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   10 </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="http://libxmlplusplus.sourceforge.net/">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="http://bugzilla.gnome.org/buglist.cgi?product=libxml2">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>
   11   <li><a href="#General1">General overview</a></li>
   12   <li><a href="#basic">The basic buffer type</a></li>
   13   <li><a href="#Input">Input I/O handlers</a></li>
   14   <li><a href="#Output">Output I/O handlers</a></li>
   15   <li><a href="#entities">The entities loader</a></li>
   16   <li><a href="#Example2">Example of customized I/O</a></li>
   17 </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
   18 the interfaces to the libxml2 I/O system. This consists of 4 main parts:</p><ul>
   19   <li>Entities loader, this is a routine which tries to fetch the entities
   20     (files) based on their PUBLIC and SYSTEM identifiers. The default loader
   21     don't look at the public identifier since libxml2 do not maintain a
   22     catalog. You can redefine you own entity loader by using
   23     <code>xmlGetExternalEntityLoader()</code> and
   24     <code>xmlSetExternalEntityLoader()</code>. <a href="#entities">Check the
   25     example</a>.</li>
   26   <li>Input I/O buffers which are a commodity structure used by the parser(s)
   27     input layer to handle fetching the information to feed the parser. This
   28     provides buffering and is also a placeholder where the encoding
   29     converters to UTF8 are piggy-backed.</li>
   30   <li>Output I/O buffers are similar to the Input ones and fulfill similar
   31     task but when generating a serialization from a tree.</li>
   32   <li>A mechanism to register sets of I/O callbacks and associate them with
   33     specific naming schemes like the protocol part of the URIs.
   34     <p>This affect the default I/O operations and allows to use specific I/O
   35     handlers for certain names.</p>
   36   </li>
   37 </ul><p>The general mechanism used when loading http://rpmfind.net/xml.html for
   38 example in the HTML parser is the following:</p><ol>
   39   <li>The default entity loader calls <code>xmlNewInputFromFile()</code> with
   40     the parsing context and the URI string.</li>
   41   <li>the URI string is checked against the existing registered handlers
   42     using their match() callback function, if the HTTP module was compiled
   43     in, it is registered and its match() function will succeeds</li>
   44   <li>the open() function of the handler is called and if successful will
   45     return an I/O Input buffer</li>
   46   <li>the parser will the start reading from this buffer and progressively
   47     fetch information from the resource, calling the read() function of the
   48     handler until the resource is exhausted</li>
   49   <li>if an encoding change is detected it will be installed on the input
   50     buffer, providing buffering and efficient use of the conversion
   51   routines</li>
   52   <li>once the parser has finished, the close() function of the handler is
   53     called once and the Input buffer and associated resources are
   54   deallocated.</li>
   55 </ol><p>The user defined callbacks are checked first to allow overriding of the
   56 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
   57 <code>xmlBuffer</code> type define in <code><a href="http://xmlsoft.org/html/libxml-tree.html">tree.h</a> </code>which is a
   58 resizable memory buffer. The buffer allocation strategy can be selected to be
   59 either best-fit or use an exponential doubling one (CPU vs. memory use
   60 trade-off). The values are <code>XML_BUFFER_ALLOC_EXACT</code> and
   61 <code>XML_BUFFER_ALLOC_DOUBLEIT</code>, and can be set individually or on a
   62 system wide basis using <code>xmlBufferSetAllocationScheme()</code>. A number
   63 of functions allows to manipulate buffers with names starting with the
   64 <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
   65 <code>xmlParserInputBuffer</code> containing a context associated to the
   66 resource (file descriptor, or pointer to a protocol handler), the read() and
   67 close() callbacks to use and an xmlBuffer. And extra xmlBuffer and a charset
   68 encoding handler are also present to support charset conversion when
   69 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
   70 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
   71 the parser. Creating an input from a filename or an URI string is done
   72 through the xmlNewInputFromFile() routine.  The default entity loader do not
   73 handle the PUBLIC identifier associated with an entity (if any). So it just
   74 calls xmlNewInputFromFile() with the SYSTEM identifier (which is mandatory in
   75 XML).</p><p>If you want to hook up a catalog mechanism then you simply need to
   76 override the default entity loader, here is an example:</p><pre>#include &lt;libxml/xmlIO.h&gt;
   78 xmlExternalEntityLoader defaultLoader = NULL;
   80 xmlParserInputPtr
   81 xmlMyExternalEntityLoader(const char *URL, const char *ID,
   82                                xmlParserCtxtPtr ctxt) {
   83     xmlParserInputPtr ret;
   84     const char *fileID = NULL;
   85     /* lookup for the fileID depending on ID */
   87     ret = xmlNewInputFromFile(ctxt, fileID);
   88     if (ret != NULL)
   89         return(ret);
   90     if (defaultLoader != NULL)
   91         ret = defaultLoader(URL, ID, ctxt);
   92     return(ret);
   93 }
   95 int main(..) {
   96     ...
   98     /*
   99      * Install our own entity loader
  100      */
  101     defaultLoader = xmlGetExternalEntityLoader();
  102     xmlSetExternalEntityLoader(xmlMyExternalEntityLoader);
  104     ...
  105 }</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
  106 real use case</a>,  xmlDocDump() closes the FILE * passed by the application
  107 and this was a problem. The <a href="http://xmlsoft.org/messages/0711.html">solution</a> was to redefine a
  108 new output handler with the closing call deactivated:</p><ol>
  109   <li>First define a new I/O output allocator where the output don't close
  110     the file:
  111     <pre>xmlOutputBufferPtr
  112 xmlOutputBufferCreateOwn(FILE *file, xmlCharEncodingHandlerPtr encoder) {
  113     xmlOutputBufferPtr ret;
  115     if (xmlOutputCallbackInitialized == 0)
  116         xmlRegisterDefaultOutputCallbacks();
  118     if (file == NULL) return(NULL);
  119     ret = xmlAllocOutputBuffer(encoder);
  120     if (ret != NULL) {
  121         ret-&gt;context = file;
  122         ret-&gt;writecallback = xmlFileWrite;
  123         ret-&gt;closecallback = NULL;  /* No close callback */
  124     }
  125     return(ret);
  126 } </pre>
  127   </li>
  128   <li>And then use it to save the document:
  129     <pre>FILE *f;
  130 xmlOutputBufferPtr output;
  131 xmlDocPtr doc;
  132 int res;
  134 f = ...
  135 doc = ....
  137 output = xmlOutputBufferCreateOwn(f, NULL);
  138 res = xmlSaveFileTo(output, doc, NULL);
  139     </pre>
  140   </li>
  141 </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>