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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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Daniel Veillard96984452000-08-31 13:50:12 +0000[diff] [blame]73<p>Table of Content:</p>
74<ol>
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame^]75<li><a href="#General1">General overview</a></li>
76<li><a href="#basic">The basic buffer type</a></li>
77<li><a href="#Input">Input I/O handlers</a></li>
78<li><a href="#Output">Output I/O handlers</a></li>
79<li><a href="#entities">The entities loader</a></li>
80<li><a href="#Example2">Example of customized I/O</a></li>
Daniel Veillard96984452000-08-31 13:50:12 +0000[diff] [blame]81</ol>
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame^]82<h3><a name="General1">General overview</a></h3>
83<p>The module <code><a href="http://xmlsoft.org/html/libxml-xmlio.html">xmlIO.h</a></code> provides
84the interfaces to the libxml I/O system. This consists of 4 main parts:</p>
Daniel Veillard96984452000-08-31 13:50:12 +0000[diff] [blame]85<ul>
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame^]86<li>Entities loader, this is a routine which tries to fetch the entities
Daniel Veillard2bb89092000-08-31 14:57:50 +0000[diff] [blame]87 (files) based on their PUBLIC and SYSTEM identifiers. The default loader
88 don't look at the public identifier since libxml do not maintain a
89 catalog. You can redefine you own entity loader by using
90 <code>xmlGetExternalEntityLoader()</code> and
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame^]91 <code>xmlSetExternalEntityLoader()</code>. <a href="#entities">Check the example</a>.</li>
92<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]93 input layer to handle fetching the informations to feed the parser. This
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame^]94 provides buffering and is also a placeholder where the encoding
95 convertors to UTF8 are piggy-backed.</li>
96<li>Output I/O buffers are similar to the Input ones and fulfill similar
Daniel Veillard96984452000-08-31 13:50:12 +0000[diff] [blame]97 task but when generating a serialization from a tree.</li>
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame^]98<li>A mechanism to register sets of I/O callbacks and associate them with
Daniel Veillard96984452000-08-31 13:50:12 +0000[diff] [blame]99 specific naming schemes like the protocol part of the URIs.
100 <p>This affect the default I/O operations and allows to use specific I/O
101 handlers for certain names.</p>
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame^]102</li>
Daniel Veillard96984452000-08-31 13:50:12 +0000[diff] [blame]103</ul>
Daniel Veillard2bb89092000-08-31 14:57:50 +0000[diff] [blame]104<p>The general mechanism used when loading http://rpmfind.net/xml.html for
Daniel Veillard96984452000-08-31 13:50:12 +0000[diff] [blame]105example in the HTML parser is the following:</p>
106<ol>
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame^]107<li>The default entity loader calls <code>xmlNewInputFromFile()</code> with
Daniel Veillard2bb89092000-08-31 14:57:50 +0000[diff] [blame]108 the parsing context and the URI string.</li>
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame^]109<li>the URI string is checked against the existing registered handlers
110 using their match() callback function, if the HTTP module was compiled
111 in, it is registered and its match() function will succeeds</li>
112<li>the open() function of the handler is called and if successful will
Daniel Veillard96984452000-08-31 13:50:12 +0000[diff] [blame]113 return an I/O Input buffer</li>
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame^]114<li>the parser will the start reading from this buffer and progressively
Daniel Veillard96984452000-08-31 13:50:12 +0000[diff] [blame]115 fetch information from the resource, calling the read() function of the
116 handler until the resource is exhausted</li>
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame^]117<li>if an encoding change is detected it will be installed on the input
Daniel Veillard96984452000-08-31 13:50:12 +0000[diff] [blame]118 buffer, providing buffering and efficient use of the conversion
119 routines</li>
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame^]120<li>once the parser has finished, the close() function of the handler is
Daniel Veillard96984452000-08-31 13:50:12 +0000[diff] [blame]121 called once and the Input buffer and associed resources are
122 deallocated.</li>
123</ol>
Daniel Veillard96984452000-08-31 13:50:12 +0000[diff] [blame]124<p>The user defined callbacks are checked first to allow overriding of the
125default libxml I/O routines.</p>
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame^]126<h3><a name="basic">The basic buffer type</a></h3>
Daniel Veillard96984452000-08-31 13:50:12 +0000[diff] [blame]127<p>All the buffer manipulation handling is done using the
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame^]128<code>xmlBuffer</code> type define in <code><a href="http://xmlsoft.org/html/libxml-tree.html">tree.h</a></code>which is a
129resizable memory buffer. The buffer allocation strategy can be selected to be
130either best-fit or use an exponential doubling one (CPU vs. memory use
Daniel Veillard96984452000-08-31 13:50:12 +0000[diff] [blame]131tradeoff). The values are <code>XML_BUFFER_ALLOC_EXACT</code> and
132<code>XML_BUFFER_ALLOC_DOUBLEIT</code>, and can be set individually or on a
133system wide basis using <code>xmlBufferSetAllocationScheme()</code>. A number
134of functions allows to manipulate buffers with names starting with the
135<code>xmlBuffer...</code> prefix.</p>
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame^]136<h3><a name="Input">Input I/O handlers</a></h3>
Daniel Veillard96984452000-08-31 13:50:12 +0000[diff] [blame]137<p>An Input I/O handler is a simple structure
138<code>xmlParserInputBuffer</code> containing a context associated to the
139resource (file descriptor, or pointer to a protocol handler), the read() and
140close() callbacks to use and an xmlBuffer. And extra xmlBuffer and a charset
141encoding handler are also present to support charset conversion when
142needed.</p>
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame^]143<h3><a name="Output">Output I/O handlers</a></h3>
Daniel Veillard96984452000-08-31 13:50:12 +0000[diff] [blame]144<p>An Output handler <code>xmlOutputBuffer</code> is completely similar to an
145Input one except the callbacks are write() and close().</p>
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame^]146<h3><a name="entities">The entities loader</a></h3>
Daniel Veillard2bb89092000-08-31 14:57:50 +0000[diff] [blame]147<p>The entity loader resolves requests for new entities and create inputs for
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame^]148the parser. Creating an input from a filename or an URI string is done
149through the xmlNewInputFromFile() routine. The default entity loader do not
150handle the PUBLIC identifier associated with an entity (if any). So it just
151calls xmlNewInputFromFile() with the SYSTEM identifier (which is mandatory in
Daniel Veillard2bb89092000-08-31 14:57:50 +0000[diff] [blame]152XML).</p>
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame^]153<p>If you want to hook up a catalog mechanism then you simply need to
154override the default entity loader, here is an example:</p>
Daniel Veillard2bb89092000-08-31 14:57:50 +0000[diff] [blame]155<pre>#include &lt;libxml/xmlIO.h&gt;
156
157xmlExternalEntityLoader defaultLoader = NULL;
158
159xmlParserInputPtr
160xmlMyExternalEntityLoader(const char *URL, const char *ID,
161 xmlParserCtxtPtr ctxt) {
162 xmlParserInputPtr ret;
163 const char *fileID = NULL;
164 /* lookup for the fileID depending on ID */
165
166 ret = xmlNewInputFromFile(ctxt, fileID);
167 if (ret != NULL)
168 return(ret);
169 if (defaultLoader != NULL)
170 ret = defaultLoader(URL, ID, ctxt);
171 return(ret);
172}
173
174int main(..) {
175 ...
176
177 /*
178 * Install our own entity loader
179 */
180 defaultLoader = xmlGetExternalEntityLoader();
181 xmlSetExternalEntityLoader(xmlMyExternalEntityLoader);
182
183 ...
184}</pre>
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame^]185<h3><a name="Example2">Example of customized I/O</a></h3>
Daniel Veillard96984452000-08-31 13:50:12 +0000[diff] [blame]186<p>This example come from <a href="http://xmlsoft.org/messages/0708.html">a
187real use case</a>, xmlDocDump() closes the FILE * passed by the application
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame^]188and 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]189new output handler with the closing call deactivated:</p>
190<ol>
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame^]191<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]192 file:
Daniel Veillarda7ad4522000-08-31 14:19:54 +0000[diff] [blame]193 <pre>xmlOutputBufferPtr
194xmlOutputBufferCreateOwn(FILE *file, xmlCharEncodingHandlerPtr encoder) {
195    xmlOutputBufferPtr ret;
196    
197    if (xmlOutputCallbackInitialized == 0)
198        xmlRegisterDefaultOutputCallbacks();
199
200    if (file == NULL) return(NULL);
201    ret = xmlAllocOutputBuffer(encoder);
202    if (ret != NULL) {
203        ret-&gt;context = file;
204        ret-&gt;writecallback = xmlFileWrite;
205        ret-&gt;closecallback = NULL; /* No close callback */
206    }
207    return(ret); <br>
Daniel Veillard2bb89092000-08-31 14:57:50 +0000[diff] [blame]208
209
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame^]210
Daniel Veillard96984452000-08-31 13:50:12 +0000[diff] [blame]211} </pre>
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame^]212</li>
213<li>And then use it to save the document:
Daniel Veillard96984452000-08-31 13:50:12 +0000[diff] [blame]214 <pre>FILE *f;
Daniel Veillarda7ad4522000-08-31 14:19:54 +0000[diff] [blame]215xmlOutputBufferPtr output;
216xmlDocPtr doc;
Daniel Veillard96984452000-08-31 13:50:12 +0000[diff] [blame]217int res;
Daniel Veillarda7ad4522000-08-31 14:19:54 +0000[diff] [blame]218
Daniel Veillard96984452000-08-31 13:50:12 +0000[diff] [blame]219f = ...
220doc = ....
Daniel Veillard96984452000-08-31 13:50:12 +0000[diff] [blame]221
Daniel Veillarda7ad4522000-08-31 14:19:54 +0000[diff] [blame]222output = xmlOutputBufferCreateOwn(f, NULL);
223res = xmlSaveFileTo(output, doc, NULL);
Daniel Veillard96984452000-08-31 13:50:12 +0000[diff] [blame]224 </pre>
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame^]225</li>
Daniel Veillard96984452000-08-31 13:50:12 +0000[diff] [blame]226</ol>
Daniel Veillardc5d64342001-06-24 12:13:24 +0000[diff] [blame]227<p><a href="mailto:daniel@veillard.com">Daniel Veillard</a></p>
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame^]228</td></tr></table></td></tr></table></td></tr></table></td>
229</tr></table></td></tr></table>
Daniel Veillard96984452000-08-31 13:50:12 +0000[diff] [blame]230</body>
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