142 lines
12 KiB
HTML
142 lines
12 KiB
HTML
<?xml version="1.0" encoding="UTF-8"?>
|
||
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
|
||
<html xmlns="http://www.w3.org/1999/xhtml"><head><meta http-equiv="Content-Type" content="text/html; charset=UTF-8" /><link rel="SHORTCUT ICON" href="/favicon.ico" /><style type="text/css">
|
||
TD {font-family: Verdana,Arial,Helvetica}
|
||
BODY {font-family: Verdana,Arial,Helvetica; margin-top: 2em; margin-left: 0em; margin-right: 0em}
|
||
H1 {font-family: Verdana,Arial,Helvetica}
|
||
H2 {font-family: Verdana,Arial,Helvetica}
|
||
H3 {font-family: Verdana,Arial,Helvetica}
|
||
A:link, A:visited, A:active { text-decoration: underline }
|
||
</style><title>I/O Interfaces</title></head><body bgcolor="#8b7765" text="#000000" link="#a06060" vlink="#000000"><table border="0" width="100%" cellpadding="5" cellspacing="0" align="center"><tr><td width="120"><a href="http://swpat.ffii.org/"><img src="epatents.png" alt="Action against software patents" /></a></td><td width="180"><a href="http://www.gnome.org/"><img src="gnome2.png" alt="Gnome2 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><div align="left"><a href="http://xmlsoft.org/"><img src="Libxml2-Logo-180x168.gif" alt="Made with Libxml2 Logo" /></a></div></td><td><table border="0" width="90%" cellpadding="2" cellspacing="0" align="center" bgcolor="#000000"><tr><td><table width="100%" border="0" cellspacing="1" cellpadding="3" bgcolor="#fffacd"><tr><td align="center"><h1>The XML C parser and toolkit of Gnome</h1><h2>I/O Interfaces</h2></td></tr></table></td></tr></table></td></tr></table><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><td valign="top" width="200" bgcolor="#8b7765"><table border="0" cellspacing="0" cellpadding="1" width="100%" bgcolor="#000000"><tr><td><table width="100%" border="0" cellspacing="1" cellpadding="3"><tr><td colspan="1" bgcolor="#eecfa1" align="center"><center><b>Developer Menu</b></center></td></tr><tr><td bgcolor="#fffacd"><form action="search.php" enctype="application/x-www-form-urlencoded" method="get"><input name="query" type="text" size="20" value="" /><input name="submit" type="submit" value="Search ..." /></form><ul><li><a href="index.html" style="font-weight:bold">Main Menu</a></li><li><a href="html/index.html" style="font-weight:bold">Reference Manual</a></li><li><a href="examples/index.html" style="font-weight:bold">Code Examples</a></li><li><a href="guidelines.html">XML Guidelines</a></li><li><a href="tutorial/index.html">Tutorial</a></li><li><a href="xmlreader.html">The Reader Interface</a></li><li><a href="ChangeLog.html">ChangeLog</a></li><li><a href="XSLT.html">XSLT</a></li><li><a href="python.html">Python and bindings</a></li><li><a href="architecture.html">libxml2 architecture</a></li><li><a href="tree.html">The tree output</a></li><li><a href="interface.html">The SAX interface</a></li><li><a href="xmlmem.html">Memory Management</a></li><li><a href="xmlio.html">I/O Interfaces</a></li><li><a href="library.html">The parser interfaces</a></li><li><a href="entities.html">Entities or no entities</a></li><li><a href="namespaces.html">Namespaces</a></li><li><a href="upgrade.html">Upgrading 1.x code</a></li><li><a href="threads.html">Thread safety</a></li><li><a href="DOM.html">DOM Principles</a></li><li><a href="example.html">A real example</a></li><li><a href="xml.html">flat page</a>, <a href="site.xsl">stylesheet</a></li></ul></td></tr></table><table width="100%" border="0" cellspacing="1" cellpadding="3"><tr><td colspan="1" bgcolor="#eecfa1" align="center"><center><b>API Indexes</b></center></td></tr><tr><td bgcolor="#fffacd"><ul><li><a href="APIchunk0.html">Alphabetic</a></li><li><a href="APIconstructors.html">Constructors</a></li><li><a href="APIfunctions.html">Functions/Types</a></li><li><a href="APIfiles.html">Modules</a></li><li><a href="APIsymbols.html">Symbols</a></li></ul></td></tr></table><table width="100%" border="0" cellspacing="1" cellpadding="3"><tr><td colspan="1" bgcolor="#eecfa1" align="center"><center><b>Related links</b></center></td></tr><tr><td bgcolor="#fffacd"><ul><li><a href="http://mail.gnome.org/archives/xml/">Mail archive</a></li><li><a href="http://xmlsoft.org/XSLT/">XSLT libxslt</a></li><li><a href="http://phd.cs.unibo.it/gdome2/">DOM gdome2</a></li><li><a href="http://www.aleksey.com/xmlsec/">XML-DSig xmlsec</a></li><li><a href="ftp://xmlsoft.org/">FTP</a></li><li><a href="http://www.zlatkovic.com/projects/libxml/">Windows binaries</a></li><li><a href="http://opencsw.org/packages/libxml2">Solaris binaries</a></li><li><a href="http://www.explain.com.au/oss/libxml2xslt.html">MacOsX binaries</a></li><li><a href="http://lxml.de/">lxml Python bindings</a></li><li><a href="http://cpan.uwinnipeg.ca/dist/XML-LibXML">Perl bindings</a></li><li><a href="https://libxmlplusplus.github.io/libxmlplusplus/">C++ bindings</a></li><li><a href="http://www.zend.com/php5/articles/php5-xmlphp.php#Heading4">PHP bindings</a></li><li><a href="http://sourceforge.net/projects/libxml2-pas/">Pascal bindings</a></li><li><a href="http://libxml.rubyforge.org/">Ruby bindings</a></li><li><a href="http://tclxml.sourceforge.net/">Tcl bindings</a></li><li><a href="https://gitlab.gnome.org/GNOME/libxml2/issues">Bug Tracker</a></li></ul></td></tr></table></td></tr></table></td><td valign="top" bgcolor="#8b7765"><table border="0" cellspacing="0" cellpadding="1" width="100%"><tr><td><table border="0" cellspacing="0" cellpadding="1" width="100%" bgcolor="#000000"><tr><td><table border="0" cellpadding="3" cellspacing="1" width="100%"><tr><td bgcolor="#fffacd"><p>Table of Content:</p><ol>
|
||
<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 <code><a href="http://xmlsoft.org/html/libxml-xmlio.html">xmlIO.h</a></code> 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 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 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
|
||
<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
|
||
trade-off). 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><h3><a name="Input" id="Input">Input I/O handlers</a></h3><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><h3><a name="Output" id="Output">Output I/O handlers</a></h3><p>An Output handler <code>xmlOutputBuffer</code> 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="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 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>
|