xmlio.html 5.54 KB
Newer Older
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN"
                      "http://www.w3.org/TR/REC-html40/loose.dtd">
<html>
<head>
  <title>Libxml Input/Output handling</title>
  <meta name="GENERATOR" content="amaya V3.2.1">
  <meta http-equiv="Content-Type" content="text/html">
</head>

<body bgcolor="#ffffff">
<h1 align="center">Libxml Input/Output handling</h1>

<p>Location: <a
href="http://xmlsoft.org/xmlio.html">http://xmlsoft.org/xmlio.html</a></p>

<p>Libxml home page: <a href="http://xmlsoft.org/">http://xmlsoft.org/</a></p>

<p>Mailing-list archive:  <a
href="http://xmlsoft.org/messages/">http://xmlsoft.org/messages/</a></p>

Daniel Veillard's avatar
Daniel Veillard committed
21
<p>Version: $Revision: 1.1 $</p>
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110

<p>Table of Content:</p>
<ol>
  <li><a href="#General">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="#Example">Example of customized I/O</a></li>
</ol>

<h2><a name="General">General overview</a></h2>

<p>The module <code><a
href="http://xmlsoft.org/html/gnome-xml-xmlio.html">xmlIO.h</a></code>
provides the interfaces to the libxml I/O system. This consists of 3 main
parts:</p>
<ul>
  <li>Input I/O buffers which are a commodity structure used by the parser(s)
    input layer to handle fetching the informations to feed the parser. This
    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
    task but when generating a serialization from a tree.</li>
  <li>A mechanism to register sets of I/O callbacks and associate them with
    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>
  </li>
</ul>

<p> The general mechanism used when loading http://rpmfind.net/xml.html for
example in the HTML parser is the following:</p>
<ol>
  <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 macth() function will succeed</li>
  <li>the open() function of the handler is called and if successful will
    return an I/O Input buffer</li>
  <li>the parser will the start reading from this buffer and progressively
    fetch information from the resource, calling the read() function of the
    handler until the resource is exhausted</li>
  <li>if an encoding change is detected it will be installed on the input
    buffer, providing buffering and efficient use of the conversion
  routines</li>
  <li>once the parser has finished, the close() function of the handler is
    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>

<h2><a name="basic">The basic buffer type</a></h2>

<p>All the buffer manipulation handling is done using the
<code>xmlBuffer</code> type define in <code><a
href="http://xmlsoft.org/html/gnome-xml-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
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>

<h2><a name="Input">Input I/O handlers</a></h2>

<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>

<h2><a name="Output">Output I/O handlers</a></h2>

<p>An Output handler <code>xmlOutputBuffer</code> is completely similar to an
Input one except the callbacks are write() and close().</p>

<h2><a name="Example">Example of customized I/O</a></h2>

<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
and this was a problem. The <a
href="http://xmlsoft.org/messages/0711.html">solution</a> was to redefine a
new output handler with the closing call deactivated:</p>
<ol>
  <li>First define a new I/O ouput allocator where the output don't close the
    file:
Daniel Veillard's avatar
Daniel Veillard committed
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
    <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>
126
127
128
129
} </pre>
  </li>
  <li>And then use it to save the document:
    <pre>FILE *f;
Daniel Veillard's avatar
Daniel Veillard committed
130
131
xmlOutputBufferPtr output;
xmlDocPtr doc;
132
int res;
Daniel Veillard's avatar
Daniel Veillard committed
133

134
135
f = ...
doc = ....
Daniel Veillard's avatar
Daniel Veillard committed
136

137
138
139
140
141
142
143
144
output = xmlOutputBufferCreateOwn(f, NULL);
res = xmlSaveFileTo(output, doc, NULL);
    </pre>
  </li>
</ol>

<p><a href="mailto:Daniel.Veillard@w3.org">Daniel Veillard</a></p>

Daniel Veillard's avatar
Daniel Veillard committed
145
<p>$Id: xmlio.html,v 1.1 2000/08/31 13:50:12 veillard Exp $</p>
146
147
</body>
</html>