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

# DbEnv::lock_vec |

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    #include <db_cxx.h>

    int
    DbEnv::lock_vec(u_int32_t locker, u_int32_t flags,
        DB_LOCKREQ list[], int nlist, DB_LOCKREQ **elistp);

# Description

The DbEnv::lock_vec method atomically obtains and releases one or more locks from the lock table. The DbEnv::lock_vec method is intended to support acquisition or trading of multiple locks under one lock table semaphore, as is needed for lock coupling or in multigranularity locking for lock escalation.

The locker argument specified to DbEnv::lock_vec is an unsigned 32-bit integer quantity. It represents the entity requesting or releasing the locks.

The flags value must be set to 0 or the following value:

DB_LOCK_NOWAIT
  If a lock cannot be granted because the requested lock conflicts with an existing lock, return DB_LOCK_NOTGRANTED or throw a DbLockNotGrantedException immediately instead of waiting for the lock to become available. In this case, if non-NULL, elistp identifies the request that was not granted, or, if an exception is thrown, the index of the request that was not granted can be found by calling DbLockNotGrantedException.get_index.

The list array provided to DbEnv::lock_vec is typedef'd as DB_LOCKREQ. A DB_LOCKREQ structure has at least the following fields. In order to ensure compatibility with future releases of Berkeley DB, all fields of the DB_LOCKREQ structure that are not explicitly set should be initialized to 0 before the first time the structure is used. Do this by declaring the structure external or static, or by calling memset(3).

lockop_t op;
  The operation to be performed, which must be set to one of the following values:

  DB_LOCK_GET
    Get the lock defined by the values of the mode and obj structure fields, for the specified locker. Upon return from DbEnv::lock_vec, if the lock field is non-NULL, a reference to the acquired lock is stored there. (This reference is invalidated by any call to DbEnv::lock_vec or DbEnv::lock_put that releases the lock.)

  DB_LOCK_GET_TIMEOUT
    Identical to DB_LOCK_GET except that the value in the timeout structure field overrides any previously specified timeout value for this lock. A value of 0 turns off any previously specified timeout.

  DB_LOCK_PUT
    The lock to which the lock structure field refers is released. The locker argument, and mode and obj fields are ignored.

  DB_LOCK_PUT_ALL
    All locks held by the specified locker are released. The lock, mode, and obj structure fields are ignored. Locks acquired in operations performed by the current call to DbEnv::lock_vec which appear before the DB_LOCK_PUT_ALL operation are released; those acquired in operations appearing after the DB_LOCK_PUT_ALL operation are not released.

  DB_LOCK_PUT_OBJ
    All locks held on the object obj are released. The locker argument and the lock and mode structure fields are ignored. Locks acquired in operations performed by the current call to DbEnv::lock_vec that appear before the DB_LOCK_PUT_OBJ operation are released; those acquired in operations appearing after the DB_LOCK_PUT_OBJ operation are not released.

  DB_LOCK_TIMEOUT
    Cause the specified locker to timeout immediately. If the database environment has not configured automatic deadlock detection, the transaction will timeout the next time deadlock detection is performed. As transactions acquire locks on behalf of a single locker ID, timing out the locker ID associated with a transaction will time out the transaction itself.

DB_LOCK lock;
  A lock reference.

const lockmode_t mode;
  The lock mode, used as an index into the environment's lock conflict matrix. When using the default lock conflict matrix, mode must be set to one of the following values:

  DB_LOCK_READ
    read (shared)
  DB_LOCK_WRITE
    write (exclusive)
  DB_LOCK_IWRITE
    intention to write (shared)
  DB_LOCK_IREAD
    intention to read (shared)
  DB_LOCK_IWR
    intention to read and write (shared)

  See DbEnv::set_lk_conflicts and Standard Lock Modes for more information on the lock conflict matrix.

const Dbt obj;
  An untyped byte string that specifies the object to be locked or released. Applications using the locking subsystem directly while also doing locking via the Berkeley DB access methods must take care not to inadvertently lock objects that happen to be equal to the unique file IDs used to lock files. See Access method locking conventions for more information.

u_int32_t timeout;
  The lock timeout value.

The nlist argument specifies the number of elements in the list array.

If any of the requested locks cannot be acquired, or any of the locks to be released cannot be released, the operations before the failing operation are guaranteed to have completed successfully, and DbEnv::lock_vec returns a non-zero value. In addition, if elistp is not NULL, it is set to point to the DB_LOCKREQ entry that was being processed when the error occurred.

Otherwise, the DbEnv::lock_vec 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.

# Errors

The DbEnv::lock_vec method may fail and throw an exception or return a non-zero error for the following conditions:

EINVAL
  An invalid flag value or parameter was specified.

ENOMEM
  The maximum number of locks has been reached.

If the operation was selected to resolve a deadlock, the DbEnv::lock_vec method will fail and and either return DB_LOCK_DEADLOCK or throw a DbDeadlockException exception.

The DbEnv::lock_vec 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::lock_vec 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.

# Class

DbEnv, DbLock

# See Also

Locking Subsystem and Related Methods`

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