Berkeley DB: Db::exists

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Berkeley DB: Db::exists

Db::exists

#include <db_cxx.h> int Db::exists(DbTxn *txnid, Dbt *key, u_int32_t flags); Description: Db::exists The Db::exists method returns if the specified key appears in the database. The Db::exists method will return DB_NOTFOUND if the specified key is not in the database. The Db::exists method will return DB_KEYEMPTY if the database is a Queue or Recno database and the specified key exists, but was never explicitly created by the application or was later deleted. Unless otherwise specified, the Db::exists 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 flagsThe flags parameter must be set to 0 or one of the following values: In addition, the following flags may be set by bitwise inclusively OR'ing them into the flags parameter: DB_READ_COMMITTEDConfigure a transactional read operation to have degree 2 isolation (the read is not repeatable). DB_READ_UNCOMMITTEDConfigure a transactional read operation to have degree 1 isolation, reading modified but not yet committed data. Silently ignored if the DB_READ_UNCOMMITTED flag was not specified when the underlying database was opened. DB_RMWAcquire write locks instead of read locks when doing the read, if locking is configured. Setting this flag can eliminate deadlock during a read-modify-write cycle by acquiring the write lock during the read part of the cycle so that another thread of control acquiring a read lock for the same item, in its own read-modify-write cycle, will not result in deadlock. Because the Db::exists method will not hold locks across Berkeley DB calls in non-transactional operations, the DB_RMW flag to the Db::exists call is meaningful only in the presence of transactions. keyThe key Dbt operated on. txnidIf the operation is part of an application-specified transaction, the txnid parameter is a transaction handle returned from DbEnv::txn_begin; if the operation is part of a Berkeley DB Concurrent Data Store group, the txnid parameter is a handle returned from DbEnv::cdsgroup_begin; otherwise NULL. Class Db See Also Databases and Related Methods