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+<!-- keywords: sqlite3_unlock_notify -->
+<div class=nosearch>
+<a href="../c3ref/intro.html"><h2>SQLite C Interface</h2></a>
+<h2>Unlock Notification</h2>
+</div>
+<blockquote><pre>
+int sqlite3_unlock_notify(
+  sqlite3 *pBlocked,                          /* Waiting connection */
+  void (*xNotify)(void **apArg, int nArg),    /* Callback function to invoke */
+  void *pNotifyArg                            /* Argument to pass to xNotify */
+);
+</pre></blockquote>
+<p>
+When running in shared-cache mode, a database operation may fail with
+an <a href="../rescode.html#locked">SQLITE_LOCKED</a> error if the required locks on the shared-cache or
+individual tables within the shared-cache cannot be obtained. See
+<a href="../sharedcache.html">SQLite Shared-Cache Mode</a> for a description of shared-cache locking.
+This API may be used to register a callback that SQLite will invoke
+when the connection currently holding the required lock relinquishes it.
+This API is only available if the library was compiled with the
+<a href="../compile.html#enable_unlock_notify">SQLITE_ENABLE_UNLOCK_NOTIFY</a> C-preprocessor symbol defined.</p>
+
+<p>See Also: <a href="../unlock_notify.html">Using the SQLite Unlock Notification Feature</a>.</p>
+
+<p>Shared-cache locks are released when a database connection concludes
+its current transaction, either by committing it or rolling it back.</p>
+
+<p>When a connection (known as the blocked connection) fails to obtain a
+shared-cache lock and SQLITE_LOCKED is returned to the caller, the
+identity of the database connection (the blocking connection) that
+has locked the required resource is stored internally. After an
+application receives an SQLITE_LOCKED error, it may call the
+sqlite3_unlock_notify() method with the blocked connection handle as
+the first argument to register for a callback that will be invoked
+when the blocking connections current transaction is concluded. The
+callback is invoked from within the <a href="../c3ref/step.html">sqlite3_step</a> or <a href="../c3ref/close.html">sqlite3_close</a>
+call that concludes the blocking connection's transaction.</p>
+
+<p>If sqlite3_unlock_notify() is called in a multi-threaded application,
+there is a chance that the blocking connection will have already
+concluded its transaction by the time sqlite3_unlock_notify() is invoked.
+If this happens, then the specified callback is invoked immediately,
+from within the call to sqlite3_unlock_notify().</p>
+
+<p>If the blocked connection is attempting to obtain a write-lock on a
+shared-cache table, and more than one other connection currently holds
+a read-lock on the same table, then SQLite arbitrarily selects one of
+the other connections to use as the blocking connection.</p>
+
+<p>There may be at most one unlock-notify callback registered by a
+blocked connection. If sqlite3_unlock_notify() is called when the
+blocked connection already has a registered unlock-notify callback,
+then the new callback replaces the old. If sqlite3_unlock_notify() is
+called with a NULL pointer as its second argument, then any existing
+unlock-notify callback is canceled. The blocked connections
+unlock-notify callback may also be canceled by closing the blocked
+connection using <a href="../c3ref/close.html">sqlite3_close()</a>.</p>
+
+<p>The unlock-notify callback is not reentrant. If an application invokes
+any sqlite3_xxx API functions from within an unlock-notify callback, a
+crash or deadlock may be the result.</p>
+
+<p>Unless deadlock is detected (see below), sqlite3_unlock_notify() always
+returns SQLITE_OK.</p>
+
+<p><b>Callback Invocation Details</b></p>
+
+<p>When an unlock-notify callback is registered, the application provides a
+single void* pointer that is passed to the callback when it is invoked.
+However, the signature of the callback function allows SQLite to pass
+it an array of void* context pointers. The first argument passed to
+an unlock-notify callback is a pointer to an array of void* pointers,
+and the second is the number of entries in the array.</p>
+
+<p>When a blocking connection's transaction is concluded, there may be
+more than one blocked connection that has registered for an unlock-notify
+callback. If two or more such blocked connections have specified the
+same callback function, then instead of invoking the callback function
+multiple times, it is invoked once with the set of void* context pointers
+specified by the blocked connections bundled together into an array.
+This gives the application an opportunity to prioritize any actions
+related to the set of unblocked database connections.</p>
+
+<p><b>Deadlock Detection</b></p>
+
+<p>Assuming that after registering for an unlock-notify callback a
+database waits for the callback to be issued before taking any further
+action (a reasonable assumption), then using this API may cause the
+application to deadlock. For example, if connection X is waiting for
+connection Y's transaction to be concluded, and similarly connection
+Y is waiting on connection X's transaction, then neither connection
+will proceed and the system may remain deadlocked indefinitely.</p>
+
+<p>To avoid this scenario, the sqlite3_unlock_notify() performs deadlock
+detection. If a given call to sqlite3_unlock_notify() would put the
+system in a deadlocked state, then SQLITE_LOCKED is returned and no
+unlock-notify callback is registered. The system is said to be in
+a deadlocked state if connection A has registered for an unlock-notify
+callback on the conclusion of connection B's transaction, and connection
+B has itself registered for an unlock-notify callback when connection
+A's transaction is concluded. Indirect deadlock is also detected, so
+the system is also considered to be deadlocked if connection B has
+registered for an unlock-notify callback on the conclusion of connection
+C's transaction, where connection C is waiting on connection A. Any
+number of levels of indirection are allowed.</p>
+
+<p><b>The "DROP TABLE" Exception</b></p>
+
+<p>When a call to <a href="../c3ref/step.html">sqlite3_step()</a> returns SQLITE_LOCKED, it is almost
+always appropriate to call sqlite3_unlock_notify(). There is however,
+one exception. When executing a "DROP TABLE" or "DROP INDEX" statement,
+SQLite checks if there are any currently executing SELECT statements
+that belong to the same connection. If there are, SQLITE_LOCKED is
+returned. In this case there is no "blocking connection", so invoking
+sqlite3_unlock_notify() results in the unlock-notify callback being
+invoked immediately. If the application then re-attempts the "DROP TABLE"
+or "DROP INDEX" query, an infinite loop might be the result.</p>
+
+<p>One way around this problem is to check the extended error code returned
+by an sqlite3_step() call. If there is a blocking connection, then the
+extended error code is set to SQLITE_LOCKED_SHAREDCACHE. Otherwise, in
+the special "DROP TABLE/INDEX" case, the extended error code is just
+SQLITE_LOCKED.
+</p><p>See also lists of
+  <a href="../c3ref/objlist.html">Objects</a>,
+  <a href="../c3ref/constlist.html">Constants</a>, and
+  <a href="../c3ref/funclist.html">Functions</a>.</p>
+