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The Db Handle
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<div class="sect1" lang="en" xml:lang="en">
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<div>
<div>
<h2 class="title" style="clear: both"><a id="dbexists"></a>Db::exists()</h2>
</div>
</div>
</div>
<pre class="programlisting">#include <db_cxx.h>
int
Db::exists(DbTxn *txnid, Dbt *key, u_int32_t flags);</pre>
<p>
The <code class="methodname">Db::exists()</code> method returns whether the specified key appears in the
database.
</p>
<p>
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
is not in the database. The <code class="methodname">Db::exists()</code> method will return
<a href="../../programmer_reference/program_errorret.html#program_errorret.DB_KEYEMPTY" class="olink">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.
</p>
<div class="sect2" lang="en" xml:lang="en">
<div class="titlepage">
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<div>
<h3 class="title"><a id="idm914008"></a>Parameters</h3>
</div>
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<div class="sect3" lang="en" xml:lang="en">
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<div>
<h4 class="title"><a id="idp2890768"></a>txnid</h4>
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<p>
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
<a class="xref" href="txnbegin.html" title="DbEnv::txn_begin()">DbEnv::txn_begin()</a>; if the operation is part of a
Berkeley DB Concurrent Data Store group, the <span class="bold"><strong>txnid</strong></span>
parameter is a handle returned from
<a class="xref" href="envcdsgroup_begin.html" title="DbEnv::cdsgroup_begin()">DbEnv::cdsgroup_begin()</a>; otherwise NULL.
If no transaction handle is specified, but the
operation occurs in a transactional database, the operation will be
implicitly transaction protected.
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<div class="sect3" lang="en" xml:lang="en">
<div class="titlepage">
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<h4 class="title"><a id="idp4528688"></a>key</h4>
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<p>
The key <a class="link" href="dbt.html" title="Chapter 4. The Dbt Handle">Dbt</a> operated on.
</p>
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<div class="sect3" lang="en" xml:lang="en">
<div class="titlepage">
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<div>
<h4 class="title"><a id="idp2622904"></a>flags</h4>
</div>
</div>
</div>
<p>
The <span class="bold"><strong>flags</strong></span> parameter must be set to zero or
by bitwise inclusively <span class="bold"><strong>OR</strong></span>'ing together one
or more of the following values:
</p>
<div class="itemizedlist">
<ul type="disc">
<li>
<p><a id="exists_DB_READ_COMMITTED"></a>
<code class="literal">DB_READ_COMMITTED</code>
</p>
<p>
Configure a transactional read operation to have degree 2 isolation
(the read is not repeatable).
</p>
</li>
<li>
<p><a id="exists_DB_READ_UNCOMMITTED"></a>
<code class="literal">DB_READ_UNCOMMITTED</code>
</p>
<p>
Configure a transactional read operation to have degree 1 isolation,
reading modified but not yet committed data. Silently ignored if the
<a class="link" href="dbopen.html#dbopen_DB_READ_UNCOMMITTED">DB_READ_UNCOMMITTED</a>
flag was not specified when the underlying database was opened.
</p>
</li>
<li>
<p><a id="exists_DB_RMW"></a>
<code class="literal">DB_RMW</code>
</p>
<p>
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>
<p>
Because the <code class="methodname">Db::exists()</code> method will not hold locks
across Berkeley DB calls in non-transactional operations, the
<a class="link" href="dbcget.html#dbcget_DB_RMW">DB_RMW</a> flag to the
<code class="methodname">Db::exists()</code> call is meaningful only in the presence
of transactions.
</p>
</li>
</ul>
</div>
</div>
</div>
<div class="sect2" lang="en" xml:lang="en">
<div class="titlepage">
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<h3 class="title"><a id="idp950616"></a>Class</h3>
</div>
</div>
</div>
<p>
<a class="link" href="db.html" title="Chapter 2. The Db Handle">Db</a>
</p>
</div>
<div class="sect2" lang="en" xml:lang="en">
<div class="titlepage">
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<div>
<h3 class="title"><a id="idp941448"></a>See Also</h3>
</div>
</div>
</div>
<p>
<a class="xref" href="db.html#dblist" title="Database and Related Methods">Database and Related Methods</a>
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