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<h2 class="title" style="clear: both"><a id="program_scope"></a>Berkeley DB handles</h2>
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<p>The Berkeley DB library has a number of object handles. The following table
lists those handles, their scope, and whether they are free-threaded
(that is, whether multiple threads within a process can share them).</p>
<div class="variablelist">
<dl>
<dt>
<span class="term">
<a href="../api_reference/C/env.html" class="olink">DB_ENV</a>
</span>
</dt>
<dd>
<p>
The <a href="../api_reference/C/env.html" class="olink">DB_ENV</a> handle, created by the <a href="../api_reference/C/envcreate.html" class="olink">db_env_create()</a> method, refers
to a Berkeley DB database environment — a collection
of Berkeley DB subsystems, log files and databases. <a href="../api_reference/C/env.html" class="olink">DB_ENV</a>
handles are free-threaded if the <a href="../api_reference/C/dbopen.html#open_DB_THREAD" class="olink">DB_THREAD</a> flag is specified
to the <a href="../api_reference/C/envopen.html" class="olink">DB_ENV->open()</a> method when the
environment is opened. The handle should not be closed
while any other handle remains open that is using it as a
reference (for example, <a href="../api_reference/C/db.html" class="olink">DB</a>, <a href="../api_reference/C/txn.html" class="olink">TXN</a>). Once either the
<a href="../api_reference/C/envclose.html" class="olink">DB_ENV->close()</a> or <a href="../api_reference/C/envremove.html" class="olink">DB_ENV->remove()</a> methods
are called, the handle may not be accessed again,
regardless of the method's return.
</p>
</dd>
<dt>
<span class="term">
<a href="../api_reference/C/txn.html" class="olink">TXN</a>
</span>
</dt>
<dd>
<p>
The <a href="../api_reference/C/txn.html" class="olink">TXN</a> handle, created by the <a href="../api_reference/C/txnbegin.html" class="olink">DB_ENV->txn_begin()</a> method, refers to a
single transaction. The handle is not free-threaded.
Transactions may span threads, but only serially, that is, the
application must serialize access to the <a href="../api_reference/C/txn.html" class="olink">TXN</a> handles. In the
case of nested transactions, since all child transactions are
part of the same parent transaction, they must observe the same
constraints. That is, children may execute in different threads
only if each child executes serially.
</p>
<p>
Once the <a href="../api_reference/C/txnabort.html" class="olink">DB_TXN->abort()</a> or <a href="../api_reference/C/txncommit.html" class="olink">DB_TXN->commit()</a> methods are called, the
handle may not be accessed again, regardless of the method's
return. In addition, parent transactions may not issue any
Berkeley DB operations while they have active child
transactions (child transactions that have not yet been
committed or aborted) except for <a href="../api_reference/C/txnbegin.html" class="olink">DB_ENV->txn_begin()</a>, <a href="../api_reference/C/txnabort.html" class="olink">DB_TXN->abort()</a> and
<a href="../api_reference/C/txncommit.html" class="olink">DB_TXN->commit()</a>.
</p>
</dd>
<dt>
<span class="term">
<a href="../api_reference/C/logc.html" class="olink">DB_LOGC</a>
</span>
</dt>
<dd>
<p>
The <a href="../api_reference/C/logc.html" class="olink">DB_LOGC</a> handle refers to a cursor into the log files. The
handle is not free-threaded. Once the <a href="../api_reference/C/logcclose.html" class="olink">DB_LOGC->close()</a> method is
called, the handle may not be accessed again, regardless of the
method's return.
</p>
</dd>
<dt>
<span class="term">
<a href="../api_reference/C/memp.html" class="olink">DB_MPOOLFILE</a>
</span>
</dt>
<dd>
<p>
The <a href="../api_reference/C/memp.html" class="olink">DB_MPOOLFILE</a> handle refers to an open file in the shared memory
buffer pool of the database environment. The handle is not
free-threaded. Once the <a href="../api_reference/C/mempfclose.html" class="olink">DB_MPOOLFILE->close()</a> method is called, the
handle may not be accessed again, regardless of the method's
return.
</p>
</dd>
<dt>
<span class="term">
<a href="../api_reference/C/db.html" class="olink">DB</a>
</span>
</dt>
<dd>
<p>
The <a href="../api_reference/C/db.html" class="olink">DB</a> handle, created by the <a href="../api_reference/C/dbcreate.html" class="olink">db_create()</a> method, refers to a
single Berkeley DB database, which may or may not be part of a
database environment. <a href="../api_reference/C/db.html" class="olink">DB</a> handles are free-threaded if the
<a href="../api_reference/C/dbopen.html#open_DB_THREAD" class="olink">DB_THREAD</a> flag is specified to the <a href="../api_reference/C/envopen.html" class="olink">DB_ENV->open()</a> method when the
database is opened or if the database environment in which the
database is opened is free-threaded. The handle should not be
closed while any other handle that refers to the database is in
use; for example, database handles should be left open while
cursor handles into the database remain open, or transactions
that include operations on the database have not yet been
committed or aborted. Once the <a href="../api_reference/C/dbclose.html" class="olink">DB->close()</a>, <a href="../api_reference/C/dbremove.html" class="olink">DB->remove()</a> or
<a href="../api_reference/C/dbrename.html" class="olink">DB->rename()</a> methods are called, the handle may not be accessed
again, regardless of the method's return.
</p>
</dd>
<dt>
<span class="term">
<a href="../api_reference/C/dbc.html" class="olink">DBC</a>
</span>
</dt>
<dd>
<p>
The <a href="../api_reference/C/dbc.html" class="olink">DBC</a> handle refers to a cursor into a Berkeley DB
database. The handle is not free-threaded. Cursors may span
threads, but only serially, that is, the application must
serialize access to the <a href="../api_reference/C/dbc.html" class="olink">DBC</a> handles. If the cursor is to be
used to perform operations on behalf of a transaction, the
cursor must be opened and closed within the context of that
single transaction. Once <a href="../api_reference/C/dbcclose.html" class="olink">DBC->close()</a> has been called, the
handle may not be accessed again, regardless of the method's
return.
</p>
</dd>
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