#include <db.h>
int
DB->open(DB *db, DB_TXN *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 returns a non-zero error value on failure and 0 on success.
            
            
        
         If DB->open() fails, the 
         DB->close()  method must be
         called to discard the DB 
         handle.
    
If the operation is part of an application-specified transaction, the txnid parameter is a transaction handle returned from DB_ENV->txn_begin(); if the operation is part of a Berkeley DB Concurrent Data Store group, the txnid parameter is a handle returned from DB_ENV->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.
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.
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.
                          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.
The flags parameter must be set to zero or by bitwise inclusively OR'ing together one or more of the following values:
                                        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.
                                
                                        Create the database. If the database does not already exist
                                        and the DB_CREATE flag is not specified, the 
                                        DB->open() will fail.
                                
                                        Return an error if the database already exists. The
                                        DB_EXCL flag is only meaningful 
                                        when specified with the DB_CREATE.
                                        flag.
                                
                                        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.
                                
Do not map this database into process memory (see the DB_ENV->set_mp_mmapsize() method for further information).
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.
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.
                                        Cause the DB  
                                        handle returned by DB->open() to be free-threaded; that is,
                                        concurrently usable by multiple threads in the address
                                        space.
                                
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.
                                
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.
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
                            DB_ENV->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
                                    
                         The DB->open() 
            
                 method may fail and return one of the following non-zero errors:
            
            
        
                    
A Berkeley DB Concurrent Data Store database environment configured for lock timeouts was unable to grant a lock in the allowed time.
                           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 DB_ENV->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.
                     
                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 
                
                return DB_REP_HANDLE_DEAD.
                The application will need to discard the handle and open a new one in order to
                continue processing.