Autovacuum Compaction Amount Callback

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## SQLite C Interface

## Autovacuum Compaction Amount Callback

>     int sqlite3_autovacuum_pages(
>       sqlite3 *db,
>       unsigned int(*)(void*,const char*,unsigned int,unsigned int,unsigned int),
>       void*,
>       void(*)(void*)
>     );

The sqlite3_autovacuum_pages(D,C,P,X) interface registers a callback function C that is invoked prior to each autovacuum of the database file. The callback is passed a copy of the generic data pointer (P), the schema-name of the attached database that is being autovacuumed, the size of the database file in pages, the number of free pages, and the number of bytes per page, respectively. The callback should return the number of free pages that should be removed by the autovacuum. If the callback returns zero, then no autovacuum happens. If the value returned is greater than or equal to the number of free pages, then a complete autovacuum happens.

If there are multiple ATTACH-ed database files that are being modified as part of a transaction commit, then the autovacuum pages callback is invoked separately for each file.

The callback is not reentrant. The callback function should not attempt to invoke any other SQLite interface. If it does, bad things may happen, including segmentation faults and corrupt database files. The callback function should be a simple function that does some arithmetic on its input parameters and returns a result.

The X parameter to sqlite3_autovacuum_pages(D,C,P,X) is an optional destructor for the P parameter. If X is not NULL, then X(P) is invoked whenever the database connection closes or when the callback is overwritten by another invocation of sqlite3_autovacuum_pages().

There is only one autovacuum pages callback per database connection. Each call to the sqlite3_autovacuum_pages() interface overrides all previous invocations for that database connection. If the callback argument (C) to sqlite3_autovacuum_pages(D,C,P,X) is a NULL pointer, then the autovacuum steps callback is cancelled. The return value from sqlite3_autovacuum_pages() is normally SQLITE_OK, but might be some other error code if something goes wrong. The current implementation will only return SQLITE_OK or SQLITE_MISUSE, but other return codes might be added in future releases.

If no autovacuum pages callback is specified (the usual case) or a NULL pointer is provided for the callback, then the default behavior is to vacuum all free pages. So, in other words, the default behavior is the same as if the callback function were something like this:

>         unsigned int demonstration_autovac_pages_callback(
>           void *pClientData,
>           const char *zSchema,
>           unsigned int nDbPage,
>           unsigned int nFreePage,
>           unsigned int nBytePerPage
>         ){
>           return nFreePage;
>         }

See also lists of Objects, Constants, and Functions.