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14<title>I/O Interfaces</title>
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Daniel Veillard8f40f1e2002-08-28 21:18:45 +000019<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>
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22<h1>The XML C library for Gnome</h1>
23<h2>I/O Interfaces</h2>
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Daniel Veillard96984452000-08-31 13:50:12 +000096<p>Table of Content:</p>
97<ol>
Daniel Veillardb8cfbd12001-10-25 10:53:28 +000098<li><a href="#General1">General overview</a></li>
Daniel Veillard0b28e882002-07-24 23:47:05 +000099 <li><a href="#basic">The basic buffer type</a></li>
100 <li><a href="#Input">Input I/O handlers</a></li>
101 <li><a href="#Output">Output I/O handlers</a></li>
102 <li><a href="#entities">The entities loader</a></li>
103 <li><a href="#Example2">Example of customized I/O</a></li>
Daniel Veillard96984452000-08-31 13:50:12 +0000104</ol>
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000105<h3><a name="General1">General overview</a></h3>
106<p>The module <code><a href="http://xmlsoft.org/html/libxml-xmlio.html">xmlIO.h</a></code> provides
107the interfaces to the libxml I/O system. This consists of 4 main parts:</p>
Daniel Veillard96984452000-08-31 13:50:12 +0000108<ul>
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000109<li>Entities loader, this is a routine which tries to fetch the entities
Daniel Veillard2bb89092000-08-31 14:57:50 +0000110 (files) based on their PUBLIC and SYSTEM identifiers. The default loader
111 don't look at the public identifier since libxml do not maintain a
112 catalog. You can redefine you own entity loader by using
113 <code>xmlGetExternalEntityLoader()</code> and
Daniel Veillard9c466822001-10-25 12:03:39 +0000114 <code>xmlSetExternalEntityLoader()</code>. <a href="#entities">Check the
115 example</a>.</li>
Daniel Veillard0b28e882002-07-24 23:47:05 +0000116 <li>Input I/O buffers which are a commodity structure used by the parser(s)
Daniel Veillard96984452000-08-31 13:50:12 +0000117 input layer to handle fetching the informations to feed the parser. This
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000118 provides buffering and is also a placeholder where the encoding
Daniel Veillard63d83142002-05-20 06:51:05 +0000119 converters to UTF8 are piggy-backed.</li>
Daniel Veillard0b28e882002-07-24 23:47:05 +0000120 <li>Output I/O buffers are similar to the Input ones and fulfill similar
Daniel Veillard96984452000-08-31 13:50:12 +0000121 task but when generating a serialization from a tree.</li>
Daniel Veillard0b28e882002-07-24 23:47:05 +0000122 <li>A mechanism to register sets of I/O callbacks and associate them with
Daniel Veillard96984452000-08-31 13:50:12 +0000123 specific naming schemes like the protocol part of the URIs.
124 <p>This affect the default I/O operations and allows to use specific I/O
125 handlers for certain names.</p>
Daniel Veillard0b28e882002-07-24 23:47:05 +0000126 </li>
Daniel Veillard96984452000-08-31 13:50:12 +0000127</ul>
Daniel Veillard2bb89092000-08-31 14:57:50 +0000128<p>The general mechanism used when loading http://rpmfind.net/xml.html for
Daniel Veillard96984452000-08-31 13:50:12 +0000129example in the HTML parser is the following:</p>
130<ol>
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000131<li>The default entity loader calls <code>xmlNewInputFromFile()</code> with
Daniel Veillard2bb89092000-08-31 14:57:50 +0000132 the parsing context and the URI string.</li>
Daniel Veillard0b28e882002-07-24 23:47:05 +0000133 <li>the URI string is checked against the existing registered handlers
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000134 using their match() callback function, if the HTTP module was compiled
135 in, it is registered and its match() function will succeeds</li>
Daniel Veillard0b28e882002-07-24 23:47:05 +0000136 <li>the open() function of the handler is called and if successful will
Daniel Veillard96984452000-08-31 13:50:12 +0000137 return an I/O Input buffer</li>
Daniel Veillard0b28e882002-07-24 23:47:05 +0000138 <li>the parser will the start reading from this buffer and progressively
Daniel Veillard96984452000-08-31 13:50:12 +0000139 fetch information from the resource, calling the read() function of the
140 handler until the resource is exhausted</li>
Daniel Veillard0b28e882002-07-24 23:47:05 +0000141 <li>if an encoding change is detected it will be installed on the input
Daniel Veillard96984452000-08-31 13:50:12 +0000142 buffer, providing buffering and efficient use of the conversion
143 routines</li>
Daniel Veillard0b28e882002-07-24 23:47:05 +0000144 <li>once the parser has finished, the close() function of the handler is
Daniel Veillard63d83142002-05-20 06:51:05 +0000145 called once and the Input buffer and associated resources are
Daniel Veillard96984452000-08-31 13:50:12 +0000146 deallocated.</li>
147</ol>
Daniel Veillard96984452000-08-31 13:50:12 +0000148<p>The user defined callbacks are checked first to allow overriding of the
149default libxml I/O routines.</p>
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000150<h3><a name="basic">The basic buffer type</a></h3>
Daniel Veillard96984452000-08-31 13:50:12 +0000151<p>All the buffer manipulation handling is done using the
Daniel Veillard0b28e882002-07-24 23:47:05 +0000152<code>xmlBuffer</code> type define in <code><a href="http://xmlsoft.org/html/libxml-tree.html">tree.h</a> </code>which is a
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000153resizable memory buffer. The buffer allocation strategy can be selected to be
154either best-fit or use an exponential doubling one (CPU vs. memory use
Daniel Veillard63d83142002-05-20 06:51:05 +0000155trade-off). The values are <code>XML_BUFFER_ALLOC_EXACT</code> and
Daniel Veillard96984452000-08-31 13:50:12 +0000156<code>XML_BUFFER_ALLOC_DOUBLEIT</code>, and can be set individually or on a
157system wide basis using <code>xmlBufferSetAllocationScheme()</code>. A number
158of functions allows to manipulate buffers with names starting with the
159<code>xmlBuffer...</code> prefix.</p>
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000160<h3><a name="Input">Input I/O handlers</a></h3>
Daniel Veillard96984452000-08-31 13:50:12 +0000161<p>An Input I/O handler is a simple structure
162<code>xmlParserInputBuffer</code> containing a context associated to the
163resource (file descriptor, or pointer to a protocol handler), the read() and
164close() callbacks to use and an xmlBuffer. And extra xmlBuffer and a charset
165encoding handler are also present to support charset conversion when
166needed.</p>
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000167<h3><a name="Output">Output I/O handlers</a></h3>
Daniel Veillard96984452000-08-31 13:50:12 +0000168<p>An Output handler <code>xmlOutputBuffer</code> is completely similar to an
169Input one except the callbacks are write() and close().</p>
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000170<h3><a name="entities">The entities loader</a></h3>
Daniel Veillard2bb89092000-08-31 14:57:50 +0000171<p>The entity loader resolves requests for new entities and create inputs for
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000172the parser. Creating an input from a filename or an URI string is done
173through the xmlNewInputFromFile() routine. The default entity loader do not
174handle the PUBLIC identifier associated with an entity (if any). So it just
175calls xmlNewInputFromFile() with the SYSTEM identifier (which is mandatory in
Daniel Veillard2bb89092000-08-31 14:57:50 +0000176XML).</p>
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000177<p>If you want to hook up a catalog mechanism then you simply need to
178override the default entity loader, here is an example:</p>
Daniel Veillard2bb89092000-08-31 14:57:50 +0000179<pre>#include &lt;libxml/xmlIO.h&gt;
180
181xmlExternalEntityLoader defaultLoader = NULL;
182
183xmlParserInputPtr
184xmlMyExternalEntityLoader(const char *URL, const char *ID,
185 xmlParserCtxtPtr ctxt) {
186 xmlParserInputPtr ret;
187 const char *fileID = NULL;
188 /* lookup for the fileID depending on ID */
189
190 ret = xmlNewInputFromFile(ctxt, fileID);
191 if (ret != NULL)
192 return(ret);
193 if (defaultLoader != NULL)
194 ret = defaultLoader(URL, ID, ctxt);
195 return(ret);
196}
197
198int main(..) {
199 ...
200
201 /*
202 * Install our own entity loader
203 */
204 defaultLoader = xmlGetExternalEntityLoader();
205 xmlSetExternalEntityLoader(xmlMyExternalEntityLoader);
206
207 ...
208}</pre>
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000209<h3><a name="Example2">Example of customized I/O</a></h3>
Daniel Veillard96984452000-08-31 13:50:12 +0000210<p>This example come from <a href="http://xmlsoft.org/messages/0708.html">a
211real use case</a>, xmlDocDump() closes the FILE * passed by the application
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000212and 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 +0000213new output handler with the closing call deactivated:</p>
214<ol>
Daniel Veillardc0801af2002-05-28 16:28:42 +0000215<li>First define a new I/O output allocator where the output don't close
216 the file:
Daniel Veillarda7ad4522000-08-31 14:19:54 +0000217 <pre>xmlOutputBufferPtr
218xmlOutputBufferCreateOwn(FILE *file, xmlCharEncodingHandlerPtr encoder) {
219    xmlOutputBufferPtr ret;
220    
221    if (xmlOutputCallbackInitialized == 0)
222        xmlRegisterDefaultOutputCallbacks();
223
224    if (file == NULL) return(NULL);
225    ret = xmlAllocOutputBuffer(encoder);
226    if (ret != NULL) {
227        ret-&gt;context = file;
228        ret-&gt;writecallback = xmlFileWrite;
229        ret-&gt;closecallback = NULL; /* No close callback */
230    }
231    return(ret); <br>
Daniel Veillard1eb24242002-03-18 11:33:03 +0000232
Daniel Veillard34ce8be2002-03-18 19:37:11 +0000233
234
Daniel Veillard19274092002-03-25 16:48:03 +0000235
Daniel Veillardb2fb8ed2002-04-01 09:33:12 +0000236
Daniel Veillarda8a89fe2002-04-12 21:03:34 +0000237
Daniel Veillardc3975342002-04-15 18:54:52 +0000238
Daniel Veillard0bfbb422002-04-26 09:21:45 +0000239
Daniel Veillard93d3a472002-04-26 14:04:55 +0000240
Daniel Veillardcf27f7c2002-04-30 07:12:39 +0000241
Daniel Veillarde6d8e202002-05-02 06:11:10 +0000242
Daniel Veillard9b6fd302002-05-13 12:06:47 +0000243
Daniel Veillardc0801af2002-05-28 16:28:42 +0000244
Daniel Veillard21473672002-06-17 07:29:22 +0000245
Daniel Veillard782afda2002-07-08 15:12:49 +0000246
Daniel Veillard42766c02002-08-22 20:52:17 +0000247
Daniel Veillarde1662542002-08-28 11:50:59 +0000248
Daniel Veillarde16b5742002-09-26 17:50:03 +0000249
Daniel Veillard321be0c2002-10-08 21:26:42 +0000250
Daniel Veillarde645e8c2002-10-22 17:35:37 +0000251
Daniel Veillarddad3f682002-11-17 16:47:27 +0000252
Daniel Veillardc14401e2002-11-20 14:28:17 +0000253
Daniel Veillardf9c4cad2002-11-22 15:57:07 +0000254
Daniel Veillard96984452000-08-31 13:50:12 +0000255} </pre>
Daniel Veillard0b28e882002-07-24 23:47:05 +0000256 </li>
257 <li>And then use it to save the document:
Daniel Veillard96984452000-08-31 13:50:12 +0000258 <pre>FILE *f;
Daniel Veillarda7ad4522000-08-31 14:19:54 +0000259xmlOutputBufferPtr output;
260xmlDocPtr doc;
Daniel Veillard96984452000-08-31 13:50:12 +0000261int res;
Daniel Veillarda7ad4522000-08-31 14:19:54 +0000262
Daniel Veillard96984452000-08-31 13:50:12 +0000263f = ...
264doc = ....
Daniel Veillard96984452000-08-31 13:50:12 +0000265
Daniel Veillarda7ad4522000-08-31 14:19:54 +0000266output = xmlOutputBufferCreateOwn(f, NULL);
267res = xmlSaveFileTo(output, doc, NULL);
Daniel Veillard96984452000-08-31 13:50:12 +0000268 </pre>
Daniel Veillard0b28e882002-07-24 23:47:05 +0000269 </li>
Daniel Veillard96984452000-08-31 13:50:12 +0000270</ol>
Daniel Veillard3f4c40f2002-02-13 09:19:28 +0000271<p><a href="bugs.html">Daniel Veillard</a></p>
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000272</td></tr></table></td></tr></table></td></tr></table></td>
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