static int Db::open(const char *fname, DBTYPE type, u_int32_t flags, int mode, DbEnv *dbenv, DbInfo *dbinfo, Db **dbpp);
The Db::open function opens the database represented by file for both reading and writing. Files never intended to be shared or preserved on disk may be created by setting the file parameter to NULL.
Note, while most of the access methods use file as the name of an underlying file on disk, this is not guaranteed. Also, calling Db::open is a reasonably expensive operation. (This is based on a model where the DBMS keeps a set of files open for a long time rather than opening and closing them on each query.)
The type argument is of type DBTYPE and must be set to one of DB_BTREE, DB_HASH, DB_RECNO or DB_UNKNOWN. If type is DB_UNKNOWN, the database must already exist and Db::open will then determine if it is of type DB_BTREE, DB_HASH or DB_RECNO.
The Btree access method is a sorted, balanced tree structure storing associated key/data pairs. Searches, insertions, and deletions in the btree will all complete in O (lg base N) where base is the average number of keys per page. Often, inserting ordered data into btrees results in pages that are half-full. This implementation has been modified to make ordered (or inverse ordered) insertion the best case, resulting in nearly perfect page space utilization.
Space freed by deleting key/data pairs from the database is never reclaimed from the filesystem, although it is reused where possible. This means that the btree storage structure is grow-only. If sufficiently many keys are deleted from a tree that shrinking the underlying database file is desirable, this can be accomplished by creating a new tree from a scan of the existing one.
The Hash access method is an extensible, dynamic hashing scheme.
The Recno access method provides support for fixed and variable length records, optionally backed by a flat text (byte stream) file. Both fixed and variable length records are accessed by their logical record number.
It is valid to create a record whose record number is more than one greater than the last record currently in the database. For example, the creation of record number 8, when records 6 and 7 do not yet exist, is not an error. However, any attempt to retrieve such records (e.g., records 6 and 7) will return DB_KEYEMPTY.
Deleting a record will not, by default, renumber records following the deleted record (see the DB_RENUMBER flag for more information). Any attempt to retrieve deleted records will return DB_KEYEMPTY.
The flags and mode arguments specify how files will be opened and/or created if they do not already exist. The flags value is specified by logically OR'ing together one or more of the following values:
All files created by the access methods 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)))). The group ownership of created files is based on the system and directory defaults, and is not further specified by Berkeley DB.
When sharing a database environment with other processes, it is necessary to provide the access methods with database environment information. See DbEnv for a description of the dbenv argument.
Additionally, there is access method specific information that may be specified when calling Db::open. See DbInfo for a description of the db_info argument.
The Db::open method copies a pointer to a Db object into the memory location referenced by dbpp. The methods of this object allow you to perform various database actions. The methods are described in the following manual pages: Db::close, Db::cursor, Db::del, Db::fd, Db::get, Db::get_type, Db::put and Db::sync.
The Db::open method may fail and throw an exception for any of the errors specified for the following Berkeley DB and C library functions: Db::cursor, Db::sync, DBcursor->c_close(3), DBmemp->pgin(3), DBmemp->pgout(3), __account_page(3), abort(3), close(3), dbenv->db_paniccall(3), dbp->h_hash(3), fcntl(3), fflush(3), fprintf(3), free(3), fstat(3), fsync(3), func(3), getenv(3), getpid(3), getuid(3), isdigit(3), DbLockTab::get, DbLockTab::id, DbLock::put, DbLockTab::vec, DbLog::compare, DbLog::flush, DbLog::put, DbLog::db_register, DbLog::db_unregister, lseek(3), malloc(3), memcmp(3), memcpy(3), memmove(3), DbMpool::close, DbMpoolFile::close, DbMpoolFile::get, DbMpoolFile::open, DbMpoolFile::put, DbMpoolFile::set, DbMpoolFile::sync, DbMpool::open, DbMpool::db_register, memset(3), mmap(3), munmap(3), open(3), pread(3), pstat_getdynamic(3), pwrite(3), read(3), realloc(3), sigfillset(3), sigprocmask(3), stat(3), strerror(3), strlen(3), sysconf(3), time(3), unlink(3), vfprintf(3), vsnprintf(3), and write(3).
In addition, the Db::open method may fail and throw an exception or return errno for the following conditions:
The DB_THREAD flag was specified and spinlocks are not implemented for this architecture.
There is a mismatch between the version number of file and the software.
A re_source file was specified with either the DB_THREAD flag or a non-NULL tx_info field in the DB_ENV argument to Db::open.