Db::open()
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## Db::open()

    #include <db_cxx.h>

    int
    Db::open(DbTxn *txnid, const char *file,
        const char *database, DBTYPE type, u_int32_t flags, int mode);

The `Db::open()` method opens the database represented by the file and database.

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 must 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.

If both the database and file parameters are 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, regardless of whether the file parameter is set to NULL.

#### 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. Whether other threads of control can access this database is driven entirely by whether the database parameter is set to NULL.

When using a Unicode build on Windows (the default), 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 zero 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_MULTIVERSION`

  Open the database with support for multiversion concurrency control. This will cause updates to the database to follow a copy-on-write protocol, which is required to support snapshot isolation. The `DB_MULTIVERSION` flag requires that the database be transactionally protected during its open and is not supported by the queue format.

- `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 transactional 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(); if the operation is part of a Berkeley DB Concurrent Data Store group, the txnid parameter is a handle returned from DbEnv::cdsgroup_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 requires the Db handle itself be transactionally protected during its open. Also note that the transaction must be committed before the handle is closed; see Berkeley DB handles for more information.

#### 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.

It is an error to specify the incorrect type for a database that already exists.

### 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

The `Db::open()` method may fail and throw a DbException exception, encapsulating one of the following non-zero errors, or return one of the following non-zero errors:

#### DbDeadlockException or DB_LOCK_DEADLOCK

A transactional database environment operation was selected to resolve a deadlock.

DbDeadlockException is thrown if your Berkeley DB API is configured to throw exceptions. Otherwise, `DB_LOCK_DEADLOCK` is returned.

#### DbLockNotGrantedException or DB_LOCK_NOTGRANTED

A Berkeley DB Concurrent Data Store database environment configured for lock timeouts was unable to grant a lock in the allowed time.

DbLockNotGrantedException is thrown if your Berkeley DB API is configured to throw exceptions. Otherwise, `DB_LOCK_NOTGRANTED` is returned.

#### ENOENT

The file or directory does not exist.

#### ENOENT

A nonexistent re_source file was specified.

#### 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.

#### DbRepHandleDeadException or DB_REP_HANDLE_DEAD

When a client synchronizes with the master, it is possible for committed transactions to be rolled back. This invalidates all the database and cursor handles opened in the replication environment. Once this occurs, an attempt to use such a handle will throw a DbRepHandleDeadException (if your application is configured to throw exceptions), or return `DB_REP_HANDLE_DEAD`. The application will need to discard the handle and open a new one in order to continue processing.

#### DbDeadlockException or DB_REP_LOCKOUT

The operation was blocked by client/master synchronization.

DbDeadlockException is thrown if your Berkeley DB API is configured to throw exceptions. Otherwise, `DB_REP_LOCKOUT` is returned.

### Class

Db

### See Also

Database and Related Methods

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