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<title>Berkeley DB Reference Guide: Berkeley DB handles</title>
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<td><h3><dl><dt>Berkeley DB Reference Guide:<dd>Programmer Notes</dl></h3></td>
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<h1 align=center>Berkeley DB handles</h1>
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<p>The Berkeley DB library has a number of object handles. The following table
lists those handles, their scope, and if they are free-threaded, that
is, if multiple threads within a process can share them.
<p><dl compact>
<p><dt>DB_ENV<dd>The DB_ENV handle is created by the <a href="../../api_c/env_create.html">db_env_create</a> function and
references a Berkeley DB database environment, a collection of
databases and Berkeley DB subsystems. DB_ENV handles are free-threaded
if the <a href="../../api_c/env_open.html#DB_THREAD">DB_THREAD</a> flag is specified to the <a href="../../api_c/env_open.html">DBENV->open</a> function
when the environment is opened. The handle should not be closed while
any other handle remains open that is using it as a reference
(e.g., DB or DB_TXN). Once either the <a href="../../api_c/env_close.html">DBENV->close</a> or
<a href="../../api_c/env_remove.html">DBENV->remove</a> functions are called, the handle may not be accessed again,
regardless of the function's return.
<p><dt>DB_TXN<dd>The DB_TXN handle is created by the <a href="../../api_c/txn_begin.html">txn_begin</a> function and
references a single transaction. The handle is not free-threaded, and
transactions may not span threads nor may transactions be used by more
than a single thread.
Once the
<a href="../../api_c/txn_abort.html">txn_abort</a> or <a href="../../api_c/txn_commit.html">txn_commit</a> functions are called, the handle may
not be accessed again, regardless of the function's return.
In addition, parent transactions may not issue
any Berkeley DB operations, except for <a href="../../api_c/txn_begin.html">txn_begin</a>, <a href="../../api_c/txn_abort.html">txn_abort</a>
and <a href="../../api_c/txn_commit.html">txn_commit</a>, while it has active child transactions (child
transactions that have not yet been committed or aborted).
<p><dt>DB_MPOOLFILE<dd>The DB_MPOOLFILE handle references an open file in the shared
memory buffer pool of the database environment. The handle is not
free-threaded. Once the <a href="../../api_c/memp_fclose.html">memp_fclose</a> function is called, the handle may
not be accessed again, regardless of the function's return.
<p><dt>DB<dd>The DB handle is created by the <a href="../../api_c/db_create.html">db_create</a> function and
references a single Berkeley DB database, which may or may not be part of a
database environment. DB handles are free-threaded if the
<a href="../../api_c/env_open.html#DB_THREAD">DB_THREAD</a> flag is specified to the <a href="../../api_c/db_open.html">DB->open</a> function 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 references the database is in use, e.g., database
handles must not be closed while cursor handles into the database remain
open, or transactions which include operations on the database have not
yet been committed or aborted. Once the <a href="../../api_c/db_close.html">DB->close</a>,
<a href="../../api_c/db_remove.html">DB->remove</a> or <a href="../../api_c/db_rename.html">DB->rename</a> functions are called, the handle may
not be accessed again, regardless of the function's return.
<p><dt>DBC<dd>The DBC handle references a cursor into a Berkeley DB database. The
handle is not free-threaded and cursors may not span threads nor may
cursors be used by more than a single thread. 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_c/dbc_close.html">DBcursor->c_close</a> has been called, the handle may not be accessed
again, regardless of the function's return.
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