1<?xml version="1.0" encoding="UTF-8" standalone="no"?> 2<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"> 3<html xmlns="http://www.w3.org/1999/xhtml"> 4 <head> 5 <meta http-equiv="Content-Type" content="text/html; charset=UTF-8" /> 6 <title>Chapter 2. Databases</title> 7 <link rel="stylesheet" href="gettingStarted.css" type="text/css" /> 8 <meta name="generator" content="DocBook XSL Stylesheets V1.73.2" /> 9 <link rel="start" href="index.html" title="Getting Started with Berkeley DB" /> 10 <link rel="up" href="index.html" title="Getting Started with Berkeley DB" /> 11 <link rel="prev" href="gettingit.html" title="Getting and Using DB" /> 12 <link rel="next" href="coredbclose.html" title="Closing Databases" /> 13 </head> 14 <body> 15 <div class="navheader"> 16 <table width="100%" summary="Navigation header"> 17 <tr> 18 <th colspan="3" align="center">Chapter 2. Databases</th> 19 </tr> 20 <tr> 21 <td width="20%" align="left"><a accesskey="p" href="gettingit.html">Prev</a> </td> 22 <th width="60%" align="center"> </th> 23 <td width="20%" align="right"> <a accesskey="n" href="coredbclose.html">Next</a></td> 24 </tr> 25 </table> 26 <hr /> 27 </div> 28 <div class="chapter" lang="en" xml:lang="en"> 29 <div class="titlepage"> 30 <div> 31 <div> 32 <h2 class="title"><a id="DB"></a>Chapter 2. Databases</h2> 33 </div> 34 </div> 35 </div> 36 <div class="toc"> 37 <p> 38 <b>Table of Contents</b> 39 </p> 40 <dl> 41 <dt> 42 <span class="sect1"> 43 <a href="DB.html#DBOpen">Opening Databases</a> 44 </span> 45 </dt> 46 <dt> 47 <span class="sect1"> 48 <a href="coredbclose.html">Closing Databases</a> 49 </span> 50 </dt> 51 <dt> 52 <span class="sect1"> 53 <a href="DBOpenFlags.html">Database Open Flags</a> 54 </span> 55 </dt> 56 <dt> 57 <span class="sect1"> 58 <a href="CoreDBAdmin.html">Administrative Methods</a> 59 </span> 60 </dt> 61 <dt> 62 <span class="sect1"> 63 <a href="dbErrorReporting.html">Error Reporting Functions</a> 64 </span> 65 </dt> 66 <dt> 67 <span class="sect1"> 68 <a href="CoreEnvUsage.html">Managing Databases in Environments</a> 69 </span> 70 </dt> 71 <dt> 72 <span class="sect1"> 73 <a href="CoreDbUsage.html">Database Example</a> 74 </span> 75 </dt> 76 </dl> 77 </div> 78 <p>In Berkeley DB, a database is a collection of <span class="emphasis"><em>records</em></span>. Records, 79 in turn, consist of key/data pairings. 80 </p> 81 <p> 82 Conceptually, you can think of a 83 84 <span>database</span> 85 as containing a two-column table where column 1 contains a key and column 2 86 contains data. Both the key and the data are managed using 87 88 <code class="classname">DBT</code> 89 90 91 <span>structures</span> 92 (see <a class="xref" href="DBEntry.html" title="Chapter 3. Database Records">Database Records</a> for details on this 93 94 <span>structure</span>). 95 So, fundamentally, using a DB 96 97 <span>database</span> 98 involves putting, getting, and deleting database records, which in turns involves efficiently 99 managing information 100 101 <span>contained in </span> 102 103 104 <code class="classname">DBT</code> 105 106 107 108 <span>structures.</span> 109 The next several chapters of this book are dedicated to those activities. 110 </p> 111 <div class="sect1" lang="en" xml:lang="en"> 112 <div class="titlepage"> 113 <div> 114 <div> 115 <h2 class="title" style="clear: both"><a id="DBOpen"></a>Opening Databases</h2> 116 </div> 117 </div> 118 </div> 119 <p> 120 To open a database, you must first use the <code class="function">db_create()</code> function to 121 initialize a <code class="classname">DB</code> handle. 122 Once you have initialized the <code class="classname">DB</code> 123 handle, you use its <code class="methodname">open()</code> method to open the database. 124 </p> 125 <p> 126 Note that by default, DB does not create databases if they do not already exist. 127 To override this behavior, specify the 128 <a class="link" href="DBOpenFlags.html" title="Database Open Flags"><code class="literal">DB_CREATE</code></a> flag on the 129 <code class="methodname">open()</code> method. 130 </p> 131 <p> 132 The following code fragment illustrates a database open: 133 134 </p> 135 <a id="c_db1"></a> 136 <pre class="programlisting">#include <db.h> 137 138... 139 140DB *dbp; /* DB structure handle */ 141u_int32_t flags; /* database open flags */ 142int ret; /* function return value */ 143 144/* Initialize the structure. This 145 * database is not opened in an environment, 146 * so the environment pointer is NULL. */ 147ret = db_create(&dbp, NULL, 0); 148if (ret != 0) { 149 /* Error handling goes here */ 150} 151 152/* Database open flags */ 153flags = DB_CREATE; /* If the database does not exist, 154 * create it.*/ 155 156/* open the database */ 157ret = dbp->open(dbp, /* DB structure pointer */ 158 NULL, /* Transaction pointer */ 159 "my_db.db", /* On-disk file that holds the database. */ 160 NULL, /* Optional logical database name */ 161 DB_BTREE, /* Database access method */ 162 flags, /* Open flags */ 163 0); /* File mode (using defaults) */ 164if (ret != 0) { 165 /* Error handling goes here */ 166}</pre> 167 </div> 168 </div> 169 <div class="navfooter"> 170 <hr /> 171 <table width="100%" summary="Navigation footer"> 172 <tr> 173 <td width="40%" align="left"><a accesskey="p" href="gettingit.html">Prev</a> </td> 174 <td width="20%" align="center"> </td> 175 <td width="40%" align="right"> <a accesskey="n" href="coredbclose.html">Next</a></td> 176 </tr> 177 <tr> 178 <td width="40%" align="left" valign="top">Getting and Using DB </td> 179 <td width="20%" align="center"> 180 <a accesskey="h" href="index.html">Home</a> 181 </td> 182 <td width="40%" align="right" valign="top"> Closing Databases</td> 183 </tr> 184 </table> 185 </div> 186 </body> 187</html> 188