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align=“center”><h1>The XML C parser and toolkit of
Gnome</h1><h2>I/O
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bgcolor=“#fffacd”><ul><li><a href=“Mail”>mail.gnome.org/archives/xml/“>Mail
archive</a></li><li><a href=”XSLT“>xmlsoft.org/XSLT/”>XSLT
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gdome2</a></li><li><a href=”XML-DSig“>www.aleksey.com/xmlsec/”>XML-DSig
xmlsec</a></li><li><a href=“FTP
<li><a href="#General1">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="#Example2">Example of customized I/O</a></li>
</ol><h3><a name=“General1” id=“General1”>General
overview</a></h3><p>The module <a
href="http://xmlsoft.org/html/libxml-xmlio.html">xmlIO.h
provides the interfaces to the libxml2 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 libxml2 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 information to feed the parser. This provides buffering and is also a placeholder where the encoding converters 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 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 associated resources are deallocated.</li>
</ol><p>The user defined callbacks are checked first to allow
overriding of the default libxml2 I/O routines.</p><h3><a
name=“basic” id=“basic”>The basic buffer
type</a></h3><p>All the buffer manipulation handling is
done using the xmlBuffer
type define in <a
href="http://xmlsoft.org/html/libxml-tree.html">tree.hwhich 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 trade-off). The values are
XML_BUFFER_ALLOC_EXACT
and
XML_BUFFER_ALLOC_DOUBLEIT
, and can be set individually or on a
system wide basis using xmlBufferSetAllocationScheme()
. A
number of functions allows to manipulate buffers with names starting with
the xmlBuffer...
prefix.</p><h3><a name=“Input”
id=“Input”>Input I/O handlers</a></h3><p>An Input I/O
handler is a simple structure xmlParserInputBuffer
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><h3><a
name=“Output” id=“Output”>Output I/O
handlers</a></h3><p>An Output handler
xmlOutputBuffer
is completely similar to an Input one except
the callbacks are write() and close().</p><h3><a
name=“entities” id=“entities”>The entities
loader</a></h3><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><h3><a name=“Example2” id=“Example2”>Example of customized I/O</a></h3><p>This example come from <a href=“a”>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=”solutionxmlsoft.org/messages/0711.html“>solution> was to redefine a new output handler with the closing call deactivated:</p><ol>
<li>First define a new I/O output 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); } </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=“bugs.html”>Daniel Veillard</a></p></td></tr></table></td></tr></table></td></tr></table></td></tr></table></td></tr></table></body></html>