Commit 96984456 authored by Daniel Veillard's avatar Daniel Veillard
Browse files

Added a doc on the I/O interfaces, Daniel

parent f121ab72
Thu Aug 31 14:59:28 CEST 2000 Daniel Veillard <>
* doc/xmlio.html doc/xml.html: added a doc on the I/O mechanism
used by libxml
Tue Aug 29 20:22:53 CEST 2000 Daniel Veillard <>
* parser.c: Fixed bug on invalid ontent characters and when using
......@@ -42,7 +42,15 @@ alt="W3C Logo"></a></p>
<li><a href="#Validation">Validation</a></li>
<li><a href="#Principles">DOM principles</a></li>
<li><a href="#real">A real example</a></li>
<li><a href="#Contributi">Contribution</a></li>
<li><a href="#Contributi">Contributions</a></li>
<p>Separate documents:</p>
<li><a href="upgrade.html">upgrade instructions for migrating to
<li><a href="encoding.html">libxml Internationalization support</a></li>
<li><a href="xmlio.html">libxml Input/Output interfaces</a></li>
<h2><a name="Introducti">Introduction</a></h2>
......@@ -1264,6 +1272,6 @@ Gnome CVS base under gnome-xml/example</p>
<p><a href="">Daniel Veillard</a></p>
<p>$Id: xml.html,v 1.49 2000/08/25 17:14:13 veillard Exp $</p>
<p>$Id: xml.html,v 1.50 2000/08/29 23:40:42 veillard Exp $</p>
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN"
<title>Libxml Input/Output handling</title>
<meta name="GENERATOR" content="amaya V3.2.1">
<meta http-equiv="Content-Type" content="text/html">
<body bgcolor="#ffffff">
<h1 align="center">Libxml Input/Output handling</h1>
<p>Location: <a
<p>Libxml home page: <a href=""></a></p>
<p>Mailing-list archive: <a
<p>Version: $Revision:$</p>
<p>Table of Content:</p>
<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="#Example">Example of customized I/O</a></li>
<h2><a name="General">General overview</a></h2>
<p>The module <code><a
provides the interfaces to the libxml I/O system. This consists of 3 main
<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>
<p> The general mechanism used when loading for
example in the HTML parser is the following:</p>
<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 macth() function will succeed</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 associed resources are
<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="">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
<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="Example">Example of customized I/O</a></h2>
<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>
<li>First define a new I/O ouput allocator where the output don't close the
<pre>xmlOutputBufferPtr <br>
xmlOutputBufferCreateOwn(FILE *file, xmlCharEncodingHandlerPtr encoder) { <br>
    xmlOutputBufferPtr ret; <br>
    if (xmlOutputCallbackInitialized == 0) <br>
        xmlRegisterDefaultOutputCallbacks(); <br>
    if (file == NULL) return(NULL); <br>
<pre>    ret = xmlAllocOutputBuffer(encoder); <br>
    if (ret != NULL) { <br>
        ret-&gt;context = file; <br>
        ret-&gt;writecallback = xmlFileWrite; <br>
        ret-&gt;closecallback = NULL; /* No close callback */ <br>
    } <br>
<pre>    return(ret); <br>
} </pre>
<li>And then use it to save the document:
<pre>FILE *f;
xmlOutputBufferPtr output;</pre>
<pre>xmlDocPtr doc;
int res;
f = ...
doc = ....
output = xmlOutputBufferCreateOwn(f, NULL);
res = xmlSaveFileTo(output, doc, NULL);
<p><a href="">Daniel Veillard</a></p>
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