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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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87<tr><td colspan="1" bgcolor="#eecfa1" align="center"><center><b>API Indexes</b></center></td></tr>
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Daniel Veillard96984452000-08-31 13:50:12 +000098<p>Table of Content:</p>
99<ol>
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000100<li><a href="#General1">General overview</a></li>
Daniel Veillard0b28e882002-07-24 23:47:05 +0000101 <li><a href="#basic">The basic buffer type</a></li>
102 <li><a href="#Input">Input I/O handlers</a></li>
103 <li><a href="#Output">Output I/O handlers</a></li>
104 <li><a href="#entities">The entities loader</a></li>
105 <li><a href="#Example2">Example of customized I/O</a></li>
Daniel Veillard96984452000-08-31 13:50:12 +0000106</ol>
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000107<h3><a name="General1">General overview</a></h3>
108<p>The module <code><a href="http://xmlsoft.org/html/libxml-xmlio.html">xmlIO.h</a></code> provides
109the interfaces to the libxml I/O system. This consists of 4 main parts:</p>
Daniel Veillard96984452000-08-31 13:50:12 +0000110<ul>
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000111<li>Entities loader, this is a routine which tries to fetch the entities
Daniel Veillard2bb89092000-08-31 14:57:50 +0000112 (files) based on their PUBLIC and SYSTEM identifiers. The default loader
113 don't look at the public identifier since libxml do not maintain a
114 catalog. You can redefine you own entity loader by using
115 <code>xmlGetExternalEntityLoader()</code> and
Daniel Veillard9c466822001-10-25 12:03:39 +0000116 <code>xmlSetExternalEntityLoader()</code>. <a href="#entities">Check the
117 example</a>.</li>
Daniel Veillard0b28e882002-07-24 23:47:05 +0000118 <li>Input I/O buffers which are a commodity structure used by the parser(s)
Daniel Veillard96984452000-08-31 13:50:12 +0000119 input layer to handle fetching the informations to feed the parser. This
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000120 provides buffering and is also a placeholder where the encoding
Daniel Veillard63d83142002-05-20 06:51:05 +0000121 converters to UTF8 are piggy-backed.</li>
Daniel Veillard0b28e882002-07-24 23:47:05 +0000122 <li>Output I/O buffers are similar to the Input ones and fulfill similar
Daniel Veillard96984452000-08-31 13:50:12 +0000123 task but when generating a serialization from a tree.</li>
Daniel Veillard0b28e882002-07-24 23:47:05 +0000124 <li>A mechanism to register sets of I/O callbacks and associate them with
Daniel Veillard96984452000-08-31 13:50:12 +0000125 specific naming schemes like the protocol part of the URIs.
126 <p>This affect the default I/O operations and allows to use specific I/O
127 handlers for certain names.</p>
Daniel Veillard0b28e882002-07-24 23:47:05 +0000128 </li>
Daniel Veillard96984452000-08-31 13:50:12 +0000129</ul>
Daniel Veillard2bb89092000-08-31 14:57:50 +0000130<p>The general mechanism used when loading http://rpmfind.net/xml.html for
Daniel Veillard96984452000-08-31 13:50:12 +0000131example in the HTML parser is the following:</p>
132<ol>
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000133<li>The default entity loader calls <code>xmlNewInputFromFile()</code> with
Daniel Veillard2bb89092000-08-31 14:57:50 +0000134 the parsing context and the URI string.</li>
Daniel Veillard0b28e882002-07-24 23:47:05 +0000135 <li>the URI string is checked against the existing registered handlers
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000136 using their match() callback function, if the HTTP module was compiled
137 in, it is registered and its match() function will succeeds</li>
Daniel Veillard0b28e882002-07-24 23:47:05 +0000138 <li>the open() function of the handler is called and if successful will
Daniel Veillard96984452000-08-31 13:50:12 +0000139 return an I/O Input buffer</li>
Daniel Veillard0b28e882002-07-24 23:47:05 +0000140 <li>the parser will the start reading from this buffer and progressively
Daniel Veillard96984452000-08-31 13:50:12 +0000141 fetch information from the resource, calling the read() function of the
142 handler until the resource is exhausted</li>
Daniel Veillard0b28e882002-07-24 23:47:05 +0000143 <li>if an encoding change is detected it will be installed on the input
Daniel Veillard96984452000-08-31 13:50:12 +0000144 buffer, providing buffering and efficient use of the conversion
145 routines</li>
Daniel Veillard0b28e882002-07-24 23:47:05 +0000146 <li>once the parser has finished, the close() function of the handler is
Daniel Veillard63d83142002-05-20 06:51:05 +0000147 called once and the Input buffer and associated resources are
Daniel Veillard96984452000-08-31 13:50:12 +0000148 deallocated.</li>
149</ol>
Daniel Veillard96984452000-08-31 13:50:12 +0000150<p>The user defined callbacks are checked first to allow overriding of the
151default libxml I/O routines.</p>
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000152<h3><a name="basic">The basic buffer type</a></h3>
Daniel Veillard96984452000-08-31 13:50:12 +0000153<p>All the buffer manipulation handling is done using the
Daniel Veillard0b28e882002-07-24 23:47:05 +0000154<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 +0000155resizable memory buffer. The buffer allocation strategy can be selected to be
156either best-fit or use an exponential doubling one (CPU vs. memory use
Daniel Veillard63d83142002-05-20 06:51:05 +0000157trade-off). The values are <code>XML_BUFFER_ALLOC_EXACT</code> and
Daniel Veillard96984452000-08-31 13:50:12 +0000158<code>XML_BUFFER_ALLOC_DOUBLEIT</code>, and can be set individually or on a
159system wide basis using <code>xmlBufferSetAllocationScheme()</code>. A number
160of functions allows to manipulate buffers with names starting with the
161<code>xmlBuffer...</code> prefix.</p>
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000162<h3><a name="Input">Input I/O handlers</a></h3>
Daniel Veillard96984452000-08-31 13:50:12 +0000163<p>An Input I/O handler is a simple structure
164<code>xmlParserInputBuffer</code> containing a context associated to the
165resource (file descriptor, or pointer to a protocol handler), the read() and
166close() callbacks to use and an xmlBuffer. And extra xmlBuffer and a charset
167encoding handler are also present to support charset conversion when
168needed.</p>
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000169<h3><a name="Output">Output I/O handlers</a></h3>
Daniel Veillard96984452000-08-31 13:50:12 +0000170<p>An Output handler <code>xmlOutputBuffer</code> is completely similar to an
171Input one except the callbacks are write() and close().</p>
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000172<h3><a name="entities">The entities loader</a></h3>
Daniel Veillard2bb89092000-08-31 14:57:50 +0000173<p>The entity loader resolves requests for new entities and create inputs for
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000174the parser. Creating an input from a filename or an URI string is done
175through the xmlNewInputFromFile() routine. The default entity loader do not
176handle the PUBLIC identifier associated with an entity (if any). So it just
177calls xmlNewInputFromFile() with the SYSTEM identifier (which is mandatory in
Daniel Veillard2bb89092000-08-31 14:57:50 +0000178XML).</p>
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000179<p>If you want to hook up a catalog mechanism then you simply need to
180override the default entity loader, here is an example:</p>
Daniel Veillard2bb89092000-08-31 14:57:50 +0000181<pre>#include &lt;libxml/xmlIO.h&gt;
182
183xmlExternalEntityLoader defaultLoader = NULL;
184
185xmlParserInputPtr
186xmlMyExternalEntityLoader(const char *URL, const char *ID,
187 xmlParserCtxtPtr ctxt) {
188 xmlParserInputPtr ret;
189 const char *fileID = NULL;
190 /* lookup for the fileID depending on ID */
191
192 ret = xmlNewInputFromFile(ctxt, fileID);
193 if (ret != NULL)
194 return(ret);
195 if (defaultLoader != NULL)
196 ret = defaultLoader(URL, ID, ctxt);
197 return(ret);
198}
199
200int main(..) {
201 ...
202
203 /*
204 * Install our own entity loader
205 */
206 defaultLoader = xmlGetExternalEntityLoader();
207 xmlSetExternalEntityLoader(xmlMyExternalEntityLoader);
208
209 ...
210}</pre>
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000211<h3><a name="Example2">Example of customized I/O</a></h3>
Daniel Veillard96984452000-08-31 13:50:12 +0000212<p>This example come from <a href="http://xmlsoft.org/messages/0708.html">a
213real use case</a>, xmlDocDump() closes the FILE * passed by the application
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000214and 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 +0000215new output handler with the closing call deactivated:</p>
216<ol>
Daniel Veillardc0801af2002-05-28 16:28:42 +0000217<li>First define a new I/O output allocator where the output don't close
218 the file:
Daniel Veillarda7ad4522000-08-31 14:19:54 +0000219 <pre>xmlOutputBufferPtr
220xmlOutputBufferCreateOwn(FILE *file, xmlCharEncodingHandlerPtr encoder) {
221    xmlOutputBufferPtr ret;
222    
223    if (xmlOutputCallbackInitialized == 0)
224        xmlRegisterDefaultOutputCallbacks();
225
226    if (file == NULL) return(NULL);
227    ret = xmlAllocOutputBuffer(encoder);
228    if (ret != NULL) {
229        ret-&gt;context = file;
230        ret-&gt;writecallback = xmlFileWrite;
231        ret-&gt;closecallback = NULL; /* No close callback */
232    }
233    return(ret); <br>
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236
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Daniel Veillard9b6fd302002-05-13 12:06:47 +0000245
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Daniel Veillard321be0c2002-10-08 21:26:42 +0000252
Daniel Veillarde645e8c2002-10-22 17:35:37 +0000253
Daniel Veillarddad3f682002-11-17 16:47:27 +0000254
Daniel Veillardc14401e2002-11-20 14:28:17 +0000255
Daniel Veillardf9c4cad2002-11-22 15:57:07 +0000256
Daniel Veillard9b4bb4d2002-12-11 19:28:47 +0000257
Daniel Veillardc1eed322002-12-12 11:01:32 +0000258
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Daniel Veillard7b4b2f92003-01-06 13:11:20 +0000260
261
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264
Daniel Veillard831e8fd2003-01-25 11:45:34 +0000265
Daniel Veillard96984452000-08-31 13:50:12 +0000266} </pre>
Daniel Veillard0b28e882002-07-24 23:47:05 +0000267 </li>
268 <li>And then use it to save the document:
Daniel Veillard96984452000-08-31 13:50:12 +0000269 <pre>FILE *f;
Daniel Veillarda7ad4522000-08-31 14:19:54 +0000270xmlOutputBufferPtr output;
271xmlDocPtr doc;
Daniel Veillard96984452000-08-31 13:50:12 +0000272int res;
Daniel Veillarda7ad4522000-08-31 14:19:54 +0000273
Daniel Veillard96984452000-08-31 13:50:12 +0000274f = ...
275doc = ....
Daniel Veillard96984452000-08-31 13:50:12 +0000276
Daniel Veillarda7ad4522000-08-31 14:19:54 +0000277output = xmlOutputBufferCreateOwn(f, NULL);
278res = xmlSaveFileTo(output, doc, NULL);
Daniel Veillard96984452000-08-31 13:50:12 +0000279 </pre>
Daniel Veillard0b28e882002-07-24 23:47:05 +0000280 </li>
Daniel Veillard96984452000-08-31 13:50:12 +0000281</ol>
Daniel Veillard3f4c40f2002-02-13 09:19:28 +0000282<p><a href="bugs.html">Daniel Veillard</a></p>
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000283</td></tr></table></td></tr></table></td></tr></table></td>
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