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<!--$Id: dbc_get.so,v 10.66 2001/07/12 21:13:02 bostic Exp $-->
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<title>Berkeley DB: Dbc::get</title>
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#include <db_cxx.h>
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Dbc::get(Dbt *key, Dbt *data, u_int32_t flags);
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Dbc::pget(Dbt *key, Dbt *pkey, Dbt *data, u_int32_t flags);
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<p>The Dbc::get method retrieves key/data pairs from the database. The
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address and length of the key
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are returned in the object to which <b>key</b> refers (except for the
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case of the DB_SET flag, in which the <b>key</b> object is
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unchanged), and the address
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and length of the data are returned in the object to which <b>data</b>
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<p>When called on a cursor opened on a database that has been made into a
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secondary index using the <a href="../api_cxx/db_associate.html">Db::associate</a> method, the Dbc::get
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and Dbc::pget methods return the key from the secondary index and the
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data item from the primary database. In addition, the Dbc::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 Dbc::pget interface will always fail and
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<p>Modifications to the database during a sequential scan will be reflected
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in the scan; that is, records inserted behind a cursor will not be
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returned while records inserted in front of a cursor will be returned.
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<p>In Queue and Recno databases, missing entries (that is, entries that
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were never explicitly created or that were created and then deleted)
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will be skipped during a sequential scan.
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<p>The <b>flags</b> value must be set to one of the following values:
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<p><dt><a name="DB_CURRENT">DB_CURRENT</a><dd>Return the key/data pair to which the cursor refers.
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<p>If the cursor key/data pair was deleted, Dbc::get will return
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<a href="../ref/program/errorret.html#DB_KEYEMPTY">DB_KEYEMPTY</a>.
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<p>If the cursor is not yet initialized, the Dbc::get method either returns EINVAL or throws an exception that encapsulates EINVAL.
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<p><dt><a name="DB_FIRST">DB_FIRST</a>, <a name="DB_LAST">DB_LAST</a><dd>The cursor is set to refer to the first (last) key/data pair of the
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database, and that pair is returned. In the presence of duplicate key
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values, the first (last) data item in the set of duplicates is returned.
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<p>If the database is a Queue or Recno database, Dbc::get using the
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DB_FIRST (DB_LAST) flags will ignore any keys that exist
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but were never explicitly created by the application, or were created and
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<p>If the database is empty, Dbc::get will return <a href="../ref/program/errorret.html#DB_NOTFOUND">DB_NOTFOUND</a>.
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<p><dt><a name="DB_GET_BOTH">DB_GET_BOTH</a><dd>identical to the DB_SET flag, except that both the key and the data
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arguments must be matched by the key and data item in the database.
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<p>When used with the Dbc::pget version of this interface on a secondary
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index handle, both the secondary and primary keys must be matched by the
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secondary and primary key item in the database. It is an error to use the
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DB_GET_BOTH flag with the Dbc::get version of this interface and
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a cursor that has been opened on a secondary index handle.
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<p><dt><a name="DB_GET_RECNO">DB_GET_RECNO</a><dd>Return the record number associated with the cursor. The record number
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will be returned in <b>data</b>, as described in <a href="../api_cxx/dbt_class.html">Dbt</a>. The
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<b>key</b> parameter is ignored.
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<p>For DB_GET_RECNO to be specified, the underlying database must be
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of type Btree, and it must have been created with the <a href="../api_cxx/db_set_flags.html#DB_RECNUM">DB_RECNUM</a>
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<p><dt><a name="DB_JOIN_ITEM">DB_JOIN_ITEM</a><dd>Do not use the data value found in all of the cursors as a lookup key for
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the primary database, but simply return it in the key parameter instead.
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The data parameter is left unchanged.
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<p>For DB_JOIN_ITEM to be specified, the underlying cursor must have
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been returned from the <a href="../api_cxx/db_join.html">Db::join</a> method.
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<p><dt><a name="DB_NEXT">DB_NEXT</a>, <a name="DB_PREV">DB_PREV</a><dd>If the cursor is not yet initialized, DB_NEXT (DB_PREV)
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is identical to DB_FIRST (DB_LAST). Otherwise, the cursor
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is moved to the next (previous) key/data pair of the database, and that
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pair is returned. In the presence of duplicate key values, the value of
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the key may not change.
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<p>If the database is a Queue or Recno database, Dbc::get using the
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DB_NEXT (DB_PREV) flag will skip any keys that exist
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but were never explicitly created by the application, or those that were
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created and later deleted.
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<p>If the cursor is already on the last (first) record in the database,
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Dbc::get will return <a href="../ref/program/errorret.html#DB_NOTFOUND">DB_NOTFOUND</a>.
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<p><dt><a name="DB_NEXT_DUP">DB_NEXT_DUP</a><dd>If the next key/data pair of the database is a duplicate data record for
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the current key/data pair, the cursor is moved to the next key/data pair
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of the database, and that pair is returned. Otherwise, Dbc::get
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will return <a href="../ref/program/errorret.html#DB_NOTFOUND">DB_NOTFOUND</a>.
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<p>If the cursor is not yet initialized, the Dbc::get method either returns EINVAL or throws an exception that encapsulates EINVAL.
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<p><dt><a name="DB_NEXT_NODUP">DB_NEXT_NODUP</a>, <a name="DB_PREV_NODUP">DB_PREV_NODUP</a><dd>If the cursor is not yet initialized, DB_NEXT_NODUP
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(DB_PREV_NODUP) is identical to DB_FIRST
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(DB_LAST). Otherwise, the cursor is moved to the next (previous)
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non-duplicate key of the database, and that key/data pair is returned.
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<p>If the database is a Queue or Recno database, Dbc::get using the
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DB_NEXT_NODUP (DB_PREV_NODUP) flags will ignore any keys
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that exist but were never explicitly created by the application, or those
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that were created and later deleted.
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<p>If no non-duplicate key/data pairs occur after (before) the cursor
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position in the database, Dbc::get will return <a href="../ref/program/errorret.html#DB_NOTFOUND">DB_NOTFOUND</a>.
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<p><dt><a name="DB_SET">DB_SET</a><dd>Move the cursor to the specified key/data pair of the database, and
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return the datum associated with the given key.
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<p>In the presence of duplicate key values, Dbc::get will return the
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first data item for the given key.
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<p>If the database is a Queue or Recno database, and the specified key exists,
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but was never explicitly created by the application or was later deleted,
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Dbc::get will return <a href="../ref/program/errorret.html#DB_KEYEMPTY">DB_KEYEMPTY</a>.
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<p>If no matching keys are found, Dbc::get will return
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<a href="../ref/program/errorret.html#DB_NOTFOUND">DB_NOTFOUND</a>.
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<p><dt><a name="DB_SET_RANGE">DB_SET_RANGE</a><dd>The DB_SET_RANGE flag is identical to the DB_SET flag,
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except that the key is returned as well as the data item; and, in the case
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of the Btree access method, the returned key/data pair is the smallest
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key greater than or equal to the specified key (as determined by the
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comparison method), permitting partial key matches and range
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<p><dt><a name="DB_SET_RECNO">DB_SET_RECNO</a><dd>Move the cursor to the specific numbered record of the database, and
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return the associated key/data pair. The <b>data</b> field of the
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must be a pointer to a memory location from which a <a href="../api_cxx/dbt_class.html#db_recno_t">db_recno_t</a>
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may be read, as described in <a href="../api_cxx/dbt_class.html">Dbt</a>. This memory location will be
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read to determine 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 <a href="../api_cxx/db_set_flags.html#DB_RECNUM">DB_RECNUM</a>
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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 the
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first data item associated with the key cannot fit into the buffer, the
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size field of the <b>data</b> argument is set to the length needed to
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retrieve it, and the error ENOMEM is returned. Subsequent calls with both the
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DB_NEXT_DUP and DB_MULTIPLE flags specified will return
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additional data items associated with the current key or
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<a href="../ref/program/errorret.html#DB_NOTFOUND">DB_NOTFOUND</a> if there is no additional data items to return.
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<p>If DB_MULTIPLE is specified for the Queue and Recno access
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methods, the buffer will be filled with as many data records as
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possible. The record number of the first record will be returned in
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the <b>key</b> argument. The record number of each subsequent returned
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record must be calculated from this value.
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<p>The buffer to which the <b>data</b> argument refers should be large
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relative to the page size of the underlying database, aligned for
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unsigned integer access, and be a multiple of 1024 bytes in size.
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<p>The DB_MULTIPLE flag may only be used with the
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DB_CURRENT, DB_FIRST, DB_GET_BOTH,
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DB_NEXT, DB_NEXT_DUP, DB_NEXT_NODUP,
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DB_SET, DB_SET_RANGE, and DB_SET_RECNO
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<p>The DB_MULTIPLE flag may not be used when accessing databases
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made 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_MULTIPLE_KEY">DB_MULTIPLE_KEY</a><dd>Return multiple key and data pairs. The buffer to which the
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<b>data</b> argument refers is filled with key and data pairs. If the
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first key and data pair cannot fit into the buffer, the size field of
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the <b>data</b> argument is set to the length needed to retrieve them,
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and the error ENOMEM is returned.
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<p>The buffer to which the <b>data</b> argument refers should be large
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relative to the page size of the underlying database, aligned for
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unsigned integer access, and be a multiple of 1024 bytes in size.
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<p>The DB_MULTIPLE_KEY flag may only be used with the
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DB_CURRENT, DB_FIRST, DB_GET_BOTH,
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DB_NEXT, DB_NEXT_NODUP, DB_SET,
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DB_SET_RANGE, and DB_SET_RECNO options. The
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DB_MULTIPLE_KEY flag may not be used when accessing databases
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made 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>Otherwise, the Dbc::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>If Dbc::get fails for any reason, the state of the cursor will be
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<p>The Dbc::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_LOCK_DEADLOCK<dd>The operation was selected to resolve a deadlock.
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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>The specified cursor was not currently initialized.
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<p>The Dbc::pget interface was called with a cursor that does not
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refer to a secondary index.
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<p>The Dbc::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 Dbc::get method may fail and either
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return <a href="../ref/program/errorret.html#DB_RUNRECOVERY">DB_RUNRECOVERY</a> or throw an exception encapsulating
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<a href="../ref/program/errorret.html#DB_RUNRECOVERY">DB_RUNRECOVERY</a>, in which case all subsequent Berkeley DB calls will fail
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<a href="../api_cxx/dbc_class.html">Dbc</a>
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<a href="../api_cxx/dbc_close.html">Dbc::close</a>,
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<a href="../api_cxx/dbc_count.html">Dbc::count</a>,
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<a href="../api_cxx/dbc_del.html">Dbc::del</a>,
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<a href="../api_cxx/dbc_dup.html">Dbc::dup</a>,
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<a href="../api_cxx/dbc_get.html">Dbc::get</a>,
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<a href="../api_cxx/dbc_get.html">Dbc::pget</a>,
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<a href="../api_cxx/dbc_put.html">Dbc::put</a>.
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