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<th width="60%" align="center">Chapter 5.
The DbEnv Handle
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<div class="sect1" lang="en" xml:lang="en">
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<div>
<h2 class="title" style="clear: both"><a id="envcreate"></a>DbEnv</h2>
</div>
</div>
</div>
<pre class="programlisting">#include <db_cxx.h>
class DbEnv {
public:
DbEnv(u_int32 flags);
~DbEnv();
DB_ENV *DbEnv::get_DB_ENV();
const DB_ENV *DbEnv::get_const_DB_ENV() const;
static DbEnv *DbEnv::get_DbEnv(DB_ENV *dbenv);
static const DbEnv *DbEnv::get_const_DbEnv(const DB_ENV *dbenv);
...
}; </pre>
<p>
The <code class="classname">DbEnv</code> object is the handle for a Berkeley DB environment —
a collection including support for some or all of caching, locking, logging and transaction subsystems, as well as
databases and log files. Methods of the <code class="classname">DbEnv</code> handle are used to configure the environment
as well as to operate on subsystems and databases in the environment.
</p>
<p>
<code class="classname">DbEnv</code> handles are free-threaded if the
<a class="link" href="envopen.html#envopen_DB_THREAD">DB_THREAD</a> flag is specified to the
<a class="xref" href="envopen.html" title="DbEnv::open()">DbEnv::open()</a> method when the environment is opened.
The <code class="classname">DbEnv</code> handle should not be closed while any other handle
remains open that is using it as a reference (for example,
<a class="link" href="db.html" title="Chapter 2. The Db Handle">Db</a> or <a class="link" href="txn.html" title="Chapter 13. The DbTxn Handle">DbTxn</a>).
Once either the
<a class="xref" href="envclose.html" title="DbEnv::close()">DbEnv::close()</a> or
<a class="xref" href="envremove.html" title="DbEnv::remove()">DbEnv::remove()</a>
methods are called, the handle may not be accessed again, regardless of the method's return.
</p>
<p>
The constructor creates the <code class="classname">DbEnv</code> object.
The constructor allocates memory internally; calling the
<a class="xref" href="envclose.html" title="DbEnv::close()">DbEnv::close()</a> or
<a class="xref" href="envremove.html" title="DbEnv::remove()">DbEnv::remove()</a>
methods will free that memory.
</p>
<p>
Before the handle may be used, you must open it using the
<a class="xref" href="envopen.html" title="DbEnv::open()">DbEnv::open()</a>
method.
</p>
<p>
The <span class="bold"><strong>flags</strong></span> parameter must be set to 0.
</p>
<div class="itemizedlist">
<ul type="disc">
<li>
<p><a id="env_DB_CXX_NO_EXCEPTIONS"></a>
<code class="literal">DB_CXX_NO_EXCEPTIONS</code>
</p>
<p>
The Berkeley DB C++ API supports two different error behaviors. By default, whenever an error
occurs, an exception is thrown that encapsulates the error information. This
generally allows for cleaner logic for transaction processing because a try block can
surround a single transaction. However, if
<code class="literal">DB_CXX_NO_EXCEPTIONS</code> is specified, exceptions
are not thrown; instead, each individual function returns an error code.
</p>
</li>
</ul>
</div>
<p>
Each <code class="classname">DbEnv</code> object has an associated <code class="literal">DB_ENV</code>
structure, which is used by the underlying implementation of Berkeley DB and its C-language
API. The <code class="methodname">DbEnv::get_DB_ENV()</code> method returns a pointer to this
struct. Given a <code class="literal">const DbEnv</code> object,
<code class="methodname">DbEnv::get_const_DB_ENV()</code> returns a const pointer to the same
struct.
</p>
<p>
Given a <code class="literal">DB_ENV</code> struct, the <code class="methodname">DbEnv::get_DbEnv()</code>
method returns the corresponding <code class="classname">DbEnv</code> object,
if there is one. If the <code class="literal">DB_ENV</code> struct was not associated with a
<code class="classname">DbEnv</code> (that is, it was not
returned from a call to <code class="methodname">DbEnv::get_DB_ENV()</code>), then the result of
<code class="methodname">DbEnv::get_DbEnv()</code> is
undefined. Given a <code class="literal">const DB_ENV</code> struct,
<code class="methodname">DbEnv::get_const_Db_Env()</code> returns the associated
<code class="literal">const DbEnv</code> object, if there is one.
</p>
<p>
These methods may be useful for Berkeley DB applications including both C and C++ language
software. It should not be necessary to use these calls in a purely C++ application.
</p>
<div class="sect2" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h3 class="title"><a id="idp6652256"></a>Class</h3>
</div>
</div>
</div>
<p>
<a class="link" href="env.html" title="Chapter 5. The DbEnv Handle">DbEnv</a>
</p>
</div>
<div class="sect2" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h3 class="title"><a id="idp6623920"></a>See Also</h3>
</div>
</div>
</div>
<p>
<a class="xref" href="env.html#envlist" title="Database Environments and Related Methods">Database Environments and Related Methods</a>
</p>
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