1<!--$Id: db_exists.so,v 1.1 2007/07/12 18:27:42 bostic Exp $--> 2<!--Copyright (c) 1997,2008 Oracle. All rights reserved.--> 3<!--See the file LICENSE for redistribution information.--> 4<html> 5<head> 6<title>Berkeley DB: Db::exists</title> 7<meta name="description" content="Berkeley DB: An embedded database programmatic toolkit."> 8<meta name="keywords" content="embedded,database,programmatic,toolkit,btree,hash,hashing,transaction,transactions,locking,logging,access method,access methods,Java,C,C++"> 9</head> 10<body bgcolor=white> 11<table width="100%"><tr valign=top> 12<td> 13<b>Db::exists</b> 14</td> 15<td align=right> 16<a href="../api_cxx/api_core.html"><img src="../images/api.gif" alt="API"></a> 17<a href="../ref/toc.html"><img src="../images/ref.gif" alt="Ref"></a></td> 18</tr></table> 19<hr size=1 noshade> 20<tt> 21<b><pre> 22#include <db_cxx.h> 23<p> 24int 25Db::exists(DbTxn *txnid, Dbt *key, u_int32_t flags); 26</pre></b> 27<hr size=1 noshade> 28<b>Description: Db::exists</b> 29<p>The Db::exists method returns if the specified key appears in the 30database.</p> 31<p>The Db::exists method will return <a href="../ref/program/errorret.html#DB_NOTFOUND">DB_NOTFOUND</a> if the specified key is not in the database. 32The Db::exists method will return <a href="../ref/program/errorret.html#DB_KEYEMPTY">DB_KEYEMPTY</a> if the database is a Queue or Recno database and 33the specified key exists, but was never explicitly created by the 34application or was later deleted. 35Unless otherwise specified, the Db::exists method 36either returns a non-zero error value 37or throws an exception that encapsulates a non-zero error value on 38failure, and returns 0 on success. 39</p> 40<b>Parameters</b> <br> 41 <b>flags</b><ul compact><li>The <b>flags</b> parameter must be set to 0 or 42one of the following values: 43In addition, the following flags may be set by 44bitwise inclusively <b>OR</b>'ing them into the <b>flags</b> parameter: 45<br> 46<b><a name="DB_READ_COMMITTED">DB_READ_COMMITTED</a></b><ul compact><li>Configure a transactional read operation to have degree 2 isolation (the 47read is not repeatable).</ul> 48<b><a name="DB_READ_UNCOMMITTED">DB_READ_UNCOMMITTED</a></b><ul compact><li>Configure a transactional read operation to have degree 1 isolation, 49reading modified but not yet committed data. Silently ignored if the 50<a href="../api_cxx/db_open.html#DB_READ_UNCOMMITTED">DB_READ_UNCOMMITTED</a> flag was not specified when the underlying 51database was opened.</ul> 52<b><a name="DB_RMW">DB_RMW</a></b><ul compact><li>Acquire write locks instead of read locks when doing the read, if 53locking is configured. Setting this flag can eliminate deadlock during 54a read-modify-write cycle by acquiring the write lock during the read 55part of the cycle so that another thread of control acquiring a read 56lock for the same item, in its own read-modify-write cycle, will not 57result in deadlock. 58<p>Because the Db::exists method will not hold locks across 59Berkeley DB calls in non-transactional operations, the <a href="../api_cxx/dbc_get.html#DB_RMW">DB_RMW</a> flag to 60the Db::exists call is meaningful only in the presence of 61transactions.</p></ul> 62<br></ul> 63 <b>key</b><ul compact><li>The key <a href="../api_cxx/dbt_class.html">Dbt</a> operated on.</ul> 64 <b>txnid</b><ul compact><li>If the operation is part of an application-specified transaction, the 65<b>txnid</b> parameter is a transaction handle returned from 66<a href="../api_cxx/txn_begin.html">DbEnv::txn_begin</a>; if the operation is part of a Berkeley DB Concurrent Data Store group, the 67<b>txnid</b> parameter is a handle returned from 68<a href="../api_cxx/env_cdsgroup_begin.html">DbEnv::cdsgroup_begin</a>; otherwise NULL. 69</ul> 70<br> 71<hr size=1 noshade> 72<br><b>Class</b> 73<a href="../api_cxx/db_class.html">Db</a> 74<br><b>See Also</b> 75<a href="../api_cxx/db_list.html">Databases and Related Methods</a> 76</tt> 77<table width="100%"><tr><td><br></td><td align=right> 78<a href="../api_cxx/api_core.html"><img src="../images/api.gif" alt="API"></a><a href="../ref/toc.html"><img src="../images/ref.gif" alt="Ref"></a> 79</td></tr></table> 80<p><font size=1>Copyright (c) 1996,2008 Oracle. All rights reserved.</font> 81</body> 82</html> 83
