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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 Veillard9c466822001-10-25 12:03:39 +0000[diff] [blame^]91 <code>xmlSetExternalEntityLoader()</code>. <a href="#entities">Check the
92 example</a>.</li>
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame]93<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]94 input layer to handle fetching the informations to feed the parser. This
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame]95 provides buffering and is also a placeholder where the encoding
96 convertors to UTF8 are piggy-backed.</li>
97<li>Output I/O buffers are similar to the Input ones and fulfill similar
Daniel Veillard96984452000-08-31 13:50:12 +0000[diff] [blame]98 task but when generating a serialization from a tree.</li>
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame]99<li>A mechanism to register sets of I/O callbacks and associate them with
Daniel Veillard96984452000-08-31 13:50:12 +0000[diff] [blame]100 specific naming schemes like the protocol part of the URIs.
101 <p>This affect the default I/O operations and allows to use specific I/O
102 handlers for certain names.</p>
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame]103</li>
Daniel Veillard96984452000-08-31 13:50:12 +0000[diff] [blame]104</ul>
Daniel Veillard2bb89092000-08-31 14:57:50 +0000[diff] [blame]105<p>The general mechanism used when loading http://rpmfind.net/xml.html for
Daniel Veillard96984452000-08-31 13:50:12 +0000[diff] [blame]106example in the HTML parser is the following:</p>
107<ol>
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame]108<li>The default entity loader calls <code>xmlNewInputFromFile()</code> with
Daniel Veillard2bb89092000-08-31 14:57:50 +0000[diff] [blame]109 the parsing context and the URI string.</li>
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame]110<li>the URI string is checked against the existing registered handlers
111 using their match() callback function, if the HTTP module was compiled
112 in, it is registered and its match() function will succeeds</li>
113<li>the open() function of the handler is called and if successful will
Daniel Veillard96984452000-08-31 13:50:12 +0000[diff] [blame]114 return an I/O Input buffer</li>
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame]115<li>the parser will the start reading from this buffer and progressively
Daniel Veillard96984452000-08-31 13:50:12 +0000[diff] [blame]116 fetch information from the resource, calling the read() function of the
117 handler until the resource is exhausted</li>
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame]118<li>if an encoding change is detected it will be installed on the input
Daniel Veillard96984452000-08-31 13:50:12 +0000[diff] [blame]119 buffer, providing buffering and efficient use of the conversion
120 routines</li>
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame]121<li>once the parser has finished, the close() function of the handler is
Daniel Veillard96984452000-08-31 13:50:12 +0000[diff] [blame]122 called once and the Input buffer and associed resources are
123 deallocated.</li>
124</ol>
Daniel Veillard96984452000-08-31 13:50:12 +0000[diff] [blame]125<p>The user defined callbacks are checked first to allow overriding of the
126default libxml I/O routines.</p>
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame]127<h3><a name="basic">The basic buffer type</a></h3>
Daniel Veillard96984452000-08-31 13:50:12 +0000[diff] [blame]128<p>All the buffer manipulation handling is done using the
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame]129<code>xmlBuffer</code> type define in <code><a href="http://xmlsoft.org/html/libxml-tree.html">tree.h</a></code>which is a
130resizable memory buffer. The buffer allocation strategy can be selected to be
131either best-fit or use an exponential doubling one (CPU vs. memory use
Daniel Veillard96984452000-08-31 13:50:12 +0000[diff] [blame]132tradeoff). The values are <code>XML_BUFFER_ALLOC_EXACT</code> and
133<code>XML_BUFFER_ALLOC_DOUBLEIT</code>, and can be set individually or on a
134system wide basis using <code>xmlBufferSetAllocationScheme()</code>. A number
135of functions allows to manipulate buffers with names starting with the
136<code>xmlBuffer...</code> prefix.</p>
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame]137<h3><a name="Input">Input I/O handlers</a></h3>
Daniel Veillard96984452000-08-31 13:50:12 +0000[diff] [blame]138<p>An Input I/O handler is a simple structure
139<code>xmlParserInputBuffer</code> containing a context associated to the
140resource (file descriptor, or pointer to a protocol handler), the read() and
141close() callbacks to use and an xmlBuffer. And extra xmlBuffer and a charset
142encoding handler are also present to support charset conversion when
143needed.</p>
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame]144<h3><a name="Output">Output I/O handlers</a></h3>
Daniel Veillard96984452000-08-31 13:50:12 +0000[diff] [blame]145<p>An Output handler <code>xmlOutputBuffer</code> is completely similar to an
146Input one except the callbacks are write() and close().</p>
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame]147<h3><a name="entities">The entities loader</a></h3>
Daniel Veillard2bb89092000-08-31 14:57:50 +0000[diff] [blame]148<p>The entity loader resolves requests for new entities and create inputs for
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame]149the parser. Creating an input from a filename or an URI string is done
150through the xmlNewInputFromFile() routine. The default entity loader do not
151handle the PUBLIC identifier associated with an entity (if any). So it just
152calls xmlNewInputFromFile() with the SYSTEM identifier (which is mandatory in
Daniel Veillard2bb89092000-08-31 14:57:50 +0000[diff] [blame]153XML).</p>
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame]154<p>If you want to hook up a catalog mechanism then you simply need to
155override the default entity loader, here is an example:</p>
Daniel Veillard2bb89092000-08-31 14:57:50 +0000[diff] [blame]156<pre>#include &lt;libxml/xmlIO.h&gt;
157
158xmlExternalEntityLoader defaultLoader = NULL;
159
160xmlParserInputPtr
161xmlMyExternalEntityLoader(const char *URL, const char *ID,
162 xmlParserCtxtPtr ctxt) {
163 xmlParserInputPtr ret;
164 const char *fileID = NULL;
165 /* lookup for the fileID depending on ID */
166
167 ret = xmlNewInputFromFile(ctxt, fileID);
168 if (ret != NULL)
169 return(ret);
170 if (defaultLoader != NULL)
171 ret = defaultLoader(URL, ID, ctxt);
172 return(ret);
173}
174
175int main(..) {
176 ...
177
178 /*
179 * Install our own entity loader
180 */
181 defaultLoader = xmlGetExternalEntityLoader();
182 xmlSetExternalEntityLoader(xmlMyExternalEntityLoader);
183
184 ...
185}</pre>
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame]186<h3><a name="Example2">Example of customized I/O</a></h3>
Daniel Veillard96984452000-08-31 13:50:12 +0000[diff] [blame]187<p>This example come from <a href="http://xmlsoft.org/messages/0708.html">a
188real use case</a>, xmlDocDump() closes the FILE * passed by the application
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame]189and 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]190new output handler with the closing call deactivated:</p>
191<ol>
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame]192<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]193 file:
Daniel Veillarda7ad4522000-08-31 14:19:54 +0000[diff] [blame]194 <pre>xmlOutputBufferPtr
195xmlOutputBufferCreateOwn(FILE *file, xmlCharEncodingHandlerPtr encoder) {
196    xmlOutputBufferPtr ret;
197    
198    if (xmlOutputCallbackInitialized == 0)
199        xmlRegisterDefaultOutputCallbacks();
200
201    if (file == NULL) return(NULL);
202    ret = xmlAllocOutputBuffer(encoder);
203    if (ret != NULL) {
204        ret-&gt;context = file;
205        ret-&gt;writecallback = xmlFileWrite;
206        ret-&gt;closecallback = NULL; /* No close callback */
207    }
208    return(ret); <br>
Daniel Veillard2bb89092000-08-31 14:57:50 +0000[diff] [blame]209
210
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame]211
Daniel Veillard9c466822001-10-25 12:03:39 +0000[diff] [blame^]212
213
Daniel Veillard96984452000-08-31 13:50:12 +0000[diff] [blame]214} </pre>
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame]215</li>
216<li>And then use it to save the document:
Daniel Veillard96984452000-08-31 13:50:12 +0000[diff] [blame]217 <pre>FILE *f;
Daniel Veillarda7ad4522000-08-31 14:19:54 +0000[diff] [blame]218xmlOutputBufferPtr output;
219xmlDocPtr doc;
Daniel Veillard96984452000-08-31 13:50:12 +0000[diff] [blame]220int res;
Daniel Veillarda7ad4522000-08-31 14:19:54 +0000[diff] [blame]221
Daniel Veillard96984452000-08-31 13:50:12 +0000[diff] [blame]222f = ...
223doc = ....
Daniel Veillard96984452000-08-31 13:50:12 +0000[diff] [blame]224
Daniel Veillarda7ad4522000-08-31 14:19:54 +0000[diff] [blame]225output = xmlOutputBufferCreateOwn(f, NULL);
226res = xmlSaveFileTo(output, doc, NULL);
Daniel Veillard96984452000-08-31 13:50:12 +0000[diff] [blame]227 </pre>
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame]228</li>
Daniel Veillard96984452000-08-31 13:50:12 +0000[diff] [blame]229</ol>
Daniel Veillardc5d64342001-06-24 12:13:24 +0000[diff] [blame]230<p><a href="mailto:daniel@veillard.com">Daniel Veillard</a></p>
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame]231</td></tr></table></td></tr></table></td></tr></table></td>
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Daniel Veillard96984452000-08-31 13:50:12 +0000[diff] [blame]233</body>
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