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<title>Berkeley DB Reference Guide: Creating an Environment</title>
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<td><h3><dl><dt>Berkeley DB Reference Guide:<dd>Environment</dl></h3></td>
<td width="1%"><a href="../../ref/env/intro.html"><img src="../../images/prev.gif" alt="Prev"></a><a href="../../ref/toc.html"><img src="../../images/ref.gif" alt="Ref"></a><a href="../../ref/env/naming.html"><img src="../../images/next.gif" alt="Next"></a>
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<h1 align=center>Creating an Environment</h1>
<p>The <a href="../../api_c/env_open.html">DBENV->open</a> function is the standard function for creating or
joining a database environment. Transaction-protected or multi-process
applications should call <a href="../../api_c/env_open.html">DBENV->open</a> before making any other calls
to the Berkeley DB library. Applications must obtain an environment handle from
the <a href="../../api_c/env_create.html">db_env_create</a> function before calling <a href="../../api_c/env_open.html">DBENV->open</a>.
There are a large number of options that you can set to customize
<a href="../../api_c/env_open.html">DBENV->open</a> for your environment. These options fall into four
broad categories:
<p><dl compact>
<p><dt>Subsystem Initialization:<dd>These flags indicate which Berkeley DB subsystems will be initialized for the
environment, and, what operations will happen automatically when
databases are accessed within the environment. The flags include
<a href="../../api_c/env_open.html#DB_JOINENV">DB_JOINENV</a>, <a href="../../api_c/env_open.html#DB_INIT_CDB">DB_INIT_CDB</a>, <a href="../../api_c/env_open.html#DB_INIT_LOCK">DB_INIT_LOCK</a>,
<a href="../../api_c/env_open.html#DB_INIT_LOG">DB_INIT_LOG</a>, <a href="../../api_c/env_open.html#DB_INIT_MPOOL">DB_INIT_MPOOL</a> and <a href="../../api_c/env_open.html#DB_INIT_TXN">DB_INIT_TXN</a>.
The <a href="../../api_c/env_open.html#DB_INIT_CDB">DB_INIT_CDB</a> flag does initialization for Berkeley DB Concurrent Data Store
applications, see <a href="../../ref/cam/intro.html">Building Berkeley DB Concurrent Data Store
applications</a> for more information. The rest of the flags initialize
a single subsystem, e.g., when <a href="../../api_c/env_open.html#DB_INIT_LOCK">DB_INIT_LOCK</a> is specified,
applications reading and writing databases opened in this environment
will be using locking to ensure that they do not overwrite each other's
changes.
<p><dt>Recovery options:<dd>These flags indicate what recovery is to be performed on the environment
before it is opened for normal use, and include <a href="../../api_c/env_open.html#DB_RECOVER">DB_RECOVER</a> and
<a href="../../api_c/env_open.html#DB_RECOVER_FATAL">DB_RECOVER_FATAL</a>.
<p><dt>Naming options:<dd>These flags modify how file naming happens in the environment, and include
<a href="../../api_c/env_open.html#DB_USE_ENVIRON">DB_USE_ENVIRON</a> and <a href="../../api_c/env_open.html#DB_USE_ENVIRON_ROOT">DB_USE_ENVIRON_ROOT</a>.
<p><dt>Miscellaneous:<dd>Finally, there are a number of miscellaneous flags such as <a href="../../api_c/env_open.html#DB_CREATE">DB_CREATE</a>
which causes underlying files to be created as necessary. See the
<a href="../../api_c/env_open.html">DBENV->open</a> manual pages for further information.
</dl>
<p>Most applications either specify only the <a href="../../api_c/env_open.html#DB_INIT_MPOOL">DB_INIT_MPOOL</a> flag or
they specify all four subsystem initialization flags
(<a href="../../api_c/env_open.html#DB_INIT_MPOOL">DB_INIT_MPOOL</a>, <a href="../../api_c/env_open.html#DB_INIT_LOCK">DB_INIT_LOCK</a>, <a href="../../api_c/env_open.html#DB_INIT_LOG">DB_INIT_LOG</a> and
<a href="../../api_c/env_open.html#DB_INIT_TXN">DB_INIT_TXN</a>). The former configuration is for applications that
simply want to use the basic Access Method interfaces with a shared
underlying buffer pool, but don't care about recoverability after
application or system failure. The latter is for applications that need
recoverability. There are situations where other combinations of the
initialization flags make sense, but they are rare.
<p>The <a href="../../api_c/env_open.html#DB_RECOVER">DB_RECOVER</a> flag is specified by applications that want to
perform any necessary database recovery when they start running, i.e., if
there was a system or application failure the last time they ran, they
want the databases to be made consistent before they start running again.
It is not an error to specify this flag when no recovery needs to be
done.
<p>The <a href="../../api_c/env_open.html#DB_RECOVER_FATAL">DB_RECOVER_FATAL</a> flag is more special-purpose. It performs
catastrophic database recovery, and normally requires that some initial
arrangements be made, i.e., archived log files be brought back into the
filesystem. Applications should not normally specify this flag. Instead,
under these rare conditions, the <a href="../../utility/db_recover.html">db_recover</a> utility should be
used.
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