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<title>Berkeley DB Reference Guide: Standard lock modes</title>
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<td><h3><dl><dt>Berkeley DB Reference Guide:<dd>Locking Subsystem</dl></h3></td>
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<p>
<h1 align=center>Standard lock modes</h1>
<p>The Berkeley DB locking protocol is described by a conflict matrix. A conflict
matrix is an n x n array where 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.
<p>The Berkeley DB include files declare two commonly used conflict arrays:
<p><dl compact>
<p><dt>const u_int8_t db_rw_conflicts[ ];<dd>This is a conflict matrix for a simple scheme using shared and exclusive
lock modes.
<p><dt>const u_int8_t db_riw_conflicts[ ];<dd>This is a conflict matrix that involves various intent lock modes (e.g.,
intent shared) that are used for multigranularity locking.
</dl>
<p>The number of modes associated with each matrix are DB_LOCK_RW_N and
DB_LOCK_RIW_N, respectively.
<p>In addition, the Berkeley DB include file defines the type <b>db_lockmode_t</b>,
which is the type of the lock modes used with the standard tables above:
<p><dl compact>
<p><dt>DB_LOCK_NG<dd>not granted (always 0)
<p><dt>DB_LOCK_READ<dd>read (shared)
<p><dt>DB_LOCK_WRITE<dd>write (exclusive)
</dl>
<p>As an example, consider the basic multiple-reader/single writer conflict
matrix described by <b>db_rw_conflicts</b>. In the following
example (and in the appropriate file), a 1 represents a conflict (i.e.,
do not grant the lock if the indicated lock is held) and a 0 indicates
that it is OK to grant the lock.
<p>The rows indicate the lock that is held and the columns indicate the lock
that is requested.
<p><blockquote><pre> Notheld Read Write
Notheld 0 0 0
Read* 0 0 1
Write** 0 1 1
</pre></blockquote>
<p><dl compact>
<p><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.
<p><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.
</dl>
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