void *sqlite3_wal_hook( sqlite3*, int(*)(void *,sqlite3*,const char*,int), void* );
The sqlite3_wal_hook() function is used to register a callback that will be invoked each time a database connection commits data to a write-ahead log (i.e. whenever a transaction is committed in journal_mode=WAL mode).
The callback is invoked by SQLite after the commit has taken place and the associated write-lock on the database released, so the implementation may read, write or checkpoint the database as required.
The first parameter passed to the callback function when it is invoked is a copy of the third parameter passed to sqlite3_wal_hook() when registering the callback. The second is a copy of the database handle. The third parameter is the name of the database that was written to - either "main" or the name of an ATTACH-ed database. The fourth parameter is the number of pages currently in the write-ahead log file, including those that were just committed.
The callback function should normally return SQLITE_OK. If an error code is returned, that error will propagate back up through the SQLite code base to cause the statement that provoked the callback to report an error, though the commit will have still occurred. If the callback returns SQLITE_ROW or SQLITE_DONE, or if it returns a value that does not correspond to any valid SQLite error code, the results are undefined.
A single database handle may have at most a single write-ahead log callback registered at one time. Calling sqlite3_wal_hook() replaces any previously registered write-ahead log callback. Note that the sqlite3_wal_autocheckpoint() interface and the wal_autocheckpoint pragma both invoke sqlite3_wal_hook() and will those overwrite any prior sqlite3_wal_hook() settings.
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