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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>
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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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49<li><a href="library.html">The parser interfaces</a></li>
50<li><a href="entities.html">Entities or no entities</a></li>
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Daniel Veillard3bf65be2002-01-23 12:36:34 +0000[diff] [blame]63<tr><td colspan="1" bgcolor="#eecfa1" align="center"><center><b>API Indexes</b></center></td></tr>
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Daniel Veillardf8592562002-01-23 17:58:17 +0000[diff] [blame]65<li><a href="APIchunk0.html">Alphabetic</a></li>
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Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame]73<tr><td colspan="1" bgcolor="#eecfa1" align="center"><center><b>Related links</b></center></td></tr>
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Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame]75<li><a href="http://mail.gnome.org/archives/xml/">Mail archive</a></li>
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Daniel Veillard2d347fa2002-03-17 10:34:11 +0000[diff] [blame]78<li><a href="http://www.aleksey.com/xmlsec/">XML-DSig xmlsec</a></li>
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame]79<li><a href="ftp://xmlsoft.org/">FTP</a></li>
80<li><a href="http://www.fh-frankfurt.de/~igor/projects/libxml/">Windows binaries</a></li>
Daniel Veillarddb9dfd92001-11-26 17:25:02 +0000[diff] [blame]81<li><a href="http://garypennington.net/libxml2/">Solaris binaries</a></li>
Daniel Veillard2d347fa2002-03-17 10:34:11 +0000[diff] [blame]82<li><a href="http://bugzilla.gnome.org/buglist.cgi?product=libxml&amp;product=libxml2">Bug Tracker</a></li>
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Daniel Veillard96984452000-08-31 13:50:12 +0000[diff] [blame]87<p>Table of Content:</p>
88<ol>
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame]89<li><a href="#General1">General overview</a></li>
90<li><a href="#basic">The basic buffer type</a></li>
91<li><a href="#Input">Input I/O handlers</a></li>
92<li><a href="#Output">Output I/O handlers</a></li>
93<li><a href="#entities">The entities loader</a></li>
94<li><a href="#Example2">Example of customized I/O</a></li>
Daniel Veillard96984452000-08-31 13:50:12 +0000[diff] [blame]95</ol>
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame]96<h3><a name="General1">General overview</a></h3>
97<p>The module <code><a href="http://xmlsoft.org/html/libxml-xmlio.html">xmlIO.h</a></code> provides
98the interfaces to the libxml I/O system. This consists of 4 main parts:</p>
Daniel Veillard96984452000-08-31 13:50:12 +0000[diff] [blame]99<ul>
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame]100<li>Entities loader, this is a routine which tries to fetch the entities
Daniel Veillard2bb89092000-08-31 14:57:50 +0000[diff] [blame]101 (files) based on their PUBLIC and SYSTEM identifiers. The default loader
102 don't look at the public identifier since libxml do not maintain a
103 catalog. You can redefine you own entity loader by using
104 <code>xmlGetExternalEntityLoader()</code> and
Daniel Veillard9c466822001-10-25 12:03:39 +0000[diff] [blame]105 <code>xmlSetExternalEntityLoader()</code>. <a href="#entities">Check the
106 example</a>.</li>
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame]107<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]108 input layer to handle fetching the informations to feed the parser. This
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame]109 provides buffering and is also a placeholder where the encoding
110 convertors to UTF8 are piggy-backed.</li>
111<li>Output I/O buffers are similar to the Input ones and fulfill similar
Daniel Veillard96984452000-08-31 13:50:12 +0000[diff] [blame]112 task but when generating a serialization from a tree.</li>
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame]113<li>A mechanism to register sets of I/O callbacks and associate them with
Daniel Veillard96984452000-08-31 13:50:12 +0000[diff] [blame]114 specific naming schemes like the protocol part of the URIs.
115 <p>This affect the default I/O operations and allows to use specific I/O
116 handlers for certain names.</p>
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame]117</li>
Daniel Veillard96984452000-08-31 13:50:12 +0000[diff] [blame]118</ul>
Daniel Veillard2bb89092000-08-31 14:57:50 +0000[diff] [blame]119<p>The general mechanism used when loading http://rpmfind.net/xml.html for
Daniel Veillard96984452000-08-31 13:50:12 +0000[diff] [blame]120example in the HTML parser is the following:</p>
121<ol>
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame]122<li>The default entity loader calls <code>xmlNewInputFromFile()</code> with
Daniel Veillard2bb89092000-08-31 14:57:50 +0000[diff] [blame]123 the parsing context and the URI string.</li>
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame]124<li>the URI string is checked against the existing registered handlers
125 using their match() callback function, if the HTTP module was compiled
126 in, it is registered and its match() function will succeeds</li>
127<li>the open() function of the handler is called and if successful will
Daniel Veillard96984452000-08-31 13:50:12 +0000[diff] [blame]128 return an I/O Input buffer</li>
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame]129<li>the parser will the start reading from this buffer and progressively
Daniel Veillard96984452000-08-31 13:50:12 +0000[diff] [blame]130 fetch information from the resource, calling the read() function of the
131 handler until the resource is exhausted</li>
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame]132<li>if an encoding change is detected it will be installed on the input
Daniel Veillard96984452000-08-31 13:50:12 +0000[diff] [blame]133 buffer, providing buffering and efficient use of the conversion
134 routines</li>
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame]135<li>once the parser has finished, the close() function of the handler is
Daniel Veillard96984452000-08-31 13:50:12 +0000[diff] [blame]136 called once and the Input buffer and associed resources are
137 deallocated.</li>
138</ol>
Daniel Veillard96984452000-08-31 13:50:12 +0000[diff] [blame]139<p>The user defined callbacks are checked first to allow overriding of the
140default libxml I/O routines.</p>
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame]141<h3><a name="basic">The basic buffer type</a></h3>
Daniel Veillard96984452000-08-31 13:50:12 +0000[diff] [blame]142<p>All the buffer manipulation handling is done using the
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame]143<code>xmlBuffer</code> type define in <code><a href="http://xmlsoft.org/html/libxml-tree.html">tree.h</a></code>which is a
144resizable memory buffer. The buffer allocation strategy can be selected to be
145either best-fit or use an exponential doubling one (CPU vs. memory use
Daniel Veillard96984452000-08-31 13:50:12 +0000[diff] [blame]146tradeoff). The values are <code>XML_BUFFER_ALLOC_EXACT</code> and
147<code>XML_BUFFER_ALLOC_DOUBLEIT</code>, and can be set individually or on a
148system wide basis using <code>xmlBufferSetAllocationScheme()</code>. A number
149of functions allows to manipulate buffers with names starting with the
150<code>xmlBuffer...</code> prefix.</p>
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame]151<h3><a name="Input">Input I/O handlers</a></h3>
Daniel Veillard96984452000-08-31 13:50:12 +0000[diff] [blame]152<p>An Input I/O handler is a simple structure
153<code>xmlParserInputBuffer</code> containing a context associated to the
154resource (file descriptor, or pointer to a protocol handler), the read() and
155close() callbacks to use and an xmlBuffer. And extra xmlBuffer and a charset
156encoding handler are also present to support charset conversion when
157needed.</p>
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame]158<h3><a name="Output">Output I/O handlers</a></h3>
Daniel Veillard96984452000-08-31 13:50:12 +0000[diff] [blame]159<p>An Output handler <code>xmlOutputBuffer</code> is completely similar to an
160Input one except the callbacks are write() and close().</p>
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame]161<h3><a name="entities">The entities loader</a></h3>
Daniel Veillard2bb89092000-08-31 14:57:50 +0000[diff] [blame]162<p>The entity loader resolves requests for new entities and create inputs for
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame]163the parser. Creating an input from a filename or an URI string is done
164through the xmlNewInputFromFile() routine. The default entity loader do not
165handle the PUBLIC identifier associated with an entity (if any). So it just
166calls xmlNewInputFromFile() with the SYSTEM identifier (which is mandatory in
Daniel Veillard2bb89092000-08-31 14:57:50 +0000[diff] [blame]167XML).</p>
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame]168<p>If you want to hook up a catalog mechanism then you simply need to
169override the default entity loader, here is an example:</p>
Daniel Veillard2bb89092000-08-31 14:57:50 +0000[diff] [blame]170<pre>#include &lt;libxml/xmlIO.h&gt;
171
172xmlExternalEntityLoader defaultLoader = NULL;
173
174xmlParserInputPtr
175xmlMyExternalEntityLoader(const char *URL, const char *ID,
176 xmlParserCtxtPtr ctxt) {
177 xmlParserInputPtr ret;
178 const char *fileID = NULL;
179 /* lookup for the fileID depending on ID */
180
181 ret = xmlNewInputFromFile(ctxt, fileID);
182 if (ret != NULL)
183 return(ret);
184 if (defaultLoader != NULL)
185 ret = defaultLoader(URL, ID, ctxt);
186 return(ret);
187}
188
189int main(..) {
190 ...
191
192 /*
193 * Install our own entity loader
194 */
195 defaultLoader = xmlGetExternalEntityLoader();
196 xmlSetExternalEntityLoader(xmlMyExternalEntityLoader);
197
198 ...
199}</pre>
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame]200<h3><a name="Example2">Example of customized I/O</a></h3>
Daniel Veillard96984452000-08-31 13:50:12 +0000[diff] [blame]201<p>This example come from <a href="http://xmlsoft.org/messages/0708.html">a
202real use case</a>, xmlDocDump() closes the FILE * passed by the application
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame]203and 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]204new output handler with the closing call deactivated:</p>
205<ol>
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame]206<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]207 file:
Daniel Veillarda7ad4522000-08-31 14:19:54 +0000[diff] [blame]208 <pre>xmlOutputBufferPtr
209xmlOutputBufferCreateOwn(FILE *file, xmlCharEncodingHandlerPtr encoder) {
210    xmlOutputBufferPtr ret;
211    
212    if (xmlOutputCallbackInitialized == 0)
213        xmlRegisterDefaultOutputCallbacks();
214
215    if (file == NULL) return(NULL);
216    ret = xmlAllocOutputBuffer(encoder);
217    if (ret != NULL) {
218        ret-&gt;context = file;
219        ret-&gt;writecallback = xmlFileWrite;
220        ret-&gt;closecallback = NULL; /* No close callback */
221    }
222    return(ret); <br>
Daniel Veillard1eb24242002-03-18 11:33:03 +0000[diff] [blame]223
Daniel Veillard34ce8be2002-03-18 19:37:11 +0000[diff] [blame^]224
225
Daniel Veillard96984452000-08-31 13:50:12 +0000[diff] [blame]226} </pre>
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame]227</li>
228<li>And then use it to save the document:
Daniel Veillard96984452000-08-31 13:50:12 +0000[diff] [blame]229 <pre>FILE *f;
Daniel Veillarda7ad4522000-08-31 14:19:54 +0000[diff] [blame]230xmlOutputBufferPtr output;
231xmlDocPtr doc;
Daniel Veillard96984452000-08-31 13:50:12 +0000[diff] [blame]232int res;
Daniel Veillarda7ad4522000-08-31 14:19:54 +0000[diff] [blame]233
Daniel Veillard96984452000-08-31 13:50:12 +0000[diff] [blame]234f = ...
235doc = ....
Daniel Veillard96984452000-08-31 13:50:12 +0000[diff] [blame]236
Daniel Veillarda7ad4522000-08-31 14:19:54 +0000[diff] [blame]237output = xmlOutputBufferCreateOwn(f, NULL);
238res = xmlSaveFileTo(output, doc, NULL);
Daniel Veillard96984452000-08-31 13:50:12 +0000[diff] [blame]239 </pre>
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame]240</li>
Daniel Veillard96984452000-08-31 13:50:12 +0000[diff] [blame]241</ol>
Daniel Veillard3f4c40f2002-02-13 09:19:28 +0000[diff] [blame]242<p><a href="bugs.html">Daniel Veillard</a></p>
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame]243</td></tr></table></td></tr></table></td></tr></table></td>
244</tr></table></td></tr></table>
Daniel Veillard96984452000-08-31 13:50:12 +0000[diff] [blame]245</body>
246</html>