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5 <title>Libxml Input/Output handling</title>
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11<h1 align="center">Libxml Input/Output handling</h1>
12
13<p>Location: <a
14href="http://xmlsoft.org/xmlio.html">http://xmlsoft.org/xmlio.html</a></p>
15
16<p>Libxml home page: <a href="http://xmlsoft.org/">http://xmlsoft.org/</a></p>
17
18<p>Mailing-list archive: <a
19href="http://xmlsoft.org/messages/">http://xmlsoft.org/messages/</a></p>
20
21<p>Version: $Revision:$</p>
22
23<p>Table of Content:</p>
24<ol>
25 <li><a href="#General">General overview</a></li>
26 <li><a href="#basic">The basic buffer type</a></li>
27 <li><a href="#Input">Input I/O handlers</a></li>
28 <li><a href="#Output">Output I/O handlers</a></li>
29 <li><a href="#Example">Example of customized I/O</a></li>
30</ol>
31
32<h2><a name="General">General overview</a></h2>
33
34<p>The module <code><a
35href="http://xmlsoft.org/html/gnome-xml-xmlio.html">xmlIO.h</a></code>
36provides the interfaces to the libxml I/O system. This consists of 3 main
37parts:</p>
38<ul>
39 <li>Input I/O buffers which are a commodity structure used by the parser(s)
40 input layer to handle fetching the informations to feed the parser. This
41 provides buffering and is also a placeholder where the encoding convertors
42 to UTF8 are piggy-backed.</li>
43 <li>Output I/O buffers are similar to the Input ones and fulfill similar
44 task but when generating a serialization from a tree.</li>
45 <li>A mechanism to register sets of I/O callbacks and associate them with
46 specific naming schemes like the protocol part of the URIs.
47 <p>This affect the default I/O operations and allows to use specific I/O
48 handlers for certain names.</p>
49 </li>
50</ul>
51
52<p> The general mechanism used when loading http://rpmfind.net/xml.html for
53example in the HTML parser is the following:</p>
54<ol>
55 <li>the URI string is checked against the existing registered handlers using
56 their match() callback function, if the HTTP module was compiled in, it is
57 registered and its macth() function will succeed</li>
58 <li>the open() function of the handler is called and if successful will
59 return an I/O Input buffer</li>
60 <li>the parser will the start reading from this buffer and progressively
61 fetch information from the resource, calling the read() function of the
62 handler until the resource is exhausted</li>
63 <li>if an encoding change is detected it will be installed on the input
64 buffer, providing buffering and efficient use of the conversion
65 routines</li>
66 <li>once the parser has finished, the close() function of the handler is
67 called once and the Input buffer and associed resources are
68 deallocated.</li>
69</ol>
70
71<p>The user defined callbacks are checked first to allow overriding of the
72default libxml I/O routines.</p>
73
74<h2><a name="basic">The basic buffer type</a></h2>
75
76<p>All the buffer manipulation handling is done using the
77<code>xmlBuffer</code> type define in <code><a
78href="http://xmlsoft.org/html/gnome-xml-tree.html">tree.h</a> </code>which is
79a resizable memory buffer. The buffer allocation strategy can be selected to
80be either best-fit or use an exponential doubling one (CPU vs. memory use
81tradeoff). The values are <code>XML_BUFFER_ALLOC_EXACT</code> and
82<code>XML_BUFFER_ALLOC_DOUBLEIT</code>, and can be set individually or on a
83system wide basis using <code>xmlBufferSetAllocationScheme()</code>. A number
84of functions allows to manipulate buffers with names starting with the
85<code>xmlBuffer...</code> prefix.</p>
86
87<h2><a name="Input">Input I/O handlers</a></h2>
88
89<p>An Input I/O handler is a simple structure
90<code>xmlParserInputBuffer</code> containing a context associated to the
91resource (file descriptor, or pointer to a protocol handler), the read() and
92close() callbacks to use and an xmlBuffer. And extra xmlBuffer and a charset
93encoding handler are also present to support charset conversion when
94needed.</p>
95
96<h2><a name="Output">Output I/O handlers</a></h2>
97
98<p>An Output handler <code>xmlOutputBuffer</code> is completely similar to an
99Input one except the callbacks are write() and close().</p>
100
101<h2><a name="Example">Example of customized I/O</a></h2>
102
103<p>This example come from <a href="http://xmlsoft.org/messages/0708.html">a
104real use case</a>, xmlDocDump() closes the FILE * passed by the application
105and this was a problem. The <a
106href="http://xmlsoft.org/messages/0711.html">solution</a> was to redefine a
107new output handler with the closing call deactivated:</p>
108<ol>
109 <li>First define a new I/O ouput allocator where the output don't close the
110 file:
111 <pre>xmlOutputBufferPtr <br>
112xmlOutputBufferCreateOwn(FILE *file, xmlCharEncodingHandlerPtr encoder) { <br>
113    xmlOutputBufferPtr ret; <br>
114     <br>
115    if (xmlOutputCallbackInitialized == 0) <br>
116        xmlRegisterDefaultOutputCallbacks(); <br>
117     <br>
118    if (file == NULL) return(NULL); <br>
119 </pre>
120 <pre>    ret = xmlAllocOutputBuffer(encoder); <br>
121    if (ret != NULL) { <br>
122        ret-&gt;context = file; <br>
123        ret-&gt;writecallback = xmlFileWrite; <br>
124        ret-&gt;closecallback = NULL; /* No close callback */ <br>
125    } <br>
126 </pre>
127 <pre>    return(ret); <br>
128} </pre>
129 </li>
130 <li>And then use it to save the document:
131 <pre>FILE *f;
132xmlOutputBufferPtr output;</pre>
133 <pre>xmlDocPtr doc;
134int res;
135<br>
136f = ...
137doc = ....
138output = xmlOutputBufferCreateOwn(f, NULL);
139<br>
140res = xmlSaveFileTo(output, doc, NULL);
141
142 </pre>
143 </li>
144</ol>
145
146<p><a href="mailto:Daniel.Veillard@w3.org">Daniel Veillard</a></p>
147
148<p>$Id$</p>
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