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## SQLite C Interface

## Checkpoint a database

>     int sqlite3_wal_checkpoint_v2(
>       sqlite3 *db,                    /* Database handle */
>       const char *zDb,                /* Name of attached database (or NULL) */
>       int eMode,                      /* SQLITE_CHECKPOINT_* value */
>       int *pnLog,                     /* OUT: Size of WAL log in frames */
>       int *pnCkpt                     /* OUT: Total number of frames checkpointed */
>     );

The sqlite3_wal_checkpoint_v2(D,X,M,L,C) interface runs a checkpoint operation on database X of database connection D in mode M. Status information is written back into integers pointed to by L and C. The M parameter must be a valid checkpoint mode:

SQLITE_CHECKPOINT_PASSIVE
  Checkpoint as many frames as possible without waiting for any database readers or writers to finish, then sync the database file if all frames in the log were checkpointed. The busy-handler callback is never invoked in the SQLITE_CHECKPOINT_PASSIVE mode. On the other hand, passive mode might leave the checkpoint unfinished if there are concurrent readers or writers.

SQLITE_CHECKPOINT_FULL
  This mode blocks (it invokes the busy-handler callback) until there is no database writer and all readers are reading from the most recent database snapshot. It then checkpoints all frames in the log file and syncs the database file. This mode blocks new database writers while it is pending, but new database readers are allowed to continue unimpeded.

SQLITE_CHECKPOINT_RESTART
  This mode works the same way as SQLITE_CHECKPOINT_FULL with the addition that after checkpointing the log file it blocks (calls the busy-handler callback) until all readers are reading from the database file only. This ensures that the next writer will restart the log file from the beginning. Like SQLITE_CHECKPOINT_FULL, this mode blocks new database writer attempts while it is pending, but does not impede readers.

SQLITE_CHECKPOINT_TRUNCATE
  This mode works the same way as SQLITE_CHECKPOINT_RESTART with the addition that it also truncates the log file to zero bytes just prior to a successful return.

SQLITE_CHECKPOINT_NOOP
  This mode always checkpoints zero frames. The only reason to invoke a NOOP checkpoint is to access the values returned by sqlite3_wal_checkpoint_v2() via output parameters *pnLog and *pnCkpt.

If pnLog is not NULL, then *pnLog is set to the total number of frames in the log file or to -1 if the checkpoint could not run because of an error or because the database is not in WAL mode. If pnCkpt is not NULL,then *pnCkpt is set to the total number of checkpointed frames in the log file (including any that were already checkpointed before the function was called) or to -1 if the checkpoint could not run due to an error or because the database is not in WAL mode. Note that upon successful completion of an SQLITE_CHECKPOINT_TRUNCATE, the log file will have been truncated to zero bytes and so both *pnLog and *pnCkpt will be set to zero.

All calls obtain an exclusive "checkpoint" lock on the database file. If any other process is running a checkpoint operation at the same time, the lock cannot be obtained and SQLITE_BUSY is returned. Even if there is a busy-handler configured, it will not be invoked in this case.

The SQLITE_CHECKPOINT_FULL, RESTART and TRUNCATE modes also obtain the exclusive "writer" lock on the database file. If the writer lock cannot be obtained immediately, and a busy-handler is configured, it is invoked and the writer lock retried until either the busy-handler returns 0 or the lock is successfully obtained. The busy-handler is also invoked while waiting for database readers as described above. If the busy-handler returns 0 before the writer lock is obtained or while waiting for database readers, the checkpoint operation proceeds from that point in the same way as SQLITE_CHECKPOINT_PASSIVE - checkpointing as many frames as possible without blocking any further. SQLITE_BUSY is returned in this case.

If parameter zDb is NULL or points to a zero length string, then the specified operation is attempted on all WAL databases attached to database connection db. In this case the values written to output parameters *pnLog and *pnCkpt are undefined. If an SQLITE_BUSY error is encountered when processing one or more of the attached WAL databases, the operation is still attempted on any remaining attached databases and SQLITE_BUSY is returned at the end. If any other error occurs while processing an attached database, processing is abandoned and the error code is returned to the caller immediately. If no error (SQLITE_BUSY or otherwise) is encountered while processing the attached databases, SQLITE_OK is returned.

If database zDb is the name of an attached database that is not in WAL mode, SQLITE_OK is returned and both *pnLog and *pnCkpt set to -1. If zDb is not NULL (or a zero length string) and is not the name of any attached database, SQLITE_ERROR is returned to the caller.

Unless it returns SQLITE_MISUSE, the sqlite3_wal_checkpoint_v2() interface sets the error information that is queried by sqlite3_errcode() and sqlite3_errmsg().

The PRAGMA wal_checkpoint command can be used to invoke this interface from SQL.

See also lists of Objects, Constants, and Functions.