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13<title>I/O Interfaces</title>
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21<h1>The XML C library for Gnome</h1>
22<h2>I/O Interfaces</h2>
23</td></tr></table></td></tr></table></td>
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Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame]30<li><a href="index.html">Home</a></li>
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Daniel Veillard8acca112002-01-21 09:52:27 +0000[diff] [blame^]63<tr><td bgcolor="#fffacd"><ul>
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame]64<li><a href="http://mail.gnome.org/archives/xml/">Mail archive</a></li>
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Daniel Veillard96984452000-08-31 13:50:12 +0000[diff] [blame]75<p>Table of Content:</p>
76<ol>
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame]77<li><a href="#General1">General overview</a></li>
78<li><a href="#basic">The basic buffer type</a></li>
79<li><a href="#Input">Input I/O handlers</a></li>
80<li><a href="#Output">Output I/O handlers</a></li>
81<li><a href="#entities">The entities loader</a></li>
82<li><a href="#Example2">Example of customized I/O</a></li>
Daniel Veillard96984452000-08-31 13:50:12 +0000[diff] [blame]83</ol>
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame]84<h3><a name="General1">General overview</a></h3>
85<p>The module <code><a href="http://xmlsoft.org/html/libxml-xmlio.html">xmlIO.h</a></code> provides
86the interfaces to the libxml I/O system. This consists of 4 main parts:</p>
Daniel Veillard96984452000-08-31 13:50:12 +0000[diff] [blame]87<ul>
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame]88<li>Entities loader, this is a routine which tries to fetch the entities
Daniel Veillard2bb89092000-08-31 14:57:50 +0000[diff] [blame]89 (files) based on their PUBLIC and SYSTEM identifiers. The default loader
90 don't look at the public identifier since libxml do not maintain a
91 catalog. You can redefine you own entity loader by using
92 <code>xmlGetExternalEntityLoader()</code> and
Daniel Veillard9c466822001-10-25 12:03:39 +0000[diff] [blame]93 <code>xmlSetExternalEntityLoader()</code>. <a href="#entities">Check the
94 example</a>.</li>
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame]95<li>Input I/O buffers which are a commodity structure used by the parser(s)
Daniel Veillard96984452000-08-31 13:50:12 +0000[diff] [blame]96 input layer to handle fetching the informations to feed the parser. This
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame]97 provides buffering and is also a placeholder where the encoding
98 convertors to UTF8 are piggy-backed.</li>
99<li>Output I/O buffers are similar to the Input ones and fulfill similar
Daniel Veillard96984452000-08-31 13:50:12 +0000[diff] [blame]100 task but when generating a serialization from a tree.</li>
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame]101<li>A mechanism to register sets of I/O callbacks and associate them with
Daniel Veillard96984452000-08-31 13:50:12 +0000[diff] [blame]102 specific naming schemes like the protocol part of the URIs.
103 <p>This affect the default I/O operations and allows to use specific I/O
104 handlers for certain names.</p>
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame]105</li>
Daniel Veillard96984452000-08-31 13:50:12 +0000[diff] [blame]106</ul>
Daniel Veillard2bb89092000-08-31 14:57:50 +0000[diff] [blame]107<p>The general mechanism used when loading http://rpmfind.net/xml.html for
Daniel Veillard96984452000-08-31 13:50:12 +0000[diff] [blame]108example in the HTML parser is the following:</p>
109<ol>
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame]110<li>The default entity loader calls <code>xmlNewInputFromFile()</code> with
Daniel Veillard2bb89092000-08-31 14:57:50 +0000[diff] [blame]111 the parsing context and the URI string.</li>
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame]112<li>the URI string is checked against the existing registered handlers
113 using their match() callback function, if the HTTP module was compiled
114 in, it is registered and its match() function will succeeds</li>
115<li>the open() function of the handler is called and if successful will
Daniel Veillard96984452000-08-31 13:50:12 +0000[diff] [blame]116 return an I/O Input buffer</li>
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame]117<li>the parser will the start reading from this buffer and progressively
Daniel Veillard96984452000-08-31 13:50:12 +0000[diff] [blame]118 fetch information from the resource, calling the read() function of the
119 handler until the resource is exhausted</li>
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame]120<li>if an encoding change is detected it will be installed on the input
Daniel Veillard96984452000-08-31 13:50:12 +0000[diff] [blame]121 buffer, providing buffering and efficient use of the conversion
122 routines</li>
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame]123<li>once the parser has finished, the close() function of the handler is
Daniel Veillard96984452000-08-31 13:50:12 +0000[diff] [blame]124 called once and the Input buffer and associed resources are
125 deallocated.</li>
126</ol>
Daniel Veillard96984452000-08-31 13:50:12 +0000[diff] [blame]127<p>The user defined callbacks are checked first to allow overriding of the
128default libxml I/O routines.</p>
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame]129<h3><a name="basic">The basic buffer type</a></h3>
Daniel Veillard96984452000-08-31 13:50:12 +0000[diff] [blame]130<p>All the buffer manipulation handling is done using the
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame]131<code>xmlBuffer</code> type define in <code><a href="http://xmlsoft.org/html/libxml-tree.html">tree.h</a></code>which is a
132resizable memory buffer. The buffer allocation strategy can be selected to be
133either best-fit or use an exponential doubling one (CPU vs. memory use
Daniel Veillard96984452000-08-31 13:50:12 +0000[diff] [blame]134tradeoff). The values are <code>XML_BUFFER_ALLOC_EXACT</code> and
135<code>XML_BUFFER_ALLOC_DOUBLEIT</code>, and can be set individually or on a
136system wide basis using <code>xmlBufferSetAllocationScheme()</code>. A number
137of functions allows to manipulate buffers with names starting with the
138<code>xmlBuffer...</code> prefix.</p>
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame]139<h3><a name="Input">Input I/O handlers</a></h3>
Daniel Veillard96984452000-08-31 13:50:12 +0000[diff] [blame]140<p>An Input I/O handler is a simple structure
141<code>xmlParserInputBuffer</code> containing a context associated to the
142resource (file descriptor, or pointer to a protocol handler), the read() and
143close() callbacks to use and an xmlBuffer. And extra xmlBuffer and a charset
144encoding handler are also present to support charset conversion when
145needed.</p>
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame]146<h3><a name="Output">Output I/O handlers</a></h3>
Daniel Veillard96984452000-08-31 13:50:12 +0000[diff] [blame]147<p>An Output handler <code>xmlOutputBuffer</code> is completely similar to an
148Input one except the callbacks are write() and close().</p>
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame]149<h3><a name="entities">The entities loader</a></h3>
Daniel Veillard2bb89092000-08-31 14:57:50 +0000[diff] [blame]150<p>The entity loader resolves requests for new entities and create inputs for
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame]151the parser. Creating an input from a filename or an URI string is done
152through the xmlNewInputFromFile() routine. The default entity loader do not
153handle the PUBLIC identifier associated with an entity (if any). So it just
154calls xmlNewInputFromFile() with the SYSTEM identifier (which is mandatory in
Daniel Veillard2bb89092000-08-31 14:57:50 +0000[diff] [blame]155XML).</p>
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame]156<p>If you want to hook up a catalog mechanism then you simply need to
157override the default entity loader, here is an example:</p>
Daniel Veillard2bb89092000-08-31 14:57:50 +0000[diff] [blame]158<pre>#include &lt;libxml/xmlIO.h&gt;
159
160xmlExternalEntityLoader defaultLoader = NULL;
161
162xmlParserInputPtr
163xmlMyExternalEntityLoader(const char *URL, const char *ID,
164 xmlParserCtxtPtr ctxt) {
165 xmlParserInputPtr ret;
166 const char *fileID = NULL;
167 /* lookup for the fileID depending on ID */
168
169 ret = xmlNewInputFromFile(ctxt, fileID);
170 if (ret != NULL)
171 return(ret);
172 if (defaultLoader != NULL)
173 ret = defaultLoader(URL, ID, ctxt);
174 return(ret);
175}
176
177int main(..) {
178 ...
179
180 /*
181 * Install our own entity loader
182 */
183 defaultLoader = xmlGetExternalEntityLoader();
184 xmlSetExternalEntityLoader(xmlMyExternalEntityLoader);
185
186 ...
187}</pre>
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame]188<h3><a name="Example2">Example of customized I/O</a></h3>
Daniel Veillard96984452000-08-31 13:50:12 +0000[diff] [blame]189<p>This example come from <a href="http://xmlsoft.org/messages/0708.html">a
190real use case</a>, xmlDocDump() closes the FILE * passed by the application
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame]191and this was a problem. The <a href="http://xmlsoft.org/messages/0711.html">solution</a> was to redefine a
Daniel Veillard96984452000-08-31 13:50:12 +0000[diff] [blame]192new output handler with the closing call deactivated:</p>
193<ol>
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame]194<li>First define a new I/O ouput allocator where the output don't close the
Daniel Veillard96984452000-08-31 13:50:12 +0000[diff] [blame]195 file:
Daniel Veillarda7ad4522000-08-31 14:19:54 +0000[diff] [blame]196 <pre>xmlOutputBufferPtr
197xmlOutputBufferCreateOwn(FILE *file, xmlCharEncodingHandlerPtr encoder) {
198    xmlOutputBufferPtr ret;
199    
200    if (xmlOutputCallbackInitialized == 0)
201        xmlRegisterDefaultOutputCallbacks();
202
203    if (file == NULL) return(NULL);
204    ret = xmlAllocOutputBuffer(encoder);
205    if (ret != NULL) {
206        ret-&gt;context = file;
207        ret-&gt;writecallback = xmlFileWrite;
208        ret-&gt;closecallback = NULL; /* No close callback */
209    }
210    return(ret); <br>
Daniel Veillard2bb89092000-08-31 14:57:50 +0000[diff] [blame]211
212
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame]213
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215
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Daniel Veillard52dcab32001-10-30 12:51:17 +0000[diff] [blame]218
Daniel Veillarded421aa2001-11-04 21:22:45 +0000[diff] [blame]219
Daniel Veillard43d3f612001-11-10 11:57:23 +0000[diff] [blame]220
Daniel Veillarda4871052001-11-26 13:19:48 +0000[diff] [blame]221
Daniel Veillard1aadc442001-11-28 13:10:32 +0000[diff] [blame]222
Daniel Veillardef90ba72001-12-07 14:24:22 +0000[diff] [blame]223
Daniel Veillard9ae4b7a2001-12-13 14:24:09 +0000[diff] [blame]224
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Daniel Veillard35e937a2002-01-19 22:21:54 +0000[diff] [blame]227
Daniel Veillard96984452000-08-31 13:50:12 +0000[diff] [blame]228} </pre>
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame]229</li>
230<li>And then use it to save the document:
Daniel Veillard96984452000-08-31 13:50:12 +0000[diff] [blame]231 <pre>FILE *f;
Daniel Veillarda7ad4522000-08-31 14:19:54 +0000[diff] [blame]232xmlOutputBufferPtr output;
233xmlDocPtr doc;
Daniel Veillard96984452000-08-31 13:50:12 +0000[diff] [blame]234int res;
Daniel Veillarda7ad4522000-08-31 14:19:54 +0000[diff] [blame]235
Daniel Veillard96984452000-08-31 13:50:12 +0000[diff] [blame]236f = ...
237doc = ....
Daniel Veillard96984452000-08-31 13:50:12 +0000[diff] [blame]238
Daniel Veillarda7ad4522000-08-31 14:19:54 +0000[diff] [blame]239output = xmlOutputBufferCreateOwn(f, NULL);
240res = xmlSaveFileTo(output, doc, NULL);
Daniel Veillard96984452000-08-31 13:50:12 +0000[diff] [blame]241 </pre>
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame]242</li>
Daniel Veillard96984452000-08-31 13:50:12 +0000[diff] [blame]243</ol>
Daniel Veillardc5d64342001-06-24 12:13:24 +0000[diff] [blame]244<p><a href="mailto:daniel@veillard.com">Daniel Veillard</a></p>
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame]245</td></tr></table></td></tr></table></td></tr></table></td>
246</tr></table></td></tr></table>
Daniel Veillard96984452000-08-31 13:50:12 +0000[diff] [blame]247</body>
248</html>