<?xml version="1.0" encoding="UTF-8" standalone="no"?>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=UTF-8" />
<title>DB->exists()</title>
<link rel="stylesheet" href="apiReference.css" type="text/css" />
<meta name="generator" content="DocBook XSL Stylesheets V1.73.2" />
<link rel="start" href="index.html" title="Berkeley DB C API Reference" />
<link rel="up" href="db.html" title="Chapter 2. The DB Handle" />
<link rel="prev" href="dberr.html" title="DB->err()" />
<link rel="next" href="dbfd.html" title="DB->fd()" />
</head>
<body>
<div class="navheader">
<table width="100%" summary="Navigation header">
<tr>
<th colspan="3" align="center">DB->exists()</th>
</tr>
<tr>
<td width="20%" align="left"><a accesskey="p" href="dberr.html">Prev</a> </td>
<th width="60%" align="center">Chapter 2.
The DB Handle
</th>
<td width="20%" align="right"> <a accesskey="n" href="dbfd.html">Next</a></td>
</tr>
</table>
<hr />
</div>
<div class="sect1" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h2 class="title" style="clear: both"><a id="dbexists"></a>DB->exists()</h2>
</div>
</div>
</div>
<pre class="programlisting">#include <db.h>
int
DB->exists(DB *db, DB_TXN *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">
<div>
<div>
<h3 class="title"><a id="id1634051"></a>Parameters</h3>
</div>
</div>
</div>
<div class="sect3" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h4 class="title"><a id="id1634130"></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 class="sect3" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h4 class="title"><a id="id1634222"></a>key</h4>
</div>
</div>
</div>
<p>
The key <a class="link" href="dbt.html" title="Chapter 4. The DBT Handle">DBT</a> operated on.
</p>
</div>
<div class="sect3" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h4 class="title"><a id="id1634305"></a>txnid</h4>
</div>
</div>
</div>
<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="DB_ENV->txn_begin()">DB_ENV->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="DB_ENV->cdsgroup_begin()">DB_ENV->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.
</p>
</div>
</div>
<div class="sect2" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h3 class="title"><a id="id1634446"></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">
<div>
<div>
<h3 class="title"><a id="id1634296"></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>
</p>
</div>
</div>
<div class="navfooter">
<hr />
<table width="100%" summary="Navigation footer">
<tr>
<td width="40%" align="left"><a accesskey="p" href="dberr.html">Prev</a> </td>
<td width="20%" align="center">
<a accesskey="u" href="db.html">Up</a>
</td>
<td width="40%" align="right"> <a accesskey="n" href="dbfd.html">Next</a></td>
</tr>
<tr>
<td width="40%" align="left" valign="top">DB->err() </td>
<td width="20%" align="center">
<a accesskey="h" href="index.html">Home</a>
</td>
<td width="40%" align="right" valign="top"> DB->fd()</td>
</tr>
</table>
</div>
</body>
</html>
distrib
>
Mandriva
>
2010.1
>
x86_64
>
media
>
main-release
>
by-pkgid
>
a00abf5f94bf470fd4cc9e10b2d135e0
>
files
>
88
