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<title>Flags for the xShmLock VFS method</title>
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<!-- keywords: SQLITE_SHM_EXCLUSIVE SQLITE_SHM_LOCK SQLITE_SHM_SHARED SQLITE_SHM_UNLOCK -->
<div class=nosearch>
<a href="intro.html"><h2>SQLite C Interface</h2></a>
<h2>Flags for the xShmLock VFS method</h2>
</div>
<blockquote><pre>
#define SQLITE_SHM_UNLOCK 1
#define SQLITE_SHM_LOCK 2
#define SQLITE_SHM_SHARED 4
#define SQLITE_SHM_EXCLUSIVE 8
</pre></blockquote>
<p>
These integer constants define the various locking operations
allowed by the xShmLock method of <a href="../c3ref/io_methods.html">sqlite3_io_methods</a>. The
following are the only legal combinations of flags to the
xShmLock method:</p>
<p><ul>
<li> SQLITE_SHM_LOCK | SQLITE_SHM_SHARED
<li> SQLITE_SHM_LOCK | SQLITE_SHM_EXCLUSIVE
<li> SQLITE_SHM_UNLOCK | SQLITE_SHM_SHARED
<li> SQLITE_SHM_UNLOCK | SQLITE_SHM_EXCLUSIVE
</ul></p>
<p>When unlocking, the same SHARED or EXCLUSIVE flag must be supplied as
was given on the corresponding lock.</p>
<p>The xShmLock method can transition between unlocked and SHARED or
between unlocked and EXCLUSIVE. It cannot transition between SHARED
and EXCLUSIVE.
</p><p>See also lists of
<a href="objlist.html">Objects</a>,
<a href="constlist.html">Constants</a>, and
<a href="funclist.html">Functions</a>.</p>
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