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Berkeley DB Reference Guide:
  Programmer Notes

### Berkeley DB handles

The Berkeley DB library has a number of object handles. The following table lists those handles, their scope, and whether they are free-threaded (that is, whether multiple threads within a process can share them).

DB_ENV
  The DB_ENV handle, created by the db_env_create method, refers to a Berkeley DB database environment -- a collection of Berkeley DB subsystems, log files and databases. DB_ENV handles are free-threaded if the DB_THREAD flag is specified to the DB_ENV->open method when the environment is opened. The handle should not be closed while any other handle remains open that is using it as a reference (for example, DB or DB_TXN). Once either the DB_ENV->close or DB_ENV->remove methods are called, the handle may not be accessed again, regardless of the method's return.
DB_TXN
  The DB_TXN handle, created by the DB_ENV->txn_begin method, refers to a single transaction. The handle is not free-threaded. Transactions may span threads, but only serially, that is, the application must serialize access to the DB_TXN handles. In the case of nested transactions, since all child transactions are part of the same parent transaction, they must observe the same constraints. That is, children may execute in different threads only if each child executes serially.

  Once the DB_TXN->abort or DB_TXN->commit methods are called, the handle may not be accessed again, regardless of the method's return. In addition, parent transactions may not issue any Berkeley DB operations while they have active child transactions (child transactions that have not yet been committed or aborted) except for DB_ENV->txn_begin, DB_TXN->abort and DB_TXN->commit.

DB_LOGC
  The DB_LOGC handle refers to a cursor into the log files. The handle is not free-threaded. Once the DB_LOGC->close method is called, the handle may not be accessed again, regardless of the method's return.
DB_MPOOLFILE
  The DB_MPOOLFILE handle refers to an open file in the shared memory buffer pool of the database environment. The handle is not free-threaded. Once the DB_MPOOLFILE->close method is called, the handle may not be accessed again, regardless of the method's return.
DB
  The DB handle, created by the db_create method, refers to a single Berkeley DB database, which may or may not be part of a database environment. DB handles are free-threaded if the DB_THREAD flag is specified to the DB->open method when the database is opened or if the database environment in which the database is opened is free-threaded. The handle should not be closed while any other handle that refers to the database is in use; for example, database handles must not be closed while cursor handles into the database remain open, or transactions that include operations on the database have not yet been committed or aborted. Once the DB->close, DB->remove, or DB->rename methods are called, the handle may not be accessed again, regardless of the method's return.
DBC
  The DBC handle refers to a cursor into a Berkeley DB database. The handle is not free-threaded. Cursors may span threads, but only serially, that is, the application must serialize access to the DBC handles. If the cursor is to be used to perform operations on behalf of a transaction, the cursor must be opened and closed within the context of that single transaction. Once DBcursor->c_close has been called, the handle may not be accessed again, regardless of the method's return.

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