Berkeley DB: Db::get
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++

# Db::get |

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    #include <db_cxx.h>

    int
    Db::get(DbTxn *txnid, Dbt *key, Dbt *data, u_int32_t flags);

# Description

The Db::get method retrieves key/data pairs from the database. The address and length of the data associated with the specified key are returned in the structure referenced by data.

In the presence of duplicate key values, Db::get will return the first data item for the designated key. Duplicates are sorted by insert order except where this order has been overridden by cursor operations. Retrieval of duplicates requires the use of cursor operations. See Dbc::get for details.

If the file is being accessed under transaction protection, the txnid parameter is a transaction ID returned from DbEnv::txn_begin, otherwise, NULL.

The flags parameter must be set to 0 or one of the following values:

DB_GET_BOTH
  Retrieve the key/data pair only if both the key and data match the arguments.

DB_SET_RECNO
  Retrieve the specified numbered key/data pair from a database. Upon return, both the key and data items will have been filled in, not just the data item as is done for all other uses of the Db::get method.

  The data field of the specified key must be a pointer to a logical record number (i.e., a db_recno_t). This record number determines the record to be retrieved.

  For DB_SET_RECNO to be specified, the underlying database must be of type Btree and it must have been created with the DB_RECNUM flag.

In addition, the following flag may be set by bitwise inclusively OR'ing it into the flags parameter:

DB_RMW
  Acquire write locks instead of read locks when doing the retrieval. Setting this flag may decrease the likelihood of deadlock during a read-modify-write cycle by immediately 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.

  As the Db::get interface will not hold locks across Berkeley DB interface calls in non-transactional environments, the DB_RMW flag to the Db::get call is only meaningful in the presence of transactions.

If the database is a Queue or Recno database and the requested key exists, but was never explicitly created by the application or was later deleted, the Db::get method returns DB_KEYEMPTY.

Otherwise, if the requested key is not in the database, the Db::get function returns DB_NOTFOUND.

Otherwise, the Db::get 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 Db::get method may fail and throw an exception or return a non-zero error for the following conditions:

DB_LOCK_DEADLOCK
  The operation was selected to resolve a deadlock.

ENOMEM
  There was insufficient memory to return the requested item.

EINVAL
  An invalid flag value or parameter was specified.

  A record number of 0 was specified.

  The DB_THREAD flag was specified to the Db::open method and none of the DB_DBT_MALLOC, DB_DBT_REALLOC or DB_DBT_USERMEM flags were set in the Dbt.

The Db::get 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 Db::get method may fail and either return DB_RUNRECOVERY or throw an exception encapsulating DB_RUNRECOVERY, in which case all subsequent Berkeley DB calls will fail in the same way.

### Class

Db

# See Also

Db::close, Db::cursor, Db::del, Db::err, Db::fd, Db::get, Db::get_byteswapped, Db::get_type, Db::join, Db::key_range, Db::open, Db::put, Db::remove, Db::set_bt_compare, Db::set_bt_minkey, Db::set_bt_prefix, Db::set_cachesize, Db::set_dup_compare, Db::set_errcall, Db::set_errfile, Db::set_errpfx, Db::set_flags, Db::set_h_ffactor, Db::set_h_hash, Db::set_h_nelem, Db::set_lorder, Db::set_malloc, Db::set_pagesize, Db::set_paniccall, Db::set_realloc, Db::set_re_delim, Db::set_re_len, Db::set_re_pad, Db::set_re_source, Db::stat, Db::sync, Db::upgrade and Db::verify.`

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