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14<title>I/O Interfaces</title>
Daniel Veillard96984452000-08-31 13:50:12 +0000[diff] [blame]15</head>
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22<h1>The XML C library for Gnome</h1>
23<h2>I/O Interfaces</h2>
24</td></tr></table></td></tr></table></td>
25</tr></table>
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29<tr><td colspan="1" bgcolor="#eecfa1" align="center"><center><b>Main Menu</b></center></td></tr>
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Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame]42<li><a href="architecture.html">libxml architecture</a></li>
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46<li><a href="xmlmem.html">Memory Management</a></li>
47<li><a href="encoding.html">Encodings support</a></li>
48<li><a href="xmlio.html">I/O Interfaces</a></li>
49<li><a href="catalog.html">Catalog support</a></li>
50<li><a href="library.html">The parser interfaces</a></li>
51<li><a href="entities.html">Entities or no entities</a></li>
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53<li><a href="upgrade.html">Upgrading 1.x code</a></li>
Daniel Veillard52dcab32001-10-30 12:51:17 +0000[diff] [blame]54<li><a href="threads.html">Thread safety</a></li>
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Daniel Veillard3bf65be2002-01-23 12:36:34 +0000[diff] [blame]64<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]66<li><a href="APIchunk0.html">Alphabetic</a></li>
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Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame]74<tr><td colspan="1" bgcolor="#eecfa1" align="center"><center><b>Related links</b></center></td></tr>
Daniel Veillard8acca112002-01-21 09:52:27 +0000[diff] [blame]75<tr><td bgcolor="#fffacd"><ul>
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame]76<li><a href="http://mail.gnome.org/archives/xml/">Mail archive</a></li>
77<li><a href="http://xmlsoft.org/XSLT/">XSLT libxslt</a></li>
Daniel Veillard4a859202002-01-08 11:49:22 +0000[diff] [blame]78<li><a href="http://phd.cs.unibo.it/gdome2/">DOM gdome2</a></li>
Daniel Veillard2d347fa2002-03-17 10:34:11 +0000[diff] [blame]79<li><a href="http://www.aleksey.com/xmlsec/">XML-DSig xmlsec</a></li>
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame]80<li><a href="ftp://xmlsoft.org/">FTP</a></li>
81<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]82<li><a href="http://garypennington.net/libxml2/">Solaris binaries</a></li>
Daniel Veillard2d347fa2002-03-17 10:34:11 +0000[diff] [blame]83<li><a href="http://bugzilla.gnome.org/buglist.cgi?product=libxml&amp;product=libxml2">Bug Tracker</a></li>
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame]84</ul></td></tr>
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Daniel Veillard96984452000-08-31 13:50:12 +0000[diff] [blame]88<p>Table of Content:</p>
89<ol>
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame]90<li><a href="#General1">General overview</a></li>
91<li><a href="#basic">The basic buffer type</a></li>
92<li><a href="#Input">Input I/O handlers</a></li>
93<li><a href="#Output">Output I/O handlers</a></li>
94<li><a href="#entities">The entities loader</a></li>
95<li><a href="#Example2">Example of customized I/O</a></li>
Daniel Veillard96984452000-08-31 13:50:12 +0000[diff] [blame]96</ol>
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame]97<h3><a name="General1">General overview</a></h3>
98<p>The module <code><a href="http://xmlsoft.org/html/libxml-xmlio.html">xmlIO.h</a></code> provides
99the interfaces to the libxml I/O system. This consists of 4 main parts:</p>
Daniel Veillard96984452000-08-31 13:50:12 +0000[diff] [blame]100<ul>
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame]101<li>Entities loader, this is a routine which tries to fetch the entities
Daniel Veillard2bb89092000-08-31 14:57:50 +0000[diff] [blame]102 (files) based on their PUBLIC and SYSTEM identifiers. The default loader
103 don't look at the public identifier since libxml do not maintain a
104 catalog. You can redefine you own entity loader by using
105 <code>xmlGetExternalEntityLoader()</code> and
Daniel Veillard9c466822001-10-25 12:03:39 +0000[diff] [blame]106 <code>xmlSetExternalEntityLoader()</code>. <a href="#entities">Check the
107 example</a>.</li>
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame]108<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]109 input layer to handle fetching the informations to feed the parser. This
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame]110 provides buffering and is also a placeholder where the encoding
111 convertors to UTF8 are piggy-backed.</li>
112<li>Output I/O buffers are similar to the Input ones and fulfill similar
Daniel Veillard96984452000-08-31 13:50:12 +0000[diff] [blame]113 task but when generating a serialization from a tree.</li>
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame]114<li>A mechanism to register sets of I/O callbacks and associate them with
Daniel Veillard96984452000-08-31 13:50:12 +0000[diff] [blame]115 specific naming schemes like the protocol part of the URIs.
116 <p>This affect the default I/O operations and allows to use specific I/O
117 handlers for certain names.</p>
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame]118</li>
Daniel Veillard96984452000-08-31 13:50:12 +0000[diff] [blame]119</ul>
Daniel Veillard2bb89092000-08-31 14:57:50 +0000[diff] [blame]120<p>The general mechanism used when loading http://rpmfind.net/xml.html for
Daniel Veillard96984452000-08-31 13:50:12 +0000[diff] [blame]121example in the HTML parser is the following:</p>
122<ol>
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame]123<li>The default entity loader calls <code>xmlNewInputFromFile()</code> with
Daniel Veillard2bb89092000-08-31 14:57:50 +0000[diff] [blame]124 the parsing context and the URI string.</li>
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame]125<li>the URI string is checked against the existing registered handlers
126 using their match() callback function, if the HTTP module was compiled
127 in, it is registered and its match() function will succeeds</li>
128<li>the open() function of the handler is called and if successful will
Daniel Veillard96984452000-08-31 13:50:12 +0000[diff] [blame]129 return an I/O Input buffer</li>
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame]130<li>the parser will the start reading from this buffer and progressively
Daniel Veillard96984452000-08-31 13:50:12 +0000[diff] [blame]131 fetch information from the resource, calling the read() function of the
132 handler until the resource is exhausted</li>
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame]133<li>if an encoding change is detected it will be installed on the input
Daniel Veillard96984452000-08-31 13:50:12 +0000[diff] [blame]134 buffer, providing buffering and efficient use of the conversion
135 routines</li>
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame]136<li>once the parser has finished, the close() function of the handler is
Daniel Veillard96984452000-08-31 13:50:12 +0000[diff] [blame]137 called once and the Input buffer and associed resources are
138 deallocated.</li>
139</ol>
Daniel Veillard96984452000-08-31 13:50:12 +0000[diff] [blame]140<p>The user defined callbacks are checked first to allow overriding of the
141default libxml I/O routines.</p>
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame]142<h3><a name="basic">The basic buffer type</a></h3>
Daniel Veillard96984452000-08-31 13:50:12 +0000[diff] [blame]143<p>All the buffer manipulation handling is done using the
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame]144<code>xmlBuffer</code> type define in <code><a href="http://xmlsoft.org/html/libxml-tree.html">tree.h</a></code>which is a
145resizable memory buffer. The buffer allocation strategy can be selected to be
146either best-fit or use an exponential doubling one (CPU vs. memory use
Daniel Veillard96984452000-08-31 13:50:12 +0000[diff] [blame]147tradeoff). The values are <code>XML_BUFFER_ALLOC_EXACT</code> and
148<code>XML_BUFFER_ALLOC_DOUBLEIT</code>, and can be set individually or on a
149system wide basis using <code>xmlBufferSetAllocationScheme()</code>. A number
150of functions allows to manipulate buffers with names starting with the
151<code>xmlBuffer...</code> prefix.</p>
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame]152<h3><a name="Input">Input I/O handlers</a></h3>
Daniel Veillard96984452000-08-31 13:50:12 +0000[diff] [blame]153<p>An Input I/O handler is a simple structure
154<code>xmlParserInputBuffer</code> containing a context associated to the
155resource (file descriptor, or pointer to a protocol handler), the read() and
156close() callbacks to use and an xmlBuffer. And extra xmlBuffer and a charset
157encoding handler are also present to support charset conversion when
158needed.</p>
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame]159<h3><a name="Output">Output I/O handlers</a></h3>
Daniel Veillard96984452000-08-31 13:50:12 +0000[diff] [blame]160<p>An Output handler <code>xmlOutputBuffer</code> is completely similar to an
161Input one except the callbacks are write() and close().</p>
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame]162<h3><a name="entities">The entities loader</a></h3>
Daniel Veillard2bb89092000-08-31 14:57:50 +0000[diff] [blame]163<p>The entity loader resolves requests for new entities and create inputs for
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame]164the parser. Creating an input from a filename or an URI string is done
165through the xmlNewInputFromFile() routine. The default entity loader do not
166handle the PUBLIC identifier associated with an entity (if any). So it just
167calls xmlNewInputFromFile() with the SYSTEM identifier (which is mandatory in
Daniel Veillard2bb89092000-08-31 14:57:50 +0000[diff] [blame]168XML).</p>
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame]169<p>If you want to hook up a catalog mechanism then you simply need to
170override the default entity loader, here is an example:</p>
Daniel Veillard2bb89092000-08-31 14:57:50 +0000[diff] [blame]171<pre>#include &lt;libxml/xmlIO.h&gt;
172
173xmlExternalEntityLoader defaultLoader = NULL;
174
175xmlParserInputPtr
176xmlMyExternalEntityLoader(const char *URL, const char *ID,
177 xmlParserCtxtPtr ctxt) {
178 xmlParserInputPtr ret;
179 const char *fileID = NULL;
180 /* lookup for the fileID depending on ID */
181
182 ret = xmlNewInputFromFile(ctxt, fileID);
183 if (ret != NULL)
184 return(ret);
185 if (defaultLoader != NULL)
186 ret = defaultLoader(URL, ID, ctxt);
187 return(ret);
188}
189
190int main(..) {
191 ...
192
193 /*
194 * Install our own entity loader
195 */
196 defaultLoader = xmlGetExternalEntityLoader();
197 xmlSetExternalEntityLoader(xmlMyExternalEntityLoader);
198
199 ...
200}</pre>
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame]201<h3><a name="Example2">Example of customized I/O</a></h3>
Daniel Veillard96984452000-08-31 13:50:12 +0000[diff] [blame]202<p>This example come from <a href="http://xmlsoft.org/messages/0708.html">a
203real use case</a>, xmlDocDump() closes the FILE * passed by the application
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame]204and 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]205new output handler with the closing call deactivated:</p>
206<ol>
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame]207<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]208 file:
Daniel Veillarda7ad4522000-08-31 14:19:54 +0000[diff] [blame]209 <pre>xmlOutputBufferPtr
210xmlOutputBufferCreateOwn(FILE *file, xmlCharEncodingHandlerPtr encoder) {
211    xmlOutputBufferPtr ret;
212    
213    if (xmlOutputCallbackInitialized == 0)
214        xmlRegisterDefaultOutputCallbacks();
215
216    if (file == NULL) return(NULL);
217    ret = xmlAllocOutputBuffer(encoder);
218    if (ret != NULL) {
219        ret-&gt;context = file;
220        ret-&gt;writecallback = xmlFileWrite;
221        ret-&gt;closecallback = NULL; /* No close callback */
222    }
223    return(ret); <br>
Daniel Veillard1eb24242002-03-18 11:33:03 +0000[diff] [blame]224
Daniel Veillard34ce8be2002-03-18 19:37:11 +0000[diff] [blame]225
226
Daniel Veillard19274092002-03-25 16:48:03 +0000[diff] [blame]227
Daniel Veillardb2fb8ed2002-04-01 09:33:12 +0000[diff] [blame]228
Daniel Veillarda8a89fe2002-04-12 21:03:34 +0000[diff] [blame^]229
Daniel Veillard96984452000-08-31 13:50:12 +0000[diff] [blame]230} </pre>
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame]231</li>
232<li>And then use it to save the document:
Daniel Veillard96984452000-08-31 13:50:12 +0000[diff] [blame]233 <pre>FILE *f;
Daniel Veillarda7ad4522000-08-31 14:19:54 +0000[diff] [blame]234xmlOutputBufferPtr output;
235xmlDocPtr doc;
Daniel Veillard96984452000-08-31 13:50:12 +0000[diff] [blame]236int res;
Daniel Veillarda7ad4522000-08-31 14:19:54 +0000[diff] [blame]237
Daniel Veillard96984452000-08-31 13:50:12 +0000[diff] [blame]238f = ...
239doc = ....
Daniel Veillard96984452000-08-31 13:50:12 +0000[diff] [blame]240
Daniel Veillarda7ad4522000-08-31 14:19:54 +0000[diff] [blame]241output = xmlOutputBufferCreateOwn(f, NULL);
242res = xmlSaveFileTo(output, doc, NULL);
Daniel Veillard96984452000-08-31 13:50:12 +0000[diff] [blame]243 </pre>
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame]244</li>
Daniel Veillard96984452000-08-31 13:50:12 +0000[diff] [blame]245</ol>
Daniel Veillard3f4c40f2002-02-13 09:19:28 +0000[diff] [blame]246<p><a href="bugs.html">Daniel Veillard</a></p>
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame]247</td></tr></table></td></tr></table></td></tr></table></td>
248</tr></table></td></tr></table>
Daniel Veillard96984452000-08-31 13:50:12 +0000[diff] [blame]249</body>
250</html>