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<title>I/O Interfaces</title>
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<h1>The XML C library for Gnome</h1>
<h2>I/O Interfaces</h2>
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<p>Table of Content:</p>
<ol>
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<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>
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</ol>
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<h3><a name="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 libxml I/O system. This consists of 4 main parts:</p>
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<ul>
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<li>Entities loader, this is a routine which tries to fetch the entities
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    (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
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    <code>xmlSetExternalEntityLoader()</code>. <a href="#entities">Check the
    example</a>.</li>
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<li>Input I/O buffers which are a commodity structure used by the parser(s)
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    input layer to handle fetching the informations to feed the parser. This
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    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
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    task but when generating a serialization from a tree.</li>
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<li>A mechanism to register sets of I/O callbacks and associate them with
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    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>
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</li>
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</ul>
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<p>The general mechanism used when loading http://rpmfind.net/xml.html for
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example in the HTML parser is the following:</p>
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<li>The default entity loader calls <code>xmlNewInputFromFile()</code> with
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    the parsing context and the URI string.</li>
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<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
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    return an I/O Input buffer</li>
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<li>the parser will the start reading from this buffer and progressively
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    fetch information from the resource, calling the read() function of the
    handler until the resource is exhausted</li>
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<li>if an encoding change is detected it will be installed on the input
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    buffer, providing buffering and efficient use of the conversion
  routines</li>
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<li>once the parser has finished, the close() function of the handler is
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    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>
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<h3><a name="basic">The basic buffer type</a></h3>
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<p>All the buffer manipulation handling is done using the
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<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
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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>
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<h3><a name="Input">Input I/O handlers</a></h3>
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<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>
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<h3><a name="Output">Output I/O handlers</a></h3>
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<p>An Output handler <code>xmlOutputBuffer</code> is completely similar to an
Input one except the callbacks are write() and close().</p>
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<h3><a name="entities">The entities loader</a></h3>
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<p>The entity loader resolves requests for new entities and create inputs for
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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
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XML).</p>
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<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>
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<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>
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<h3><a name="Example2">Example of customized I/O</a></h3>
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<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
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and this was a problem. The <a href="http://xmlsoft.org/messages/0711.html">solution</a> was to redefine a
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new output handler with the closing call deactivated:</p>
<ol>
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<li>First define a new I/O ouput allocator where the output don't close the
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    file:
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    <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>
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} </pre>
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</li>
<li>And then use it to save the document:
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    <pre>FILE *f;
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xmlOutputBufferPtr output;
xmlDocPtr doc;
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int res;
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f = ...
doc = ....
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output = xmlOutputBufferCreateOwn(f, NULL);
res = xmlSaveFileTo(output, doc, NULL);
    </pre>
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</li>
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</ol>
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<p><a href="mailto:daniel@veillard.com">Daniel Veillard</a></p>
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