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+<!-- keywords: {busy handler} {busy-handler callback} sqlite3_busy_handler -->
+<div class=nosearch>
+<a href="intro.html"><h2>SQLite C Interface</h2></a>
+<h2>Register A Callback To Handle SQLITE_BUSY Errors</h2>
+</div>
+<blockquote><pre>
+int sqlite3_busy_handler(sqlite3*,int(*)(void*,int),void*);
+</pre></blockquote>
+<p>
+The sqlite3_busy_handler(D,X,P) routine sets a callback function X
+that might be invoked with argument P whenever
+an attempt is made to access a database table associated with
+<a href="../c3ref/sqlite3.html">database connection</a> D when another thread
+or process has the table locked.
+The sqlite3_busy_handler() interface is used to implement
+<a href="../c3ref/busy_timeout.html">sqlite3_busy_timeout()</a> and <a href="../pragma.html#pragma_busy_timeout">PRAGMA busy_timeout</a>.</p>
+
+<p>If the busy callback is NULL, then <a href="../rescode.html#busy">SQLITE_BUSY</a>
+is returned immediately upon encountering the lock.  If the busy callback
+is not NULL, then the callback might be invoked with two arguments.</p>
+
+<p>The first argument to the busy handler is a copy of the void* pointer which
+is the third argument to sqlite3_busy_handler().  The second argument to
+the busy handler callback is the number of times that the busy handler has
+been invoked previously for the same locking event.  If the
+busy callback returns 0, then no additional attempts are made to
+access the database and <a href="../rescode.html#busy">SQLITE_BUSY</a> is returned
+to the application.
+If the callback returns non-zero, then another attempt
+is made to access the database and the cycle repeats.</p>
+
+<p>The presence of a busy handler does not guarantee that it will be invoked
+when there is lock contention. If SQLite determines that invoking the busy
+handler could result in a deadlock, it will go ahead and return <a href="../rescode.html#busy">SQLITE_BUSY</a>
+to the application instead of invoking the
+busy handler.
+Consider a scenario where one process is holding a read lock that
+it is trying to promote to a reserved lock and
+a second process is holding a reserved lock that it is trying
+to promote to an exclusive lock.  The first process cannot proceed
+because it is blocked by the second and the second process cannot
+proceed because it is blocked by the first.  If both processes
+invoke the busy handlers, neither will make any progress.  Therefore,
+SQLite returns <a href="../rescode.html#busy">SQLITE_BUSY</a> for the first process, hoping that this
+will induce the first process to release its read lock and allow
+the second process to proceed.</p>
+
+<p>The default busy callback is NULL.</p>
+
+<p>There can only be a single busy handler defined for each
+<a href="../c3ref/sqlite3.html">database connection</a>.  Setting a new busy handler clears any
+previously set handler.  Note that calling <a href="../c3ref/busy_timeout.html">sqlite3_busy_timeout()</a>
+or evaluating <a href="../pragma.html#pragma_busy_timeout">PRAGMA busy_timeout=N</a> will change the
+busy handler and thus clear any previously set busy handler.</p>
+
+<p>The busy callback should not take any actions which modify the
+database connection that invoked the busy handler.  In other words,
+the busy handler is not reentrant.  Any such actions
+result in undefined behavior.</p>
+
+<p>A busy handler must not close the database connection
+or <a href="../c3ref/stmt.html">prepared statement</a> that invoked the busy handler.
+</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>
+