doc/xmlio.html - platform/external/libxml2 - Gitiles

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   <title>Libxml Input/Output handling</title>
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 <body bgcolor="#ffffff">
 <h1 align="center">Libxml Input/Output handling</h1>

 <p>Location: <a
 href="http://xmlsoft.org/xmlio.html">http://xmlsoft.org/xmlio.html</a></p>

 <p>Libxml home page: <a href="http://xmlsoft.org/">http://xmlsoft.org/</a></p>

 <p>Mailing-list archive:  <a
 href="http://xmlsoft.org/messages/">http://xmlsoft.org/messages/</a></p>

 <p>Version: $Revision: 1.4 $</p>

 <p>Table of Content:</p>
 <ol>
   <li><a href="#General">General overview</a></li>
   <li><a href="#basic">The basic buffer type</a></li>
   <li><a href="#Input">Input I/O handlers</a></li>
   <li><a href="#Output">Output I/O handlers</a></li>
   <li><a href="#entities">The entities loader</a></li>
   <li><a href="#Example">Example of customized I/O</a></li>
 </ol>

 <h2><a name="General">General overview</a></h2>

 <p>The module <code><a
 href="http://xmlsoft.org/html/libxml-xmlio.html">xmlIO.h</a></code>
 provides the interfaces to the libxml I/O system. This consists of 4 main
 parts:</p>
 <ul>
   <li>Entities loader, this is a routine which tries to fetch the entities
     (files) based on their PUBLIC and SYSTEM identifiers. The default loader
     don't look at the public identifier since libxml do not maintain a
     catalog. You can redefine you own entity loader by using
     <code>xmlGetExternalEntityLoader()</code> and
     <code>xmlSetExternalEntityLoader()</code>. <a href="#entities">Check the
     example</a>.</li>
   <li>Input I/O buffers which are a commodity structure used by the parser(s)
     input layer to handle fetching the informations to feed the parser. This
     provides buffering and is also a placeholder where the encoding convertors
     to UTF8 are piggy-backed.</li>
   <li>Output I/O buffers are similar to the Input ones and fulfill similar
     task but when generating a serialization from a tree.</li>
   <li>A mechanism to register sets of I/O callbacks and associate them with
     specific naming schemes like the protocol part of the URIs.
     <p>This affect the default I/O operations and allows to use specific I/O
     handlers for certain names.</p>
   </li>
 </ul>

 <p>The general mechanism used when loading http://rpmfind.net/xml.html for
 example in the HTML parser is the following:</p>
 <ol>
   <li>The default entity loader calls <code>xmlNewInputFromFile()</code> with
     the parsing context and the URI string.</li>
   <li>the URI string is checked against the existing registered handlers using
     their match() callback function, if the HTTP module was compiled in, it is
     registered and its match() function will succeeds</li>
   <li>the open() function of the handler is called and if successful will
     return an I/O Input buffer</li>
   <li>the parser will the start reading from this buffer and progressively
     fetch information from the resource, calling the read() function of the
     handler until the resource is exhausted</li>
   <li>if an encoding change is detected it will be installed on the input
     buffer, providing buffering and efficient use of the conversion
   routines</li>
   <li>once the parser has finished, the close() function of the handler is
     called once and the Input buffer and associed resources are
   deallocated.</li>
 </ol>

 <p>The user defined callbacks are checked first to allow overriding of the
 default libxml I/O routines.</p>

 <h2><a name="basic">The basic buffer type</a></h2>

 <p>All the buffer manipulation handling is done using the
 <code>xmlBuffer</code> type define in <code><a
 href="http://xmlsoft.org/html/libxml-tree.html">tree.h</a> </code>which is
 a resizable memory buffer. The buffer allocation strategy can be selected to
 be either best-fit or use an exponential doubling one (CPU vs. memory use
 tradeoff). The values are <code>XML_BUFFER_ALLOC_EXACT</code> and
 <code>XML_BUFFER_ALLOC_DOUBLEIT</code>, and can be set individually or on a
 system wide basis using <code>xmlBufferSetAllocationScheme()</code>. A number
 of functions allows to manipulate buffers with names starting with the
 <code>xmlBuffer...</code> prefix.</p>

 <h2><a name="Input">Input I/O handlers</a></h2>

 <p>An Input I/O handler is a simple structure
 <code>xmlParserInputBuffer</code> containing a context associated to the
 resource (file descriptor, or pointer to a protocol handler), the read() and
 close() callbacks to use and an xmlBuffer. And extra xmlBuffer and a charset
 encoding handler are also present to support charset conversion when
 needed.</p>

 <h2><a name="Output">Output I/O handlers</a></h2>

 <p>An Output handler <code>xmlOutputBuffer</code> is completely similar to an
 Input one except the callbacks are write() and close().</p>

 <h2><a name="entities">The entities loader</a></h2>

 <p>The entity loader resolves requests for new entities and create inputs for
 the parser. Creating an input from a filename or an URI string is done through
 the xmlNewInputFromFile() routine.  The default entity loader do not handle
 the PUBLIC identifier associated with an entity (if any). So it just calls
 xmlNewInputFromFile() with the SYSTEM identifier (which is mandatory in
 XML).</p>

 <p>If you want to hook up a catalog mechanism then you simply need to override
 the default entity loader, here is an example:</p>
 <pre>#include &lt;libxml/xmlIO.h&gt;

 xmlExternalEntityLoader defaultLoader = NULL;

 xmlParserInputPtr
 xmlMyExternalEntityLoader(const char *URL, const char *ID,
                                xmlParserCtxtPtr ctxt) {
     xmlParserInputPtr ret;
     const char *fileID = NULL;
     /* lookup for the fileID depending on ID */

     ret = xmlNewInputFromFile(ctxt, fileID);
     if (ret != NULL)
         return(ret);
     if (defaultLoader != NULL)
         ret = defaultLoader(URL, ID, ctxt);
     return(ret);
 }

 int main(..) {
     ...

     /*
      * Install our own entity loader
      */
     defaultLoader = xmlGetExternalEntityLoader();
     xmlSetExternalEntityLoader(xmlMyExternalEntityLoader);

     ...
 }</pre>

 <h2><a name="Example">Example of customized I/O</a></h2>

 <p>This example come from <a href="http://xmlsoft.org/messages/0708.html">a
 real use case</a>,  xmlDocDump() closes the FILE * passed by the application
 and this was a problem. The <a
 href="http://xmlsoft.org/messages/0711.html">solution</a> was to redefine a
 new output handler with the closing call deactivated:</p>
 <ol>
   <li>First define a new I/O ouput allocator where the output don't close the
     file:
     <pre>xmlOutputBufferPtr
 xmlOutputBufferCreateOwn(FILE *file, xmlCharEncodingHandlerPtr encoder) {
     xmlOutputBufferPtr ret;

     if (xmlOutputCallbackInitialized == 0)
         xmlRegisterDefaultOutputCallbacks();

     if (file == NULL) return(NULL);
     ret = xmlAllocOutputBuffer(encoder);
     if (ret != NULL) {
         ret-&gt;context = file;
         ret-&gt;writecallback = xmlFileWrite;
         ret-&gt;closecallback = NULL;  /* No close callback */
     }
     return(ret); <br>


 } </pre>
   </li>
   <li>And then use it to save the document:
     <pre>FILE *f;
 xmlOutputBufferPtr output;
 xmlDocPtr doc;
 int res;

 f = ...
 doc = ....

 output = xmlOutputBufferCreateOwn(f, NULL);
 res = xmlSaveFileTo(output, doc, NULL);
     </pre>
   </li>
 </ol>

 <p><a href="mailto:daniel@veillard.com">Daniel Veillard</a></p>

 <p>$Id: xmlio.html,v 1.4 2001/01/29 08:22:12 veillard Exp $</p>
 </body>
 </html>
	<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN"
	"http://www.w3.org/TR/REC-html40/loose.dtd">
	<html>
	<head>
	<title>Libxml Input/Output handling</title>
	<meta name="GENERATOR" content="amaya V3.2.1">
	<meta http-equiv="Content-Type" content="text/html">
	</head>

	<body bgcolor="#ffffff">
	<h1 align="center">Libxml Input/Output handling</h1>

	<p>Location: <a
	href="http://xmlsoft.org/xmlio.html">http://xmlsoft.org/xmlio.html</a></p>

	<p>Libxml home page: <a href="http://xmlsoft.org/">http://xmlsoft.org/</a></p>

	<p>Mailing-list archive: <a
	href="http://xmlsoft.org/messages/">http://xmlsoft.org/messages/</a></p>

	<p>Version: $Revision: 1.4 $</p>

	<p>Table of Content:</p>
	<ol>
	<li><a href="#General">General overview</a></li>
	<li><a href="#basic">The basic buffer type</a></li>
	<li><a href="#Input">Input I/O handlers</a></li>
	<li><a href="#Output">Output I/O handlers</a></li>
	<li><a href="#entities">The entities loader</a></li>
	<li><a href="#Example">Example of customized I/O</a></li>
	</ol>

	<h2><a name="General">General overview</a></h2>

	<p>The module <code><a
	href="http://xmlsoft.org/html/libxml-xmlio.html">xmlIO.h</a></code>
	provides the interfaces to the libxml I/O system. This consists of 4 main
	parts:</p>
	<ul>
	<li>Entities loader, this is a routine which tries to fetch the entities
	(files) based on their PUBLIC and SYSTEM identifiers. The default loader
	don't look at the public identifier since libxml do not maintain a
	catalog. You can redefine you own entity loader by using
	<code>xmlGetExternalEntityLoader()</code> and
	<code>xmlSetExternalEntityLoader()</code>. <a href="#entities">Check the
	example</a>.</li>
	<li>Input I/O buffers which are a commodity structure used by the parser(s)
	input layer to handle fetching the informations to feed the parser. This
	provides buffering and is also a placeholder where the encoding convertors
	to UTF8 are piggy-backed.</li>
	<li>Output I/O buffers are similar to the Input ones and fulfill similar
	task but when generating a serialization from a tree.</li>
	<li>A mechanism to register sets of I/O callbacks and associate them with
	specific naming schemes like the protocol part of the URIs.
	<p>This affect the default I/O operations and allows to use specific I/O
	handlers for certain names.</p>
	</li>
	</ul>

	<p>The general mechanism used when loading http://rpmfind.net/xml.html for
	example in the HTML parser is the following:</p>
	<ol>
	<li>The default entity loader calls <code>xmlNewInputFromFile()</code> with
	the parsing context and the URI string.</li>
	<li>the URI string is checked against the existing registered handlers using
	their match() callback function, if the HTTP module was compiled in, it is
	registered and its match() function will succeeds</li>
	<li>the open() function of the handler is called and if successful will
	return an I/O Input buffer</li>
	<li>the parser will the start reading from this buffer and progressively
	fetch information from the resource, calling the read() function of the
	handler until the resource is exhausted</li>
	<li>if an encoding change is detected it will be installed on the input
	buffer, providing buffering and efficient use of the conversion
	routines</li>
	<li>once the parser has finished, the close() function of the handler is
	called once and the Input buffer and associed resources are
	deallocated.</li>
	</ol>

	<p>The user defined callbacks are checked first to allow overriding of the
	default libxml I/O routines.</p>

	<h2><a name="basic">The basic buffer type</a></h2>

	<p>All the buffer manipulation handling is done using the
	<code>xmlBuffer</code> type define in <code><a
	href="http://xmlsoft.org/html/libxml-tree.html">tree.h</a> </code>which is
	a resizable memory buffer. The buffer allocation strategy can be selected to
	be either best-fit or use an exponential doubling one (CPU vs. memory use
	tradeoff). The values are <code>XML_BUFFER_ALLOC_EXACT</code> and
	<code>XML_BUFFER_ALLOC_DOUBLEIT</code>, and can be set individually or on a
	system wide basis using <code>xmlBufferSetAllocationScheme()</code>. A number
	of functions allows to manipulate buffers with names starting with the
	<code>xmlBuffer...</code> prefix.</p>

	<h2><a name="Input">Input I/O handlers</a></h2>

	<p>An Input I/O handler is a simple structure
	<code>xmlParserInputBuffer</code> containing a context associated to the
	resource (file descriptor, or pointer to a protocol handler), the read() and
	close() callbacks to use and an xmlBuffer. And extra xmlBuffer and a charset
	encoding handler are also present to support charset conversion when
	needed.</p>

	<h2><a name="Output">Output I/O handlers</a></h2>

	<p>An Output handler <code>xmlOutputBuffer</code> is completely similar to an
	Input one except the callbacks are write() and close().</p>

	<h2><a name="entities">The entities loader</a></h2>

	<p>The entity loader resolves requests for new entities and create inputs for
	the parser. Creating an input from a filename or an URI string is done through
	the xmlNewInputFromFile() routine. The default entity loader do not handle
	the PUBLIC identifier associated with an entity (if any). So it just calls
	xmlNewInputFromFile() with the SYSTEM identifier (which is mandatory in
	XML).</p>

	<p>If you want to hook up a catalog mechanism then you simply need to override
	the default entity loader, here is an example:</p>
	<pre>#include <libxml/xmlIO.h>

	xmlExternalEntityLoader defaultLoader = NULL;

	xmlParserInputPtr
	xmlMyExternalEntityLoader(const char URL, const char ID,
	xmlParserCtxtPtr ctxt) {
	xmlParserInputPtr ret;
	const char *fileID = NULL;
	/* lookup for the fileID depending on ID */

	ret = xmlNewInputFromFile(ctxt, fileID);
	if (ret != NULL)
	return(ret);
	if (defaultLoader != NULL)
	ret = defaultLoader(URL, ID, ctxt);
	return(ret);
	}

	int main(..) {
	...

	/*
	* Install our own entity loader
	*/
	defaultLoader = xmlGetExternalEntityLoader();
	xmlSetExternalEntityLoader(xmlMyExternalEntityLoader);

	...
	}</pre>

	<h2><a name="Example">Example of customized I/O</a></h2>

	<p>This example come from <a href="http://xmlsoft.org/messages/0708.html">a
	real use case</a>, xmlDocDump() closes the FILE * passed by the application
	and this was a problem. The <a
	href="http://xmlsoft.org/messages/0711.html">solution</a> was to redefine a
	new output handler with the closing call deactivated:</p>
	<ol>
	<li>First define a new I/O ouput allocator where the output don't close the
	file:
	<pre>xmlOutputBufferPtr
	xmlOutputBufferCreateOwn(FILE *file, xmlCharEncodingHandlerPtr encoder) {
	xmlOutputBufferPtr ret;

	if (xmlOutputCallbackInitialized == 0)
	xmlRegisterDefaultOutputCallbacks();

	if (file == NULL) return(NULL);
	ret = xmlAllocOutputBuffer(encoder);
	if (ret != NULL) {
	ret->context = file;
	ret->writecallback = xmlFileWrite;
	ret->closecallback = NULL; /* No close callback */
	}
	return(ret); <br>


	} </pre>
	</li>
	<li>And then use it to save the document:
	<pre>FILE *f;
	xmlOutputBufferPtr output;
	xmlDocPtr doc;
	int res;

	f = ...
	doc = ....

	output = xmlOutputBufferCreateOwn(f, NULL);
	res = xmlSaveFileTo(output, doc, NULL);
	</pre>
	</li>
	</ol>

	<p><a href="mailto:daniel@veillard.com">Daniel Veillard</a></p>

	<p>$Id: xmlio.html,v 1.4 2001/01/29 08:22:12 veillard Exp $</p>
	</body>
	</html>