" bgcolor="#eecfa1" align="center"><center><b>API Indexes</b></center></td></tr><tr><td bgcolor="#fffacd"><ul><li><a href="APIchunk0.html">Alphabetic</a></li><li><a href="APIconstructors.html">Constructors</a></li><li><a href="APIfunctions.html">Functions/Types</a></li><li><a href="APIfiles.html">Modules</a></li><li><a href="APIsymbols.html">Symbols</a></li></ul></td></tr></table><table width="100%" border="0" cellspacing="1" cellpadding="3"><tr><td colspan="1" bgcolor="#eecfa1" align="center"><center><b>Related links</b></center></td></tr><tr><td bgcolor="#fffacd"><ul><li><a href="http://mail.gnome.org/archives/xml/">Mail archive</a></li><li><a href="http://xmlsoft.org/XSLT/">XSLT libxslt</a></li><li><a href="http://phd.cs.unibo.it/gdome2/">DOM gdome2</a></li><li><a href="http://www.aleksey.com/xmlsec/">XML-DSig xmlsec</a></li><li><a href="ftp://xmlsoft.org/">FTP</a></li><li><a href="http://www.zlatkovic.com/projects/libxml/">Windows binaries</a></li><li><a href="http://opencsw.org/packages/libxml2">Solaris binaries</a></li><li><a href="http://www.explain.com.au/oss/libxml2xslt.html">MacOsX binaries</a></li><li><a href="http://lxml.de/">lxml Python bindings</a></li><li><a href="http://cpan.uwinnipeg.ca/dist/XML-LibXML">Perl bindings</a></li><li><a href="https://libxmlplusplus.github.io/libxmlplusplus/">C++ bindings</a></li><li><a href="http://www.zend.com/php5/articles/php5-xmlphp.php#Heading4">PHP bindings</a></li><li><a href="http://sourceforge.net/projects/libxml2-pas/">Pascal bindings</a></li><li><a href="http://libxml.rubyforge.org/">Ruby bindings</a></li><li><a href="http://tclxml.sourceforge.net/">Tcl bindings</a></li><li><a href="https://gitlab.gnome.org/GNOME/libxml2/issues">Bug Tracker</a></li></ul></td></tr></table></td></tr></table></td><td valign="top" bgcolor="#8b7765"><table border="0" cellspacing="0" cellpadding="1" width="100%"><tr><td><table border="0" cellspacing="0" cellpadding="1" width="100%" bgcolor="#000000"><tr><td><table border="0" cellpadding="3" cellspacing="1" width="100%"><tr><td bgcolor="#fffacd"><p>Table of Content:</p><ol>
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| 11 | <li><a href="#General1">General overview</a></li>
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| 12 | <li><a href="#basic">The basic buffer type</a></li>
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| 13 | <li><a href="#Input">Input I/O handlers</a></li>
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| 14 | <li><a href="#Output">Output I/O handlers</a></li>
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| 15 | <li><a href="#entities">The entities loader</a></li>
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| 16 | <li><a href="#Example2">Example of customized I/O</a></li>
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| 17 | </ol><h3><a name="General1" id="General1">General overview</a></h3><p>The module <code><a href="http://xmlsoft.org/html/libxml-xmlio.html">xmlIO.h</a></code> provides
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| 18 | the interfaces to the libxml2 I/O system. This consists of 4 main parts:</p><ul>
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| 19 | <li>Entities loader, this is a routine which tries to fetch the entities
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| 20 | (files) based on their PUBLIC and SYSTEM identifiers. The default loader
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| 21 | don't look at the public identifier since libxml2 do not maintain a
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| 22 | catalog. You can redefine you own entity loader by using
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| 23 | <code>xmlGetExternalEntityLoader()</code> and
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| 24 | <code>xmlSetExternalEntityLoader()</code>. <a href="#entities">Check the
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| 25 | example</a>.</li>
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| 26 | <li>Input I/O buffers which are a commodity structure used by the parser(s)
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| 27 | input layer to handle fetching the information to feed the parser. This
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| 28 | provides buffering and is also a placeholder where the encoding
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| 29 | converters to UTF8 are piggy-backed.</li>
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| 30 | <li>Output I/O buffers are similar to the Input ones and fulfill similar
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| 31 | task but when generating a serialization from a tree.</li>
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| 32 | <li>A mechanism to register sets of I/O callbacks and associate them with
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| 33 | specific naming schemes like the protocol part of the URIs.
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| 34 | <p>This affect the default I/O operations and allows to use specific I/O
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| 35 | handlers for certain names.</p>
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| 36 | </li>
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| 37 | </ul><p>The general mechanism used when loading http://rpmfind.net/xml.html for
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| 38 | example in the HTML parser is the following:</p><ol>
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| 39 | <li>The default entity loader calls <code>xmlNewInputFromFile()</code> with
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| 40 | the parsing context and the URI string.</li>
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| 41 | <li>the URI string is checked against the existing registered handlers
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| 42 | using their match() callback function, if the HTTP module was compiled
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| 43 | in, it is registered and its match() function will succeeds</li>
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| 44 | <li>the open() function of the handler is called and if successful will
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| 45 | return an I/O Input buffer</li>
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| 46 | <li>the parser will the start reading from this buffer and progressively
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| 47 | fetch information from the resource, calling the read() function of the
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| 48 | handler until the resource is exhausted</li>
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| 49 | <li>if an encoding change is detected it will be installed on the input
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| 50 | buffer, providing buffering and efficient use of the conversion
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| 51 | routines</li>
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| 52 | <li>once the parser has finished, the close() function of the handler is
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| 53 | called once and the Input buffer and associated resources are
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| 54 | deallocated.</li>
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| 55 | </ol><p>The user defined callbacks are checked first to allow overriding of the
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| 56 | default libxml2 I/O routines.</p><h3><a name="basic" id="basic">The basic buffer type</a></h3><p>All the buffer manipulation handling is done using the
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| 57 | <code>xmlBuffer</code> type define in <code><a href="http://xmlsoft.org/html/libxml-tree.html">tree.h</a> </code>which is a
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| 58 | resizable memory buffer. The buffer allocation strategy can be selected to be
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| 59 | either best-fit or use an exponential doubling one (CPU vs. memory use
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| 60 | trade-off). The values are <code>XML_BUFFER_ALLOC_EXACT</code> and
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| 61 | <code>XML_BUFFER_ALLOC_DOUBLEIT</code>, and can be set individually or on a
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| 62 | system wide basis using <code>xmlBufferSetAllocationScheme()</code>. A number
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| 63 | of functions allows to manipulate buffers with names starting with the
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| 64 | <code>xmlBuffer...</code> prefix.</p><h3><a name="Input" id="Input">Input I/O handlers</a></h3><p>An Input I/O handler is a simple structure
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| 65 | <code>xmlParserInputBuffer</code> containing a context associated to the
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| 66 | resource (file descriptor, or pointer to a protocol handler), the read() and
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| 67 | close() callbacks to use and an xmlBuffer. And extra xmlBuffer and a charset
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| 68 | encoding handler are also present to support charset conversion when
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| 69 | needed.</p><h3><a name="Output" id="Output">Output I/O handlers</a></h3><p>An Output handler <code>xmlOutputBuffer</code> is completely similar to an
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| 70 | Input one except the callbacks are write() and close().</p><h3><a name="entities" id="entities">The entities loader</a></h3><p>The entity loader resolves requests for new entities and create inputs for
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| 71 | the parser. Creating an input from a filename or an URI string is done
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| 72 | through the xmlNewInputFromFile() routine. The default entity loader do not
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| 73 | handle the PUBLIC identifier associated with an entity (if any). So it just
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| 74 | calls xmlNewInputFromFile() with the SYSTEM identifier (which is mandatory in
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| 75 | XML).</p><p>If you want to hook up a catalog mechanism then you simply need to
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| 76 | override the default entity loader, here is an example:</p><pre>#include <libxml/xmlIO.h>
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| 77 |
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| 78 | xmlExternalEntityLoader defaultLoader = NULL;
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| 79 |
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| 80 | xmlParserInputPtr
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| 81 | xmlMyExternalEntityLoader(const char *URL, const char *ID,
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| 82 | xmlParserCtxtPtr ctxt) {
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| 83 | xmlParserInputPtr ret;
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| 84 | const char *fileID = NULL;
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| 85 | /* lookup for the fileID depending on ID */
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| 86 |
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| 87 | ret = xmlNewInputFromFile(ctxt, fileID);
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| 88 | if (ret != NULL)
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| 89 | return(ret);
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| 90 | if (defaultLoader != NULL)
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| 91 | ret = defaultLoader(URL, ID, ctxt);
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| 92 | return(ret);
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| 93 | }
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| 94 |
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| 95 | int main(..) {
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| 96 | ...
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| 97 |
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| 98 | /*
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| 99 | * Install our own entity loader
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| 100 | */
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| 101 | defaultLoader = xmlGetExternalEntityLoader();
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| 102 | xmlSetExternalEntityLoader(xmlMyExternalEntityLoader);
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| 103 |
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| 104 | ...
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| 105 | }</pre><h3><a name="Example2" id="Example2">Example of customized I/O</a></h3><p>This example come from <a href="http://xmlsoft.org/messages/0708.html">a
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| 106 | real use case</a>, xmlDocDump() closes the FILE * passed by the application
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| 107 | and this was a problem. The <a href="http://xmlsoft.org/messages/0711.html">solution</a> was to redefine a
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| 108 | new output handler with the closing call deactivated:</p><ol>
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| 109 | <li>First define a new I/O output allocator where the output don't close
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| 110 | the file:
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| 111 | <pre>xmlOutputBufferPtr
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| 112 | xmlOutputBufferCreateOwn(FILE *file, xmlCharEncodingHandlerPtr encoder) {
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| 113 | xmlOutputBufferPtr ret;
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| 114 |
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| 115 | if (xmlOutputCallbackInitialized == 0)
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| 116 | xmlRegisterDefaultOutputCallbacks();
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| 117 |
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| 118 | if (file == NULL) return(NULL);
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| 119 | ret = xmlAllocOutputBuffer(encoder);
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| 120 | if (ret != NULL) {
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| 121 | ret->context = file;
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| 122 | ret->writecallback = xmlFileWrite;
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| 123 | ret->closecallback = NULL; /* No close callback */
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| 124 | }
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| 125 | return(ret);
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| 126 | } </pre>
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| 127 | </li>
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| 128 | <li>And then use it to save the document:
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| 129 | <pre>FILE *f;
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| 130 | xmlOutputBufferPtr output;
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| 131 | xmlDocPtr doc;
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| 132 | int res;
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| 133 |
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| 134 | f = ...
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| 135 | doc = ....
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| 136 |
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| 137 | output = xmlOutputBufferCreateOwn(f, NULL);
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| 138 | res = xmlSaveFileTo(output, doc, NULL);
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| 139 | </pre>
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| 140 | </li>
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| 141 | </ol><p><a href="bugs.html">Daniel Veillard</a></p></td></tr></table></td></tr></table></td></tr></table></td></tr></table></td></tr></table></body></html>
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