Berkeley DB: DbInfo::set_re_source
Полезная информация


#include <db_cxx.h>

void DbInfo::set_re_source(char *);


Set the underlying source file for the Recno access method. The purpose of the re_source value is to provide fast access and modification to databases that are normally stored as flat text files.

If the re_source field is explicitly set, it specifies an underlying flat text database file that is read to initialize a transient record number index. In the case of variable length records, the records are separated by the byte value re_delim. For example, standard UNIX byte stream files can be interpreted as a sequence of variable length records separated by <newline> characters.

In addition, when cached data would normally be written back to the underlying database file (e.g., the Db::close or Db::sync methods are called), the in-memory copy of the database will be written back to the re_source file.

By default, the backing source file is read lazily, i.e., records are not read from the file until they are requested by the application. If multiple processes (not threads) are accessing a recno database concurrently and either inserting or deleting records, the backing source file must be read in its entirety before more than a single process accesses the database, and only that process should specify the backing source file as part of the Db::open call.

See the DB_SNAPSHOT flag for more information.

Reading and writing the backing source file specified by re_source cannot be transactionally protected because it involves filesystem operations that are not part of the Db transaction methodology. For this reason, if a temporary database is used to hold the records, i.e., a NULL was specified as the file argument to Db::open, it is possible to lose the contents of the re_source file, e.g., if the system crashes at the right instant. If a file is used to hold the database, i.e., a file name was specified as the file argument to Db::open, normal database recovery on that file can be used to prevent information loss, although it is still possible that the contents of re_source will be lost if the system crashes.

The re_source file must already exist (but may be zero-length) when Db::open is called.

For all of the above reasons, the re_source field is generally used to specify databases that are read-only for Db applications, and that are either generated on the fly by software tools, or modified using a different mechanism, e.g., a text editor.



See Also

DbInfo::set_bt_compare, DbInfo::set_bt_maxkey, DbInfo::set_bt_minkey, DbInfo::set_bt_prefix, DbInfo::set_cachesize, DbInfo::set_compare, DbInfo::set_flags, DbInfo::set_h_ffactor, DbInfo::set_h_hash, DbInfo::set_h_nelem, DbInfo::set_lorder, DbInfo::set_malloc, DbInfo::set_pagesize, DbInfo::set_re_delim, DbInfo::set_re_len, DbInfo::set_re_pad and DbInfo::set_re_source.