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SQLITE3_BACKUP_INIT(3) Library Functions Manual SQLITE3_BACKUP_INIT(3)
NAME
sqlite3_backup_init, sqlite3_backup_step, sqlite3_backup_finish,
sqlite3_backup_remaining, sqlite3_backup_pagecount - online backup API
SYNOPSIS
#include <sqlite3.h>
sqlite3_backup *
sqlite3_backup_init(sqlite3 *pDest, const char *zDestName,
sqlite3 *pSource, const char *zSourceName);
int
sqlite3_backup_step(sqlite3_backup *p, int nPage);
int
sqlite3_backup_finish(sqlite3_backup *p);
int
sqlite3_backup_remaining(sqlite3_backup *p);
int
sqlite3_backup_pagecount(sqlite3_backup *p);
DESCRIPTION
The backup API copies the content of one database into another. It is
useful either for creating backups of databases or for copying in-memory
databases to or from persistent files.
SQLite holds a write transaction open on the destination database file
for the duration of the backup operation. The source database is read-
locked only while it is being read; it is not locked continuously for the
entire backup operation. Thus, the backup may be performed on a live
source database without preventing other database connections from
reading or writing to the source database while the backup is underway.
To perform a backup operation:
1. sqlite3_backup_init() is called once to initialize the backup,
2. sqlite3_backup_step() is called one or more times to transfer the
data between the two databases, and finally
3. sqlite3_backup_finish() is called to release all resources
associated with the backup operation.
There should be exactly one call to sqlite3_backup_finish() for each
successful call to sqlite3_backup_init().
sqlite3_backup_init()
The D and N arguments to sqlite3_backup_init(D,N,S,M) are the database
connection associated with the destination database and the database
name, respectively. The database name is "main" for the main database,
"temp" for the temporary database, or the name specified after the AS
keyword in an ATTACH statement for an attached database. The S and M
arguments passed to sqlite3_backup_init(D,N,S,M) identify the database
connection and database name of the source database, respectively. The
source and destination database connections (parameters S and D) must be
different or else sqlite3_backup_init(D,N,S,M) will fail with an error.
A call to sqlite3_backup_init() will fail, returning NULL, if there is
already a read or read-write transaction open on the destination
database.
If an error occurs within sqlite3_backup_init(D,N,S,M), then NULL is
returned and an error code and error message are stored in the
destination database connection D. The error code and message for the
failed call to sqlite3_backup_init() can be retrieved using the
sqlite3_errcode(), sqlite3_errmsg(), and/or sqlite3_errmsg16() functions.
A successful call to sqlite3_backup_init() returns a pointer to an
sqlite3_backup object. The sqlite3_backup object may be used with the
sqlite3_backup_step() and sqlite3_backup_finish() functions to perform
the specified backup operation.
sqlite3_backup_step()
Function sqlite3_backup_step(B,N) will copy up to N pages between the
source and destination databases specified by sqlite3_backup object B.
If N is negative, all remaining source pages are copied. If
sqlite3_backup_step(B,N) successfully copies N pages and there are still
more pages to be copied, then the function returns SQLITE_OK. If
sqlite3_backup_step(B,N) successfully finishes copying all pages from
source to destination, then it returns SQLITE_DONE. If an error occurs
while running sqlite3_backup_step(B,N), then an error code is returned.
As well as SQLITE_OK and SQLITE_DONE, a call to sqlite3_backup_step() may
return SQLITE_READONLY, SQLITE_NOMEM, SQLITE_BUSY, SQLITE_LOCKED, or an
SQLITE_IOERR_XXX extended error code.
The sqlite3_backup_step() might return SQLITE_READONLY if
1. the destination database was opened read-only, or
2. the destination database is using write-ahead-log journaling and the
destination and source page sizes differ, or
3. the destination database is an in-memory database and the
destination and source page sizes differ.
If sqlite3_backup_step() cannot obtain a required file-system lock, then
the busy-handler function is invoked (if one is specified). If the busy-
handler returns non-zero before the lock is available, then SQLITE_BUSY
is returned to the caller. In this case the call to
sqlite3_backup_step() can be retried later. If the source database
connection is being used to write to the source database when
sqlite3_backup_step() is called, then SQLITE_LOCKED is returned
immediately. Again, in this case the call to sqlite3_backup_step() can
be retried later on. If SQLITE_IOERR_XXX, SQLITE_NOMEM, or
SQLITE_READONLY is returned, then there is no point in retrying the call
to sqlite3_backup_step(). These errors are considered fatal. The
application must accept that the backup operation has failed and pass the
backup operation handle to the sqlite3_backup_finish() to release
associated resources.
The first call to sqlite3_backup_step() obtains an exclusive lock on the
destination file. The exclusive lock is not released until either
sqlite3_backup_finish() is called or the backup operation is complete and
sqlite3_backup_step() returns SQLITE_DONE. Every call to
sqlite3_backup_step() obtains a shared lock on the source database that
lasts for the duration of the sqlite3_backup_step() call. Because the
source database is not locked between calls to sqlite3_backup_step(), the
source database may be modified mid-way through the backup process. If
the source database is modified by an external process or via a database
connection other than the one being used by the backup operation, then
the backup will be automatically restarted by the next call to
sqlite3_backup_step(). If the source database is modified by the using
the same database connection as is used by the backup operation, then the
backup database is automatically updated at the same time.
sqlite3_backup_finish()
When sqlite3_backup_step() has returned SQLITE_DONE, or when the
application wishes to abandon the backup operation, the application
should destroy the sqlite3_backup by passing it to
sqlite3_backup_finish(). The sqlite3_backup_finish() interfaces releases
all resources associated with the sqlite3_backup object. If
sqlite3_backup_step() has not yet returned SQLITE_DONE, then any active
write-transaction on the destination database is rolled back. The
sqlite3_backup object is invalid and may not be used following a call to
sqlite3_backup_finish().
The value returned by sqlite3_backup_finish is SQLITE_OK if no
sqlite3_backup_step() errors occurred, regardless or whether or not
sqlite3_backup_step() completed. If an out-of-memory condition or IO
error occurred during any prior sqlite3_backup_step() call on the same
sqlite3_backup object, then sqlite3_backup_finish() returns the
corresponding error code.
A return of SQLITE_BUSY or SQLITE_LOCKED from sqlite3_backup_step() is
not a permanent error and does not affect the return value of
sqlite3_backup_finish().
sqlite3_backup_remaining() and sqlite3_backup_pagecount()
The sqlite3_backup_remaining() routine returns the number of pages still
to be backed up at the conclusion of the most recent
sqlite3_backup_step(). The sqlite3_backup_pagecount() routine returns
the total number of pages in the source database at the conclusion of the
most recent sqlite3_backup_step(). The values returned by these
functions are only updated by sqlite3_backup_step(). If the source
database is modified in a way that changes the size of the source
database or the number of pages remaining, those changes are not
reflected in the output of sqlite3_backup_pagecount() and
sqlite3_backup_remaining() until after the next sqlite3_backup_step().
Concurrent Usage of Database Handles
The source database connection may be used by the application for other
purposes while a backup operation is underway or being initialized. If
SQLite is compiled and configured to support threadsafe database
connections, then the source database connection may be used concurrently
from within other threads.
However, the application must guarantee that the destination database
connection is not passed to any other API (by any thread) after
sqlite3_backup_init() is called and before the corresponding call to
sqlite3_backup_finish(). SQLite does not currently check to see if the
application incorrectly accesses the destination database connection and
so no error code is reported, but the operations may malfunction
nevertheless. Use of the destination database connection while a backup
is in progress might also cause a mutex deadlock.
If running in shared cache mode, the application must guarantee that the
shared cache used by the destination database is not accessed while the
backup is running. In practice this means that the application must
guarantee that the disk file being backed up to is not accessed by any
connection within the process, not just the specific connection that was
passed to sqlite3_backup_init().
The sqlite3_backup object itself is partially threadsafe. Multiple
threads may safely make multiple concurrent calls to
sqlite3_backup_step(). However, the sqlite3_backup_remaining() and
sqlite3_backup_pagecount() APIs are not strictly speaking threadsafe. If
they are invoked at the same time as another thread is invoking
sqlite3_backup_step() it is possible that they return invalid values.
IMPLEMENTATION NOTES
These declarations were extracted from the interface documentation at
line 9007.
SQLITE_API sqlite3_backup *sqlite3_backup_init(
sqlite3 *pDest, /* Destination database handle */
const char *zDestName, /* Destination database name */
sqlite3 *pSource, /* Source database handle */
const char *zSourceName /* Source database name */
);
SQLITE_API int sqlite3_backup_step(sqlite3_backup *p, int nPage);
SQLITE_API int sqlite3_backup_finish(sqlite3_backup *p);
SQLITE_API int sqlite3_backup_remaining(sqlite3_backup *p);
SQLITE_API int sqlite3_backup_pagecount(sqlite3_backup *p);
SEE ALSO
sqlite3(3), sqlite3_backup(3), sqlite3_busy_handler(3),
sqlite3_errcode(3), SQLITE_ERROR_MISSING_COLLSEQ(3), SQLITE_OK(3)
NetBSD 10.99 August 24, 2023 NetBSD 10.99