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13<title>I/O Interfaces</title>
Daniel Veillard96984452000-08-31 13:50:12 +0000[diff] [blame]14</head>
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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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28<tr><td colspan="1" bgcolor="#eecfa1" align="center"><center><b>Main Menu</b></center></td></tr>
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Daniel Veillard3bf65be2002-01-23 12:36:34 +0000[diff] [blame]62<tr><td colspan="1" bgcolor="#eecfa1" align="center"><center><b>API Indexes</b></center></td></tr>
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Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame]74<li><a href="http://mail.gnome.org/archives/xml/">Mail archive</a></li>
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Daniel Veillarddb9dfd92001-11-26 17:25:02 +0000[diff] [blame]79<li><a href="http://garypennington.net/libxml2/">Solaris binaries</a></li>
Daniel Veillardc6271d22001-10-27 07:50:58 +0000[diff] [blame]80<li><a href="http://bugzilla.gnome.org/buglist.cgi?product=libxml">Bug Tracker</a></li>
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Daniel Veillard96984452000-08-31 13:50:12 +0000[diff] [blame]85<p>Table of Content:</p>
86<ol>
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame]87<li><a href="#General1">General overview</a></li>
88<li><a href="#basic">The basic buffer type</a></li>
89<li><a href="#Input">Input I/O handlers</a></li>
90<li><a href="#Output">Output I/O handlers</a></li>
91<li><a href="#entities">The entities loader</a></li>
92<li><a href="#Example2">Example of customized I/O</a></li>
Daniel Veillard96984452000-08-31 13:50:12 +0000[diff] [blame]93</ol>
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame]94<h3><a name="General1">General overview</a></h3>
95<p>The module <code><a href="http://xmlsoft.org/html/libxml-xmlio.html">xmlIO.h</a></code> provides
96the interfaces to the libxml I/O system. This consists of 4 main parts:</p>
Daniel Veillard96984452000-08-31 13:50:12 +0000[diff] [blame]97<ul>
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame]98<li>Entities loader, this is a routine which tries to fetch the entities
Daniel Veillard2bb89092000-08-31 14:57:50 +0000[diff] [blame]99 (files) based on their PUBLIC and SYSTEM identifiers. The default loader
100 don't look at the public identifier since libxml do not maintain a
101 catalog. You can redefine you own entity loader by using
102 <code>xmlGetExternalEntityLoader()</code> and
Daniel Veillard9c466822001-10-25 12:03:39 +0000[diff] [blame]103 <code>xmlSetExternalEntityLoader()</code>. <a href="#entities">Check the
104 example</a>.</li>
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame]105<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]106 input layer to handle fetching the informations to feed the parser. This
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame]107 provides buffering and is also a placeholder where the encoding
108 convertors to UTF8 are piggy-backed.</li>
109<li>Output I/O buffers are similar to the Input ones and fulfill similar
Daniel Veillard96984452000-08-31 13:50:12 +0000[diff] [blame]110 task but when generating a serialization from a tree.</li>
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame]111<li>A mechanism to register sets of I/O callbacks and associate them with
Daniel Veillard96984452000-08-31 13:50:12 +0000[diff] [blame]112 specific naming schemes like the protocol part of the URIs.
113 <p>This affect the default I/O operations and allows to use specific I/O
114 handlers for certain names.</p>
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame]115</li>
Daniel Veillard96984452000-08-31 13:50:12 +0000[diff] [blame]116</ul>
Daniel Veillard2bb89092000-08-31 14:57:50 +0000[diff] [blame]117<p>The general mechanism used when loading http://rpmfind.net/xml.html for
Daniel Veillard96984452000-08-31 13:50:12 +0000[diff] [blame]118example in the HTML parser is the following:</p>
119<ol>
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame]120<li>The default entity loader calls <code>xmlNewInputFromFile()</code> with
Daniel Veillard2bb89092000-08-31 14:57:50 +0000[diff] [blame]121 the parsing context and the URI string.</li>
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame]122<li>the URI string is checked against the existing registered handlers
123 using their match() callback function, if the HTTP module was compiled
124 in, it is registered and its match() function will succeeds</li>
125<li>the open() function of the handler is called and if successful will
Daniel Veillard96984452000-08-31 13:50:12 +0000[diff] [blame]126 return an I/O Input buffer</li>
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame]127<li>the parser will the start reading from this buffer and progressively
Daniel Veillard96984452000-08-31 13:50:12 +0000[diff] [blame]128 fetch information from the resource, calling the read() function of the
129 handler until the resource is exhausted</li>
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame]130<li>if an encoding change is detected it will be installed on the input
Daniel Veillard96984452000-08-31 13:50:12 +0000[diff] [blame]131 buffer, providing buffering and efficient use of the conversion
132 routines</li>
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame]133<li>once the parser has finished, the close() function of the handler is
Daniel Veillard96984452000-08-31 13:50:12 +0000[diff] [blame]134 called once and the Input buffer and associed resources are
135 deallocated.</li>
136</ol>
Daniel Veillard96984452000-08-31 13:50:12 +0000[diff] [blame]137<p>The user defined callbacks are checked first to allow overriding of the
138default libxml I/O routines.</p>
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame]139<h3><a name="basic">The basic buffer type</a></h3>
Daniel Veillard96984452000-08-31 13:50:12 +0000[diff] [blame]140<p>All the buffer manipulation handling is done using the
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame]141<code>xmlBuffer</code> type define in <code><a href="http://xmlsoft.org/html/libxml-tree.html">tree.h</a></code>which is a
142resizable memory buffer. The buffer allocation strategy can be selected to be
143either best-fit or use an exponential doubling one (CPU vs. memory use
Daniel Veillard96984452000-08-31 13:50:12 +0000[diff] [blame]144tradeoff). The values are <code>XML_BUFFER_ALLOC_EXACT</code> and
145<code>XML_BUFFER_ALLOC_DOUBLEIT</code>, and can be set individually or on a
146system wide basis using <code>xmlBufferSetAllocationScheme()</code>. A number
147of functions allows to manipulate buffers with names starting with the
148<code>xmlBuffer...</code> prefix.</p>
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame]149<h3><a name="Input">Input I/O handlers</a></h3>
Daniel Veillard96984452000-08-31 13:50:12 +0000[diff] [blame]150<p>An Input I/O handler is a simple structure
151<code>xmlParserInputBuffer</code> containing a context associated to the
152resource (file descriptor, or pointer to a protocol handler), the read() and
153close() callbacks to use and an xmlBuffer. And extra xmlBuffer and a charset
154encoding handler are also present to support charset conversion when
155needed.</p>
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame]156<h3><a name="Output">Output I/O handlers</a></h3>
Daniel Veillard96984452000-08-31 13:50:12 +0000[diff] [blame]157<p>An Output handler <code>xmlOutputBuffer</code> is completely similar to an
158Input one except the callbacks are write() and close().</p>
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame]159<h3><a name="entities">The entities loader</a></h3>
Daniel Veillard2bb89092000-08-31 14:57:50 +0000[diff] [blame]160<p>The entity loader resolves requests for new entities and create inputs for
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame]161the parser. Creating an input from a filename or an URI string is done
162through the xmlNewInputFromFile() routine. The default entity loader do not
163handle the PUBLIC identifier associated with an entity (if any). So it just
164calls xmlNewInputFromFile() with the SYSTEM identifier (which is mandatory in
Daniel Veillard2bb89092000-08-31 14:57:50 +0000[diff] [blame]165XML).</p>
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame]166<p>If you want to hook up a catalog mechanism then you simply need to
167override the default entity loader, here is an example:</p>
Daniel Veillard2bb89092000-08-31 14:57:50 +0000[diff] [blame]168<pre>#include &lt;libxml/xmlIO.h&gt;
169
170xmlExternalEntityLoader defaultLoader = NULL;
171
172xmlParserInputPtr
173xmlMyExternalEntityLoader(const char *URL, const char *ID,
174 xmlParserCtxtPtr ctxt) {
175 xmlParserInputPtr ret;
176 const char *fileID = NULL;
177 /* lookup for the fileID depending on ID */
178
179 ret = xmlNewInputFromFile(ctxt, fileID);
180 if (ret != NULL)
181 return(ret);
182 if (defaultLoader != NULL)
183 ret = defaultLoader(URL, ID, ctxt);
184 return(ret);
185}
186
187int main(..) {
188 ...
189
190 /*
191 * Install our own entity loader
192 */
193 defaultLoader = xmlGetExternalEntityLoader();
194 xmlSetExternalEntityLoader(xmlMyExternalEntityLoader);
195
196 ...
197}</pre>
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame]198<h3><a name="Example2">Example of customized I/O</a></h3>
Daniel Veillard96984452000-08-31 13:50:12 +0000[diff] [blame]199<p>This example come from <a href="http://xmlsoft.org/messages/0708.html">a
200real use case</a>, xmlDocDump() closes the FILE * passed by the application
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame]201and 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]202new output handler with the closing call deactivated:</p>
203<ol>
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame]204<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]205 file:
Daniel Veillarda7ad4522000-08-31 14:19:54 +0000[diff] [blame]206 <pre>xmlOutputBufferPtr
207xmlOutputBufferCreateOwn(FILE *file, xmlCharEncodingHandlerPtr encoder) {
208    xmlOutputBufferPtr ret;
209    
210    if (xmlOutputCallbackInitialized == 0)
211        xmlRegisterDefaultOutputCallbacks();
212
213    if (file == NULL) return(NULL);
214    ret = xmlAllocOutputBuffer(encoder);
215    if (ret != NULL) {
216        ret-&gt;context = file;
217        ret-&gt;writecallback = xmlFileWrite;
218        ret-&gt;closecallback = NULL; /* No close callback */
219    }
220    return(ret); <br>
Daniel Veillard2bb89092000-08-31 14:57:50 +0000[diff] [blame]221
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Daniel Veillard5b16f582002-02-20 11:38:46 +0000[diff] [blame^]243
Daniel Veillard96984452000-08-31 13:50:12 +0000[diff] [blame]244} </pre>
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame]245</li>
246<li>And then use it to save the document:
Daniel Veillard96984452000-08-31 13:50:12 +0000[diff] [blame]247 <pre>FILE *f;
Daniel Veillarda7ad4522000-08-31 14:19:54 +0000[diff] [blame]248xmlOutputBufferPtr output;
249xmlDocPtr doc;
Daniel Veillard96984452000-08-31 13:50:12 +0000[diff] [blame]250int res;
Daniel Veillarda7ad4522000-08-31 14:19:54 +0000[diff] [blame]251
Daniel Veillard96984452000-08-31 13:50:12 +0000[diff] [blame]252f = ...
253doc = ....
Daniel Veillard96984452000-08-31 13:50:12 +0000[diff] [blame]254
Daniel Veillarda7ad4522000-08-31 14:19:54 +0000[diff] [blame]255output = xmlOutputBufferCreateOwn(f, NULL);
256res = xmlSaveFileTo(output, doc, NULL);
Daniel Veillard96984452000-08-31 13:50:12 +0000[diff] [blame]257 </pre>
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame]258</li>
Daniel Veillard96984452000-08-31 13:50:12 +0000[diff] [blame]259</ol>
Daniel Veillard3f4c40f2002-02-13 09:19:28 +0000[diff] [blame]260<p><a href="bugs.html">Daniel Veillard</a></p>
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame]261</td></tr></table></td></tr></table></td></tr></table></td>
262</tr></table></td></tr></table>
Daniel Veillard96984452000-08-31 13:50:12 +0000[diff] [blame]263</body>
264</html>