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);
    int
    Db::pget(DbTxn *txnid, Dbt *key, Dbt *pkey, 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 to which data refers.

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.

When called on a database that has been made into a secondary index using the Db::associate method, the Db::get and Db::pget methods return the key from the secondary index and the data item from the primary database. In addition, the Db::pget method returns the key from the primary database. In databases that are not secondary indices, the Db::pget interface will always fail and return EINVAL.

If the operation is to be transaction-protected, the txnid parameter is a transaction handle returned from DbEnv::txn_begin; otherwise, NULL.

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

DB_CONSUME
  Return the record number and data from the available record closest to the head of the queue, and delete the record. The cursor will be positioned on the deleted record. The record number will be returned in key, as described in Dbt. The data will be returned in the data parameter. A record is available if it is not deleted and is not currently locked. The underlying database must be of type Queue for DB_CONSUME to be specified.

DB_CONSUME_WAIT
  The DB_CONSUME_WAIT flag is the same as the DB_CONSUME flag, except that if the Queue database is empty, the thread of control will wait until there is data in the queue before returning. The underlying database must be of type Queue for DB_CONSUME_WAIT to be specified.

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

  When used with the Db::pget version of this interface on a secondary index handle, return the secondary key/primary key/data tuple only if both the primary and secondary keys match the arguments. It is an error to use the DB_GET_BOTH flag with the Db::get version of this interface and a secondary index handle.

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 (that is, 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 flags may be set by bitwise inclusively OR'ing them into the flags parameter:

DB_DIRTY_READ
  Read modified but not yet committed data. Silently ignored if the DB_DIRTY_READ flag was not specified when the underlying database was opened.

DB_MULTIPLE
  Return multiple data items. The buffer to which the data argument refers is filled with the specified key's data items. If all of the data items associated with the key cannot fit into the buffer, the size field of the data argument is set to the length needed for the specified items, and the error ENOMEM is returned. The buffer to which the data argument refers should be large relative to the page size of the underlying database, aligned for unsigned integer access, and be a multiple of 1024 bytes in size.

  The DB_MULTIPLE flag may only be used alone, or with the DB_GET_BOTH and DB_SET_RECNO options. The DB_MULTIPLE flag may not be used when accessing databases made into secondary indices using the Db::associate method.

  See DB_MULTIPLE_INIT for more information.

DB_RMW
  Acquire write locks instead of read locks when doing the retrieval. 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::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 meaningful only in the presence of transactions.

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, the Db::get method returns DB_KEYEMPTY.

Otherwise, if the specified 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.

DB_SECONDARY_BAD
  A secondary index references a nonexistent primary key.

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::pget interface was called with a Db handle that does not refer to a secondary index.

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::associate, Db::close, Db::cursor, Db::del, Db::err, Db::errx Db::fd, Db::get, Db::pget, Db::get_byteswapped, Db::get_type, Db::join, Db::key_range, Db::open, Db::put, Db::remove, Db::rename, Db::set_alloc, Db::set_append_recno, 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_feedback, Db::set_flags, Db::set_h_ffactor, Db::set_h_hash, Db::set_h_nelem, Db::set_lorder, Db::set_pagesize, Db::set_paniccall, Db::set_q_extentsize, Db::set_re_delim, Db::set_re_len, Db::set_re_pad, Db::set_re_source, Db::stat, Db::sync, Db::truncate, Db::upgrade, and Db::verify.`

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