DB->associate(DB *primary, DB *secondary, int (*callback)(DB *, const DBT *, const DBT *, DBT *), u_int32_t flags);
The DB->associate function is used to declare one database a secondary index for a primary database. After a secondary database has been "associated" with a primary database, all updates to the primary will be automatically reflected in the secondary and all reads from the secondary will return corresponding data from the primary. Note that as primary keys must be unique for secondary indices to work, the primary database must be configured without support for duplicate data items. See Secondary indices for more information.
The primary argument should be a database handle for the primary database that is to be indexed. The secondary argument should be an open database handle of either a newly created and empty database that is to be used to store a secondary index, or of a database that was previously associated with the same primary and contains a secondary index. Note that it is not safe to associate as a secondary database a handle that is in use by another thread of control or has open cursors. If the handle was opened with the DB_THREAD flag it is safe to use it in multiple threads of control after the DB->associate function has returned. Note also that either secondary keys must be unique or the secondary database must be configured with support for duplicate data items.
The callback argument should refer to a callback function that creates a secondary key from a given primary key and data pair. When called, the first argument will be the secondary DB handle; the second and third arguments will be DBTs containing a primary key and datum respectively; and the fourth argument will be a zeroed DBT in which the callback function should fill in data and size fields that describe the secondary key.
If the callback function needs to allocate memory for the data field rather than simply pointing into the primary key or datum, the flags field of the returned DBT should be set to DB_DBT_APPMALLOC, which indicates that Berkeley DB should free the memory when it is done with it.
If any key/data pair in the primary yields a null secondary key and should be left out of the secondary index, the callback function may optionally return DB_DONOTINDEX. Otherwise, the callback function should return 0 in case of success or any other integer error code in case of failure; the error code will be returned from the Berkeley DB interface call that initiated the callback. Note that if the callback function returns DB_DONOTINDEX for any key/data pairs in the primary database, the secondary index will not contain any reference to those key/data pairs, and such operations as cursor iterations and range queries will reflect only the corresponding subset of the database. If this is not desirable, the application should ensure that the callback function is well-defined for all possible values and never returns DB_DONOTINDEX.
The callback argument may be NULL if and only if both the primary and secondary database handles were opened with the DB_RDONLY flag.
The flags value must be set to 0 or the following value:
If the secondary database has been opened in an environment configured with transactions, each put necessary for its creation will be done in the context of a transaction created for the purpose.
Note that care should be taken not to use a newly-created secondary database in another thread of control until the DB->associate call has returned successfully in the first thread.
The DB->associate function may fail and return a non-zero error for the following conditions:
The secondary database handle has already been associated with this or another database handle.
The secondary database handle is not open.
The primary database has been configured to allow duplicates.
The DB->associate 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 DB->associate function may fail and return DB_RUNRECOVERY, in which case all subsequent Berkeley DB calls will fail in the same way.
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