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</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=""><img src="epatents.png" alt="Action against software patents" /></a></td><td width="180"><a href=""><img src="gnome2.png" alt="Gnome2 Logo" /></a><a href=""><img src="w3c.png" alt="W3C Logo" /></a><a href=""><img src="redhat.gif" alt="Red Hat Logo" /></a><div align="left"><a href=""><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="">Mail archive</a></li><li><a href="">XSLT libxslt</a></li><li><a href="">DOM gdome2</a></li><li><a href="">XML-DSig xmlsec</a></li><li><a href="">FTP</a></li><li><a href="">Windows binaries</a></li><li><a href="">Solaris binaries</a></li><li><a href="">MacOsX binaries</a></li><li><a href="">lxml Python bindings</a></li><li><a href="">Perl bindings</a></li><li><a href="">C++ bindings</a></li><li><a href="">PHP bindings</a></li><li><a href="">Pascal bindings</a></li><li><a href="">Ruby bindings</a></li><li><a href="">Tcl bindings</a></li><li><a href="">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="">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
<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>
</ul><p>The general mechanism used when loading 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
<li>once the parser has finished, the close() function of the handler is
called once and the Input buffer and associated resources are
</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="">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 &lt;libxml/xmlIO.h&gt;
xmlExternalEntityLoader defaultLoader = NULL;
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)
if (defaultLoader != NULL)
ret = defaultLoader(URL, ID, ctxt);
int main(..) {
* Install our own entity loader
defaultLoader = xmlGetExternalEntityLoader();
}</pre><h3><a name="Example2" id="Example2">Example of customized I/O</a></h3><p>This example come from <a href="">a
real use case</a>, xmlDocDump() closes the FILE * passed by the application
and this was a problem. The <a href="">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:
xmlOutputBufferCreateOwn(FILE *file, xmlCharEncodingHandlerPtr encoder) {
    xmlOutputBufferPtr ret;
    if (xmlOutputCallbackInitialized == 0)
    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 */
} </pre>
<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);
</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>