#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.
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.
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.
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.
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.
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.
Initialize the shared memory buffer pool subsystem. This subsystem should be used whenever an application is using any Berkeley DB access method.
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.
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:
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.
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:
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.
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:
Cause Berkeley DB subsystems to create any underlying files, as necessary.
Lock shared Berkeley DB environment files and memory-mapped databases into memory.
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.
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.
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.
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.
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.
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.
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:
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.
The version of the Berkeley DB library doesn't match the version that created the database environment.
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.
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.