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The DbMpoolFile Handle
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<h2 class="title" style="clear: both"><a id="mempfget"></a>DbMpoolFile::get()</h2>
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<pre class="programlisting">#include <db_cxx.h>
int
DbMpoolFile::get(db_pgno_t *pgnoaddr,
DbTxn *txnid, u_int32_t flags, void **pagep);</pre>
<p>
The <code class="methodname">DbMpoolFile::get()</code> method returns pages from the cache.
</p>
<p>
All pages returned by <code class="methodname">DbMpoolFile::get()</code> will be retained (that is,
<span class="emphasis"><em>latched</em></span>) in the cache until a subsequent call to
<a class="xref" href="mempput.html" title="DbMpoolFile::put()">DbMpoolFile::put()</a>.
There is no deadlock detection amoung latches so care must be taken in the application if the DB_MPOOL_DIRTY
or DB_MPOOL_EDIT flags are used as these get exlusive latches on the pages.
</p>
<p>
The returned page is <span class="bold"><strong>size_t</strong></span> type
aligned.
</p>
<p>
Fully or partially created pages have all their bytes set to a nul
byte, unless the
<a class="xref" href="mempset_clear_len.html" title="DbMpoolFile::set_clear_len()">DbMpoolFile::set_clear_len()</a>
method was called to specify other behavior before the file was
opened.
</p>
<p><a id="fget_DB_PAGE_NOTFOUND"></a>
The <code class="methodname">DbMpoolFile::get()</code> method will return
<code class="literal">DB_PAGE_NOTFOUND</code> if the requested page does not exist and DB_MPOOL_CREATE
was not set. Unless otherwise specified, the <code class="methodname">DbMpoolFile::get()</code>
<span>
<span>
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.
</span>
</span>
</p>
<div class="sect2" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h3 class="title"><a id="id1693956"></a>Parameters</h3>
</div>
</div>
</div>
<div class="sect3" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h4 class="title"><a id="id1693846"></a>flags</h4>
</div>
</div>
</div>
<p>
The <span class="bold"><strong>flags</strong></span> parameter must be set to 0
or by bitwise inclusively <span class="bold"><strong>OR</strong></span>'ing
together one or more of the following values:
</p>
<div class="itemizedlist">
<ul type="disc">
<li>
<p><a id="mpoolfget_DB_MPOOL_CREATE"></a>
<code class="literal">DB_MPOOL_CREATE</code>
</p>
<p>
If the specified page does not exist, create it. In this case, the
<a class="link" href="mempregister.html" title="DbEnv::memp_register()">pgin</a> method, if
specified, is called.
</p>
</li>
<li>
<p><a id="fget_DB_MPOOL_DIRTY"></a>
<code class="literal">DB_MPOOL_DIRTY</code>
</p>
<p>
The page will be modified and must be written to the source file
before being evicted from the cache. For files open with the
<a class="link" href="dbopen.html#dbopen_DB_MULTIVERSION">DB_MULTIVERSION</a>
flag set, a new copy of the page will be made if this is the first
time the specified transaction is modifying it.
A page fetched with the <code class="literal">DB_MPOOL_DIRTY</code> flag will be
<span class="bold"><strong>excluively latched</strong></span> until
a subsequent call to <a class="xref" href="mempput.html" title="DbMpoolFile::put()">DbMpoolFile::put()</a>.
</p>
</li>
<li>
<p><a id="fget_DB_MPOOL_EDIT"></a>
<code class="literal">DB_MPOOL_EDIT</code>
</p>
<p>
The page will be modified and must be written to the source file
before being evicted from the cache. No copy of the page will be made,
regardless of the
<a class="link" href="dbopen.html#dbopen_DB_MULTIVERSION">DB_MULTIVERSION</a>
setting. This flag is only intended for use in situations where a
transaction handle is not available, such as during aborts or
recovery.
A page fetched with the <code class="literal">DB_MPOOL_EDIT</code> flag will be
<span class="bold"><strong>excluively latched</strong></span> until
a subsequent call to <a class="xref" href="mempput.html" title="DbMpoolFile::put()">DbMpoolFile::put()</a>.
</p>
</li>
<li>
<p><a id="fget_DB_MPOOL_LAST"></a>
<code class="literal">DB_MPOOL_LAST</code>
</p>
<p>
Return the last page of the source file, and copy its page number into
the memory location to which <span class="bold"><strong>pgnoaddr</strong></span>
refers.
</p>
</li>
<li>
<p><a id="mpoolfget_DB_MPOOL_NEW"></a>
<code class="literal">DB_MPOOL_NEW</code>
</p>
<p>
Create a new page in the file, and copy its page number into the
memory location to which <span class="bold"><strong>pgnoaddr</strong></span>
refers. In this case, the <code class="literal">pgin_fcn</code> callback, if specified on
<a class="xref" href="mempregister.html" title="DbEnv::memp_register()">DbEnv::memp_register()</a>, is
<span class="bold"><strong>not</strong></span> called.
</p>
</li>
</ul>
</div>
<p>
The <code class="literal">DB_MPOOL_CREATE</code>, <code class="literal">DB_MPOOL_LAST</code>, and
<code class="literal">DB_MPOOL_NEW</code> flags are mutually exclusive.
</p>
</div>
<div class="sect3" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h4 class="title"><a id="id1694300"></a>pagep</h4>
</div>
</div>
</div>
<p>
The <span class="bold"><strong>pagep</strong></span> parameter references memory
into which a pointer to the returned page is copied.
</p>
</div>
<div class="sect3" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h4 class="title"><a id="id1694153"></a>pgnoaddr</h4>
</div>
</div>
</div>
<p>
If the <span class="bold"><strong>flags</strong></span> parameter is set to
<code class="literal">DB_MPOOL_LAST</code> or <code class="literal">DB_MPOOL_NEW</code>, the
page number of the created page is copied into the memory location to
which the <span class="bold"><strong>pgnoaddr</strong></span> parameter refers.
Otherwise, the <span class="bold"><strong>pgnoaddr</strong></span> parameter is the
page to create or retrieve.
</p>
<div class="note" style="margin-left: 0.5in; margin-right: 0.5in;">
<h3 class="title">Note</h3>
<p>
Page numbers begin at 0; that is, the first page in the file is page number 0,
not page number 1.
</p>
</div>
</div>
<div class="sect3" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h4 class="title"><a id="id1694424"></a>txnid</h4>
</div>
</div>
</div>
<p>
If the operation is part of an application-specified transaction, the
<span class="bold"><strong>txnid</strong></span> parameter is a transaction
handle returned from <a class="xref" href="txnbegin.html" title="DbEnv::txn_begin()">DbEnv::txn_begin()</a>;
otherwise NULL. A transaction is required if the file is open for multiversion
concurrency control by passing
<a class="link" href="dbopen.html#dbopen_DB_MULTIVERSION">DB_MULTIVERSION</a>
to
<a class="xref" href="mempfopen.html" title="DbMpoolFile::open()">DbMpoolFile::open()</a>
and the DB_MPOOL_DIRTY, DB_MPOOL_CREATE or DB_MPOOL_NEW flags were
specified. Otherwise it is ignored.
</p>
</div>
</div>
<div class="sect2" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h3 class="title"><a id="id1694406"></a>Errors</h3>
</div>
</div>
</div>
<p>
The <code class="methodname">DbMpoolFile::get()</code> <span>
<span>
method may fail and throw a <a class="link" href="dbexception.html" title="Chapter 6. The DbException Class">DbException</a>
exception, encapsulating one of the following non-zero errors, or return one
of the following non-zero errors:
</span>
</span>
</p>
<div class="sect3" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h4 class="title"><a id="id1694141"></a>EACCES</h4>
</div>
</div>
</div>
<p>
The <code class="literal">DB_MPOOL_DIRTY</code> or
<code class="literal">DB_MPOOL_EDIT</code> flag was set and the source file was
not opened for writing.
</p>
</div>
<div class="sect3" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h4 class="title"><a id="id1694229"></a>EAGAIN</h4>
</div>
</div>
</div>
<p>
The page reference count has overflowed. (This should never happen
unless there is a bug in the application.)
</p>
</div>
<div class="sect3" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h4 class="title"><a id="id1694344"></a>EINVAL</h4>
</div>
</div>
</div>
<p>
If the <code class="literal">DB_MPOOL_NEW</code> flag was set, and the source
file was not opened for writing; more than one of
<code class="literal">DB_MPOOL_CREATE</code>, <code class="literal">DB_MPOOL_LAST</code>,
and <code class="literal">DB_MPOOL_NEW</code> was set; or if an invalid flag
value or parameter was specified.
</p>
</div>
<div class="sect3" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h4 class="title"><a id="id1694007"></a>DB_LOCK_DEADLOCK</h4>
</div>
</div>
</div>
<p>
For transactions configured with
<a class="link" href="txnbegin.html#txnbegin_DB_TXN_SNAPSHOT">DB_TXN_SNAPSHOT</a>,
the page has been modified since the transaction began.
</p>
</div>
<div class="sect3" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h4 class="title"><a id="id1694510"></a>ENOMEM</h4>
</div>
</div>
</div>
<p>
The cache is full, and no more pages will fit in the cache.
</p>
</div>
</div>
<div class="sect2" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h3 class="title"><a id="id1694265"></a>Class</h3>
</div>
</div>
</div>
<p>
<a class="link" href="env.html" title="Chapter 5. The DbEnv Handle">DbEnv</a>, <a class="link" href="memp.html" title="Chapter 9. The DbMpoolFile Handle">DbMpoolFile</a>
</p>
</div>
<div class="sect2" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h3 class="title"><a id="id1693932"></a>See Also</h3>
</div>
</div>
</div>
<p>
<a class="xref" href="memp.html#memplist" title="Memory Pools and Related Methods">Memory Pools and Related Methods</a>
</p>
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