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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 (for example, 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; that is, 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 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 <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 transaction-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, it is
possible to lose the contents of the <b>re_source</b> 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 <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 previous reasons, the <b>re_source</b> 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.
<p>The Db::set_re_source method configures operations performed using the specified
<a href="../api_cxx/db_class.html">Db</a> handle, not all operations performed on the underlying
database.
<p>The Db::set_re_source interface may not be called after the <a href="../api_cxx/db_open.html">Db::open</a>
interface is called.
If the database already exists when
<a href="../api_cxx/db_open.html">Db::open</a> 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.
<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>The Db::set_re_source method may fail and throw an exception or return a non-zero error for the following conditions:
<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>
<p>The Db::set_re_source method may fail and throw an exception or return a non-zero error for errors specified for other Berkeley DB and C library or system methods.
If a catastrophic error has occurred, the Db::set_re_source method may fail and
either return <a href="../ref/program/errorret.html#DB_RUNRECOVERY">DB_RUNRECOVERY</a> or throw a
<a href="../api_cxx/runrec_class.html">DbRunRecoveryException</a>,
in which case all subsequent Berkeley DB calls will fail in the same way.
<h1>Class</h1>
<a href="../api_cxx/db_class.html">Db</a>
<h1>See Also</h1>
<a href="../api_cxx/db_list.html">Databases and Related Methods</a>
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