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<b>DbMpoolFile::get</b>
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<b><pre>
#include <db_cxx.h>
<p>
int
DbMpoolFile::get(db_pgno_t *pgnoaddr,
DbTxn *txnid, u_int32_t flags, void **pagep);
</pre></b>
<hr size=1 noshade>
<b>Description: DbMpoolFile::get</b>
<p>The DbMpoolFile::get method returns pages from the cache.</p>
<p>All pages returned by DbMpoolFile::get will be retained (that is,
<i>pinned</i>), in the pool until a subsequent call to
<a href="../../db4-devel-4.7.25/api_cxx/memp_fput.html">DbMpoolFile::put</a>.</p>
<p>The returned page is <b>size_t</b> type aligned.</p>
<p>Fully or partially created pages have all their bytes set to a nul byte,
unless the <a href="../../db4-devel-4.7.25/api_cxx/memp_set_clear_len.html">DbMpoolFile::set_clear_len</a> method was called to specify other
behavior before the file was opened.</p>
<a name="2"><!--meow--></a>
<p>The DbMpoolFile::get method
will either return DB_PAGE_NOTFOUND or
throw an exception that encapsulates DB_PAGE_NOTFOUND if the requested page does not exist and DB_MPOOL_CREATE was
not set.
Unless otherwise specified, the DbMpoolFile::get 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.
</p>
<b>Parameters</b> <br>
<b>flags</b><ul compact><li>The <b>flags</b> parameter must be set to 0 or by bitwise inclusively <b>OR</b>'ing together one
or more of the following values:
<br>
<b><a name="DB_MPOOL_CREATE">DB_MPOOL_CREATE</a></b><ul compact><li>If the specified page does not exist, create it. In this case, the
<a href="memp_register.html#pgin">pgin</a> method, if specified, is
called.</ul>
<b><a name="DB_MPOOL_DIRTY">DB_MPOOL_DIRTY</a></b><ul compact><li>The page will be modified and must be written to the source file before
being evicted from the pool. For files open with the
<a href="../../db4-devel-4.7.25/api_cxx/db_open.html#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.</ul>
<b><a name="DB_MPOOL_EDIT">DB_MPOOL_EDIT</a></b><ul compact><li>The page will be modified and must be written to the source file before
being evicted from the pool. No copy of the page will be made, regardless
of the <a href="../../db4-devel-4.7.25/api_cxx/db_open.html#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.</ul>
<b><a name="DB_MPOOL_LAST">DB_MPOOL_LAST</a></b><ul compact><li>Return the last page of the source file, and copy its page number into
the memory location to which <b>pgnoaddr</b> refers.</ul>
<b><a name="DB_MPOOL_NEW">DB_MPOOL_NEW</a></b><ul compact><li>Create a new page in the file, and copy its page number into the memory
location to which <b>pgnoaddr</b> refers. In this case, the
<a href="memp_register.html#pgin">pgin</a> method, if specified, is
<b>not</b> called.</ul>
<br>
<p>The DB_MPOOL_CREATE, DB_MPOOL_LAST, and
DB_MPOOL_NEW flags are mutually exclusive.</p></ul>
<b>pagep</b><ul compact><li>The <b>pagep</b> parameter references memory into which
a pointer to the returned page is copied.</ul>
<b>pgnoaddr</b><ul compact><li>If the <b>flags</b> parameter is set to DB_MPOOL_LAST or
DB_MPOOL_NEW, the page number of the created page is copied
into the memory location to which the <b>pgnoaddr</b> parameter
refers. Otherwise, the <b>pgnoaddr</b> parameter is the page to
create or retrieve.
<p><b>Page numbers begin at 0; that is, the first page in the file is page
number 0, not page number 1.</b></p></ul>
<b>txnid</b><ul compact><li>If the operation is part of an application-specified
transaction, the <b>txnid</b> parameter is a transaction handle
returned from <a href="../../db4-devel-4.7.25/api_cxx/txn_begin.html">DbEnv::txn_begin</a>; otherwise NULL. A transaction is
required if the file is open for multiversion concurrency control by
passing <a href="../../db4-devel-4.7.25/api_cxx/db_open.html#DB_MULTIVERSION">DB_MULTIVERSION</a> to <a href="../../db4-devel-4.7.25/api_cxx/memp_fopen.html">DbMpoolFile::open</a> and the
DB_MPOOL_DIRTY, DB_MPOOL_CREATE or
DB_MPOOL_NEW flags were specified. Otherwise it is ignored.</ul>
<br>
<br><b>Errors</b>
<p>The DbMpoolFile::get method
may fail and throw
<a href="../../db4-devel-4.7.25/api_cxx/except_class.html">DbException</a>,
encapsulating one of the following non-zero errors, or return one of
the following non-zero errors:</p>
<br>
<b>EACCES</b><ul compact><li>The DB_MPOOL_DIRTY or DB_MPOOL_EDIT flag was set and
the source file was not opened for writing.</ul>
<br>
<br>
<b>EAGAIN</b><ul compact><li>The page reference count has overflowed. (This should never happen
unless there is a bug in the application.)</ul>
<br>
<br>
<b>EINVAL</b><ul compact><li>If the DB_MPOOL_NEW flag was set, and the source file was not
opened for writing;
more than one of DB_MPOOL_CREATE, DB_MPOOL_LAST, and
DB_MPOOL_NEW was set.; or if an
invalid flag value or parameter was specified.</ul>
<br>
<br>
<b>DB_LOCK_DEADLOCK</b><ul compact><li>For transactions configured with <a href="../../db4-devel-4.7.25/api_cxx/txn_begin.html#DB_TXN_SNAPSHOT">DB_TXN_SNAPSHOT</a>, the page has
been modified since the transaction began.</ul>
<br>
<p>If the cache is full, and no more pages will fit in the pool, the DbMpoolFile::get method will fail and
either return ENOMEM or
throw a DbMemoryException.</p>
<hr size=1 noshade>
<br><b>Class</b>
<a href="../../db4-devel-4.7.25/api_cxx/env_class.html">DbEnv</a>, <a href="../../db4-devel-4.7.25/api_cxx/mempfile_class.html">DbMpoolFile</a>
<br><b>See Also</b>
<a href="../../db4-devel-4.7.25/api_cxx/memp_list.html">Memory Pools and Related Methods</a>
</tt>
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