DbEnv::open()
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## DbEnv::open()

    #include <db_cxx.h>

    int
    DbEnv::open(const char *db_home, u_int32_t flags, int mode);

The `DbEnv::open()` method opens a 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 `DbEnv::open()` method 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 `DbEnv::open()` fails, the DbEnv::close() method must be called to discard the DbEnv handle.

### Parameters

#### db_home

The db_home parameter is the database environment's home directory. For more information on db_home, and filename resolution in general, see Berkeley DB File Naming. The environment variable DB_HOME may be used as the path of the database home, as described in Berkeley DB File Naming.

When using a Unicode build on Windows (the default), the db_home argument will be interpreted as a UTF-8 string, which is equivalent to ASCII for Latin characters.

#### flags

The flags parameter specifies the subsystems that are initialized and how the application's environment affects Berkeley DB file naming, among other things. The flags parameter must be set to 0 or by bitwise inclusively OR'ing together one or more of the values described in this section.

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 choice of subsystems initialized for a Berkeley DB database environment is specified by the thread of control initially creating the environment. Any subsequent thread of control joining the environment will automatically be configured to use the same subsystems as were created in the environment (unless the thread of control requests a subsystem not available in the environment, which will fail). Applications joining an environment, able to adapt to whatever subsystems have been configured in the environment, should open the environment without specifying any subsystem flags. Applications joining an environment, requiring specific subsystems from their environments, should open the environment specifying those specific subsystem flags.

- `DB_INIT_CDB`

  Initialize locking for the Berkeley DB Concurrent Data Store product. In this mode, Berkeley DB provides multiple reader/single writer access. The only other subsystem that should be specified with the `DB_INIT_CDB` flag is `DB_INIT_MPOOL`.

- `DB_INIT_LOCK`

  Initialize the locking subsystem. This subsystem should be used when multiple processes or threads are going to be reading and writing a Berkeley DB database, so that they do not interfere with each other. If all threads are accessing the database(s) read-only, locking is unnecessary. When the DB_INIT_LOCK flag is specified, it is usually necessary to run a deadlock detector, as well. See db_deadlock and DbEnv::lock_detect() for more information.

- `DB_INIT_LOG`

  Initialize the logging subsystem. This subsystem should be used when recovery from application or system failure is necessary. If the log region is being created and log files are already present, the log files are reviewed; subsequent log writes are appended to the end of the log, rather than overwriting current log entries.

- `DB_INIT_MPOOL`

  Initialize the shared memory buffer pool subsystem. This subsystem should be used whenever an application is using any Berkeley DB access method.

- `DB_INIT_REP`

  Initialize the replication subsystem. This subsystem should be used whenever an application plans on using replication. The `DB_INIT_REP` flag requires the `DB_INIT_TXN` and `DB_INIT_LOCK` flags also be configured.

- `DB_INIT_TXN`

  Initialize the transaction subsystem. This subsystem should be used when recovery and atomicity of multiple operations are important. The `DB_INIT_TXN` flag implies the `DB_INIT_LOG` flag.

The second group of flags govern what recovery, if any, is performed when the environment is initialized:

- `DB_RECOVER`

  Run normal recovery on this environment before opening it for normal use. If this flag is set, the `DB_CREATE` and `DB_INIT_TXN` flags must also be set, because the regions will be removed and re-created, and transactions are required for application recovery.

- `DB_RECOVER_FATAL`

  Run catastrophic recovery on this environment before opening it for normal use. If this flag is set, the `DB_CREATE` and `DB_INIT_TXN` flags must also be set, because the regions will be removed and re-created, and transactions are required for application recovery.

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 the `DbEnv::open()` method.

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 third group of flags govern file-naming extensions in the environment:

- `DB_USE_ENVIRON`

  The Berkeley DB process' environment may be permitted to specify information to be used when naming files; see Berkeley DB File Naming. Because permitting users to specify which files are used can create security problems, environment information will be used in file naming for all users only if the `DB_USE_ENVIRON` flag is set.

- `DB_USE_ENVIRON_ROOT`

  The Berkeley DB process' environment may be permitted to specify information to be used when naming files; see Berkeley DB File Naming. Because permitting users to specify which files are used can create security problems, if the `DB_USE_ENVIRON_ROOT` flag is set, environment information will be used in file naming only for users with appropriate permissions (for example, users with a user-ID of 0 on `UNIX` systems).

Finally, there are a few additional unrelated flags:

- `DB_CREATE`

  Cause Berkeley DB subsystems to create any underlying files, as necessary.

- `DB_LOCKDOWN`

  Lock shared Berkeley DB environment files and memory-mapped databases into memory.

- `DB_FAILCHK`

  Internally call the DbEnv::failchk() method as part of opening the environment. When `DB_FAILCHK` is specified, a check is made to ensure all `DbEnv::failchk()` prerequisites are meet.

  If the `DB_FAILCHK` flag is used in conjunction with the `DB_REGISTER` flag, then a check will be made to see if the environment needs recovery. If recovery is needed, a call will be made to the `DbEnv::failchk()` method to release any database reads locks held by the thread of control that exited and, if needed, to abort the unresolved transaction. If `DbEnv::failchk()` determines environment recovery is still required, the recovery actions for `DB_REGISTER` will be followed.

  If the `DB_FAILCHK` flag is not used in conjunction with the `DB_REGISTER` flag, then make an internal call to `DbEnv::failchk()` as the last step of opening the environment. If `DbEnv::failchk()` determines database environment recovery is required, DB_RUNRECOVERY will be returned.

- `DB_PRIVATE`

  Allocate region memory from the heap instead of from memory backed by the filesystem or system shared memory.

  This flag implies the environment will only be accessed by a single process (although that process may be multithreaded). This flag has two effects on the Berkeley DB environment. First, all underlying data structures are allocated from per-process memory instead of from shared memory that is accessible to more than a single process. Second, mutexes are only configured to work between threads.

  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 Berkeley DB utilities (for example, db_archive, db_checkpoint or db_stat) are expected to access the environment, the `DB_PRIVATE` flag should not be specified.

  See Shared Memory Regions for more information.

- `DB_REGISTER`

  Check to see if recovery needs to be performed before opening the database environment. (For this check to be accurate, all processes using the environment must specify `DB_REGISTER` when opening the environment.) If recovery needs to be performed for any reason (including the initial use of the `DB_REGISTER` flag), and `DB_RECOVER` is also specified, recovery will be performed and the open will proceed normally. If recovery needs to be performed and `DB_RECOVER` is not specified, DB_RUNRECOVERY will be returned. If recovery does not need to be performed, the `DB_RECOVER` flag will be ignored. See Architecting Transactional Data Store applications for more information.

- `DB_SYSTEM_MEM`

  Allocate region memory from system shared memory instead of from heap memory or memory backed by the filesystem.

  See Shared Memory Regions for more information.

- `DB_THREAD`

  Cause the DbEnv handle returned by `DbEnv::open()` to be free-threaded; that is, concurrently usable by multiple threads in the address space. The `DB_THREAD` flag should be specified if the DbEnv handle will be concurrently used by more than one thread in the process, or if any Db handles opened in the scope of the DbEnv handle will be concurrently used by more than one thread in the process.

  This flag is required when using the Replication Manager.

#### mode

On Windows systems, the mode parameter is ignored.

On UNIX systems or in IEEE/ANSI Std 1003.1 (POSIX) environments, 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)). 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 Berkeley DB are created with mode mode, unmodified by the process' umask value. If mode is 0, Berkeley DB will use a default mode of readable and writable by both owner and group.

### Errors

The `DbEnv::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:

#### DB_RUNRECOVERY

Either the `DB_REGISTER` flag was specified, a failure occurred, and no recovery flag was specified, or the `DB_FAILCHK` flag was specified and recovery was deemed necessary.

#### DB_VERSION_MISMATCH

The version of the Berkeley DB library doesn't match the version that created the database environment.

#### EAGAIN

The shared memory region was locked and (repeatedly) unavailable.

#### EINVAL

If 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; or if an invalid flag value or parameter was specified.

#### ENOSPC

HP-UX only: Due to the constraints of the PA-RISC memory architecture, HP-UX does not allow a process to map a file into its address space multiple times. For this reason, each Berkeley DB environment may be opened only once by a process on HP-UX; that is, calls to `DbEnv::open()` will fail if the specified Berkeley DB environment has been opened and not subsequently closed.

#### ENOENT

The file or directory does not exist.

### Class

DbEnv

### See Also

Database Environments and Related Methods

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