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<p>The Berkeley DB locking protocol is described by a conflict matrix. A
conflict matrix is an NxN array in which N is the number of different
lock modes supported, and the (i, j)th entry of the array indicates
whether a lock of mode i conflicts with a lock of mode j. In addition,
Berkeley DB defines the type <span class="bold"><strong>db_lockmode_t</strong></span>, which is the type of a
lock mode within a conflict matrix.</p>
<p>The following is an example of a conflict matrix. The actual conflict
matrix used by Berkeley DB to support the underlying access methods is more
complicated, but this matrix shows the lock mode relationships available
to applications using the Berkeley DB Locking subsystem interfaces directly.</p>
<div class="variablelist">
<dl>
<dt>
<span class="term">DB_LOCK_NG</span>
</dt>
<dd>not granted (always 0)</dd>
<dt>
<span class="term">DB_LOCK_READ</span>
</dt>
<dd>read (shared)</dd>
<dt>
<span class="term">DB_LOCK_WRITE</span>
</dt>
<dd>write (exclusive)</dd>
<dt>
<span class="term">DB_LOCK_IWRITE</span>
</dt>
<dd>intention to write (shared)</dd>
<dt>
<span class="term">DB_LOCK_IREAD</span>
</dt>
<dd>intention to read (shared)</dd>
<dt>
<span class="term">DB_LOCK_IWR</span>
</dt>
<dd>intention to read and write (shared)</dd>
</dl>
</div>
<p>In a conflict matrix, the rows indicate the lock that is held, and the
columns indicate the lock that is requested. A 1 represents a conflict
(that is, do not grant the lock if the indicated lock is held), and a
0 indicates that it is OK to grant the lock.</p>
<pre class="programlisting"> Notheld Read Write IWrite IRead IRW
Notheld 0 0 0 0 0 0
Read* 0 0 1 1 0 1
Write** 0 1 1 1 1 1
Intent Write 0 1 1 0 0 0
Intent Read 0 0 1 0 0 0
Intent RW 0 1 1 0 0 0</pre>
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<dt>
<span class="term">*</span>
</dt>
<dd>In this case, suppose that there is a read lock held on an object. A new
request for a read lock would be granted, but a request for a write lock
would not.</dd>
<dt>
<span class="term">**</span>
</dt>
<dd>In this case, suppose that there is a write lock held on an object. A
new request for either a read or write lock would be denied.</dd>
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