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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://codespeak.net/lxml/">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><li><a href="#General1">General overview</a></li>
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>
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
17: 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
18: (files) based on their PUBLIC and SYSTEM identifiers. The default loader
19: don't look at the public identifier since libxml2 do not maintain a
20: catalog. You can redefine you own entity loader by using
21: <code>xmlGetExternalEntityLoader()</code> and
22: <code>xmlSetExternalEntityLoader()</code>. <a href="#entities">Check the
23: example</a>.</li>
24: <li>Input I/O buffers which are a commodity structure used by the parser(s)
25: input layer to handle fetching the information to feed the parser. This
26: provides buffering and is also a placeholder where the encoding
27: converters to UTF8 are piggy-backed.</li>
28: <li>Output I/O buffers are similar to the Input ones and fulfill similar
29: task but when generating a serialization from a tree.</li>
30: <li>A mechanism to register sets of I/O callbacks and associate them with
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>
34: </li>
35: </ul><p>The general mechanism used when loading http://rpmfind.net/xml.html for
36: example in the HTML parser is the following:</p><ol><li>The default entity loader calls <code>xmlNewInputFromFile()</code> with
37: the parsing context and the URI string.</li>
38: <li>the URI string is checked against the existing registered handlers
39: using their match() callback function, if the HTTP module was compiled
40: in, it is registered and its match() function will succeeds</li>
41: <li>the open() function of the handler is called and if successful will
42: return an I/O Input buffer</li>
43: <li>the parser will the start reading from this buffer and progressively
44: fetch information from the resource, calling the read() function of the
45: handler until the resource is exhausted</li>
46: <li>if an encoding change is detected it will be installed on the input
47: buffer, providing buffering and efficient use of the conversion
48: routines</li>
49: <li>once the parser has finished, the close() function of the handler is
50: called once and the Input buffer and associated resources are
51: deallocated.</li>
52: </ol><p>The user defined callbacks are checked first to allow overriding of the
53: 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
54: <code>xmlBuffer</code> type define in <code><a href="http://xmlsoft.org/html/libxml-tree.html">tree.h</a> </code>which is a
55: resizable memory buffer. The buffer allocation strategy can be selected to be
56: either best-fit or use an exponential doubling one (CPU vs. memory use
57: trade-off). The values are <code>XML_BUFFER_ALLOC_EXACT</code> and
58: <code>XML_BUFFER_ALLOC_DOUBLEIT</code>, and can be set individually or on a
59: system wide basis using <code>xmlBufferSetAllocationScheme()</code>. A number
60: of functions allows to manipulate buffers with names starting with the
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
62: <code>xmlParserInputBuffer</code> containing a context associated to the
63: resource (file descriptor, or pointer to a protocol handler), the read() and
64: close() callbacks to use and an xmlBuffer. And extra xmlBuffer and a charset
65: encoding handler are also present to support charset conversion when
66: 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
67: 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
68: the parser. Creating an input from a filename or an URI string is done
69: through the xmlNewInputFromFile() routine. The default entity loader do not
70: handle the PUBLIC identifier associated with an entity (if any). So it just
71: calls xmlNewInputFromFile() with the SYSTEM identifier (which is mandatory in
72: XML).</p><p>If you want to hook up a catalog mechanism then you simply need to
73: override the default entity loader, here is an example:</p><pre>#include <libxml/xmlIO.h>
74:
75: xmlExternalEntityLoader defaultLoader = NULL;
76:
77: xmlParserInputPtr
78: xmlMyExternalEntityLoader(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:
92: int main(..) {
93: ...
94:
95: /*
96: * Install our own entity loader
97: */
98: defaultLoader = xmlGetExternalEntityLoader();
99: xmlSetExternalEntityLoader(xmlMyExternalEntityLoader);
100:
101: ...
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
103: real use case</a>, xmlDocDump() closes the FILE * passed by the application
104: and this was a problem. The <a href="http://xmlsoft.org/messages/0711.html">solution</a> was to redefine a
105: 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
106: the file:
107: <pre>xmlOutputBufferPtr
108: xmlOutputBufferCreateOwn(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->context = file;
118: ret->writecallback = xmlFileWrite;
119: ret->closecallback = NULL; /* No close callback */
120: }
121: return(ret);
122: } </pre>
123: </li>
124: <li>And then use it to save the document:
125: <pre>FILE *f;
126: xmlOutputBufferPtr output;
127: xmlDocPtr doc;
128: int res;
129:
130: f = ...
131: doc = ....
132:
133: output = xmlOutputBufferCreateOwn(f, NULL);
134: res = xmlSaveFileTo(output, doc, NULL);
135: </pre>
136: </li>
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>
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