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>DB->exists()</title> 7 <link rel="stylesheet" href="apiReference.css" type="text/css" /> 8 <meta name="generator" content="DocBook XSL Stylesheets V1.73.2" /> 9 <link rel="start" href="index.html" title="Berkeley DB C API Reference" /> 10 <link rel="up" href="db.html" title="Chapter��2.�� The DB Handle" /> 11 <link rel="prev" href="dberr.html" title="DB->err()" /> 12 <link rel="next" href="dbfd.html" title="DB->fd()" /> 13 </head> 14 <body> 15 <div class="navheader"> 16 <table width="100%" summary="Navigation header"> 17 <tr> 18 <th colspan="3" align="center">DB->exists()</th> 19 </tr> 20 <tr> 21 <td width="20%" align="left"><a accesskey="p" href="dberr.html">Prev</a>��</td> 22 <th width="60%" align="center">Chapter��2.�� 23 The DB Handle 24 </th> 25 <td width="20%" align="right">��<a accesskey="n" href="dbfd.html">Next</a></td> 26 </tr> 27 </table> 28 <hr /> 29 </div> 30 <div class="sect1" lang="en" xml:lang="en"> 31 <div class="titlepage"> 32 <div> 33 <div> 34 <h2 class="title" style="clear: both"><a id="dbexists"></a>DB->exists()</h2> 35 </div> 36 </div> 37 </div> 38 <pre class="programlisting">#include <db.h> 39 40int 41DB->exists(DB *db, DB_TXN *txnid, DBT *key, u_int32_t flags); </pre> 42 <p> 43 The <code class="methodname">DB->exists()</code> method returns whether the specified key appears in the 44 database. 45 </p> 46 <p> 47 The <code class="methodname">DB->exists()</code> method will return <a href="../../programmer_reference/program_errorret.html#program_errorret.DB_NOTFOUND" class="olink">DB_NOTFOUND</a> if the specified key 48 is not in the database. The <code class="methodname">DB->exists()</code> method will return 49 <a href="../../programmer_reference/program_errorret.html#program_errorret.DB_KEYEMPTY" class="olink">DB_KEYEMPTY</a> if the database 50 is a Queue or Recno database and the specified key exists, but was never explicitly created 51 by the application or was later deleted. 52 </p> 53 <div class="sect2" lang="en" xml:lang="en"> 54 <div class="titlepage"> 55 <div> 56 <div> 57 <h3 class="title"><a id="id1634166"></a>Parameters</h3> 58 </div> 59 </div> 60 </div> 61 <div class="sect3" lang="en" xml:lang="en"> 62 <div class="titlepage"> 63 <div> 64 <div> 65 <h4 class="title"><a id="id1634245"></a>flags</h4> 66 </div> 67 </div> 68 </div> 69 <p> 70 The <span class="bold"><strong>flags</strong></span> parameter must be set to zero or 71 by bitwise inclusively <span class="bold"><strong>OR</strong></span>'ing together one 72 or more of the following values: 73 </p> 74 <div class="itemizedlist"> 75 <ul type="disc"> 76 <li> 77 <p><a id="exists_DB_READ_COMMITTED"></a> 78 <code class="literal">DB_READ_COMMITTED</code> 79 </p> 80 <p> 81 Configure a transactional read operation to have degree 2 isolation 82 (the read is not repeatable). 83 </p> 84 </li> 85 <li> 86 <p><a id="exists_DB_READ_UNCOMMITTED"></a> 87 <code class="literal">DB_READ_UNCOMMITTED</code> 88 </p> 89 <p> 90 Configure a transactional read operation to have degree 1 isolation, 91 reading modified but not yet committed data. Silently ignored if the 92 <a class="link" href="dbopen.html#dbopen_DB_READ_UNCOMMITTED">DB_READ_UNCOMMITTED</a> 93 flag was not specified when the underlying database was opened. 94 </p> 95 </li> 96 <li> 97 <p><a id="exists_DB_RMW"></a> 98 <code class="literal">DB_RMW</code> 99 </p> 100 <p> 101 Acquire write locks instead of read locks when doing the read, if locking is 102 configured. Setting this flag can eliminate deadlock during a read-modify-write 103 cycle by acquiring the write lock during the read part of the cycle so that 104 another thread of control acquiring a read lock for the same item, in its own 105 read-modify-write cycle, will not result in deadlock. 106 </p> 107 <p> 108 Because the <code class="methodname">DB->exists()</code> method will not hold locks 109 across Berkeley DB calls in non-transactional operations, the 110 <a class="link" href="dbcget.html#dbcget_DB_RMW">DB_RMW</a> flag to the 111 <code class="methodname">DB->exists()</code> call is meaningful only in the presence 112 of transactions. 113 </p> 114 </li> 115 </ul> 116 </div> 117 </div> 118 <div class="sect3" lang="en" xml:lang="en"> 119 <div class="titlepage"> 120 <div> 121 <div> 122 <h4 class="title"><a id="id1634337"></a>key</h4> 123 </div> 124 </div> 125 </div> 126 <p> 127 The key <a class="link" href="dbt.html" title="Chapter��4.�� The DBT Handle">DBT</a> operated on. 128 </p> 129 </div> 130 <div class="sect3" lang="en" xml:lang="en"> 131 <div class="titlepage"> 132 <div> 133 <div> 134 <h4 class="title"><a id="id1634420"></a>txnid</h4> 135 </div> 136 </div> 137 </div> 138 <p> 139 If the operation is part of an application-specified transaction, the <span class="bold"><strong>txnid</strong></span> parameter is a transaction handle returned from 140 <a class="xref" href="txnbegin.html" title="DB_ENV->txn_begin()">DB_ENV->txn_begin()</a>; if the operation is part of a 141 Berkeley DB Concurrent Data Store group, the <span class="bold"><strong>txnid</strong></span> 142 parameter is a handle returned from 143 <a class="xref" href="envcdsgroup_begin.html" title="DB_ENV->cdsgroup_begin()">DB_ENV->cdsgroup_begin()</a>; otherwise NULL. 144 If no transaction handle is specified, but the 145 operation occurs in a transactional database, the operation will be 146 implicitly transaction protected. 147 </p> 148 </div> 149 </div> 150 <div class="sect2" lang="en" xml:lang="en"> 151 <div class="titlepage"> 152 <div> 153 <div> 154 <h3 class="title"><a id="id1634561"></a>Class</h3> 155 </div> 156 </div> 157 </div> 158 <p> 159 <a class="link" href="db.html" title="Chapter��2.�� The DB Handle">DB</a> 160 </p> 161 </div> 162 <div class="sect2" lang="en" xml:lang="en"> 163 <div class="titlepage"> 164 <div> 165 <div> 166 <h3 class="title"><a id="id1634410"></a>See Also</h3> 167 </div> 168 </div> 169 </div> 170 <p> 171 <a class="xref" href="db.html#dblist" title="Database and Related Methods">Database and Related Methods</a> 172 </p> 173 </div> 174 </div> 175 <div class="navfooter"> 176 <hr /> 177 <table width="100%" summary="Navigation footer"> 178 <tr> 179 <td width="40%" align="left"><a accesskey="p" href="dberr.html">Prev</a>��</td> 180 <td width="20%" align="center"> 181 <a accesskey="u" href="db.html">Up</a> 182 </td> 183 <td width="40%" align="right">��<a accesskey="n" href="dbfd.html">Next</a></td> 184 </tr> 185 <tr> 186 <td width="40%" align="left" valign="top">DB->err()��</td> 187 <td width="20%" align="center"> 188 <a accesskey="h" href="index.html">Home</a> 189 </td> 190 <td width="40%" align="right" valign="top">��DB->fd()</td> 191 </tr> 192 </table> 193 </div> 194 </body> 195</html> 196