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<!--$Id: db_exists.so,v 1.1 2007/07/12 18:27:42 bostic Exp $--> <!--Copyright (c) 1997,2008 Oracle. All rights reserved.--> <!--See the file LICENSE for redistribution information.--> <html> <head> <title>Berkeley DB: Db::exists</title> <meta name="description" content="Berkeley DB: An embedded database programmatic toolkit."> <meta name="keywords" content="embedded,database,programmatic,toolkit,btree,hash,hashing,transaction,transactions,locking,logging,access method,access methods,Java,C,C++"> </head> <body bgcolor=white> <table width="100%"><tr valign=top> <td> <b>Db::exists</b> </td> <td align=right> <a href="../../db4-devel-4.7.25/api_cxx/api_core.html"><img src="../../db4-devel-4.7.25/images/api.gif" alt="API"></a> <a href="../../db4-devel-4.7.25/ref/toc.html"><img src="../../db4-devel-4.7.25/images/ref.gif" alt="Ref"></a></td> </tr></table> <hr size=1 noshade> <tt> <b><pre> #include <db_cxx.h> <p> int Db::exists(DbTxn *txnid, Dbt *key, u_int32_t flags); </pre></b> <hr size=1 noshade> <b>Description: Db::exists</b> <p>The Db::exists method returns if the specified key appears in the database.</p> <p>The Db::exists method will return <a href="../../db4-devel-4.7.25/ref/program/errorret.html#DB_NOTFOUND">DB_NOTFOUND</a> if the specified key is not in the database. The Db::exists method will return <a href="../../db4-devel-4.7.25/ref/program/errorret.html#DB_KEYEMPTY">DB_KEYEMPTY</a> if the database is a Queue or Recno database and the specified key exists, but was never explicitly created by the application or was later deleted. Unless otherwise specified, the Db::exists method either returns a non-zero error value or throws an exception that encapsulates a non-zero error value on failure, and returns 0 on success. </p> <b>Parameters</b> <br> <b>flags</b><ul compact><li>The <b>flags</b> parameter must be set to 0 or one of the following values: In addition, the following flags may be set by bitwise inclusively <b>OR</b>'ing them into the <b>flags</b> parameter: <br> <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 read is not repeatable).</ul> <b><a name="DB_READ_UNCOMMITTED">DB_READ_UNCOMMITTED</a></b><ul compact><li>Configure a transactional read operation to have degree 1 isolation, reading modified but not yet committed data. Silently ignored if the <a href="../../db4-devel-4.7.25/api_cxx/db_open.html#DB_READ_UNCOMMITTED">DB_READ_UNCOMMITTED</a> flag was not specified when the underlying database was opened.</ul> <b><a name="DB_RMW">DB_RMW</a></b><ul compact><li>Acquire write locks instead of read locks when doing the read, if locking is configured. Setting this flag can eliminate deadlock during a read-modify-write cycle by acquiring the write lock during the read part of the cycle so that another thread of control acquiring a read lock for the same item, in its own read-modify-write cycle, will not result in deadlock. <p>Because the Db::exists method will not hold locks across Berkeley DB calls in non-transactional operations, the <a href="../../db4-devel-4.7.25/api_cxx/dbc_get.html#DB_RMW">DB_RMW</a> flag to the Db::exists call is meaningful only in the presence of transactions.</p></ul> <br></ul> <b>key</b><ul compact><li>The key <a href="../../db4-devel-4.7.25/api_cxx/dbt_class.html">Dbt</a> operated on.</ul> <b>txnid</b><ul compact><li>If the operation is part of an application-specified transaction, the <b>txnid</b> parameter is a transaction handle returned from <a href="../../db4-devel-4.7.25/api_cxx/txn_begin.html">DbEnv::txn_begin</a>; if the operation is part of a Berkeley DB Concurrent Data Store group, the <b>txnid</b> parameter is a handle returned from <a href="../../db4-devel-4.7.25/api_cxx/env_cdsgroup_begin.html">DbEnv::cdsgroup_begin</a>; otherwise NULL. </ul> <br> <hr size=1 noshade> <br><b>Class</b> <a href="../../db4-devel-4.7.25/api_cxx/db_class.html">Db</a> <br><b>See Also</b> <a href="../../db4-devel-4.7.25/api_cxx/db_list.html">Databases and Related Methods</a> </tt> <table width="100%"><tr><td><br></td><td align=right> <a href="../../db4-devel-4.7.25/api_cxx/api_core.html"><img src="../../db4-devel-4.7.25/images/api.gif" alt="API"></a><a href="../../db4-devel-4.7.25/ref/toc.html"><img src="../../db4-devel-4.7.25/images/ref.gif" alt="Ref"></a> </td></tr></table> <p><font size=1>Copyright (c) 1996,2008 Oracle. All rights reserved.</font> </body> </html>
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