83 lines
4.0 KiB
HTML
83 lines
4.0 KiB
HTML
<!--$Id: db_exists.html 63573 2008-05-23 21:43:21Z trent.nelson $-->
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<!--Copyright (c) 1997,2008 Oracle. All rights reserved.-->
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<!--See the file LICENSE for redistribution information.-->
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<html>
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<head>
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<title>Berkeley DB: Db::exists</title>
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<meta name="description" content="Berkeley DB: An embedded database programmatic toolkit.">
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<meta name="keywords" content="embedded,database,programmatic,toolkit,btree,hash,hashing,transaction,transactions,locking,logging,access method,access methods,Java,C,C++">
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</head>
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<body bgcolor=white>
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<table width="100%"><tr valign=top>
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<td>
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<b>Db::exists</b>
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</td>
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<td align=right>
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<a href="../api_cxx/api_core.html"><img src="../images/api.gif" alt="API"></a>
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<a href="../ref/toc.html"><img src="../images/ref.gif" alt="Ref"></a></td>
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</tr></table>
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<hr size=1 noshade>
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<tt>
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<b><pre>
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#include <db_cxx.h>
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<p>
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int
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Db::exists(DbTxn *txnid, Dbt *key, u_int32_t flags);
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</pre></b>
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<hr size=1 noshade>
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<b>Description: Db::exists</b>
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<p>The Db::exists method returns if the specified key appears in the
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database.</p>
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<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.
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The 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
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the specified key exists, but was never explicitly created by the
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application or was later deleted.
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Unless otherwise specified, the Db::exists method
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either returns a non-zero error value
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or throws an exception that encapsulates a non-zero error value on
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failure, and returns 0 on success.
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</p>
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<b>Parameters</b> <br>
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<b>flags</b><ul compact><li>The <b>flags</b> parameter must be set to 0 or
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one of the following values:
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In addition, the following flags may be set by
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bitwise inclusively <b>OR</b>'ing them into the <b>flags</b> parameter:
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<br>
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<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
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read is not repeatable).</ul>
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<b><a name="DB_READ_UNCOMMITTED">DB_READ_UNCOMMITTED</a></b><ul compact><li>Configure a transactional read operation to have degree 1 isolation,
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reading modified but not yet committed data. Silently ignored if the
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<a href="../api_cxx/db_open.html#DB_READ_UNCOMMITTED">DB_READ_UNCOMMITTED</a> flag was not specified when the underlying
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database was opened.</ul>
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<b><a name="DB_RMW">DB_RMW</a></b><ul compact><li>Acquire write locks instead of read locks when doing the read, if
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locking is configured. Setting this flag can eliminate deadlock during
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a read-modify-write cycle by acquiring the write lock during the read
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part of the cycle so that another thread of control acquiring a read
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lock for the same item, in its own read-modify-write cycle, will not
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result in deadlock.
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<p>Because the Db::exists method will not hold locks across
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Berkeley DB calls in non-transactional operations, the <a href="../api_cxx/dbc_get.html#DB_RMW">DB_RMW</a> flag to
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the Db::exists call is meaningful only in the presence of
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transactions.</p></ul>
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<br></ul>
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<b>key</b><ul compact><li>The key <a href="../api_cxx/dbt_class.html">Dbt</a> operated on.</ul>
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<b>txnid</b><ul compact><li>If the operation is part of an application-specified transaction, the
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<b>txnid</b> parameter is a transaction handle returned from
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<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
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<b>txnid</b> parameter is a handle returned from
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<a href="../api_cxx/env_cdsgroup_begin.html">DbEnv::cdsgroup_begin</a>; otherwise NULL.
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</ul>
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<br>
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<hr size=1 noshade>
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<br><b>Class</b>
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<a href="../api_cxx/db_class.html">Db</a>
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<br><b>See Also</b>
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<a href="../api_cxx/db_list.html">Databases and Related Methods</a>
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</tt>
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<table width="100%"><tr><td><br></td><td align=right>
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<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>
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</td></tr></table>
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<p><font size=1>Copyright (c) 1996,2008 Oracle. All rights reserved.</font>
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</body>
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</html>
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