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<?xml version="1.0" encoding="ISO-8859-1"?><!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"><html xmlns="http://www.w3.org/1999/xhtml"><head><meta http-equiv="Content-Type" content="text/html; charset=ISO-8859-1" /><link rel="SHORTCUT ICON" href="/favicon.ico" /><style type="text/css">TD {font-family: Verdana,Arial,Helvetica}BODY {font-family: Verdana,Arial,Helvetica; margin-top: 2em; margin-left: 0em; margin-right: 0em}H1 {font-family: Verdana,Arial,Helvetica}H2 {font-family: Verdana,Arial,Helvetica}H3 {font-family: Verdana,Arial,Helvetica}A:link, A:visited, A:active { text-decoration: underline }</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 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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="http://bugzilla.gnome.org/buglist.cgi?product=libxml2">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><li><a href="#General1">General overview</a></li> <li><a href="#basic">The basic buffer type</a></li> <li><a href="#Input">Input I/O handlers</a></li> <li><a href="#Output">Output I/O handlers</a></li> <li><a href="#entities">The entities loader</a></li> <li><a href="#Example2">Example of customized I/O</a></li></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> providesthe interfaces to the libxml2 I/O system. This consists of 4 main parts:</p><ul><li>Entities loader, this is a routine which tries to fetch the entities (files) based on their PUBLIC and SYSTEM identifiers. The default loader don't look at the public identifier since libxml2 do not maintain a catalog. You can redefine you own entity loader by using <code>xmlGetExternalEntityLoader()</code> and <code>xmlSetExternalEntityLoader()</code>. <a href="#entities">Check the example</a>.</li> <li>Input I/O buffers which are a commodity structure used by the parser(s) input layer to handle fetching the information to feed the parser. This provides buffering and is also a placeholder where the encoding converters to UTF8 are piggy-backed.</li> <li>Output I/O buffers are similar to the Input ones and fulfill similar task but when generating a serialization from a tree.</li> <li>A mechanism to register sets of I/O callbacks and associate them with specific naming schemes like the protocol part of the URIs. <p>This affect the default I/O operations and allows to use specific I/O handlers for certain names.</p> </li></ul><p>The general mechanism used when loading http://rpmfind.net/xml.html forexample in the HTML parser is the following:</p><ol><li>The default entity loader calls <code>xmlNewInputFromFile()</code> with the parsing context and the URI string.</li> <li>the URI string is checked against the existing registered handlers using their match() callback function, if the HTTP module was compiled in, it is registered and its match() function will succeeds</li> <li>the open() function of the handler is called and if successful will return an I/O Input buffer</li> <li>the parser will the start reading from this buffer and progressively fetch information from the resource, calling the read() function of the handler until the resource is exhausted</li> <li>if an encoding change is detected it will be installed on the input buffer, providing buffering and efficient use of the conversion routines</li> <li>once the parser has finished, the close() function of the handler is called once and the Input buffer and associated resources are deallocated.</li></ol><p>The user defined callbacks are checked first to allow overriding of thedefault 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<code>xmlBuffer</code> type define in <code><a href="http://xmlsoft.org/html/libxml-tree.html">tree.h</a> </code>which is aresizable memory buffer. The buffer allocation strategy can be selected to beeither best-fit or use an exponential doubling one (CPU vs. memory usetrade-off). The values are <code>XML_BUFFER_ALLOC_EXACT</code> and<code>XML_BUFFER_ALLOC_DOUBLEIT</code>, and can be set individually or on asystem wide basis using <code>xmlBufferSetAllocationScheme()</code>. A numberof functions allows to manipulate buffers with names starting with the<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<code>xmlParserInputBuffer</code> containing a context associated to theresource (file descriptor, or pointer to a protocol handler), the read() andclose() callbacks to use and an xmlBuffer. And extra xmlBuffer and a charsetencoding handler are also present to support charset conversion whenneeded.</p><h3><a name="Output" id="Output">Output I/O handlers</a></h3><p>An Output handler <code>xmlOutputBuffer</code> is completely similar to anInput 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 forthe parser. Creating an input from a filename or an URI string is donethrough the xmlNewInputFromFile() routine. The default entity loader do nothandle the PUBLIC identifier associated with an entity (if any). So it justcalls xmlNewInputFromFile() with the SYSTEM identifier (which is mandatory inXML).</p><p>If you want to hook up a catalog mechanism then you simply need tooverride the default entity loader, here is an example:</p><pre>#include <libxml/xmlIO.h>xmlExternalEntityLoader defaultLoader = NULL;xmlParserInputPtrxmlMyExternalEntityLoader(const char *URL, const char *ID, xmlParserCtxtPtr ctxt) { xmlParserInputPtr ret; const char *fileID = NULL; /* lookup for the fileID depending on ID */ ret = xmlNewInputFromFile(ctxt, fileID); if (ret != NULL) return(ret); if (defaultLoader != NULL) ret = defaultLoader(URL, ID, ctxt); return(ret);}int main(..) { ... /* * Install our own entity loader */ defaultLoader = xmlGetExternalEntityLoader(); xmlSetExternalEntityLoader(xmlMyExternalEntityLoader); ...}</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">areal use case</a>, xmlDocDump() closes the FILE * passed by the applicationand this was a problem. The <a href="http://xmlsoft.org/messages/0711.html">solution</a> was to redefine anew output handler with the closing call deactivated:</p><ol><li>First define a new I/O output allocator where the output don't close the file: <pre>xmlOutputBufferPtrxmlOutputBufferCreateOwn(FILE *file, xmlCharEncodingHandlerPtr encoder) {牋牋xmlOutputBufferPtr ret;牋牋牋牋if (xmlOutputCallbackInitialized == 0)牋牋牋牋xmlRegisterDefaultOutputCallbacks();牋牋if (file == NULL) return(NULL);牋牋ret = xmlAllocOutputBuffer(encoder);牋牋if (ret != NULL) {牋牋牋牋ret->context = file;牋牋牋牋ret->writecallback = xmlFileWrite;牋牋牋牋ret->closecallback = NULL; /* No close callback */牋牋}牋牋return(ret);} </pre> </li> <li>And then use it to save the document: <pre>FILE *f;xmlOutputBufferPtr output;xmlDocPtr doc;int res;f = ...doc = ....output = xmlOutputBufferCreateOwn(f, NULL);res = xmlSaveFileTo(output, doc, NULL); </pre> </li></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>⌨️ 快捷键说明
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