#include <db.h>

int
DB->set_flags(DB *db, u_int32_t flags);

int
DB->get_flags(DB *db, u_int32_t *flagsp);

Description: DB->set_flags

Configure a database. Calling DB->set_flags is additive; there is no way to clear flags.

The DB->set_flags method may not be called after the DB->open method is called.

The DB->set_flags method returns a non-zero error value on failure and 0 on success.

Parameters

flags

The flags parameter must be set to 0 or by bitwise inclusively OR'ing together one or more of the following values:

General

The following flags may be specified for any Berkeley DB access method:

DB_CHKSUM

Do checksum verification of pages read into the cache from the backing filestore. Berkeley DB uses the SHA1 Secure Hash Algorithm if encryption is configured and a general hash algorithm if it is not.

Calling DB->set_flags with the DB_CHKSUM flag only affects the specified DB handle (and any other Berkeley DB handles opened within the scope of that handle).

If the database already exists when DB->open is called, the DB_CHKSUM flag will be ignored.

If creating additional databases in a file, the checksum behavior specified must be consistent with the existing databases in the file or an error will be returned.

DB_ENCRYPT

Encrypt the database using the cryptographic password specified to the DB_ENV->set_encrypt or DB->set_encrypt methods.

Calling DB->set_flags with the DB_ENCRYPT flag only affects the specified DB handle (and any other Berkeley DB handles opened within the scope of that handle).

If the database already exists when DB->open is called, the DB_ENCRYPT flag must be the same as the existing database or an error will be returned.

If creating additional databases in a file, the encryption behavior specified must be consistent with the existing databases in the file or an error will be returned.

Encrypted databases are not portable between machines of different byte orders, that is, encrypted databases created on big-endian machines cannot be read on little-endian machines, and vice versa.

DB_TXN_NOT_DURABLE

If set, Berkeley DB will not write log records for this database. This means that updates of this database exhibit the ACI (atomicity, consistency, and isolation) properties, but not D (durability); that is, database integrity will be maintained, but if the application or system fails, integrity will not persist. The database file must be verified and/or restored from backup after a failure. In order to ensure integrity after application shut down, the database handles must be closed without specifying DB_NOSYNC, or all database changes must be flushed from the database environment cache using either the DB_ENV->txn_checkpoint or DB_ENV->memp_sync methods. All database handles for a single physical file must set DB_TXN_NOT_DURABLE, including database handles for different databases in a physical file.

Calling DB->set_flags with the DB_TXN_NOT_DURABLE flag only affects the specified DB handle (and any other Berkeley DB handles opened within the scope of that handle).

Btree

The following flags may be specified for the Btree access method:

DB_DUP

Permit duplicate data items in the database; that is, insertion when the key of the key/data pair being inserted already exists in the database will be successful. The ordering of duplicates in the database is determined by the order of insertion, unless the ordering is otherwise specified by use of a cursor operation.

The DB_DUPSORT flag is preferred to DB_DUP for performance reasons. The DB_DUP flag should only be used by applications wanting to order duplicate data items manually.

Calling DB->set_flags with the DB_DUP flag affects the database, including all threads of control accessing the database.

If the database already exists when DB->open is called, the DB_DUP flag must be the same as the existing database or an error will be returned.

It is an error to specify both DB_DUP and DB_RECNUM.

DB_DUPSORT

Permit duplicate data items in the database; that is, insertion when the key of the key/data pair being inserted already exists in the database will be successful. The ordering of duplicates in the database is determined by the duplicate comparison function. If the application does not specify a comparison function using the DB->set_dup_compare method, a default lexical comparison will be used. It is an error to specify both DB_DUPSORT and DB_RECNUM.

Calling DB->set_flags with the DB_DUPSORT flag affects the database, including all threads of control accessing the database.

If the database already exists when DB->open is called, the DB_DUPSORT flag must be the same as the existing database or an error will be returned.

DB_RECNUM

Support retrieval from the Btree using record numbers. For more information, see the DB_SET_RECNO flag to the DB->get and DBcursor->c_get methods.

Logical record numbers in Btree databases are mutable in the face of record insertion or deletion. See the DB_RENUMBER flag in the Recno access method information for further discussion.

Maintaining record counts within a Btree introduces a serious point of contention, namely the page locations where the record counts are stored. In addition, the entire database must be locked during both insertions and deletions, effectively single-threading the database for those operations. Specifying DB_RECNUM can result in serious performance degradation for some applications and data sets.

It is an error to specify both DB_DUP and DB_RECNUM.

Calling DB->set_flags with the DB_RECNUM flag affects the database, including all threads of control accessing the database.

If the database already exists when DB->open is called, the DB_RECNUM flag must be the same as the existing database or an error will be returned.

DB_REVSPLITOFF

Turn off reverse splitting in the Btree. As pages are emptied in a database, the Berkeley DB Btree implementation attempts to coalesce empty pages into higher-level pages in order to keep the database as small as possible and minimize search time. This can hurt performance in applications with cyclical data demands; that is, applications where the database grows and shrinks repeatedly. For example, because Berkeley DB does page-level locking, the maximum level of concurrency in a database of two pages is far smaller than that in a database of 100 pages, so a database that has shrunk to a minimal size can cause severe deadlocking when a new cycle of data insertion begins.

Calling DB->set_flags with the DB_REVSPLITOFF flag only affects the specified DB handle (and any other Berkeley DB handles opened within the scope of that handle).

Hash

The following flags may be specified for the Hash access method:

DB_DUP

Permit duplicate data items in the database; that is, insertion when the key of the key/data pair being inserted already exists in the database will be successful. The ordering of duplicates in the database is determined by the order of insertion, unless the ordering is otherwise specified by use of a cursor operation.

The DB_DUPSORT flag is preferred to DB_DUP for performance reasons. The DB_DUP flag should only be used by applications wanting to order duplicate data items manually.

Calling DB->set_flags with the DB_DUP flag affects the database, including all threads of control accessing the database.

If the database already exists when DB->open is called, the DB_DUP flag must be the same as the existing database or an error will be returned.

DB_DUPSORT

Permit duplicate data items in the database; that is, insertion when the key of the key/data pair being inserted already exists in the database will be successful. The ordering of duplicates in the database is determined by the duplicate comparison function. If the application does not specify a comparison function using the DB->set_dup_compare method, a default lexical comparison will be used. It is an error to specify both DB_DUPSORT and DB_RECNUM.

Calling DB->set_flags with the DB_DUPSORT flag affects the database, including all threads of control accessing the database.

If the database already exists when DB->open is called, the DB_DUPSORT flag must be the same as the existing database or an error will be returned.

Queue

The following flags may be specified for the Queue access method:

DB_INORDER

The DB_INORDER flag modifies the operation of the DB_CONSUME or DB_CONSUME_WAIT flags to DB->get to return key/data pairs in order. That is, they will always return the key/data item from the head of the queue.

The default behavior of queue databases is optimized for multiple readers, and does not guarantee that record will be retrieved in the order they are added to the queue. Specifically, if a writing thread adds multiple records to an empty queue, reading threads may skip some of the initial records when the next DB->get call returns.

This flag modifies the DB->get call to verify that the record being returned is in fact the head of the queue. This will increase contention and reduce concurrency when there are many reading threads.

Calling DB->set_flags with the DB_INORDER flag only affects the specified DB handle (and any other Berkeley DB handles opened within the scope of that handle).

Recno

The following flags may be specified for the Recno access method:

DB_RENUMBER

Specifying the DB_RENUMBER flag causes the logical record numbers to be mutable, and change as records are added to and deleted from the database. For example, the deletion of record number 4 causes records numbered 5 and greater to be renumbered downward by one. If a cursor was positioned to record number 4 before the deletion, it will refer to the new record number 4, if any such record exists, after the deletion. If a cursor was positioned after record number 4 before the deletion, it will be shifted downward one logical record, continuing to refer to the same record as it did before.

Using the DB->put or DBcursor->c_put interfaces to create new records will cause the creation of multiple records if the record number is more than one greater than the largest record currently in the database. For example, creating record 28, when record 25 was previously the last record in the database, will create records 26 and 27 as well as 28. Attempts to retrieve records that were created in this manner will result in an error return of DB_KEYEMPTY.

If a created record is not at the end of the database, all records following the new record will be automatically renumbered upward by one. For example, the creation of a new record numbered 8 causes records numbered 8 and greater to be renumbered upward by one. If a cursor was positioned to record number 8 or greater before the insertion, it will be shifted upward one logical record, continuing to refer to the same record as it did before.

For these reasons, concurrent access to a Recno database with the DB_RENUMBER flag specified may be largely meaningless, although it is supported.

Calling DB->set_flags with the DB_RENUMBER flag affects the database, including all threads of control accessing the database.

If the database already exists when DB->open is called, the DB_RENUMBER flag must be the same as the existing database or an error will be returned.

DB_SNAPSHOT

This flag specifies that any specified re_source file be read in its entirety when DB->open is called. If this flag is not specified, the re_source file may be read lazily.

Calling DB->set_flags with the DB_SNAPSHOT flag only affects the specified DB handle (and any other Berkeley DB handles opened within the scope of that handle).

Errors

The DB->set_flags method may fail and return one of the following non-zero errors:

EINVAL

An invalid flag value or parameter was specified.

Description: DB->get_flags

The DB->get_flags method returns the current flags.

The DB->get_flags method may be called at any time during the life of the application.

The DB->get_flags method returns a non-zero error value on failure and 0 on success.

Parameters

flagsp

The DB->get_flags method returns the current flags in flagsp.

Class

See Also