Db::open
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#include <db_cxx.h>
int
Db::open(DbTxn *txnid, const char *file,
const char *database, DBTYPE type, u_int32_t flags, int mode);
int
Db::get_dbname(const char **filenamep, const char **dbnamep);
int
Db::get_open_flags(u_int32_t *flagsp);
int
Db::get_transactional()
Description: Db::open
The Db::open method opens the database represented by the file
and database parameters for both reading and writing.
The currently supported Berkeley DB file formats (or access
methods) are Btree, Hash, Queue, and Recno. The Btree format is a
representation of a sorted, balanced tree structure. The Hash format
is an extensible, dynamic hashing scheme. The Queue format supports
fast access to fixed-length records accessed sequentially or by logical
record number. The Recno format supports fixed- or variable-length
records, accessed sequentially or by logical record number, and
optionally backed by a flat text file.
Storage and retrieval for the Berkeley DB access methods are based on key/data
pairs; see Dbt for more information.
Calling Db::open is a relatively expensive operation, and
maintaining a set of open databases will normally be preferable to
repeatedly opening and closing the database for each new query.
The Db::open method
either returns a non-zero error value
or throws an exception that encapsulates a non-zero error value on
failure, and returns 0 on success.
If Db::open fails, the Db::close method should be called to
discard the Db handle.
Parameters
- database
- The database parameter is optional, and allows applications to
have multiple databases in a single file. Although no database
parameter needs to be specified, it is an error to attempt to open a
second database in a file that was not initially created using
a database name. Further, the database parameter is not
supported by the Queue format. Finally, when opening multiple databases
in the same physical file, it is important to consider locking and
memory cache issues; see Opening
multiple databases in a single file for more information.
In-memory databases never intended to be preserved on disk may be
created by setting the file parameter to NULL. If the
database parameter is also NULL, the database is strictly
temporary and cannot be opened by any other thread of control, thus the
database can only be accessed by sharing the single database handle that
created it, in circumstances where doing so is safe. If the
database parameter is not set to NULL, the database can be opened
by other threads of control and will be replicated to client sites in
any replication group.
- file
- The file parameter is used as the name of an underlying file that
will be used to back the database; see File naming for more information.
In-memory databases never intended to be preserved on disk may be
created by setting the file parameter to NULL.
On Windows, the file argument will be interpreted as a UTF-8
string, which is equivalent to ASCII for Latin characters.
- flags
- The flags parameter must be set to 0 or by bitwise inclusively OR'ing together one
or more of the following values:
- DB_AUTO_COMMIT
- Enclose the Db::open call within a transaction. If the call
succeeds, the open operation will be recoverable and all subsequent
database modification operations based on this handle will be
transactionally protected. If the call fails, no database will have
been created.
- DB_CREATE
- Create the database. If the database does not already exist and the
DB_CREATE flag is not specified, the Db::open will fail.
- DB_EXCL
- Return an error if the database already exists. The DB_EXCL
flag is only meaningful when specified with the DB_CREATE
flag.
- DB_NOMMAP
- Do not map this database into process memory (see the
DbEnv::set_mp_mmapsize method for further information).
- DB_RDONLY
- Open the database for reading only. Any attempt to modify items in the
database will fail, regardless of the actual permissions of any
underlying files.
- DB_READ_UNCOMMITTED
- Support read operations with degree 1 isolation. Read operations on the
database may request the return of modified but not yet committed data.
This flag must be specified on all Db handles used to perform
dirty reads or database updates, otherwise requests for dirty reads may
not be honored and the read may block.
- DB_THREAD
- Cause the Db handle returned by Db::open to be
free-threaded; that is, concurrently usable by multiple
threads in the address space.
- DB_TRUNCATE
- Physically truncate the underlying file, discarding all previous
databases it might have held. Underlying filesystem primitives are used
to implement this flag. For this reason, it is applicable only to the
file and cannot be used to discard databases within a file.
The DB_TRUNCATE flag cannot be lock or transaction-protected,
and it is an error to specify it in a locking or transaction-protected
environment.
- mode
- On Windows systems, the mode parameter is ignored.
On UNIX systems or in IEEE/ANSI Std 1003.1 (POSIX) environments, files created by the database open
are created with mode mode (as described in chmod(2))
and modified by the process' umask value at the time of creation (see
umask(2)). Created files are owned by the process owner; the
group ownership of created files is based on the system and directory
defaults, and is not further specified by Berkeley DB. System shared memory
segments created by the database open are created with mode mode, unmodified
by the process' umask value. If mode is 0, the database open will use a
default mode of readable and writable by both owner and group.
- txnid
- If the operation is part of an application-specified
transaction, the txnid parameter is a transaction handle
returned from DbEnv::txn_begin; otherwise NULL. If no transaction
handle is specified, but the
DB_AUTO_COMMIT flag is specified,
the operation will be implicitly transaction protected. Note that transactionally protected operations on a Db handle
require the Db handle itself be transactionally protected during
its open.
- type
- The type parameter is of type DBTYPE, and must be set to one of
DB_BTREE, DB_HASH, DB_QUEUE,
DB_RECNO, or DB_UNKNOWN. If type is
DB_UNKNOWN, the database must already exist and Db::open will
automatically determine its type. The Db::get_type method may be used
to determine the underlying type of databases opened using DB_UNKNOWN.
Environment Variables
If the database was opened within a database environment, the
environment variable DB_HOME may be used as the path of the
database environment home.
Db::open is affected by any database directory specified using
the DbEnv::set_data_dir method, or by setting the "set_data_dir" string
in the environment's DB_CONFIG file.
- TMPDIR
- If the file and dbenv parameters to Db::open are
NULL, the environment variable TMPDIR may be used as a
directory in which to create temporary backing files
Errors
- ENOENT
- The file or directory does not exist.
The Db::open method
may fail and throw
DbException,
encapsulating one of the following non-zero errors, or return one of
the following non-zero errors:
- DB_OLD_VERSION
- The database cannot be opened without being first upgraded.
- EEXIST
- DB_CREATE and DB_EXCL were specified and the database exists.
- EINVAL
- If an unknown database type, page size, hash function, pad byte, byte
order, or a flag value or parameter that is incompatible with the
specified database was specified;
the DB_THREAD flag was specified and fast mutexes are not
available for this architecture;
the DB_THREAD flag was specified to Db::open, but was
not specified to the DbEnv::open call for the environment in
which the Db handle was created;
a backing flat text file was specified with either the DB_THREAD
flag or the provided database environment supports transaction
processing; or if an
invalid flag value or parameter was specified.
- ENOENT
- A nonexistent re_source file was specified.
- DB_REP_HANDLE_DEAD
- The database handle has been invalidated because a replication election
unrolled a committed transaction.
- DB_REP_LOCKOUT
- The operation was blocked by client/master synchronization.
If a transactional database environment operation was selected to
resolve a deadlock, the Db::open method will fail and
either return DB_LOCK_DEADLOCK or
throw a DbDeadlockException exception.
If a Berkeley DB Concurrent Data Store database environment configured for lock timeouts was unable
to grant a lock in the allowed time, the Db::open method will fail and
either return DB_LOCK_NOTGRANTED or
throw a DbLockNotGrantedException exception.
Description: Db::get_database
The Db::get_database method returns the current filename and database
name.
Parameters
- filenamep
- The filenamep parameter references memory into which
a pointer to the current filename is copied.
- dbnamep
- The dbnamep parameter references memory into which
a pointer to the current database name is copied.
The Db::get_database method may be called at any time during the life of the
application.
The Db::get_database method
either returns a non-zero error value
or throws an exception that encapsulates a non-zero error value on
failure, and returns 0 on success.
Description: Db::get_open_flags
The Db::get_open_flags method returns the current open method flags.
The Db::get_open_flags method may not be called before the Db::open method has been
called.
The Db::get_open_flags method
either returns a non-zero error value
or throws an exception that encapsulates a non-zero error value on
failure, and returns 0 on success.
Parameters
- flagsp
- The Db::get_open_flags method returns the
current open method flags in flagsp.
Description: Db::get_transactional
The Db::get_transactional method returns non-zero if the Db
handle has been opened in a transactional mode.
The Db::get_transactional method may be called at any time during the life of the
application.
Class
Db
See Also
Databases and Related Methods
Copyright (c) 1996-2005 Sleepycat Software, Inc. - All rights reserved.