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Daniel Veillard1177ca42003-04-26 22:29:54 +0000[diff] [blame]1<?xml version="1.0" encoding="ISO-8859-1"?>
2<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
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Daniel Veillard373a4752002-02-21 14:46:29 +0000[diff] [blame]4TD {font-family: Verdana,Arial,Helvetica}
5BODY {font-family: Verdana,Arial,Helvetica; margin-top: 2em; margin-left: 0em; margin-right: 0em}
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Daniel Veillard6c57dca2003-09-03 10:10:51 +0000[diff] [blame]10</style><title>I/O Interfaces</title></head><body bgcolor="#8b7765" text="#000000" link="#000000" 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="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="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://garypennington.net/libxml2/">Solaris binaries</a></li><li><a href="http://www.zveno.com/open_source/libxml2xslt.html">MacOsX binaries</a></li><li><a href="http://sourceforge.net/projects/libxml2-pas/">Pascal 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><li><a href="#General1">General overview</a></li>
Daniel Veillard0b28e882002-07-24 23:47:05 +0000[diff] [blame]11 <li><a href="#basic">The basic buffer type</a></li>
12 <li><a href="#Input">Input I/O handlers</a></li>
13 <li><a href="#Output">Output I/O handlers</a></li>
14 <li><a href="#entities">The entities loader</a></li>
15 <li><a href="#Example2">Example of customized I/O</a></li>
Daniel Veillard1177ca42003-04-26 22:29:54 +0000[diff] [blame]16</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
Daniel Veillard8a469172003-06-12 16:05:07 +0000[diff] [blame]17the 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
Daniel Veillard2bb89092000-08-31 14:57:50 +0000[diff] [blame]18 (files) based on their PUBLIC and SYSTEM identifiers. The default loader
Daniel Veillard8a469172003-06-12 16:05:07 +0000[diff] [blame]19 don't look at the public identifier since libxml2 do not maintain a
Daniel Veillard2bb89092000-08-31 14:57:50 +0000[diff] [blame]20 catalog. You can redefine you own entity loader by using
21 <code>xmlGetExternalEntityLoader()</code> and
Daniel Veillard9c466822001-10-25 12:03:39 +0000[diff] [blame]22 <code>xmlSetExternalEntityLoader()</code>. <a href="#entities">Check the
23 example</a>.</li>
Daniel Veillard0b28e882002-07-24 23:47:05 +0000[diff] [blame]24 <li>Input I/O buffers which are a commodity structure used by the parser(s)
Daniel Veillard96984452000-08-31 13:50:12 +0000[diff] [blame]25 input layer to handle fetching the informations to feed the parser. This
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame]26 provides buffering and is also a placeholder where the encoding
Daniel Veillard63d83142002-05-20 06:51:05 +0000[diff] [blame]27 converters to UTF8 are piggy-backed.</li>
Daniel Veillard0b28e882002-07-24 23:47:05 +0000[diff] [blame]28 <li>Output I/O buffers are similar to the Input ones and fulfill similar
Daniel Veillard96984452000-08-31 13:50:12 +0000[diff] [blame]29 task but when generating a serialization from a tree.</li>
Daniel Veillard0b28e882002-07-24 23:47:05 +0000[diff] [blame]30 <li>A mechanism to register sets of I/O callbacks and associate them with
Daniel Veillard96984452000-08-31 13:50:12 +0000[diff] [blame]31 specific naming schemes like the protocol part of the URIs.
32 <p>This affect the default I/O operations and allows to use specific I/O
33 handlers for certain names.</p>
Daniel Veillard0b28e882002-07-24 23:47:05 +0000[diff] [blame]34 </li>
Daniel Veillard1177ca42003-04-26 22:29:54 +0000[diff] [blame]35</ul><p>The general mechanism used when loading http://rpmfind.net/xml.html for
36example in the HTML parser is the following:</p><ol><li>The default entity loader calls <code>xmlNewInputFromFile()</code> with
Daniel Veillard2bb89092000-08-31 14:57:50 +0000[diff] [blame]37 the parsing context and the URI string.</li>
Daniel Veillard0b28e882002-07-24 23:47:05 +0000[diff] [blame]38 <li>the URI string is checked against the existing registered handlers
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame]39 using their match() callback function, if the HTTP module was compiled
40 in, it is registered and its match() function will succeeds</li>
Daniel Veillard0b28e882002-07-24 23:47:05 +0000[diff] [blame]41 <li>the open() function of the handler is called and if successful will
Daniel Veillard96984452000-08-31 13:50:12 +0000[diff] [blame]42 return an I/O Input buffer</li>
Daniel Veillard0b28e882002-07-24 23:47:05 +0000[diff] [blame]43 <li>the parser will the start reading from this buffer and progressively
Daniel Veillard96984452000-08-31 13:50:12 +0000[diff] [blame]44 fetch information from the resource, calling the read() function of the
45 handler until the resource is exhausted</li>
Daniel Veillard0b28e882002-07-24 23:47:05 +0000[diff] [blame]46 <li>if an encoding change is detected it will be installed on the input
Daniel Veillard96984452000-08-31 13:50:12 +0000[diff] [blame]47 buffer, providing buffering and efficient use of the conversion
48 routines</li>
Daniel Veillard0b28e882002-07-24 23:47:05 +0000[diff] [blame]49 <li>once the parser has finished, the close() function of the handler is
Daniel Veillard63d83142002-05-20 06:51:05 +0000[diff] [blame]50 called once and the Input buffer and associated resources are
Daniel Veillard96984452000-08-31 13:50:12 +0000[diff] [blame]51 deallocated.</li>
Daniel Veillard1177ca42003-04-26 22:29:54 +0000[diff] [blame]52</ol><p>The user defined callbacks are checked first to allow overriding of the
Daniel Veillard8a469172003-06-12 16:05:07 +0000[diff] [blame]53default 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
Daniel Veillard0b28e882002-07-24 23:47:05 +0000[diff] [blame]54<code>xmlBuffer</code> type define in <code><a href="http://xmlsoft.org/html/libxml-tree.html">tree.h</a> </code>which is a
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame]55resizable memory buffer. The buffer allocation strategy can be selected to be
56either best-fit or use an exponential doubling one (CPU vs. memory use
Daniel Veillard63d83142002-05-20 06:51:05 +0000[diff] [blame]57trade-off). The values are <code>XML_BUFFER_ALLOC_EXACT</code> and
Daniel Veillard96984452000-08-31 13:50:12 +0000[diff] [blame]58<code>XML_BUFFER_ALLOC_DOUBLEIT</code>, and can be set individually or on a
59system wide basis using <code>xmlBufferSetAllocationScheme()</code>. A number
60of functions allows to manipulate buffers with names starting with the
Daniel Veillard1177ca42003-04-26 22:29:54 +0000[diff] [blame]61<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
Daniel Veillard96984452000-08-31 13:50:12 +0000[diff] [blame]62<code>xmlParserInputBuffer</code> containing a context associated to the
63resource (file descriptor, or pointer to a protocol handler), the read() and
64close() callbacks to use and an xmlBuffer. And extra xmlBuffer and a charset
65encoding handler are also present to support charset conversion when
Daniel Veillard1177ca42003-04-26 22:29:54 +0000[diff] [blame]66needed.</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
67Input 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
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame]68the parser. Creating an input from a filename or an URI string is done
69through the xmlNewInputFromFile() routine. The default entity loader do not
70handle the PUBLIC identifier associated with an entity (if any). So it just
71calls xmlNewInputFromFile() with the SYSTEM identifier (which is mandatory in
Daniel Veillard1177ca42003-04-26 22:29:54 +0000[diff] [blame]72XML).</p><p>If you want to hook up a catalog mechanism then you simply need to
73override the default entity loader, here is an example:</p><pre>#include &lt;libxml/xmlIO.h&gt;
Daniel Veillard2bb89092000-08-31 14:57:50 +0000[diff] [blame]74
75xmlExternalEntityLoader defaultLoader = NULL;
76
77xmlParserInputPtr
78xmlMyExternalEntityLoader(const char *URL, const char *ID,
79 xmlParserCtxtPtr ctxt) {
80 xmlParserInputPtr ret;
81 const char *fileID = NULL;
82 /* lookup for the fileID depending on ID */
83
84 ret = xmlNewInputFromFile(ctxt, fileID);
85 if (ret != NULL)
86 return(ret);
87 if (defaultLoader != NULL)
88 ret = defaultLoader(URL, ID, ctxt);
89 return(ret);
90}
91
92int main(..) {
93 ...
94
95 /*
96 * Install our own entity loader
97 */
98 defaultLoader = xmlGetExternalEntityLoader();
99 xmlSetExternalEntityLoader(xmlMyExternalEntityLoader);
100
101 ...
Daniel Veillard1177ca42003-04-26 22:29:54 +0000[diff] [blame]102}</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
Daniel Veillard96984452000-08-31 13:50:12 +0000[diff] [blame]103real use case</a>, xmlDocDump() closes the FILE * passed by the application
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame]104and this was a problem. The <a href="http://xmlsoft.org/messages/0711.html">solution</a> was to redefine a
Daniel Veillard1177ca42003-04-26 22:29:54 +0000[diff] [blame]105new output handler with the closing call deactivated:</p><ol><li>First define a new I/O output allocator where the output don't close
Daniel Veillardc0801af2002-05-28 16:28:42 +0000[diff] [blame]106 the file:
Daniel Veillarda7ad4522000-08-31 14:19:54 +0000[diff] [blame]107 <pre>xmlOutputBufferPtr
108xmlOutputBufferCreateOwn(FILE *file, xmlCharEncodingHandlerPtr encoder) {
109    xmlOutputBufferPtr ret;
110    
111    if (xmlOutputCallbackInitialized == 0)
112        xmlRegisterDefaultOutputCallbacks();
113
114    if (file == NULL) return(NULL);
115    ret = xmlAllocOutputBuffer(encoder);
116    if (ret != NULL) {
117        ret-&gt;context = file;
118        ret-&gt;writecallback = xmlFileWrite;
119        ret-&gt;closecallback = NULL; /* No close callback */
120    }
Daniel Veillardf83a2c72003-04-08 13:48:40 +0000[diff] [blame]121    return(ret);
Daniel Veillard96984452000-08-31 13:50:12 +0000[diff] [blame]122} </pre>
Daniel Veillard0b28e882002-07-24 23:47:05 +0000[diff] [blame]123 </li>
124 <li>And then use it to save the document:
Daniel Veillard96984452000-08-31 13:50:12 +0000[diff] [blame]125 <pre>FILE *f;
Daniel Veillarda7ad4522000-08-31 14:19:54 +0000[diff] [blame]126xmlOutputBufferPtr output;
127xmlDocPtr doc;
Daniel Veillard96984452000-08-31 13:50:12 +0000[diff] [blame]128int res;
Daniel Veillarda7ad4522000-08-31 14:19:54 +0000[diff] [blame]129
Daniel Veillard96984452000-08-31 13:50:12 +0000[diff] [blame]130f = ...
131doc = ....
Daniel Veillard96984452000-08-31 13:50:12 +0000[diff] [blame]132
Daniel Veillarda7ad4522000-08-31 14:19:54 +0000[diff] [blame]133output = xmlOutputBufferCreateOwn(f, NULL);
134res = xmlSaveFileTo(output, doc, NULL);
Daniel Veillard96984452000-08-31 13:50:12 +0000[diff] [blame]135 </pre>
Daniel Veillard0b28e882002-07-24 23:47:05 +0000[diff] [blame]136 </li>
Daniel Veillard1177ca42003-04-26 22:29:54 +0000[diff] [blame]137</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>