DBENV->open
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#include <db.h>
int
DBENV->open(DB_ENV *,
char *db_home, char *db_config[], u_int32_t flags, int mode);
Description
The DBENV->open function 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 and db_config arguments to DBENV->open
are 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.
As there are a large number of flags that can be specified, they have been
grouped together by functionality. The first group of flags indicate
which of the Berkeley DB subsystems should be initialized:
- DB_INIT_CDB
- Initialize locking for the Berkeley DB Concurrent Data Store product. In this mode, Berkeley DB provides
multiple reader/single writer access. No other subystems should be
specified (e.g., do not also specify DB_INIT_LOCK).
Access method calls are largely unchanged when using this flag, although
any cursors through which update operations (e.g., DBcursor->c_put,
DBcursor->c_del) will be made must have the DB_RMW value set in
the flags parameter to the cursor call that creates the cursor. See
DB->cursor for more information.
- 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, then locking is unnecessary.
When the DB_INIT_LOCK flag is specified, it is usually necessary to run
the deadlock detector, as well. See db_deadlock and
lock_detect for more information.
- DB_INIT_LOG
- Initialize the logging subsystem. This subsystem is used when recovery
from application or system failure is necessary.
The log is stored in one or more files in the environment directory.
Each file is named using the format log.NNNNNNNNNN, where
NNNNNNNNNN is the sequence number of the file within the log.
For further information, see
Log File Limits.
If the log region is being created and log files are already present, the
log files are reviewed and 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 is
used whenever the application is using any Berkeley DB access method.
- DB_INIT_TXN
- Initialize the transaction subsystem. This subsystem is used when
recovery and atomicity of multiple operations and recovery 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 flag must also be set since the regions
will be removed and recreated.
- DB_RECOVER_FATAL
- Run catastrophic recovery on this environment before opening it for
normal use. If this flag is set, the DB_CREATE flag must also be set
since the regions will be removed and recreated.
A standard part of the recovery process is to remove any 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 (e.g., 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 either specify correct configuration
information before calling the DBENV->open function, or it should remove
the environment after recovery is completed, leaving creation of the
correctly sized environment to a subsequent call to DBENV->open.
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 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. As 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. As 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 (e.g., on IEEE/ANSI Std 1003.1 (POSIX) systems, users with a
user-ID of 0).
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_NOMMAP
- Always copy read-only database files in this environment into the local
cache instead of potentially mapping them into process memory (see the
description of the DBENV->set_mp_mmapsize function for further information).
- DB_PRIVATE
- Specify that the environment will only be accessed by a single process
(although that process may be multi-threaded). 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
potentially 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, as it is likely to cause database corruption
and unpredictable behavior, e.g., if both a server application and the
Berkeley DB utility db_stat will access the environment, the
DB_PRIVATE flag should not be specified.
- DB_SYSTEM_MEM
- Allocate memory from system shared memory instead of from memory backed
by the filesystem. This flag is ignored on Microsoft Windows
architectures. See Shared Memory Regions
for more information.
- DB_THREAD
- Cause the DB_ENV handle returned by DBENV->open to be
free-threaded, that is, useable by multiple threads within a
single address space. In addition, any DB handles created in a
free-threaded environment will also be free-threaded.
- DB_TXN_NOSYNC
- Do not synchronously flush the log on transaction commit or prepare.
This means that transactions exhibit the ACI (atomicity, consistency and
isolation) properties, but not D (durability), i.e., database integrity
will be maintained but it is possible that some number of the most
recently committed transactions may be undone during recovery instead of
being redone.
The number of transactions that are potentially at risk is governed by
how often the log is checkpointed (see db_checkpoint for more
information) and how many log updates can fit on a single log page.
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)). The group ownership of created files is based on
the system and directory defaults, and is not further specified by Berkeley DB.
If mode is 0, files are created readable and writeable by both
owner and group. On Windows systems, the mode argument is ignored.
The DBENV->open
function returns a non-zero error value on failure and 0 on success.
Environment Variables
- DB_HOME
- The environment variable DB_HOME may be used as the path of
the database home as described in
Berkeley DB File Naming.
Errors
If a fatal error occurs in Berkeley DB, the DBENV->open function will fail and return
DB_RUNRECOVERY, at which point all subsequent database calls will
fail in the same way.
In addition, the DBENV->open
function may fail and return a non-zero error
for the following conditions:
- EAGAIN
- The shared memory region was locked and (repeatedly) unavailable.
- EINVAL
- An invalid flag value or parameter was specified.
The DB_THREAD flag was specified and spinlocks are not implemented for
this architecture.
The DB_HOME or TMPDIR environment variables were set but empty.
An incorrectly formatted NAME VALUE entry or line was found.
The specified log file size was too large.
The specified cache size was impossibly small.
- ENOSPC
- HP-UX only: a previously created Berkeley DB environment for this process still
exists.
In addition, the DBENV->open function may fail and
return a non-zero error
for errors specified for other Berkeley DB and C library or system functions.
See Also
DBENV->close,
db_env_create,
DBENV->open,
DBENV->remove,
DBENV->err,
db_strerror,
db_version,
DBENV->set_cachesize,
DBENV->set_errcall,
DBENV->set_errfile,
DBENV->set_errpfx,
DBENV->set_mutexlocks,
DBENV->set_pageyield,
DBENV->set_paniccall,
DBENV->set_region_init
and
DBENV->set_verbose.
Copyright Sleepycat Software