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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 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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
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
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
36example 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
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
54<code>xmlBuffer</code> type define in <code><a href="http://xmlsoft.org/html/libxml-tree.html">tree.h</a> </code>which is a
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
57trade-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
59system wide basis using <code>xmlBufferSetAllocationScheme()</code>. A number
60of 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
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
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
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
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;
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    ...
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
103real use case</a>,  xmlDocDump() closes the FILE * passed by the application
104and this was a problem. The <a href="http://xmlsoft.org/messages/0711.html">solution</a> was to redefine a
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
106    the file:
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    }
121    return(ret);
122} </pre>
123  </li>
124  <li>And then use it to save the document:
125    <pre>FILE *f;
126xmlOutputBufferPtr output;
127xmlDocPtr doc;
128int res;
129
130f = ...
131doc = ....
132
133output = xmlOutputBufferCreateOwn(f, NULL);
134res = 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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