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<title>DbMpoolFile::set_fileid()</title>
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<p>Library Version 11.2.5.2</p>
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<table width="100%" summary="Navigation header">
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<tr>
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<th colspan="3" align="center">DbMpoolFile::set_fileid()</th>
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</tr>
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<tr>
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<td width="20%" align="left"><a accesskey="p" href="mempset_clear_len.html">Prev</a> </td>
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<th width="60%" align="center">Chapter 9.
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The DbMpoolFile Handle
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</th>
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<td width="20%" align="right"> <a accesskey="n" href="mempset_flags.html">Next</a></td>
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<div class="sect1" lang="en" xml:lang="en">
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<div class="titlepage">
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<div>
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<h2 class="title" style="clear: both"><a id="mempset_fileid"></a>DbMpoolFile::set_fileid()</h2>
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</div>
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</div>
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</div>
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<pre class="programlisting">#include <db_cxx.h>
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int
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DbMpoolFile::set_fileid(u_int8_t *fileid); </pre>
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<p>
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The <code class="methodname">DbMpoolFile::set_fileid()</code> method specifies a unique identifier
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for the file. (The shared memory buffer pool functions must be able to
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uniquely identify files in order that multiple processes wanting to
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share a file will correctly identify it in the cache.)
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</p>
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<p>
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On most UNIX/POSIX systems, the <span class="bold"><strong>fileid</strong></span> field will not need
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to be set, and the memory pool functions will use the file's device and inode numbers for
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this purpose. On Windows systems, the memory pool functions use the values returned by
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<code class="methodname">GetFileInformationByHandle()</code> by default — these values are
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known to be constant between processes and over reboot in the case of NTFS (in which they
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are the NTFS MFT indices).
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</p>
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<p>
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On other filesystems (for example, FAT or NFS), these default values
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are not necessarily unique between processes or across system reboots.
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<span class="bold"><strong>Applications wanting to maintain a shared
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cache between processes or across system reboots, in which the
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cache contains pages from files stored on such filesystems, must
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specify a unique file identifier using the <code class="methodname">DbMpoolFile::set_fileid()</code>
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method, and each process opening the file must provide the same unique
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identifier.</strong></span>
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</p>
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<p>
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This call should not be necessary for most applications.
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Specifically, it is not necessary if the cache is not shared
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between processes and is reinstantiated after each system reboot, if
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the application is using the Berkeley DB access methods instead of
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calling the pool functions explicitly, or if the files in the
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cache are stored on filesystems in which the default values as
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described previously are invariant between process and across system
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reboots.
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</p>
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<p>
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The <code class="methodname">DbMpoolFile::set_fileid()</code> method configures a file in the
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cache, not only operations performed using the specified
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<a class="link" href="memp.html" title="Chapter 9. The DbMpoolFile Handle">DbMpoolFile</a> handle.
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</p>
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<p>
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The <code class="methodname">DbMpoolFile::set_fileid()</code> method may not be called after the
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<a class="xref" href="mempfopen.html" title="DbMpoolFile::open()">DbMpoolFile::open()</a>
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method is called.
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</p>
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<p>
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The <code class="methodname">DbMpoolFile::set_fileid()</code> <span>
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<span>
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method either returns a non-zero error value or throws an
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exception that encapsulates a non-zero error value on
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failure, and returns 0 on success.
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</span>
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</span>
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</p>
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<div class="sect2" lang="en" xml:lang="en">
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<div class="titlepage">
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<div>
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<div>
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<h3 class="title"><a id="id3631027"></a>Parameters</h3>
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</div>
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</div>
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</div>
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<div class="sect3" lang="en" xml:lang="en">
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<div class="titlepage">
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<div>
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<div>
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<h4 class="title"><a id="id3631013"></a>fileid</h4>
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</div>
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</div>
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</div>
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<p>
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The <span class="bold"><strong>fileid</strong></span> parameter is the unique
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identifier for the file. Unique file identifiers must be a
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<code class="literal">DB_FILE_ID_LEN</code> length array of bytes.
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</p>
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</div>
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</div>
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<div class="sect2" lang="en" xml:lang="en">
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<div class="titlepage">
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<div>
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<div>
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<h3 class="title"><a id="id3631035"></a>Class</h3>
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</div>
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</div>
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</div>
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<p>
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<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>
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</p>
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</div>
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<div class="sect2" lang="en" xml:lang="en">
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<div class="titlepage">
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<div>
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<div>
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<h3 class="title"><a id="id3631079"></a>See Also</h3>
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</div>
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</div>
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</div>
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<p>
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<a class="xref" href="memp.html#memplist" title="Memory Pools and Related Methods">Memory Pools and Related Methods</a>
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</p>
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</div>
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<hr />
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<td width="40%" align="left"><a accesskey="p" href="mempset_clear_len.html">Prev</a> </td>
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<a accesskey="u" href="memp.html">Up</a>
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</td>
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<td width="40%" align="right"> <a accesskey="n" href="mempset_flags.html">Next</a></td>
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</tr>
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<tr>
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<td width="40%" align="left" valign="top">DbMpoolFile::set_clear_len() </td>
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<td width="20%" align="center">
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<a accesskey="h" href="index.html">Home</a>
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</td>
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<td width="40%" align="right" valign="top"> DbMpoolFile::set_flags()</td>
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