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<!--$Id: db_get.so,v 10.31 2000/11/28 20:12:30 bostic Exp $-->
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<title>Berkeley DB: Db::get</title>
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<h1>Db::get</h1>
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<h3><pre>
#include &lt;db_cxx.h&gt;
<p>
int
Db::get(DbTxn *txnid, Dbt *key, Dbt *data, u_int32_t flags);
</pre></h3>
<h1>Description</h1>
<p>The Db::get method retrieves key/data pairs from the database. The
address
and length of the data associated with the specified <b>key</b> are
returned in the structure referenced by <b>data</b>.
<p>In the presence of duplicate key values, Db::get will return the
first data item for the designated key. Duplicates are sorted by insert
order except where this order has been overridden by cursor operations.
<b>Retrieval of duplicates requires the use of cursor operations.</b>
See <a href="../api_cxx/dbc_get.html">Dbc::get</a> for details.
<p>If the file is being accessed under transaction protection, the
<b>txnid</b> parameter is a transaction ID returned from
<a href="../api_cxx/txn_begin.html">DbEnv::txn_begin</a>, otherwise, NULL.
<p>The <b>flags</b> parameter must be set to 0 or one of the following
values:
<p><dl compact>
<p><dt><a name="DB_CONSUME">DB_CONSUME</a><dd>Return the record number and data from the available record closest to
the head of the queue and delete the record. The cursor will be
positioned on the deleted record. The record number will be returned
in <b>key</b> as described in <a href="../api_cxx/dbt_class.html">Dbt</a>. The data will be returned
in the <b>data</b> parameter. A record is available if it is not
deleted and is not currently locked. The underlying database must be
of type Queue for DB_CONSUME to be specified.
<p><dt><a name="DB_CONSUME_WAIT">DB_CONSUME_WAIT</a><dd>The DB_CONSUME_WAIT flag is the same as the DB_CONSUME
flag except that if the Queue database is empty, the thread of control
will wait until there is data in the queue before returning. The
underlying database must be of type Queue for DB_CONSUME_WAIT
to be specified.
<p><dt><a name="DB_GET_BOTH">DB_GET_BOTH</a><dd>Retrieve the key/data pair only if both the key and data match the
arguments.
<p><dt><a name="DB_SET_RECNO">DB_SET_RECNO</a><dd>Retrieve the specified numbered key/data pair from a database.
Upon return, both the <b>key</b> and <b>data</b> items will have been
filled in, not just the data item as is done for all other uses of the
Db::get method.
<p>The <b>data</b> field of the specified <b>key</b>
must be a pointer to a logical record number (i.e., a <b>db_recno_t</b>).
This record number determines the record to be retrieved.
<p>For DB_SET_RECNO to be specified, the underlying database must be
of type Btree and it must have been created with the DB_RECNUM flag.
</dl>
<p>In addition, the following flag may be set by bitwise inclusively <b>OR</b>'ing it into the
<b>flags</b> parameter:
<p><dl compact>
<p><dt><a name="DB_RMW">DB_RMW</a><dd>Acquire write locks instead of read locks when doing the retrieval.
Setting this flag may decrease the likelihood of deadlock during a
read-modify-write cycle by immediately 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>As the Db::get interface will not hold locks across
Berkeley DB interface calls in non-transactional environments, the
<a href="../api_cxx/dbc_get.html#DB_RMW">DB_RMW</a> flag to the Db::get call is only meaningful in
the presence of transactions.
</dl>
<p>If the database is a Queue or Recno database and the requested key exists,
but was never explicitly created by the application or was later deleted,
the Db::get method returns <a href="../ref/program/errorret.html#DB_KEYEMPTY">DB_KEYEMPTY</a>.
<p>Otherwise, if the requested key is not in the database, the
Db::get function returns <a href="../ref/program/errorret.html#DB_NOTFOUND">DB_NOTFOUND</a>.
<p>Otherwise, the Db::get 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.
<h1>Errors</h1>
<p>The Db::get method may fail and throw an exception or return a non-zero error for the following conditions:
<p><dl compact>
<p><dt>DB_LOCK_DEADLOCK<dd>The operation was selected to resolve a deadlock.
</dl>
<p><dl compact>
<p><dt>ENOMEM<dd>There was insufficient memory to return the requested item.
</dl>
<p><dl compact>
<p><dt>EINVAL<dd>An invalid flag value or parameter was specified.
<p>A record number of 0 was specified.
<p>The <a href="../api_cxx/env_open.html#DB_THREAD">DB_THREAD</a> flag was specified to the
<a href="../api_cxx/db_open.html">Db::open</a> method and none of the <a href="../api_cxx/dbt_class.html#DB_DBT_MALLOC">DB_DBT_MALLOC</a>,
<a href="../api_cxx/dbt_class.html#DB_DBT_REALLOC">DB_DBT_REALLOC</a> or <a href="../api_cxx/dbt_class.html#DB_DBT_USERMEM">DB_DBT_USERMEM</a> flags were set in the
<a href="../api_cxx/dbt_class.html">Dbt</a>.
</dl>
<p>The Db::get method may fail and throw an exception or return a non-zero error for errors specified for other Berkeley DB and C library or system methods.
If a catastrophic error has occurred, the Db::get method may fail and either
return <a href="../ref/program/errorret.html#DB_RUNRECOVERY">DB_RUNRECOVERY</a> or throw an exception encapsulating
<a href="../ref/program/errorret.html#DB_RUNRECOVERY">DB_RUNRECOVERY</a>, in which case all subsequent Berkeley DB calls will fail
in the same way.
<h3>Class</h3>
<a href="../api_cxx/db_class.html">Db</a>
<h1>See Also</h1>
<a href="../api_cxx/db_close.html">Db::close</a>,
<a href="../api_cxx/db_cursor.html">Db::cursor</a>,
<a href="../api_cxx/db_del.html">Db::del</a>,
<a href="../api_cxx/db_err.html">Db::err</a>,
<a href="../api_cxx/db_fd.html">Db::fd</a>,
<a href="../api_cxx/db_get.html">Db::get</a>,
<a href="../api_cxx/db_get_byteswapped.html">Db::get_byteswapped</a>,
<a href="../api_cxx/db_get_type.html">Db::get_type</a>,
<a href="../api_cxx/db_join.html">Db::join</a>,
<a href="../api_cxx/db_key_range.html">Db::key_range</a>,
<a href="../api_cxx/db_open.html">Db::open</a>,
<a href="../api_cxx/db_put.html">Db::put</a>,
<a href="../api_cxx/db_remove.html">Db::remove</a>,
<a href="../api_cxx/db_set_bt_compare.html">Db::set_bt_compare</a>,
<a href="../api_cxx/db_set_bt_minkey.html">Db::set_bt_minkey</a>,
<a href="../api_cxx/db_set_bt_prefix.html">Db::set_bt_prefix</a>,
<a href="../api_cxx/db_set_cachesize.html">Db::set_cachesize</a>,
<a href="../api_cxx/db_set_dup_compare.html">Db::set_dup_compare</a>,
<a href="../api_cxx/db_set_errcall.html">Db::set_errcall</a>,
<a href="../api_cxx/db_set_errfile.html">Db::set_errfile</a>,
<a href="../api_cxx/db_set_errpfx.html">Db::set_errpfx</a>,
<a href="../api_cxx/db_set_flags.html">Db::set_flags</a>,
<a href="../api_cxx/db_set_h_ffactor.html">Db::set_h_ffactor</a>,
<a href="../api_cxx/db_set_h_hash.html">Db::set_h_hash</a>,
<a href="../api_cxx/db_set_h_nelem.html">Db::set_h_nelem</a>,
<a href="../api_cxx/db_set_lorder.html">Db::set_lorder</a>,
<a href="../api_cxx/db_set_malloc.html">Db::set_malloc</a>,
<a href="../api_cxx/db_set_pagesize.html">Db::set_pagesize</a>,
<a href="../api_cxx/db_set_paniccall.html">Db::set_paniccall</a>,
<a href="../api_cxx/db_set_q_extentsize.html">Db::set_q_extentsize</a>,
<a href="../api_cxx/db_set_realloc.html">Db::set_realloc</a>,
<a href="../api_cxx/db_set_re_delim.html">Db::set_re_delim</a>,
<a href="../api_cxx/db_set_re_len.html">Db::set_re_len</a>,
<a href="../api_cxx/db_set_re_pad.html">Db::set_re_pad</a>,
<a href="../api_cxx/db_set_re_source.html">Db::set_re_source</a>,
<a href="../api_cxx/db_stat.html">Db::stat</a>,
<a href="../api_cxx/db_sync.html">Db::sync</a>,
<a href="../api_cxx/db_upgrade.html">Db::upgrade</a>
and
<a href="../api_cxx/db_verify.html">Db::verify</a>.
</tt>
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