Berkeley DB: DB->get
Полезная информация

DB->get


#include <db.h>

int DB->get(DB *db, DB_TXN *txnid, DBT *key, DBT *data, u_int32_t flags);

Description

The DB->get function 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 DBcursor->c_get for details.

If the file is being accessed under transaction protection, the txnid parameter is a transaction ID returned from 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 function.

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 value may be set by logically 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.

If the database is a recno database and the requested key exists, but was never explicitly created by the application or was later deleted, the DB->get function returns DB_KEYEMPTY. Otherwise, if the requested key is not in the database, the DB->get function returns DB_NOTFOUND. Otherwise, the DB->get function returns the value of errno on failure, and 0 on success.

Errors

If a fatal error occurs in Berkeley DB, the DB->get function may fail and return DB_RUNRECOVERY, at which point all subsequent database calls will also return DB_RUNRECOVERY.

The DB->get function may fail and return errno for any of the errors specified for the following Berkeley DB and C library functions: DB->cursor, DBcursor->c_close(3), DBcursor->c_get, fflush(3), fprintf(3), vfprintf(3), and vsnprintf(3).

In addition, the DB->get function may fail and return errno for the following conditions:

EAGAIN
A lock was unavailable.

EINVAL
An invalid flag value or parameter was specified.

The DB_THREAD flag was specified to the db_open function and neither the DB_DBT_MALLOC or DB_DBT_USERMEM flags were set in the DBT.

A record number of 0 was specified.

See Also

db_appexit, db_appinit, db_version, DB->close, DB->cursor, DB->del, DB->fd, DB->get, DB->join, db_open, DB->put, DB->stat, DB->sync, DBcursor->c_close, DBcursor->c_del, DBcursor->c_get and DBcursor->c_put.