int DbEnv::open(const char *db_home, u_int32_t flags, int mode);
The DbEnv::open method is the interface for opening the Berkeley DB environment. It provides a structure for creating a consistent environment for processes using one or more of the features of Berkeley DB.
The db_home argument to DbEnv::open (and filename resolution in general) is described in Berkeley DB File Naming.
The flags argument specifies the subsystems that are initialized and how the application's environment affects Berkeley DB file naming, among other things.
The flags value must be set to 0 or by bitwise inclusively OR'ing together one or more of the following values:
Because there are a large number of flags that can be specified, they have been grouped together by functionality. The first group of flags indicates which of the Berkeley DB subsystems should be initialized:
The second group of flags govern what recovery, if any, is performed when the environment is initialized:
A standard part of the recovery process is to remove the existing Berkeley DB environment and create a new one in which to perform recovery. If the thread of control performing recovery does not specify the correct region initialization information (for example, the correct memory pool cache size), the result can be an application running in an environment with incorrect cache and other subsystem sizes. For this reason, the thread of control performing recovery should specify correct configuration information before calling the DbEnv::open method; or it should remove the environment after recovery is completed, leaving creation of the correctly sized environment to a subsequent call to DbEnv::open.
All Berkeley DB recovery processing must be single-threaded; that is, only a single thread of control may perform recovery or access a Berkeley DB environment while recovery is being performed. Because it is not an error to specify DB_RECOVER for an environment for which no recovery is required, it is reasonable programming practice for the thread of control responsible for performing recovery and creating the environment to always specify the DB_CREATE and DB_RECOVER flags during startup.
The DbEnv::open function returns successfully if DB_RECOVER or DB_RECOVER_FATAL is specified and no log files exist, so it is necessary to ensure that all necessary log files are present before running recovery. For further information, consult db_archive and db_recover.
The third group of flags govern file-naming extensions in the environment:
Finally, there are a few additional unrelated flags:
This flag should not be specified if more than a single process is accessing the environment because it is likely to cause database corruption and unpredictable behavior. For example, if both a server application and the Berkeley DB utility db_stat are expected to access the environment, the DB_PRIVATE flag should not be specified.
On UNIX systems or in IEEE/ANSI Std 1003.1 (POSIX) environments, all files created by Berkeley DB 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)). If mode is 0, Berkeley DB will use a default mode of readable and writable by both owner and group. On Windows systems, the mode argument is ignored. The group ownership of created files is based on the system and directory defaults, and is not further specified by Berkeley DB.
The DbEnv::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.
The DbEnv::open method may fail and throw an exception or return a non-zero error for the following conditions:
The DB_THREAD flag was specified and fast mutexes are not available for this architecture.
The DB_HOME or TMPDIR environment variables were set, but empty.
An incorrectly formatted NAME VALUE entry or line was found.
If the file or directory does not exist, the DbEnv::open method will fail and and either return ENOENT or throw a FileNotFoundException exception.
The DbEnv::open method may fail and throw an exception or return a non-zero error for errors specified for other Berkeley DB and C library or system methods. If a catastrophic error has occurred, the DbEnv::open method may fail and either return DB_RUNRECOVERY or throw a DbRunRecoveryException, in which case all subsequent Berkeley DB calls will fail in the same way.
Copyright Sleepycat Software