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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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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>
11 <li><a href="#General1">General overview</a></li>
12 <li><a href="#basic">The basic buffer type</a></li>
13 <li><a href="#Input">Input I/O handlers</a></li>
14 <li><a href="#Output">Output I/O handlers</a></li>
15 <li><a href="#entities">The entities loader</a></li>
16 <li><a href="#Example2">Example of customized I/O</a></li>
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
18 the interfaces to the libxml2 I/O system. This consists of 4 main parts:</p><ul>
19 <li>Entities loader, this is a routine which tries to fetch the entities
20 (files) based on their PUBLIC and SYSTEM identifiers. The default loader
21 don't look at the public identifier since libxml2 do not maintain a
22 catalog. You can redefine you own entity loader by using
23 <code>xmlGetExternalEntityLoader()</code> and
24 <code>xmlSetExternalEntityLoader()</code>. <a href="#entities">Check the
26 <li>Input I/O buffers which are a commodity structure used by the parser(s)
27 input layer to handle fetching the information to feed the parser. This
28 provides buffering and is also a placeholder where the encoding
29 converters to UTF8 are piggy-backed.</li>
30 <li>Output I/O buffers are similar to the Input ones and fulfill similar
31 task but when generating a serialization from a tree.</li>
32 <li>A mechanism to register sets of I/O callbacks and associate them with
33 specific naming schemes like the protocol part of the URIs.
34 <p>This affect the default I/O operations and allows to use specific I/O
35 handlers for certain names.</p>
37 </ul><p>The general mechanism used when loading http://rpmfind.net/xml.html for
38 example in the HTML parser is the following:</p><ol>
39 <li>The default entity loader calls <code>xmlNewInputFromFile()</code> with
40 the parsing context and the URI string.</li>
41 <li>the URI string is checked against the existing registered handlers
42 using their match() callback function, if the HTTP module was compiled
43 in, it is registered and its match() function will succeeds</li>
44 <li>the open() function of the handler is called and if successful will
45 return an I/O Input buffer</li>
46 <li>the parser will the start reading from this buffer and progressively
47 fetch information from the resource, calling the read() function of the
48 handler until the resource is exhausted</li>
49 <li>if an encoding change is detected it will be installed on the input
50 buffer, providing buffering and efficient use of the conversion
52 <li>once the parser has finished, the close() function of the handler is
53 called once and the Input buffer and associated resources are
55 </ol><p>The user defined callbacks are checked first to allow overriding of the
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
57 <code>xmlBuffer</code> type define in <code><a href="http://xmlsoft.org/html/libxml-tree.html">tree.h</a> </code>which is a
58 resizable memory buffer. The buffer allocation strategy can be selected to be
59 either best-fit or use an exponential doubling one (CPU vs. memory use
60 trade-off). The values are <code>XML_BUFFER_ALLOC_EXACT</code> and
61 <code>XML_BUFFER_ALLOC_DOUBLEIT</code>, and can be set individually or on a
62 system wide basis using <code>xmlBufferSetAllocationScheme()</code>. A number
63 of functions allows to manipulate buffers with names starting with the
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
65 <code>xmlParserInputBuffer</code> containing a context associated to the
66 resource (file descriptor, or pointer to a protocol handler), the read() and
67 close() callbacks to use and an xmlBuffer. And extra xmlBuffer and a charset
68 encoding handler are also present to support charset conversion when
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
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
71 the parser. Creating an input from a filename or an URI string is done
72 through the xmlNewInputFromFile() routine. The default entity loader do not
73 handle the PUBLIC identifier associated with an entity (if any). So it just
74 calls xmlNewInputFromFile() with the SYSTEM identifier (which is mandatory in
75 XML).</p><p>If you want to hook up a catalog mechanism then you simply need to
76 override the default entity loader, here is an example:</p><pre>#include <libxml/xmlIO.h>
78 xmlExternalEntityLoader defaultLoader = NULL;
81 xmlMyExternalEntityLoader(const char *URL, const char *ID,
82 xmlParserCtxtPtr ctxt) {
83 xmlParserInputPtr ret;
84 const char *fileID = NULL;
85 /* lookup for the fileID depending on ID */
87 ret = xmlNewInputFromFile(ctxt, fileID);
90 if (defaultLoader != NULL)
91 ret = defaultLoader(URL, ID, ctxt);
99 * Install our own entity loader
101 defaultLoader = xmlGetExternalEntityLoader();
102 xmlSetExternalEntityLoader(xmlMyExternalEntityLoader);
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
106 real use case</a>, xmlDocDump() closes the FILE * passed by the application
107 and this was a problem. The <a href="http://xmlsoft.org/messages/0711.html">solution</a> was to redefine a
108 new output handler with the closing call deactivated:</p><ol>
109 <li>First define a new I/O output allocator where the output don't close
111 <pre>xmlOutputBufferPtr
112 xmlOutputBufferCreateOwn(FILE *file, xmlCharEncodingHandlerPtr encoder) {
113 xmlOutputBufferPtr ret;
115 if (xmlOutputCallbackInitialized == 0)
116 xmlRegisterDefaultOutputCallbacks();
118 if (file == NULL) return(NULL);
119 ret = xmlAllocOutputBuffer(encoder);
121 ret->context = file;
122 ret->writecallback = xmlFileWrite;
123 ret->closecallback = NULL; /* No close callback */
128 <li>And then use it to save the document:
130 xmlOutputBufferPtr output;
137 output = xmlOutputBufferCreateOwn(f, NULL);
138 res = xmlSaveFileTo(output, doc, NULL);
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>