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<?xml version="1.0" encoding="UTF-8" standalone="no"?>
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<tr>
<th colspan="3" align="center">DB->open()</th>
</tr>
<tr>
<td width="20%" align="left"><a accesskey="p" href="dbkey_range.html">Prev</a> </td>
<th width="60%" align="center">Chapter 2.
The DB Handle
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</tr>
</table>
<hr />
</div>
<div class="sect1" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h2 class="title" style="clear: both"><a id="dbopen"></a>DB->open()</h2>
</div>
</div>
</div>
<pre class="programlisting">#include <db.h>
int
DB->open(DB *db, DB_TXN *txnid, const char *file,
const char *database, DBTYPE type, u_int32_t flags, int mode); </pre>
<p>
The <code class="methodname">DB->open()</code> method opens the database represented by the
<span class="bold"><strong>file</strong></span> and <span class="bold"><strong>database</strong></span>.
</p>
<p>
The currently supported Berkeley DB file formats (or <span class="emphasis"><em>access
methods</em></span>) are Btree, Hash, Queue, and Recno. The Btree
format is a representation of a sorted, balanced tree structure. The
Hash format is an extensible, dynamic hashing scheme. The Queue
format supports fast access to fixed-length records accessed
sequentially or by logical record number. The Recno format supports
fixed- or variable-length records, accessed sequentially or by logical
record number, and optionally backed by a flat text file.
</p>
<p>
Storage and retrieval for the Berkeley DB access methods are based on
key/data pairs; see <a class="link" href="dbt.html" title="Chapter 4. The DBT Handle">DBT</a>
for more information.
</p>
<p>
Calling <code class="methodname">DB->open()</code> is a relatively expensive operation, and
maintaining a set of open databases will normally be preferable to
repeatedly opening and closing the database for each new query.
</p>
<p>
The <code class="methodname">DB->open()</code> <span>
<span>
method returns a non-zero error value on failure and 0 on success.
</span>
</span>
If <code class="methodname">DB->open()</code> fails, the
<a class="xref" href="dbclose.html" title="DB->close()">DB->close()</a> method must be
called to discard the <a class="link" href="db.html" title="Chapter 2. The DB Handle">DB</a>
handle.
</p>
<div class="sect2" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h3 class="title"><a id="idp50079768"></a>Parameters</h3>
</div>
</div>
</div>
<div class="sect3" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h4 class="title"><a id="idp50072448"></a>txnid</h4>
</div>
</div>
</div>
<p>
If the operation is part of an application-specified transaction, the
<span class="bold"><strong>txnid</strong></span> parameter is a transaction
handle returned from <a class="xref" href="txnbegin.html" title="DB_ENV->txn_begin()">DB_ENV->txn_begin()</a>;
if the operation is part of a Berkeley DB Concurrent Data Store group, the
<span class="bold"><strong>txnid</strong></span> parameter is a handle returned
from <a class="xref" href="envcdsgroup_begin.html" title="DB_ENV->cdsgroup_begin()">DB_ENV->cdsgroup_begin()</a>;
otherwise NULL. If no transaction handle is specified, but the
DB_AUTO_COMMIT flag is specified, the operation will be implicitly
transaction protected. Note that transactionally protected operations
on a <a class="link" href="db.html" title="Chapter 2. The DB Handle">DB</a> handle requires
the <a class="link" href="db.html" title="Chapter 2. The DB Handle">DB</a> handle itself be
transactionally protected during its open. Also note that the
transaction must be committed before the handle is closed; see <a href="../../programmer_reference/program_scope.html" class="olink">Berkeley DB handles</a> for more
information.
</p>
</div>
<div class="sect3" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h4 class="title"><a id="idp50096944"></a>file</h4>
</div>
</div>
</div>
<p>
The <span class="bold"><strong>file</strong></span> parameter is used as the
name of an underlying file that will be used to back the database; see
<a href="../../programmer_reference/env_naming.html" class="olink">File naming</a> for more
information.
</p>
<p>
In-memory databases never intended to be preserved on disk may be
created by setting the <span class="bold"><strong>file</strong></span> parameter
to NULL. Whether other threads of control can access this database is
driven entirely by whether the <span class="bold"><strong>database</strong></span>
parameter is set to NULL.
</p>
<p>
When using a Unicode build on Windows (the default), the <span class="bold"><strong>file</strong></span> argument will be interpreted as a UTF-8
string, which is equivalent to ASCII for Latin characters.
</p>
</div>
<div class="sect3" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h4 class="title"><a id="idp50019200"></a>database</h4>
</div>
</div>
</div>
<p>
The <span class="bold"><strong>database</strong></span> parameter is optional,
and allows applications to have multiple databases in a single file.
Although no <span class="bold"><strong>database</strong></span> parameter needs
to be specified, it is an error to attempt to open a second database
in a <span class="bold"><strong>file</strong></span> that was not initially
created using a <span class="bold"><strong>database</strong></span> name.
Further, the <span class="bold"><strong>database</strong></span> parameter is
not supported by the Queue format. Finally, when opening multiple
databases in the same physical file, it is important to consider
locking and memory cache issues; see
<a href="../../programmer_reference/am_opensub.html" class="olink">Opening multiple databases in a single file</a>
for more information.
</p>
<p>
If both the <span class="bold"><strong>database</strong></span> and
<span class="bold"><strong>file</strong></span> parameters are NULL,
the database is strictly temporary and cannot be opened
by any other thread of control. Thus the database can only be accessed
by sharing the single database handle that created it, in
circumstances where doing so is safe.
</p>
<p>
If the <span class="bold"><strong>database</strong></span> parameter is not
set to NULL, the database can be opened by other threads of control
and will be replicated to client sites in any replication group, regardless
of whether the <span class="bold"><strong>file</strong></span> parameter is set to
NULL.
</p>
</div>
<div class="sect3" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h4 class="title"><a id="idp50092304"></a>type</h4>
</div>
</div>
</div>
<p>
The <span class="bold"><strong>type</strong></span> parameter is of type DBTYPE,
and must be set to one of <code class="literal">DB_BTREE</code>,
<code class="literal">DB_HASH</code>, <code class="literal">DB_QUEUE</code>,
<code class="literal">DB_RECNO</code>, or <code class="literal">DB_UNKNOWN</code>. If <span class="bold"><strong>type</strong></span> is DB_UNKNOWN, the database must already
exist and <code class="methodname">DB->open()</code> will automatically determine its type. The
<a class="xref" href="dbget_type.html" title="DB->get_type()">DB->get_type()</a> method
may be used to determine the underlying type of databases opened using
DB_UNKNOWN.
</p>
<p>
It is an error to specify the incorrect <span class="bold"><strong>type</strong></span>
for a database that already exists.
</p>
</div>
<div class="sect3" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h4 class="title"><a id="idp50093848"></a>flags</h4>
</div>
</div>
</div>
<p>
The <span class="bold"><strong>flags</strong></span> parameter must be set to zero or
by bitwise inclusively <span class="bold"><strong>OR</strong></span>'ing together one
or more of the following values:
</p>
<div class="itemizedlist">
<ul type="disc">
<li>
<p><a id="open_DB_AUTO_COMMIT"></a>
<code class="literal">DB_AUTO_COMMIT</code>
</p>
<p>
Enclose the <code class="methodname">DB->open()</code> call within a transaction. If the call
succeeds, the open operation will be recoverable and all
subsequent database modification operations based on this
handle will be transactionally protected. If the call fails,
no database will have been created.
</p>
</li>
<li>
<p><a id="open_DB_CREATE"></a>
<code class="literal">DB_CREATE</code>
</p>
<p>
Create the database. If the database does not already exist
and the <code class="literal">DB_CREATE</code> flag is not specified, the
<code class="methodname">DB->open()</code> will fail.
</p>
</li>
<li>
<p><a id="open_DB_EXCL"></a>
<code class="literal">DB_EXCL</code>
</p>
<p>
Return an error if the database already exists. The
<code class="literal">DB_EXCL</code> flag is only meaningful
when specified with the <code class="literal">DB_CREATE</code>.
flag.
</p>
</li>
<li>
<p><a id="dbopen_DB_MULTIVERSION"></a>
<code class="literal">DB_MULTIVERSION</code>
</p>
<p>
Open the database with support for
<a href="../../programmer_reference/transapp_read.html" class="olink">multiversion concurrency control</a>.
This will cause updates to the database to follow a
copy-on-write protocol, which is required to support
snapshot isolation. The <code class="literal">DB_MULTIVERSION</code>
flag requires that the database be transactionally protected during its open
and is not supported by the queue format.
</p>
</li>
<li>
<p><a id="open_DB_NOMMAP"></a>
<code class="literal">DB_NOMMAP</code>
</p>
<p>
Do not map this database into process memory (see the
<a class="xref" href="envset_mp_mmapsize.html" title="DB_ENV->set_mp_mmapsize()">DB_ENV->set_mp_mmapsize()</a>
method for further information).
</p>
</li>
<li>
<p><a id="dbopen_DB_RDONLY"></a>
<code class="literal">DB_RDONLY</code>
</p>
<p>
Open the database for reading only. Any attempt to modify
items in the database will fail, regardless of the actual
permissions of any underlying files.
</p>
</li>
<li>
<p><a id="dbopen_DB_READ_UNCOMMITTED"></a>
<code class="literal">DB_READ_UNCOMMITTED</code>
</p>
<p>
Support transactional read operations with degree 1
isolation. Read operations on the database may request the
return of modified but not yet committed data. This flag
must be specified on all
<a class="link" href="db.html" title="Chapter 2. The DB Handle">DB</a>
handles used to perform dirty
reads or database updates, otherwise requests for dirty
reads may not be honored and the read may block.
</p>
</li>
<li>
<p><a id="open_DB_THREAD"></a>
<code class="literal">DB_THREAD</code>
</p>
<p>
Cause the <a class="link" href="db.html" title="Chapter 2. The DB Handle">DB</a>
handle returned by <code class="methodname">DB->open()</code> to be <span class="emphasis"><em>free-threaded</em></span>; that is,
concurrently usable by multiple threads in the address
space.
</p>
</li>
<li>
<p><a id="open_DB_TRUNCATE"></a>
<code class="literal">DB_TRUNCATE</code>
</p>
<p>
Physically truncate the underlying file, discarding all
previous databases it might have held. Underlying filesystem
primitives are used to implement this flag. For this reason,
it is applicable only to the file and cannot be used to
discard databases within a file.
</p>
<p>
The <code class="literal">DB_TRUNCATE</code> flag cannot be lock or
transaction-protected, and it is an error to specify it in a
locking or transaction-protected environment.
</p>
</li>
</ul>
</div>
</div>
<div class="sect3" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h4 class="title"><a id="idp50125584"></a>mode</h4>
</div>
</div>
</div>
<p>
On Windows systems, the mode parameter is ignored.
</p>
<p>
On UNIX systems or in IEEE/ANSI Std 1003.1 (POSIX) environments, files
created by the database open are created with mode <span class="bold"><strong>mode</strong></span> (as described in <span class="bold"><strong>chmod</strong></span>(2)) and modified by the process' umask
value at the time of creation (see <span class="bold"><strong>umask</strong></span>(2)). Created files are owned by the
process owner; the group ownership of created files is based on the
system and directory defaults, and is not further specified by
Berkeley DB. System shared memory segments created by the database
open are created with mode <span class="bold"><strong>mode</strong></span>,
unmodified by the process' umask value. If <span class="bold"><strong>mode</strong></span> is 0, the database open will use a default
mode of readable and writable by both owner and group.
</p>
</div>
</div>
<div class="sect2" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h3 class="title"><a id="idp50100120"></a>Environment Variables</h3>
</div>
</div>
</div>
<p>
If the database was opened within a database environment, the
environment variable <span class="bold"><strong>DB_HOME</strong></span> may be used as the path of the
database environment home.
</p>
<p>
<code class="methodname">DB->open()</code> is affected by any database directory specified using the
<a class="xref" href="envset_data_dir.html" title="DB_ENV->set_data_dir()">DB_ENV->set_data_dir()</a>
method, or by setting the "set_data_dir" string in
the environment's
<a href="../../programmer_reference/env_db_config.html#env_db_config.DB_CONFIG" class="olink">DB_CONFIG</a>
file.
</p>
<div class="itemizedlist">
<ul type="disc">
<li>
<p>
<span class="bold"><strong>TMPDIR</strong></span>
</p>
<p>
If the <span class="bold"><strong>file</strong></span>
and <span class="bold"><strong>dbenv</strong></span>
parameters to <code class="methodname">DB->open()</code> are <code class="literal">NULL</code>,
the environment variable
<span class="bold"><strong>TMPDIR</strong></span>
may be used as a
directory in which to create temporary backing files
</p>
</li>
</ul>
</div>
</div>
<div class="sect2" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h3 class="title"><a id="idp50145888"></a>Errors</h3>
</div>
</div>
</div>
<p>
The <code class="methodname">DB->open()</code> <span>
<span>
method may fail and return one of the following non-zero errors:
</span>
</span>
</p>
<div class="sect3" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h4 class="title"><a id="idp50092616"></a>DB_LOCK_DEADLOCK</h4>
</div>
</div>
</div>
<p>
A transactional database environment operation was selected to resolve
a deadlock.
</p>
</div>
<div class="sect3" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h4 class="title"><a id="idp50129520"></a>DB_LOCK_NOTGRANTED</h4>
</div>
</div>
</div>
<p>
A Berkeley DB Concurrent Data Store database environment configured
for lock timeouts was unable to grant a lock in the allowed time.
</p>
</div>
<div class="sect3" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h4 class="title"><a id="idp50127976"></a>ENOENT</h4>
</div>
</div>
</div>
<p>
The file or directory does not exist.
</p>
</div>
<div class="sect3" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h4 class="title"><a id="idp50133824"></a>ENOENT</h4>
</div>
</div>
</div>
<p>
A nonexistent <span class="bold"><strong>re_source</strong></span> file was specified.
</p>
</div>
<div class="sect3" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h4 class="title"><a id="idp50097656"></a>DB_OLD_VERSION</h4>
</div>
</div>
</div>
<p>
The database cannot be opened without being first upgraded.
</p>
</div>
<div class="sect3" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h4 class="title"><a id="idp50148440"></a>EEXIST</h4>
</div>
</div>
</div>
<p>
<code class="literal">DB_CREATE</code> and <code class="literal">DB_EXCL</code>
were specified and the database exists.
</p>
</div>
<div class="sect3" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h4 class="title"><a id="idp50143160"></a>EINVAL</h4>
</div>
</div>
</div>
<p>
If an unknown database type, page size, hash function, pad byte, byte order, or a flag value
or parameter that is incompatible with the specified database was specified; the
<a class="link" href="envopen.html#envopen_DB_THREAD">DB_THREAD</a>
flag was specified and fast mutexes are not available for this architecture; the
<a class="link" href="envopen.html#envopen_DB_THREAD">DB_THREAD</a>
flag was specified to <code class="methodname">DB->open()</code>, but was
not specified to the <code class="methodname">DB_ENV->open()</code> call for the
environment in which the <a class="link" href="db.html" title="Chapter 2. The DB Handle">DB</a>
handle was created; a backing flat text file was specified with either the
<a class="link" href="envopen.html#envopen_DB_THREAD">DB_THREAD</a>
flag or the provided database environment supports transaction
processing; or if an invalid flag value or parameter was specified.
</p>
</div>
<div class="sect3" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h4 class="title"><a id="idp50098056"></a> DB_REP_HANDLE_DEAD</h4>
</div>
</div>
</div>
<p>
When a client synchronizes with the master, it is possible for committed
transactions to be rolled back. This invalidates all the database and cursor
handles opened in the replication environment. Once this occurs, an attempt to use
such a handle will
return <code class="literal">DB_REP_HANDLE_DEAD</code>.
The application will need to discard the handle and open a new one in order to
continue processing.
</p>
</div>
<div class="sect3" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h4 class="title"><a id="idp50145352"></a>DB_REP_LOCKOUT</h4>
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<p>
The operation was blocked by client/master synchronization.
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<h3 class="title"><a id="idp50144568"></a>Class</h3>
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<a class="link" href="db.html" title="Chapter 2. The DB Handle">DB</a>
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<h3 class="title"><a id="idp50113216"></a>See Also</h3>
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<a class="xref" href="db.html#dblist" title="Database and Related Methods">Database and Related Methods</a>
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