160 lines
6.4 KiB
HTML
160 lines
6.4 KiB
HTML
<!DOCTYPE html>
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<html><head>
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<meta name="viewport" content="width=device-width, initial-scale=1.0">
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<meta http-equiv="content-type" content="text/html; charset=UTF-8">
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<link href="../sqlite.css" rel="stylesheet">
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<title>Register A Callback To Handle SQLITE_BUSY Errors</title>
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<!-- path=../ -->
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</head>
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<body>
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<div class=nosearch>
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<a href="../index.html">
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<img class="logo" src="../images/sqlite370_banner.gif" alt="SQLite" border="0">
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</a>
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<div class="tagline desktoponly">
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Small. Fast. Reliable.<br>Choose any three.
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</div>
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</div>
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<!-- keywords: {busy handler} {busy-handler callback} sqlite3_busy_handler -->
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<div class=nosearch>
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<a href="intro.html"><h2>SQLite C Interface</h2></a>
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<h2>Register A Callback To Handle SQLITE_BUSY Errors</h2>
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</div>
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<blockquote><pre>
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int sqlite3_busy_handler(sqlite3*,int(*)(void*,int),void*);
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</pre></blockquote>
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<p>
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The sqlite3_busy_handler(D,X,P) routine sets a callback function X
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that might be invoked with argument P whenever
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an attempt is made to access a database table associated with
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<a href="../c3ref/sqlite3.html">database connection</a> D when another thread
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or process has the table locked.
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The sqlite3_busy_handler() interface is used to implement
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<a href="../c3ref/busy_timeout.html">sqlite3_busy_timeout()</a> and <a href="../pragma.html#pragma_busy_timeout">PRAGMA busy_timeout</a>.</p>
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<p>If the busy callback is NULL, then <a href="../rescode.html#busy">SQLITE_BUSY</a>
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is returned immediately upon encountering the lock. If the busy callback
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is not NULL, then the callback might be invoked with two arguments.</p>
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<p>The first argument to the busy handler is a copy of the void* pointer which
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is the third argument to sqlite3_busy_handler(). The second argument to
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the busy handler callback is the number of times that the busy handler has
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been invoked previously for the same locking event. If the
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busy callback returns 0, then no additional attempts are made to
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access the database and <a href="../rescode.html#busy">SQLITE_BUSY</a> is returned
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to the application.
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If the callback returns non-zero, then another attempt
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is made to access the database and the cycle repeats.</p>
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<p>The presence of a busy handler does not guarantee that it will be invoked
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when there is lock contention. If SQLite determines that invoking the busy
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handler could result in a deadlock, it will go ahead and return <a href="../rescode.html#busy">SQLITE_BUSY</a>
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to the application instead of invoking the
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busy handler.
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Consider a scenario where one process is holding a read lock that
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it is trying to promote to a reserved lock and
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a second process is holding a reserved lock that it is trying
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to promote to an exclusive lock. The first process cannot proceed
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because it is blocked by the second and the second process cannot
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proceed because it is blocked by the first. If both processes
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invoke the busy handlers, neither will make any progress. Therefore,
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SQLite returns <a href="../rescode.html#busy">SQLITE_BUSY</a> for the first process, hoping that this
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will induce the first process to release its read lock and allow
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the second process to proceed.</p>
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<p>The default busy callback is NULL.</p>
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<p>There can only be a single busy handler defined for each
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<a href="../c3ref/sqlite3.html">database connection</a>. Setting a new busy handler clears any
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previously set handler. Note that calling <a href="../c3ref/busy_timeout.html">sqlite3_busy_timeout()</a>
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or evaluating <a href="../pragma.html#pragma_busy_timeout">PRAGMA busy_timeout=N</a> will change the
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busy handler and thus clear any previously set busy handler.</p>
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<p>The busy callback should not take any actions which modify the
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database connection that invoked the busy handler. In other words,
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the busy handler is not reentrant. Any such actions
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result in undefined behavior.</p>
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<p>A busy handler must not close the database connection
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or <a href="../c3ref/stmt.html">prepared statement</a> that invoked the busy handler.
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</p><p>See also lists of
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<a href="objlist.html">Objects</a>,
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<a href="constlist.html">Constants</a>, and
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<a href="funclist.html">Functions</a>.</p>
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