<?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="id1634166"></a>Parameters</h3> </div> </div> </div> <div class="sect3" lang="en" xml:lang="en"> <div class="titlepage"> <div> <div> <h4 class="title"><a id="id1634245"></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="id1634337"></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="id1634420"></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="id1634561"></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="id1634410"></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>