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<title>Berkeley DB: Db::get</title>
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#include <db_cxx.h>
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Db::get(DbTxn *txnid, Dbt *key, Dbt *data, u_int32_t flags);
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Db::pget(DbTxn *txnid, Dbt *key, Dbt *pkey, Dbt *data, u_int32_t flags);
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<p>The Db::get method retrieves key/data pairs from the database. The
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and length of the data associated with the specified <b>key</b> are
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returned in the structure to which <b>data</b> refers.
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<p>In the presence of duplicate key values, Db::get will return the
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first data item for the designated key. Duplicates are sorted by insert
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order, except where this order has been overridden by cursor operations.
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<b>Retrieval of duplicates requires the use of cursor operations.</b>
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See <a href="../api_cxx/dbc_get.html">Dbc::get</a> for details.
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<p>When called on a database that has been made into a secondary index
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using the <a href="../api_cxx/db_associate.html">Db::associate</a> method, the Db::get and
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Db::pget methods return the key from the secondary index and the data
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item from the primary database. In addition, the Db::pget method
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returns the key from the primary database. In databases that are not
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secondary indices, the Db::pget interface will always fail and
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<p>If the operation is to be transaction-protected, the <b>txnid</b>
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parameter is a transaction handle returned from <a href="../api_cxx/txn_begin.html">DbEnv::txn_begin</a>;
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<p>The <b>flags</b> value must be set to 0 or
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one of the following values:
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<p><dt><a name="DB_CONSUME">DB_CONSUME</a><dd>Return the record number and data from the available record closest to
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the head of the queue, and delete the record. The cursor will be
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positioned on the deleted record. The record number will be returned
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in <b>key</b>, as described in <a href="../api_cxx/dbt_class.html">Dbt</a>. The data will be returned
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in the <b>data</b> parameter. A record is available if it is not
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deleted and is not currently locked. The underlying database must be
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of type Queue for DB_CONSUME to be specified.
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<p><dt><a name="DB_CONSUME_WAIT">DB_CONSUME_WAIT</a><dd>The DB_CONSUME_WAIT flag is the same as the DB_CONSUME
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flag, except that if the Queue database is empty, the thread of control
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will wait until there is data in the queue before returning. The
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underlying database must be of type Queue for DB_CONSUME_WAIT
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<p><dt><a name="DB_GET_BOTH">DB_GET_BOTH</a><dd>Retrieve the key/data pair only if both the key and data match the
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<p>When used with the Db::pget version of this interface
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on a secondary index handle, return the secondary key/primary key/data
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tuple only if both the primary and secondary keys match the arguments.
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It is an error to use the DB_GET_BOTH flag with the Db::get
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version of this interface and a secondary index handle.
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<p><dt><a name="DB_SET_RECNO">DB_SET_RECNO</a><dd>Retrieve the specified numbered key/data pair from a database. Upon
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return, both the <b>key</b> and <b>data</b> items will have been
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<p>The <b>data</b> field of the specified <b>key</b>
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must be a pointer to a logical record number (that is, a <b>db_recno_t</b>).
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This record number determines the record to be retrieved.
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<p>For DB_SET_RECNO to be specified, the underlying database must be
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of type Btree, and it must have been created with the DB_RECNUM flag.
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<p>In addition, the following flags may be set by
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bitwise inclusively <b>OR</b>'ing them into the <b>flags</b> parameter:
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<p><dt><a name="DB_DIRTY_READ">DB_DIRTY_READ</a><dd>Read modified but not yet committed data. Silently ignored if the
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<a href="../api_cxx/db_open.html#DB_DIRTY_READ">DB_DIRTY_READ</a> flag was not specified when the underlying
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<p><dt><a name="DB_MULTIPLE">DB_MULTIPLE</a><dd>Return multiple data items. The buffer to which the <b>data</b>
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argument refers is filled with the specified key's data items. If all
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of the data items associated with the key cannot fit into the buffer,
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the size field of the <b>data</b> argument is set to the length needed
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for the specified items, and the error ENOMEM is returned. The buffer
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to which the <b>data</b> argument refers should be large relative to
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the page size of the underlying database, aligned for unsigned integer
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access, and be a multiple of 1024 bytes in size.
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<p>The DB_MULTIPLE flag may only be used alone, or with the
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DB_GET_BOTH and DB_SET_RECNO options. The
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DB_MULTIPLE flag may not be used when accessing databases made
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into secondary indices using the <a href="../api_cxx/db_associate.html">Db::associate</a> method.
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<p>See <a href="../api_cxx/dbt_bulk.html#DB_MULTIPLE_INIT">DB_MULTIPLE_INIT</a> for more information.
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<p><dt><a name="DB_RMW">DB_RMW</a><dd>Acquire write locks instead of read locks when doing the retrieval.
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Setting this flag can eliminate deadlock during a read-modify-write
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cycle by acquiring the write lock during the read part of the cycle so
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that another thread of control acquiring a read lock for the same item,
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in its own read-modify-write cycle, will not result in deadlock.
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<p>Because the Db::get interface will not hold locks
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across Berkeley DB interface calls in non-transactional environments, the
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<a href="../api_cxx/dbc_get.html#DB_RMW">DB_RMW</a> flag to the Db::get call is meaningful only in
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the presence of transactions.
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If the specified key is not in the database, the Db::get method will return DB_NOTFOUND.
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If the database is a Queue or Recno database and the specified key
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exists, but was never explicitly created by the application or was
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later deleted, the Db::get method will return DB_KEYEMPTY.
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Otherwise, the Db::get method either returns a non-zero error value or throws an exception that
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encapsulates a non-zero error value on failure, and returns 0 on success.
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<p>The Db::get method may fail and throw an exception or return a non-zero error for the following conditions:
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<p><dt>DB_SECONDARY_BAD<dd>A secondary index references a nonexistent primary key.
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<p><dt>ENOMEM<dd>There was insufficient memory to return the requested item.
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<p><dt>EINVAL<dd>An invalid flag value or parameter was specified.
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<p>A record number of 0 was specified.
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<p>The <a href="../api_cxx/env_open.html#DB_THREAD">DB_THREAD</a> flag was specified to the <a href="../api_cxx/db_open.html">Db::open</a> method and
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none of the <a href="../api_cxx/dbt_class.html#DB_DBT_MALLOC">DB_DBT_MALLOC</a>, <a href="../api_cxx/dbt_class.html#DB_DBT_REALLOC">DB_DBT_REALLOC</a> or
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<a href="../api_cxx/dbt_class.html#DB_DBT_USERMEM">DB_DBT_USERMEM</a> flags were set in the <a href="../api_cxx/dbt_class.html">Dbt</a>.
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<p>The Db::pget interface was called with a <a href="../api_cxx/db_class.html">Db</a> handle that
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does not refer to a secondary index.
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<p>If the operation was selected to resolve a deadlock, the
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Db::get method will fail and
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and either return <a href="../ref/program/errorret.html#DB_LOCK_DEADLOCK">DB_LOCK_DEADLOCK</a> or
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throw a <a href="../api_cxx/deadlock_class.html">DbDeadlockException</a> exception.
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<p>If the requested item could not be returned due to insufficient memory,
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the Db::get method will fail and
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either return ENOMEM or
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throw a <a href="../api_cxx/memp_class.html">DbMemoryException</a> exception.
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<p>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.
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If a catastrophic error has occurred, the Db::get method may fail and
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either return <a href="../ref/program/errorret.html#DB_RUNRECOVERY">DB_RUNRECOVERY</a> or throw a
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<a href="../api_cxx/runrec_class.html">DbRunRecoveryException</a>,
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in which case all subsequent Berkeley DB calls will fail in the same way.
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<a href="../api_cxx/db_class.html">Db</a>
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<a href="../api_cxx/db_list.html">Databases and Related Methods</a>
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