int DB->set_re_source(DB *db, char *re_source);
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 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, as specified by DB->set_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 (for example, 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; that is, 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 are 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 transaction-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, it is possible to lose the contents of the re_source file, for example, if the system crashes at the right instant. If a file is used to hold the database, 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.
It is not an error to specify a read-only re_source file when creating a database, nor is it an error to modify the resulting database. However, any attempt to write the changes to the backing source file using either the DB->sync or DB->close methods will fail, of course. Specify the DB_NOSYNC flag to the DB->close method to stop it from attempting to write the changes to the backing file; instead, they will be silently discarded.
For all of the previous reasons, the re_source field is generally used to specify databases that are read-only for Berkeley DB applications; and that are either generated on the fly by software tools or modified using a different mechanism -- for example, a text editor.
The DB->set_re_source method configures operations performed using the specified DB handle, not all operations performed on the underlying database.
The DB->set_re_source interface may not be called after the DB->open interface is called. If the database already exists when DB->open is called, the information specified to DB->set_re_source must be the same as that historically used to create the database or corruption can occur.
The DB->set_re_source method returns a non-zero error value on failure and 0 on success.
The DB->set_re_source method may fail and return a non-zero error for the following conditions:
Called after DB->open was called.
The DB->set_re_source method 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->set_re_source method may fail and return DB_RUNRECOVERY, in which case all subsequent Berkeley DB calls will fail in the same way.
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