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<!--$Id: db_set_re_source.so,v 10.17 2000/05/01 21:57:43 bostic Exp $-->
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<title>Berkeley DB: Db::set_re_source</title>
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<h1>Db::set_re_source</h1>
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<h3><pre>
#include <db_cxx.h>
<p>
int
Db::set_re_source(char *re_source);
</pre></h3>
<h1>Description</h1>
<p>Set the underlying source file for the Recno access method. The purpose
of the <b>re_source</b> value is to provide fast access and modification
to databases that are normally stored as flat text files.
<p>If the <b>re_source</b> 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 <a href="../api_cxx/db_set_re_delim.html">Db::set_re_delim</a>. For example, standard UNIX
byte stream files can be interpreted as a sequence of variable length
records separated by <newline> characters.
<p>In addition, when cached data would normally be written back to the
underlying database file (e.g., the <a href="../api_cxx/db_close.html">Db::close</a> or <a href="../api_cxx/db_sync.html">Db::sync</a>
methods are called), the in-memory copy of the database will be written
back to the <b>re_source</b> file.
<p>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.
<b>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 <a href="../api_cxx/db_open.html">Db::open</a> call. See the <a href="../api_cxx/db_set_flags.html#DB_SNAPSHOT">DB_SNAPSHOT</a>
flag for more information.</b>
<p><b>Reading and writing the backing source file specified by <b>re_source</b>
cannot be transactionally protected because it involves filesystem
operations that are not part of the Db transaction methodology.</b>
For this reason, if a temporary database is used to hold the records,
i.e., a NULL was specified as the <b>file</b> argument to <a href="../api_cxx/db_open.html">Db::open</a>,
it is possible to lose the contents of the <b>re_source</b> 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 <b>file</b> argument to <a href="../api_cxx/db_open.html">Db::open</a>, normal database
recovery on that file can be used to prevent information loss,
although it is still possible that the contents of <b>re_source</b>
will be lost if the system crashes.
<p>The <b>re_source</b> file must already exist (but may be zero-length) when
<a href="../api_cxx/db_open.html">Db::open</a> is called.
<p>It is not an error to specify a read-only <b>re_source</b> 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 <a href="../api_cxx/db_sync.html">Db::sync</a> or <a href="../api_cxx/db_close.html">Db::close</a> methods will fail, of course.
Specify the <a href="../api_cxx/db_close.html#DB_NOSYNC">DB_NOSYNC</a> flag to the <a href="../api_cxx/db_close.html">Db::close</a> method to stop it
from attempting to write the changes to the backing file, instead, they
will be silently discarded.
<p>For all of the above reasons, the <b>re_source</b> field is generally
used to specify databases that are read-only for <a href="../api_cxx/db_class.html">Db</a> applications,
and that are either generated on the fly by software tools, or modified
using a different mechanism, e.g., a text editor.
<p>The Db::set_re_source interface may only be used to configure Berkeley DB before
the <a href="../api_cxx/db_open.html">Db::open</a> interface is called.
<p>The Db::set_re_source method either returns a non-zero error value or throws an exception that
encapsulates a non-zero error value on failure, and returns 0 on success.
<h1>Errors</h1>
<p><dl compact>
<p><dt>EINVAL<dd>An invalid flag value or parameter was specified.
<p>Called after <a href="../api_cxx/db_open.html">Db::open</a> was called.
</dl>
<h3>Class</h3>
<a href="../api_cxx/db_class.html">Db</a>
<h1>See Also</h1>
<a href="../api_cxx/db_close.html">Db::close</a>,
<a href="../api_cxx/db_cursor.html">Db::cursor</a>,
<a href="../api_cxx/db_del.html">Db::del</a>,
<a href="../api_cxx/db_err.html">Db::err</a>,
<a href="../api_cxx/db_fd.html">Db::fd</a>,
<a href="../api_cxx/db_get.html">Db::get</a>,
<a href="../api_cxx/db_get_byteswapped.html">Db::get_byteswapped</a>,
<a href="../api_cxx/db_get_type.html">Db::get_type</a>,
<a href="../api_cxx/db_join.html">Db::join</a>,
<a href="../api_cxx/db_key_range.html">Db::key_range</a>,
<a href="../api_cxx/db_open.html">Db::open</a>,
<a href="../api_cxx/db_put.html">Db::put</a>,
<a href="../api_cxx/db_remove.html">Db::remove</a>,
<a href="../api_cxx/db_set_bt_compare.html">Db::set_bt_compare</a>,
<a href="../api_cxx/db_set_bt_minkey.html">Db::set_bt_minkey</a>,
<a href="../api_cxx/db_set_bt_prefix.html">Db::set_bt_prefix</a>,
<a href="../api_cxx/db_set_cachesize.html">Db::set_cachesize</a>,
<a href="../api_cxx/db_set_dup_compare.html">Db::set_dup_compare</a>,
<a href="../api_cxx/db_set_errcall.html">Db::set_errcall</a>,
<a href="../api_cxx/db_set_errfile.html">Db::set_errfile</a>,
<a href="../api_cxx/db_set_errpfx.html">Db::set_errpfx</a>,
<a href="../api_cxx/db_set_flags.html">Db::set_flags</a>,
<a href="../api_cxx/db_set_h_ffactor.html">Db::set_h_ffactor</a>,
<a href="../api_cxx/db_set_h_hash.html">Db::set_h_hash</a>,
<a href="../api_cxx/db_set_h_nelem.html">Db::set_h_nelem</a>,
<a href="../api_cxx/db_set_lorder.html">Db::set_lorder</a>,
<a href="../api_cxx/db_set_malloc.html">Db::set_malloc</a>,
<a href="../api_cxx/db_set_pagesize.html">Db::set_pagesize</a>,
<a href="../api_cxx/db_set_paniccall.html">Db::set_paniccall</a>,
<a href="../api_cxx/db_set_q_extentsize.html">Db::set_q_extentsize</a>,
<a href="../api_cxx/db_set_realloc.html">Db::set_realloc</a>,
<a href="../api_cxx/db_set_re_delim.html">Db::set_re_delim</a>,
<a href="../api_cxx/db_set_re_len.html">Db::set_re_len</a>,
<a href="../api_cxx/db_set_re_pad.html">Db::set_re_pad</a>,
<a href="../api_cxx/db_set_re_source.html">Db::set_re_source</a>,
<a href="../api_cxx/db_stat.html">Db::stat</a>,
<a href="../api_cxx/db_sync.html">Db::sync</a>,
<a href="../api_cxx/db_upgrade.html">Db::upgrade</a>
and
<a href="../api_cxx/db_verify.html">Db::verify</a>.
</tt>
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