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          <th colspan="3" align="center">DB_ENV-&gt;memp_register()</th>
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          <td width="20%" align="left"><a accesskey="p" href="mempfcreate.html">Prev</a> </td>
          <th width="60%" align="center">Chapter 8. 
              The  DB_MPOOLFILE Handle
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            <h2 class="title" style="clear: both"><a id="mempregister"></a>DB_ENV-&gt;memp_register()</h2>
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      <pre class="programlisting">#include &lt;db.h&gt;

int
DB_ENV-&gt;memp_register(DB_ENV *env, int ftype,
    int (*pgin_fcn)(DB_ENV *env, db_pgno_t pgno, void *pgaddr, 
    DBT *pgcookie), int (*pgout_fcn)(DB_ENV *env, db_pgno_t pgno, 
    void *pgaddr, DBT *pgcookie));  </pre>
      <p>
         The <code class="methodname">DB_ENV-&gt;memp_register()</code> method registers page-in and page-out
         functions for files of type <span class="bold"><strong>ftype</strong></span> in the cache.
    </p>
      <p>
         If the <span class="bold"><strong>pgin_fcn</strong></span> function is non-NULL,
         it is called each time a page is read into the cache from a file
         of type <span class="bold"><strong>ftype</strong></span>, or a page is created
         for a file of type <span class="bold"><strong>ftype</strong></span> (see the
         DB_MPOOL_CREATE flag for the 
         <a class="xref" href="mempfget.html" title="DB_MPOOLFILE-&gt;get()">DB_MPOOLFILE-&gt;get()</a>  method).
    </p>
      <p>
         If the <span class="bold"><strong>pgout_fcn</strong></span> function is
         non-NULL, it is called each time a page is written to a file of type
         <span class="bold"><strong>ftype</strong></span>.
    </p>
      <p>
         The purpose of the <code class="methodname">DB_ENV-&gt;memp_register()</code> function is to support
         processing when pages are entered into, or flushed from, the cache.  For example, this
         functionality might be used to do byte-endian conversion as pages are read from, or written
         to, the underlying file.
    </p>
      <p>
         A file type must be specified to make it possible for unrelated
         threads or processes that are sharing a cache, to evict each other's
         pages from the cache.  During initialization, applications should call
         <code class="methodname">DB_ENV-&gt;memp_register()</code> for each type of file requiring input or
         output processing that will be sharing the underlying cache.  (No
         registry is necessary for the standard Berkeley DB access method types
         because <a class="xref" href="dbopen.html" title="DB-&gt;open()">DB-&gt;open()</a> 
         registers them separately.)
    </p>
      <p>
         If a thread or process does not call <code class="methodname">DB_ENV-&gt;memp_register()</code> for a
         file type, it is impossible for it to evict pages for any file
         requiring input or output processing from the cache.  For this reason,
         <code class="methodname">DB_ENV-&gt;memp_register()</code> should always be called by each application
         sharing a cache for each type of file included in the cache, regardless
         of whether or not the application itself uses files of that type.
    </p>
      <p>
         The <code class="methodname">DB_ENV-&gt;memp_register()</code> <span>
            <span>
                  method returns a non-zero error value on failure and 0 on success.
            </span>
            
        </span>
    </p>
      <div class="sect2" lang="en" xml:lang="en">
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              <h3 class="title"><a id="id3881123"></a>Parameters</h3>
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        <div class="sect3" lang="en" xml:lang="en">
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                <h4 class="title"><a id="id3881113"></a>ftype</h4>
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          <p>
                          The <span class="bold"><strong>ftype</strong></span> parameter specifies the
                          type of file for which the page-in and page-out functions will be
                          called.
                     </p>
          <p>
                          The <span class="bold"><strong>ftype</strong></span> value for a file must be a
                          non-zero positive number less than 128 (0 and negative numbers are
                          reserved for internal use by the Berkeley DB library).
                     </p>
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        <div class="sect3" lang="en" xml:lang="en">
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              <div>
                <h4 class="title"><a id="id3881031"></a>pgin_fcn, pgout_fcn</h4>
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          <p>
                             The page-in and page-out functions.
                     </p>
          <p>
                          The <span class="bold"><strong>pgin_fcn</strong></span> and <span class="bold"><strong>pgout_fcn</strong></span> functions are called with a reference
                          to the current database environment, the page number being read or
                          written, a pointer to the page being read or written, and any
                          parameter <span class="bold"><strong>pgcookie</strong></span> that was specified
                          to the <a class="xref" href="mempset_pgcookie.html" title="DB_MPOOLFILE-&gt;set_pgcookie()">DB_MPOOLFILE-&gt;set_pgcookie()</a> 
                          method.
                     </p>
          <p>
                          The <span class="bold"><strong>pgin_fcn</strong></span> and <span class="bold"><strong>pgout_fcn</strong></span> functions should return 0 on success,
                          and a non-zero value on failure, in which case the shared Berkeley DB
                          library function calling it will also fail, returning that non-zero
                          value.  The non-zero value should be selected from values outside of
                          the Berkeley DB library namespace.
                     </p>
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              <h3 class="title"><a id="id3880830"></a>Class</h3>
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        <p>
                 <a class="link" href="env.html" title="Chapter 5.  The DB_ENV Handle">DB_ENV</a>, <a class="link" href="memp.html" title="Chapter 8.  The DB_MPOOLFILE Handle">DB_MPOOLFILE</a> 
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              <h3 class="title"><a id="id3881252"></a>See Also</h3>
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        <p>
                     <a class="xref" href="memp.html#memplist" title="Memory Pools and Related Methods">Memory Pools and Related Methods</a> 
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