Berkeley DB: DBcursor->c_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++

# DBcursor->c_get |

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    #include <db.h>

    int
    DBcursor->c_get(DBC *cursor,
        DBT *key, DBT *data, u_int32_t flags);

# Description

The DBcursor->c_get function retrieves key/data pairs from the database. The address and length of the key are returned in the object referenced by key (except for the case of the DB_SET flag where the key object is unchanged), and the address and length of the data are returned in the object referenced by data.

Modifications to the database during a sequential scan will be reflected in the scan, i.e. records inserted behind a cursor will not be returned while records inserted in front of a cursor will be returned.

In Queue and Recno databases, missing entries (i.e., entries that were never explicitly created or that were created and then deleted), will be skipped during a sequential scan.

If multiple threads or processes insert items into the same database file without using locking, the results are undefined. For more detail, see cursor stability.

The flags parameter must be set to 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. For DB_CONSUME to be specified, the underlying database must be of type Queue.

DB_CURRENT
  Return the key/data pair currently referenced by the cursor.

  If the cursor key/data pair was deleted, DBcursor->c_get will return DB_KEYEMPTY.

  If the cursor is not yet initialized, the DBcursor->c_get function will return EINVAL.

DB_FIRST, DB_LAST
  The cursor is set to reference the first (last) key/data pair of the database, and that pair is returned. In the presence of duplicate key values, the first (last) data item in the set of duplicates is returned.

  If the database is a Queue or Recno database, DBcursor->c_get using the DB_FIRST (DB_LAST) flags will ignore any keys that exist but were never explicitly created by the application or were created and later deleted.

  If the database is empty, DBcursor->c_get will return DB_NOTFOUND.

DB_GET_BOTH
  The DB_GET_BOTH flag is identical to the DB_SET flag, except that both the key and the data arguments must be matched by the key and data item in the database.

DB_GET_RECNO
  Return the record number associated with the cursor. The record number will be returned in data as described in DBT. The key parameter is ignored.

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

DB_JOIN_ITEM
  Do not use the data value found in all of the cursors as a lookup key for the primary database, but simply return it in the key parameter instead. The data parameter is left unchanged.

  For DB_JOIN_ITEM to be specified, the underlying cursor must have been returned from the DB->join function.

DB_NEXT, DB_PREV
  If the cursor is not yet initialized, DB_NEXT (DB_PREV) is identical to DB_FIRST (DB_LAST). Otherwise, the cursor is moved to the next (previous) key/data pair of the database, and that pair is returned. In the presence of duplicate key values, the value of the key may not change.

  If the database is a Queue or Recno database, DBcursor->c_get using the DB_NEXT (DB_PREV) flag will skip any keys that exist but were never explicitly created by the application or were created and later deleted.

  If the cursor is already on the last (first) record in the database, DBcursor->c_get will return DB_NOTFOUND.

DB_NEXT_DUP
  If the next key/data pair of the database is a duplicate record for the current key/data pair, the cursor is moved to the next key/data pair of the database, and that pair is returned. Otherwise, DBcursor->c_get will return DB_NOTFOUND.

  If the cursor is not yet initialized, the DBcursor->c_get function will return EINVAL.

DB_NEXT_NODUP, DB_PREV_NODUP
  If the cursor is not yet initialized, DB_NEXT_NODUP (DB_PREV_NODUP) is identical to DB_FIRST (DB_LAST). Otherwise, the cursor is moved to the next (previous) non-duplicate key/data pair of the database, and that pair is returned.

  If the database is a Queue or Recno database, DBcursor->c_get using the DB_NEXT_NODUP (DB_PREV_NODUP) flags will ignore any keys that exist but were never explicitly created by the application or were created and later deleted.

  If no non-duplicate key/data pairs occur after (before) the cursor position in the database, DBcursor->c_get will return DB_NOTFOUND.

DB_SET
  Move the cursor to the specified key/data pair of the database, and return the datum associated with the given key.

  In the presence of duplicate key values, DBcursor->c_get will return the first data item for the given key.

  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, DBcursor->c_get will return DB_KEYEMPTY.

  If no matching keys are found, DBcursor->c_get will return DB_NOTFOUND.

DB_SET_RANGE
  The DB_SET_RANGE flag is identical to the DB_SET flag, except that the key is returned as well as the data item, and, in the case of the Btree access method, the returned key/data pair is the smallest key greater than or equal to the specified key (as determined by the comparison function), permitting partial key matches and range searches.

DB_SET_RECNO
  Move the cursor to the specific numbered record of the database, and return the associated key/data pair. The data field of the specified key must be a pointer to a memory location from which a db_recno_t may be read, as described in DBT. This memory location will be read to determine 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.

Otherwise, the DBcursor->c_get function returns a non-zero error value on failure and 0 on success.

If DBcursor->c_get fails for any reason, the state of the cursor will be unchanged.

# Errors

The DBcursor->c_get function may fail and 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.

  The specified cursor was not currently initialized.

The DBcursor->c_get function may fail and return a non-zero error for errors specified for other Berkeley DB and C library or system functions. If a catastrophic error has occurred, the DBcursor->c_get function may fail and return DB_RUNRECOVERY, in which case all subsequent Berkeley DB calls will fail in the same way.

# See Also

DBcursor->c_close, DBcursor->c_count, DBcursor->c_del, DBcursor->c_dup, DBcursor->c_get and DBcursor->c_put.`

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