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</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 Examples</a></li><li><a href="guidelines.html">XML Guidelines</a></li><li><a href="tutorial/index.html">Tutorial</a></li><li><a href="xmlreader.html">The Reader Interface</a></li><li><a href="ChangeLog.html">ChangeLog</a></li><li><a href="XSLT.html">XSLT</a></li><li><a href="python.html">Python and bindings</a></li><li><a href="architecture.html">libxml2 architecture</a></li><li><a href="tree.html">The tree output</a></li><li><a href="interface.html">The SAX interface</a></li><li><a href="xmlmem.html">Memory Management</a></li><li><a href="xmlio.html">I/O Interfaces</a></li><li><a href="library.html">The parser interfaces</a></li><li><a href="entities.html">Entities or no entities</a></li><li><a href="namespaces.html">Namespaces</a></li><li><a href="upgrade.html">Upgrading 1.x code</a></li><li><a href="threads.html">Thread safety</a></li><li><a href="DOM.html">DOM Principles</a></li><li><a href="example.html">A real example</a></li><li><a href="xml.html">flat page</a>, <a 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="http://libxmlplusplus.sourceforge.net/">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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<li><a href="#General1">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="#Example2">Example of customized I/O</a></li>
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</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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the interfaces to the libxml2 I/O system. This consists of 4 main parts:</p><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 libxml2 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 information to feed the parser. This
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provides buffering and is also a placeholder where the encoding
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converters 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><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><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
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using their match() callback function, if the HTTP module was compiled
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in, it is 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 associated resources are
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deallocated.</li>
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</ol><p>The user defined callbacks are checked first to allow overriding of the
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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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<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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resizable memory buffer. The buffer allocation strategy can be selected to be
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either best-fit or use an exponential doubling one (CPU vs. memory use
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trade-off). 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><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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<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><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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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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the parser. Creating an input from a filename or an URI string is done
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through the xmlNewInputFromFile() routine. The default entity loader do not
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handle the PUBLIC identifier associated with an entity (if any). So it just
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calls xmlNewInputFromFile() with the SYSTEM identifier (which is mandatory in
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XML).</p><p>If you want to hook up a catalog mechanism then you simply need to
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override the default entity loader, here is an example:</p><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><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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real use case</a>, xmlDocDump() closes the FILE * passed by the application
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and this was a problem. The <a 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><ol>
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<li>First define a new I/O output allocator where the output don't close
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the file:
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<pre>xmlOutputBufferPtr
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xmlOutputBufferCreateOwn(FILE *file, xmlCharEncodingHandlerPtr encoder) {
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xmlOutputBufferPtr ret;
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if (xmlOutputCallbackInitialized == 0)
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xmlRegisterDefaultOutputCallbacks();
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if (file == NULL) return(NULL);
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ret = xmlAllocOutputBuffer(encoder);
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if (ret != NULL) {
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ret->context = file;
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ret->writecallback = xmlFileWrite;
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ret->closecallback = NULL; /* No close callback */
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}
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return(ret);
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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><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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