1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
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
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
|
<?xml version="1.0" encoding="UTF-8" standalone="no"?>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=UTF-8" />
<title>Replacing Records Using Cursors</title>
<link rel="stylesheet" href="gettingStarted.css" type="text/css" />
<meta name="generator" content="DocBook XSL Stylesheets V1.73.2" />
<link rel="start" href="index.html" title="Getting Started with Berkeley DB" />
<link rel="up" href="Cursors.html" title="Chapter 4. Using Cursors" />
<link rel="prev" href="DeleteEntryWCursor.html" title="Deleting Records Using Cursors" />
<link rel="next" href="CoreCursorUsage.html" title="Cursor Example" />
</head>
<body>
<div xmlns="" class="navheader">
<div class="libver">
<p>Library Version 12.1.6.1</p>
</div>
<table width="100%" summary="Navigation header">
<tr>
<th colspan="3" align="center">Replacing Records Using Cursors</th>
</tr>
<tr>
<td width="20%" align="left"><a accesskey="p" href="DeleteEntryWCursor.html">Prev</a> </td>
<th width="60%" align="center">Chapter 4. Using Cursors</th>
<td width="20%" align="right"> <a accesskey="n" href="CoreCursorUsage.html">Next</a></td>
</tr>
</table>
<hr />
</div>
<div class="sect1" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h2 class="title" style="clear: both"><a id="ReplacingEntryWCursor"></a>Replacing Records Using Cursors</h2>
</div>
</div>
</div>
<p>
You replace the data for a database record by using
<span>
<code class="methodname">DBC->put()</code>
with the <code class="literal">DB_CURRENT</code> flag.
</span>
</p>
<a id="c_cursor9"></a>
<pre class="programlisting">#include <db.h>
#include <string.h>
...
DB *dbp;
DBC *cursorp;
DBT key, data;
char *key1str = "My first string";
char *replacement_data = "replace me";
int ret;
/* Initialize our DBTs. */
memset(&key, 0, sizeof(DBT));
memset(&data, 0, sizeof(DBT));
/* Set up our DBTs */
key.data = key1str;
key.size = strlen(key1str) + 1;
/* Database open omitted */
/* Get the cursor */
dbp->cursor(dbp, NULL, &cursorp, 0);
/* Position the cursor */
ret = cursorp->get(cursorp, &key, &data, DB_SET);
if (ret == 0) {
data.data = replacement_data;
data.size = strlen(replacement_data) + 1;
cursorp->put(cursorp, &key, &data, DB_CURRENT);
}
/* Cursors must be closed */
if (cursorp != NULL)
cursorp->close(cursorp);
if (dbp != NULL)
dbp->close(dbp, 0);</pre>
<p>
Note that you cannot change a record's key using this method; the key
parameter is always ignored when you replace a record.
</p>
<p>
When replacing the data portion of a record, if you are replacing a
record that is a member of a sorted duplicates set, then the replacement
will be successful only if the new record sorts identically to the old
record. This means that if you are replacing a record that is a member
of a sorted duplicates set, and if you are using the default
lexicographic sort, then the replacement will fail due to violating the
sort order. However, if you
provide a custom sort routine that, for example, sorts based on just a
few bytes out of the data item, then potentially you can perform
a direct replacement and still not violate the restrictions described
here.
</p>
<p>
<span>Under these circumstances, if</span>
you want to replace the data contained by a duplicate record,
<span>
and you are not using a custom sort routine, then
</span>
delete the record and create a new record with the desired key and data.
</p>
</div>
<div class="navfooter">
<hr />
<table width="100%" summary="Navigation footer">
<tr>
<td width="40%" align="left"><a accesskey="p" href="DeleteEntryWCursor.html">Prev</a> </td>
<td width="20%" align="center">
<a accesskey="u" href="Cursors.html">Up</a>
</td>
<td width="40%" align="right"> <a accesskey="n" href="CoreCursorUsage.html">Next</a></td>
</tr>
<tr>
<td width="40%" align="left" valign="top">Deleting Records Using Cursors </td>
<td width="20%" align="center">
<a accesskey="h" href="index.html">Home</a>
</td>
<td width="40%" align="right" valign="top"> Cursor Example</td>
</tr>
</table>
</div>
</body>
</html>
|
