190 lines
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HTML
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<!-- keywords: sqlite3_autovacuum_pages -->
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<div class=nosearch>
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<a href="../c3ref/intro.html"><h2>SQLite C Interface</h2></a>
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<h2>Autovacuum Compaction Amount Callback</h2>
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</div>
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<blockquote><pre>
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int sqlite3_autovacuum_pages(
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sqlite3 *db,
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unsigned int(*)(void*,const char*,unsigned int,unsigned int,unsigned int),
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void*,
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void(*)(void*)
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);
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</pre></blockquote>
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<p>
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The sqlite3_autovacuum_pages(D,C,P,X) interface registers a callback
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function C that is invoked prior to each autovacuum of the database
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file. The callback is passed a copy of the generic data pointer (P),
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the schema-name of the attached database that is being autovacuumed,
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the size of the database file in pages, the number of free pages,
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and the number of bytes per page, respectively. The callback should
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return the number of free pages that should be removed by the
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autovacuum. If the callback returns zero, then no autovacuum happens.
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If the value returned is greater than or equal to the number of
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free pages, then a complete autovacuum happens.</p>
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<p><p>If there are multiple ATTACH-ed database files that are being
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modified as part of a transaction commit, then the autovacuum pages
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callback is invoked separately for each file.</p>
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<p><p><b>The callback is not reentrant.</b> The callback function should
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not attempt to invoke any other SQLite interface. If it does, bad
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things may happen, including segmentation faults and corrupt database
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files. The callback function should be a simple function that
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does some arithmetic on its input parameters and returns a result.</p>
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<p>The X parameter to sqlite3_autovacuum_pages(D,C,P,X) is an optional
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destructor for the P parameter. If X is not NULL, then X(P) is
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invoked whenever the database connection closes or when the callback
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is overwritten by another invocation of sqlite3_autovacuum_pages().</p>
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<p><p>There is only one autovacuum pages callback per database connection.
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Each call to the sqlite3_autovacuum_pages() interface overrides all
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previous invocations for that database connection. If the callback
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argument (C) to sqlite3_autovacuum_pages(D,C,P,X) is a NULL pointer,
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then the autovacuum steps callback is canceled. The return value
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from sqlite3_autovacuum_pages() is normally SQLITE_OK, but might
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be some other error code if something goes wrong. The current
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implementation will only return SQLITE_OK or SQLITE_MISUSE, but other
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return codes might be added in future releases.</p>
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<p><p>If no autovacuum pages callback is specified (the usual case) or
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a NULL pointer is provided for the callback,
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then the default behavior is to vacuum all free pages. So, in other
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words, the default behavior is the same as if the callback function
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were something like this:</p>
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<p><blockquote><pre>
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unsigned int demonstration_autovac_pages_callback(
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void *pClientData,
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const char *zSchema,
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unsigned int nDbPage,
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unsigned int nFreePage,
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unsigned int nBytePerPage
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){
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return nFreePage;
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}
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</pre></blockquote>
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</p><p>See also lists of
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<a href="../c3ref/objlist.html">Objects</a>,
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<a href="../c3ref/constlist.html">Constants</a>, and
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<a href="../c3ref/funclist.html">Functions</a>.</p>
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