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<td><dl><dt>Berkeley DB Reference Guide:<dd>Access Methods</dl></td>
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Managing record-based databases
When using fixed- or variable-length record-based databases, particularly
with flat-text backing files, there are several items that the user can
control. The Recno access method can be used to store either variable-
or fixed-length data items. By default, the Recno access method stores
variable-length data items. The Queue access method can only store
fixed-length data items.
Record Delimiters
When using the Recno access method to store variable-length records,
records read from any backing source file are separated by a specific
byte value which marks the end of one record and the beginning of the
next. This delimiting value is ignored except when reading records from
a backing source file, that is, records may be stored into the database
that include the delimiter byte. However, if such records are written
out to the backing source file and the backing source file is
subsequently read into a database, the records will be split where
delimiting bytes were found.
For example, UNIX text files can usually be interpreted as a sequence of
variable-length records separated by ASCII newline characters. This byte
value (ASCII 0x0a) is the default delimiter. Applications may specify a
different delimiting byte using the <a href="../../api_c/db_set_re_delim.html">DB-&gt;set_re_delim</a> method. If no
backing source file is being used, there is no reason to set the
delimiting byte value.
Record Length
When using the Recno or Queue access methods to store fixed-length
records, the record length must be specified. Since the Queue access
method always uses fixed-length records, the user must always set the
record length prior to creating the database. Setting the record length
is what causes the Recno access method to store fixed-length, not
variable-length, records.
The length of the records is specified by calling the
<a href="../../api_c/db_set_re_len.html">DB-&gt;set_re_len</a> method. The default length of the records is 0 bytes.
Any record read from a backing source file or otherwise stored in the
database that is shorter than the declared length will automatically be
padded as described for the <a href="../../api_c/db_set_re_pad.html">DB-&gt;set_re_pad</a> method. Any record stored
that is longer than the declared length results in an error. For
further information on backing source files, see
<a href="../../ref/am_conf/re_source.html">Flat-text backing files</a>.
Record Padding Byte Value
When storing fixed-length records in a Queue or Recno database, a pad
character may be specified by calling the <a href="../../api_c/db_set_re_pad.html">DB-&gt;set_re_pad</a> method. Any
record read from the backing source file or otherwise stored in the
database that is shorter than the expected length will automatically be
padded with this byte value. If fixed-length records are specified but
no pad value is specified, a space character (0x20 in the ASCII
character set) will be used. For further information on backing source
files, see <a href="../../ref/am_conf/re_source.html">Flat-text backing
files</a>.
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