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gerrit-public.fairphone.software / fp2-dev / platform / external / libxml2 / 6dbcaf8686cec01223029997a2d22054a2507e55 / . / doc / xmlio.html
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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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18<a href="http://www.gnome.org/"><img src="smallfootonly.gif" alt="Gnome 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>
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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>
24</tr></table>
25<table border="0" cellpadding="4" cellspacing="0" width="100%" align="center"><tr><td bgcolor="#8b7765"><table border="0" cellspacing="0" cellpadding="2" width="100%"><tr>
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28<tr><td colspan="1" bgcolor="#eecfa1" align="center"><center><b>Main Menu</b></center></td></tr>
Daniel Veillard8acca112002-01-21 09:52:27 +0000[diff] [blame]29<tr><td bgcolor="#fffacd"><ul>
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame]30<li><a href="index.html">Home</a></li>
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Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame]39<li><a href="XSLT.html">XSLT</a></li>
Daniel Veillard6dbcaf82002-02-20 14:37:47 +0000[diff] [blame^]40<li><a href="python.html">Python and bindings</a></li>
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame]41<li><a href="architecture.html">libxml architecture</a></li>
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45<li><a href="xmlmem.html">Memory Management</a></li>
46<li><a href="encoding.html">Encodings support</a></li>
47<li><a href="xmlio.html">I/O Interfaces</a></li>
48<li><a href="catalog.html">Catalog support</a></li>
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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52<li><a href="upgrade.html">Upgrading 1.x code</a></li>
Daniel Veillard52dcab32001-10-30 12:51:17 +0000[diff] [blame]53<li><a href="threads.html">Thread safety</a></li>
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame]54<li><a href="DOM.html">DOM Principles</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>
Daniel Veillard8acca112002-01-21 09:52:27 +0000[diff] [blame]74<tr><td bgcolor="#fffacd"><ul>
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame]75<li><a href="http://mail.gnome.org/archives/xml/">Mail archive</a></li>
76<li><a href="http://xmlsoft.org/XSLT/">XSLT libxslt</a></li>
Daniel Veillard4a859202002-01-08 11:49:22 +0000[diff] [blame]77<li><a href="http://phd.cs.unibo.it/gdome2/">DOM gdome2</a></li>
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame]78<li><a href="ftp://xmlsoft.org/">FTP</a></li>
79<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]80<li><a href="http://garypennington.net/libxml2/">Solaris binaries</a></li>
Daniel Veillardc6271d22001-10-27 07:50:58 +0000[diff] [blame]81<li><a href="http://bugzilla.gnome.org/buglist.cgi?product=libxml">Bug Tracker</a></li>
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame]82</ul></td></tr>
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Daniel Veillard96984452000-08-31 13:50:12 +0000[diff] [blame]86<p>Table of Content:</p>
87<ol>
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame]88<li><a href="#General1">General overview</a></li>
89<li><a href="#basic">The basic buffer type</a></li>
90<li><a href="#Input">Input I/O handlers</a></li>
91<li><a href="#Output">Output I/O handlers</a></li>
92<li><a href="#entities">The entities loader</a></li>
93<li><a href="#Example2">Example of customized I/O</a></li>
Daniel Veillard96984452000-08-31 13:50:12 +0000[diff] [blame]94</ol>
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame]95<h3><a name="General1">General overview</a></h3>
96<p>The module <code><a href="http://xmlsoft.org/html/libxml-xmlio.html">xmlIO.h</a></code> provides
97the interfaces to the libxml I/O system. This consists of 4 main parts:</p>
Daniel Veillard96984452000-08-31 13:50:12 +0000[diff] [blame]98<ul>
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame]99<li>Entities loader, this is a routine which tries to fetch the entities
Daniel Veillard2bb89092000-08-31 14:57:50 +0000[diff] [blame]100 (files) based on their PUBLIC and SYSTEM identifiers. The default loader
101 don't look at the public identifier since libxml do not maintain a
102 catalog. You can redefine you own entity loader by using
103 <code>xmlGetExternalEntityLoader()</code> and
Daniel Veillard9c466822001-10-25 12:03:39 +0000[diff] [blame]104 <code>xmlSetExternalEntityLoader()</code>. <a href="#entities">Check the
105 example</a>.</li>
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame]106<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]107 input layer to handle fetching the informations to feed the parser. This
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame]108 provides buffering and is also a placeholder where the encoding
109 convertors to UTF8 are piggy-backed.</li>
110<li>Output I/O buffers are similar to the Input ones and fulfill similar
Daniel Veillard96984452000-08-31 13:50:12 +0000[diff] [blame]111 task but when generating a serialization from a tree.</li>
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame]112<li>A mechanism to register sets of I/O callbacks and associate them with
Daniel Veillard96984452000-08-31 13:50:12 +0000[diff] [blame]113 specific naming schemes like the protocol part of the URIs.
114 <p>This affect the default I/O operations and allows to use specific I/O
115 handlers for certain names.</p>
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame]116</li>
Daniel Veillard96984452000-08-31 13:50:12 +0000[diff] [blame]117</ul>
Daniel Veillard2bb89092000-08-31 14:57:50 +0000[diff] [blame]118<p>The general mechanism used when loading http://rpmfind.net/xml.html for
Daniel Veillard96984452000-08-31 13:50:12 +0000[diff] [blame]119example in the HTML parser is the following:</p>
120<ol>
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame]121<li>The default entity loader calls <code>xmlNewInputFromFile()</code> with
Daniel Veillard2bb89092000-08-31 14:57:50 +0000[diff] [blame]122 the parsing context and the URI string.</li>
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame]123<li>the URI string is checked against the existing registered handlers
124 using their match() callback function, if the HTTP module was compiled
125 in, it is registered and its match() function will succeeds</li>
126<li>the open() function of the handler is called and if successful will
Daniel Veillard96984452000-08-31 13:50:12 +0000[diff] [blame]127 return an I/O Input buffer</li>
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame]128<li>the parser will the start reading from this buffer and progressively
Daniel Veillard96984452000-08-31 13:50:12 +0000[diff] [blame]129 fetch information from the resource, calling the read() function of the
130 handler until the resource is exhausted</li>
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame]131<li>if an encoding change is detected it will be installed on the input
Daniel Veillard96984452000-08-31 13:50:12 +0000[diff] [blame]132 buffer, providing buffering and efficient use of the conversion
133 routines</li>
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame]134<li>once the parser has finished, the close() function of the handler is
Daniel Veillard96984452000-08-31 13:50:12 +0000[diff] [blame]135 called once and the Input buffer and associed resources are
136 deallocated.</li>
137</ol>
Daniel Veillard96984452000-08-31 13:50:12 +0000[diff] [blame]138<p>The user defined callbacks are checked first to allow overriding of the
139default libxml I/O routines.</p>
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame]140<h3><a name="basic">The basic buffer type</a></h3>
Daniel Veillard96984452000-08-31 13:50:12 +0000[diff] [blame]141<p>All the buffer manipulation handling is done using the
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame]142<code>xmlBuffer</code> type define in <code><a href="http://xmlsoft.org/html/libxml-tree.html">tree.h</a></code>which is a
143resizable memory buffer. The buffer allocation strategy can be selected to be
144either best-fit or use an exponential doubling one (CPU vs. memory use
Daniel Veillard96984452000-08-31 13:50:12 +0000[diff] [blame]145tradeoff). The values are <code>XML_BUFFER_ALLOC_EXACT</code> and
146<code>XML_BUFFER_ALLOC_DOUBLEIT</code>, and can be set individually or on a
147system wide basis using <code>xmlBufferSetAllocationScheme()</code>. A number
148of functions allows to manipulate buffers with names starting with the
149<code>xmlBuffer...</code> prefix.</p>
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame]150<h3><a name="Input">Input I/O handlers</a></h3>
Daniel Veillard96984452000-08-31 13:50:12 +0000[diff] [blame]151<p>An Input I/O handler is a simple structure
152<code>xmlParserInputBuffer</code> containing a context associated to the
153resource (file descriptor, or pointer to a protocol handler), the read() and
154close() callbacks to use and an xmlBuffer. And extra xmlBuffer and a charset
155encoding handler are also present to support charset conversion when
156needed.</p>
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame]157<h3><a name="Output">Output I/O handlers</a></h3>
Daniel Veillard96984452000-08-31 13:50:12 +0000[diff] [blame]158<p>An Output handler <code>xmlOutputBuffer</code> is completely similar to an
159Input one except the callbacks are write() and close().</p>
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame]160<h3><a name="entities">The entities loader</a></h3>
Daniel Veillard2bb89092000-08-31 14:57:50 +0000[diff] [blame]161<p>The entity loader resolves requests for new entities and create inputs for
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame]162the parser. Creating an input from a filename or an URI string is done
163through the xmlNewInputFromFile() routine. The default entity loader do not
164handle the PUBLIC identifier associated with an entity (if any). So it just
165calls xmlNewInputFromFile() with the SYSTEM identifier (which is mandatory in
Daniel Veillard2bb89092000-08-31 14:57:50 +0000[diff] [blame]166XML).</p>
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame]167<p>If you want to hook up a catalog mechanism then you simply need to
168override the default entity loader, here is an example:</p>
Daniel Veillard2bb89092000-08-31 14:57:50 +0000[diff] [blame]169<pre>#include &lt;libxml/xmlIO.h&gt;
170
171xmlExternalEntityLoader defaultLoader = NULL;
172
173xmlParserInputPtr
174xmlMyExternalEntityLoader(const char *URL, const char *ID,
175 xmlParserCtxtPtr ctxt) {
176 xmlParserInputPtr ret;
177 const char *fileID = NULL;
178 /* lookup for the fileID depending on ID */
179
180 ret = xmlNewInputFromFile(ctxt, fileID);
181 if (ret != NULL)
182 return(ret);
183 if (defaultLoader != NULL)
184 ret = defaultLoader(URL, ID, ctxt);
185 return(ret);
186}
187
188int main(..) {
189 ...
190
191 /*
192 * Install our own entity loader
193 */
194 defaultLoader = xmlGetExternalEntityLoader();
195 xmlSetExternalEntityLoader(xmlMyExternalEntityLoader);
196
197 ...
198}</pre>
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame]199<h3><a name="Example2">Example of customized I/O</a></h3>
Daniel Veillard96984452000-08-31 13:50:12 +0000[diff] [blame]200<p>This example come from <a href="http://xmlsoft.org/messages/0708.html">a
201real use case</a>, xmlDocDump() closes the FILE * passed by the application
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame]202and 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]203new output handler with the closing call deactivated:</p>
204<ol>
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame]205<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]206 file:
Daniel Veillarda7ad4522000-08-31 14:19:54 +0000[diff] [blame]207 <pre>xmlOutputBufferPtr
208xmlOutputBufferCreateOwn(FILE *file, xmlCharEncodingHandlerPtr encoder) {
209    xmlOutputBufferPtr ret;
210    
211    if (xmlOutputCallbackInitialized == 0)
212        xmlRegisterDefaultOutputCallbacks();
213
214    if (file == NULL) return(NULL);
215    ret = xmlAllocOutputBuffer(encoder);
216    if (ret != NULL) {
217        ret-&gt;context = file;
218        ret-&gt;writecallback = xmlFileWrite;
219        ret-&gt;closecallback = NULL; /* No close callback */
220    }
221    return(ret); <br>
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247
248
Daniel Veillard96984452000-08-31 13:50:12 +0000[diff] [blame]249} </pre>
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame]250</li>
251<li>And then use it to save the document:
Daniel Veillard96984452000-08-31 13:50:12 +0000[diff] [blame]252 <pre>FILE *f;
Daniel Veillarda7ad4522000-08-31 14:19:54 +0000[diff] [blame]253xmlOutputBufferPtr output;
254xmlDocPtr doc;
Daniel Veillard96984452000-08-31 13:50:12 +0000[diff] [blame]255int res;
Daniel Veillarda7ad4522000-08-31 14:19:54 +0000[diff] [blame]256
Daniel Veillard96984452000-08-31 13:50:12 +0000[diff] [blame]257f = ...
258doc = ....
Daniel Veillard96984452000-08-31 13:50:12 +0000[diff] [blame]259
Daniel Veillarda7ad4522000-08-31 14:19:54 +0000[diff] [blame]260output = xmlOutputBufferCreateOwn(f, NULL);
261res = xmlSaveFileTo(output, doc, NULL);
Daniel Veillard96984452000-08-31 13:50:12 +0000[diff] [blame]262 </pre>
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame]263</li>
Daniel Veillard96984452000-08-31 13:50:12 +0000[diff] [blame]264</ol>
Daniel Veillard3f4c40f2002-02-13 09:19:28 +0000[diff] [blame]265<p><a href="bugs.html">Daniel Veillard</a></p>
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame]266</td></tr></table></td></tr></table></td></tr></table></td>
267</tr></table></td></tr></table>
Daniel Veillard96984452000-08-31 13:50:12 +0000[diff] [blame]268</body>
269</html>