Berkeley DB: DbEnv::open
Description: Berkeley DB: An embedded database programmatic toolkit.
Keywords: embedded,database,programmatic,toolkit,btree,hash,hashing,transaction,transactions,locking,logging,access method,access methods,Java,C,C++

### DbEnv::open |

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    #include <db_cxx.h>

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

    int
    DbEnv::get_home(const char **homep);

    int
    DbEnv::get_open_flags(u_int32_t *flagsp);

### Description: DbEnv::open

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 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 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 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 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:

  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 for 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_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.
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 DbException, encapsulating one of the following non-zero errors, or return one of the following non-zero errors:

DB_RUNRECOVERY
  The DB_REGISTER flag was specified, a failure has occurred, and no recovery flag was specified.

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.

### Description: DbEnv::get_home

The DbEnv::get_home method returns the database environment home directory.

The DbEnv::get_home method may be called at any time during the life of the application.

### Description: DbEnv::get_open_flags

The DbEnv::get_open_flags method returns the open method flags.

The DbEnv::get_open_flags method may not be called before the DbEnv::open method has been called.

The DbEnv::get_open_flags 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.

### Parameters

flagsp
  The DbEnv::get_open_flags method returns the open method flags in flagsp.

### Class

DbEnv

### See Also

Database Environments and Related Methods`

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