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## SQLite C Interface

## Application Defined Page Cache.

>     typedef struct sqlite3_pcache_methods sqlite3_pcache_methods;
>     struct sqlite3_pcache_methods {
>       void *pArg;
>       int (*xInit)(void*);
>       void (*xShutdown)(void*);
>       sqlite3_pcache *(*xCreate)(int szPage, int bPurgeable);
>       void (*xCachesize)(sqlite3_pcache*, int nCachesize);
>       int (*xPagecount)(sqlite3_pcache*);
>       void *(*xFetch)(sqlite3_pcache*, unsigned key, int createFlag);
>       void (*xUnpin)(sqlite3_pcache*, void*, int discard);
>       void (*xRekey)(sqlite3_pcache*, void*, unsigned oldKey, unsigned newKey);
>       void (*xTruncate)(sqlite3_pcache*, unsigned iLimit);
>       void (*xDestroy)(sqlite3_pcache*);
>     };

The sqlite3_config(SQLITE_CONFIG_PCACHE, ...) interface can register an alternative page cache implementation by passing in an instance of the sqlite3_pcache_methods structure. The majority of the heap memory used by SQLite is used by the page cache to cache data read from, or ready to be written to, the database file. By implementing a custom page cache using this API, an application can control more precisely the amount of memory consumed by SQLite, the way in which that memory is allocated and released, and the policies used to determine exactly which parts of a database file are cached and for how long.

The contents of the sqlite3_pcache_methods structure are copied to an internal buffer by SQLite within the call to sqlite3_config. Hence the application may discard the parameter after the call to sqlite3_config() returns.

The xInit() method is called once for each call to sqlite3_initialize() (usually only once during the lifetime of the process). The xInit() method is passed a copy of the sqlite3_pcache_methods.pArg value. The xInit() method can set up up global structures and/or any mutexes required by the custom page cache implementation.

The xShutdown() method is called from within sqlite3_shutdown(), if the application invokes this API. It can be used to clean up any outstanding resources before process shutdown, if required.

SQLite holds a SQLITE_MUTEX_RECURSIVE mutex when it invokes the xInit method, so the xInit method need not be threadsafe. The xShutdown method is only called from sqlite3_shutdown() so it does not need to be threadsafe either. All other methods must be threadsafe in multithreaded applications.

SQLite will never invoke xInit() more than once without an intervening call to xShutdown().

The xCreate() method is used to construct a new cache instance. SQLite will typically create one cache instance for each open database file, though this is not guaranteed. The first parameter, szPage, is the size in bytes of the pages that must be allocated by the cache. szPage will not be a power of two. szPage will the page size of the database file that is to be cached plus an increment (here called "R") of about 100 or 200. SQLite will use the extra R bytes on each page to store metadata about the underlying database page on disk. The value of R depends on the SQLite version, the target platform, and how SQLite was compiled. R is constant for a particular build of SQLite. The second argument to xCreate(), bPurgeable, is true if the cache being created will be used to cache database pages of a file stored on disk, or false if it is used for an in-memory database. The cache implementation does not have to do anything special based with the value of bPurgeable; it is purely advisory. On a cache where bPurgeable is false, SQLite will never invoke xUnpin() except to deliberately delete a page. In other words, a cache created with bPurgeable set to false will never contain any unpinned pages.

The xCachesize() method may be called at any time by SQLite to set the suggested maximum cache-size (number of pages stored by) the cache instance passed as the first argument. This is the value configured using the SQLite "PRAGMA cache_size" command. As with the bPurgeable parameter, the implementation is not required to do anything with this value; it is advisory only.

The xPagecount() method should return the number of pages currently stored in the cache.

The xFetch() method is used to fetch a page and return a pointer to it. A 'page', in this context, is a buffer of szPage bytes aligned at an 8-byte boundary. The page to be fetched is determined by the key. The mimimum key value is 1. After it has been retrieved using xFetch, the page is considered to be "pinned".

If the requested page is already in the page cache, then the page cache implementation must return a pointer to the page buffer with its content intact. If the requested page is not already in the cache, then the behavior of the cache implementation is determined by the value of the createFlag parameter passed to xFetch, according to the following table:

createFlag | Behaviour when page is not already in cache
--- | ---
0 | Do not allocate a new page. Return NULL.
1 | Allocate a new page if it easy and convenient to do so. Otherwise return NULL.
2 | Make every effort to allocate a new page. Only return NULL if allocating a new page is effectively impossible.

SQLite will normally invoke xFetch() with a createFlag of 0 or 1. If a call to xFetch() with createFlag==1 returns NULL, then SQLite will attempt to unpin one or more cache pages by spilling the content of pinned pages to disk and synching the operating system disk cache. After attempting to unpin pages, the xFetch() method will be invoked again with a createFlag of 2.

xUnpin() is called by SQLite with a pointer to a currently pinned page as its second argument. If the third parameter, discard, is non-zero, then the page should be evicted from the cache. In this case SQLite assumes that the next time the page is retrieved from the cache using the xFetch() method, it will be zeroed. If the discard parameter is zero, then the page is considered to be unpinned. The cache implementation may choose to evict unpinned pages at any time.

The cache is not required to perform any reference counting. A single call to xUnpin() unpins the page regardless of the number of prior calls to xFetch().

The xRekey() method is used to change the key value associated with the page passed as the second argument from oldKey to newKey. If the cache previously contains an entry associated with newKey, it should be discarded. Any prior cache entry associated with newKey is guaranteed not to be pinned.

When SQLite calls the xTruncate() method, the cache must discard all existing cache entries with page numbers (keys) greater than or equal to the value of the iLimit parameter passed to xTruncate(). If any of these pages are pinned, they are implicitly unpinned, meaning that they can be safely discarded.

The xDestroy() method is used to delete a cache allocated by xCreate(). All resources associated with the specified cache should be freed. After calling the xDestroy() method, SQLite considers the sqlite3_pcache* handle invalid, and will not use it with any other sqlite3_pcache_methods functions.

See also lists of Objects, Constants, and Functions.

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