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<tr>
<th colspan="3" align="center">DbEnv::open()</th>
</tr>
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<td width="20%" align="left"><a accesskey="p" href="envlsn_reset.html">Prev</a> </td>
<th width="60%" align="center">Chapter 5.
The DbEnv Handle
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</div>
<div class="sect1" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h2 class="title" style="clear: both"><a id="envopen"></a>DbEnv::open()</h2>
</div>
</div>
</div>
<pre class="programlisting">#include <db_cxx.h>
int
DbEnv::open(const char *db_home, u_int32_t flags, int mode); </pre>
<p>
The <code class="methodname">DbEnv::open()</code> method opens a Berkeley
DB environment. It provides a structure for creating a consistent
environment for processes using one or more of the features of
Berkeley DB.
</p>
<p>
The <code class="methodname">DbEnv::open()</code> method <span>
<span>
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.
</span>
</span> If
<code class="methodname">DbEnv::open()</code> fails, the
<a class="xref" href="envclose.html" title="DbEnv::close()">DbEnv::close()</a> method must be
called to discard the <a class="link" href="env.html" title="Chapter 5. The DbEnv Handle">DbEnv</a>
handle.
</p>
<div class="warning" style="margin-left: 0.5in; margin-right: 0.5in;">
<h3 class="title">Warning</h3>
<p>
Using environments with some journaling filesystems might
result in log file corruption. This can occur if the operating
system experiences an unclean shutdown when a log file is being
created. Please see
<a href="../../programmer_reference/transapp_journal.html" class="olink">Using
Recovery on Journaling Filesystems</a>
in the <em class="citetitle">Berkeley DB Programmer's Reference Guide</em> for more information.
</p>
</div>
<div class="sect2" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h3 class="title"><a id="idp7011736"></a>Parameters</h3>
</div>
</div>
</div>
<div class="sect3" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h4 class="title"><a id="idp6971408"></a>db_home</h4>
</div>
</div>
</div>
<p>
The <span class="bold"><strong>db_home</strong></span>
parameter is the database environment's home
directory. For more information on
<span class="bold"><strong>db_home</strong></span>, and filename
resolution in general, see <a href="../../programmer_reference/env_naming.html" class="olink">Berkeley DB File Naming</a>. The
environment variable
<span class="bold"><strong>DB_HOME</strong></span> may be used as the
path of the database home, as described in
<a href="../../programmer_reference/env_naming.html" class="olink">Berkeley DB File Naming</a>.
</p>
<p>
When using a Unicode build on Windows (the default), the
<span class="bold"><strong>db_home</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="idp7009408"></a>flags</h4>
</div>
</div>
</div>
<p>
The <span class="bold"><strong>flags</strong></span> parameter specifies
the subsystems that are initialized and how the application's
environment affects Berkeley DB file naming, among other things.
The <span class="bold"><strong>flags</strong></span> parameter must be set
to 0 or by bitwise inclusively
<span class="bold"><strong>OR</strong></span>'ing together one or more of the
values described in this section.
</p>
<p>
Because there are a large number of flags that can be specified,
they have been grouped together by functionality. The first
group of flags indicates which of the Berkeley DB subsystems
should be initialized.
</p>
<p>
The choice of subsystems initialized for a Berkeley DB database
environment is specified by the thread of control initially
creating the environment. Any subsequent thread of control
joining the environment will automatically be configured to use
the same subsystems as were created in the environment (unless
the thread of control requests a subsystem not available in the
environment, which will fail). Applications joining an
environment, able to adapt to whatever subsystems have been
configured in the environment, should open the environment
without specifying any subsystem flags. Applications joining an
environment, requiring specific subsystems from their
environments, should open the environment specifying those
specific subsystem flags.
</p>
<div class="itemizedlist">
<ul type="disc">
<li>
<p><a id="envopen_DB_INIT_CDB"></a>
<code class="literal">DB_INIT_CDB</code>
</p>
<p>
Initialize locking for the
<a href="../../programmer_reference/cam.html#cam_intro" class="olink">Berkeley DB Concurrent Data Store</a>
product. In this mode, Berkeley DB provides multiple
reader/single writer access. The only other subsystem
that should be specified with the <code class="literal">DB_INIT_CDB</code>
flag is <code class="literal">DB_INIT_MPOOL</code>.
</p>
</li>
<li>
<p><a id="envopen_DB_INIT_LOCK"></a>
<code class="literal">DB_INIT_LOCK</code>
</p>
<p>
Initialize the locking subsystem. This subsystem should
be used when multiple processes or threads are going to
be reading and writing a Berkeley DB database, so that
they do not interfere with each other. If all threads
are accessing the database(s) read-only, locking is
unnecessary. When the DB_INIT_LOCK flag is specified, it
is usually necessary to run a deadlock detector, as
well. See <a class="link" href="db_deadlock.html" title="db_deadlock">db_deadlock</a>
and <a class="xref" href="lockdetect.html" title="DbEnv::lock_detect()">DbEnv::lock_detect()</a>
for more information.
</p>
</li>
<li>
<p><a id="envopen_DB_INIT_LOG"></a>
<code class="literal">DB_INIT_LOG</code>
</p>
<p>
Initialize the logging subsystem. This subsystem should
be used when recovery from application or system failure
is necessary. If the log region is being created and log
files are already present, the log files are reviewed;
subsequent log writes are appended to the end of the
log, rather than overwriting current log entries.
</p>
</li>
<li>
<p><a id="envopen_DB_INIT_MPOOL"></a>
<code class="literal">DB_INIT_MPOOL</code>
</p>
<p>
Initialize the shared memory buffer pool subsystem.
This subsystem should be used whenever an application is
using any Berkeley DB access method.
</p>
</li>
<li>
<p><a id="envopen_DB_INIT_REP"></a>
<code class="literal">DB_INIT_REP</code>
</p>
<p>
Initialize the replication subsystem. This subsystem
should be used whenever an application plans on using
replication. The <code class="literal">DB_INIT_REP</code> flag
requires the <code class="literal">DB_INIT_TXN</code> and
<code class="literal">DB_INIT_LOCK</code> flags also be
configured.
</p>
<p>
You can also specify this flag in the <a href="../../programmer_reference/env_db_config.html#env_db_config.DB_CONFIG" class="olink">DB_CONFIG</a>
configuration file. The syntax is a single line
with the string "set_open_flags", one or more
whitespace characters, the string "DB_INIT_REP",
optionally one or more whitespace characters and
the string "on" or "off". If the optional string
is omitted, the default is "on"; for example,
"set_open_flags DB_INIT_REP" or "set_open_flags
DB_INIT_REP on". Because the DB_CONFIG file is
read when the database environment is opened, it
will silently overrule configuration done before
that time.
</p>
</li>
<li>
<p><a id="envopen_DB_INIT_TXN"></a>
<code class="literal">DB_INIT_TXN</code>
</p>
<p>
Initialize the transaction subsystem. This subsystem
should be used when recovery and atomicity of multiple
operations are important. The
<code class="literal">DB_INIT_TXN</code> flag implies the
<code class="literal">DB_INIT_LOG</code> flag.
</p>
</li>
</ul>
</div>
<p>
The second group of flags govern what recovery, if any, is
performed when the environment is initialized:
</p>
<div class="itemizedlist">
<ul type="disc">
<li>
<p><a id="envopen_DB_RECOVER"></a>
<code class="literal">DB_RECOVER</code>
</p>
<p>
Run normal recovery on this environment before opening
it for normal use. If this flag is set, the
<code class="literal">DB_CREATE</code> and
<code class="literal">DB_INIT_TXN</code> flags must also be set,
because the regions will be removed and re-created, and
transactions are required for application recovery.
</p>
</li>
<li>
<p><a id="envopen_DB_RECOVER_FATAL"></a>
<code class="literal">DB_RECOVER_FATAL</code>
</p>
<p>
Run catastrophic recovery on this environment before
opening it for normal use. If this flag is set, the
<code class="literal">DB_CREATE</code> and
<code class="literal">DB_INIT_TXN</code> flags must also be set,
because the regions will be removed and re-created, and
transactions are required for application recovery.
</p>
</li>
</ul>
</div>
<p>
A standard part of the recovery process is to remove the
existing Berkeley DB environment and create a new one in which
to perform recovery. If the thread of control performing
recovery does not specify the correct region initialization
information (for example, the correct memory pool cache size),
the result can be an application running in an environment with
incorrect cache and other subsystem sizes. For this reason, the
thread of control performing recovery should specify correct
configuration information before calling the
<code class="methodname">DbEnv::open()</code> method; or it should
remove the environment after recovery is completed, leaving
creation of the correctly sized environment to a subsequent call
to the <code class="methodname">DbEnv::open()</code> method.
</p>
<p>
All Berkeley DB recovery processing must be single-threaded;
that is, only a single thread of control may perform recovery or
access a Berkeley DB environment while recovery is being
performed. Because it is not an error to specify
<code class="literal">DB_RECOVER</code> for an environment for which no
recovery is required, it is reasonable programming practice for
the thread of control responsible for performing recovery and
creating the environment to always specify the
<code class="literal">DB_CREATE</code> and <code class="literal">DB_RECOVER</code>
flags during startup.
</p>
<p>
The third group of flags govern file-naming extensions in the
environment:
</p>
<div class="itemizedlist">
<ul type="disc">
<li>
<p><a id="envopen_DB_USE_ENVIRON"></a>
<code class="literal">DB_USE_ENVIRON</code>
</p>
<p>
The Berkeley DB process' environment may be permitted to
specify information to be used when naming files; see
<a href="../../programmer_reference/env_naming.html" class="olink">Berkeley DB File Naming</a>.
Because permitting users to specify which files are used
can create security problems, environment information
will be used in file naming for all users only if the
<code class="literal">DB_USE_ENVIRON</code> flag is set.
</p>
</li>
<li>
<p><a id="envopen_DB_USE_ENVIRON_ROOT"></a>
<code class="literal">DB_USE_ENVIRON_ROOT</code>
</p>
<p>
The Berkeley DB process' environment may be permitted to
specify information to be used when naming files; see
<a href="../../programmer_reference/env_naming.html" class="olink">Berkeley DB File Naming</a>.
Because permitting users to specify which files are used
can create security problems, if the
<code class="literal">DB_USE_ENVIRON_ROOT</code> flag is set,
environment information will be used in file naming only
for users with appropriate permissions (for example,
users with a user-ID of 0 on <code class="literal">UNIX</code>
systems).
</p>
</li>
</ul>
</div>
<p>
Finally, there are a few additional unrelated flags:
</p>
<div class="itemizedlist">
<ul type="disc">
<li>
<p><a id="envopen_DB_CREATE"></a>
<code class="literal">DB_CREATE</code>
</p>
<p>
Cause Berkeley DB subsystems to create any underlying
files, as necessary.
</p>
</li>
<li>
<p><a id="envopen_DB_LOCKDOWN"></a>
<code class="literal">DB_LOCKDOWN</code>
</p>
<p>
Lock shared Berkeley DB environment files and
memory-mapped databases into memory.
</p>
</li>
<li>
<p><a id="envopen_DB_FAILCHK"></a>
<code class="literal">DB_FAILCHK</code>
</p>
<p>
Internally call the <a class="xref" href="envfailchk.html" title="DbEnv::failchk()">DbEnv::failchk()</a>
method as part of opening the environment. When
<code class="literal">DB_FAILCHK</code> is specified, a check is
made to ensure all
<code class="methodname">DbEnv::failchk()</code> prerequisites
are meet.
</p>
<p>
If the <code class="literal">DB_FAILCHK</code> flag is used in
conjunction with the <code class="literal">DB_REGISTER</code>
flag, then a check will be made to see if the
environment needs recovery. If recovery is needed, a
call will be made to the
<code class="methodname">DbEnv::failchk()</code> method to
release any database reads locks held by the thread of
control that exited and, if needed, to abort the
unresolved transaction. If
<code class="methodname">DbEnv::failchk()</code> determines
environment recovery is still required, the recovery
actions for <code class="literal">DB_REGISTER</code> will be
followed.
</p>
<p>
If the <code class="literal">DB_FAILCHK</code> flag is not used in
conjunction with the <code class="literal">DB_REGISTER</code>
flag, then make an internal call to
<code class="methodname">DbEnv::failchk()</code> as the last
step of opening the environment. If
<code class="methodname">DbEnv::failchk()</code> determines
database environment recovery is
required,
<a href="../../programmer_reference/program_errorret.html#program_errorret.DB_RUNRECOVERY" class="olink">DB_RUNRECOVERY</a>
will be returned.
</p>
</li>
<li>
<p><a id="envopen_DB_PRIVATE"></a>
<code class="literal">DB_PRIVATE</code>
</p>
<p>
Allocate region memory from the heap instead of from
memory backed by the filesystem or system shared memory.
</p>
<div class="note" style="margin-left: 0.5in; margin-right: 0.5in;">
<h3 class="title">Note</h3>
<p>
Use of this flag means that the environment can only be
accessed by one environment handle. The environment
cannot be accessed by multiple processes. This is true
even if one of those processes is one of the the Berkeley DB
utilities. (For example,
<a class="link" href="db_archive.html" title="db_archive">db_archive</a>,
<a class="link" href="db_checkpoint.html" title="db_checkpoint">db_checkpoint</a> or
<a class="link" href="db_stat.html" title="db_stat">db_stat</a>.)
Nor can a single process open multiple
handles to the environment.
</p>
</div>
<p>
This flag has two effects on the Berkeley DB
environment. First, all underlying data structures are
allocated from per-process memory instead of from shared
memory that is accessible to more than a single process.
Second, mutexes are only configured to work between
threads.
</p>
<p>
See
<a href="../../programmer_reference/env_region.html" class="olink">Shared Memory Regions</a>
for more information.
</p>
<p>
You can also specify this flag in the <a href="../../programmer_reference/env_db_config.html#env_db_config.DB_CONFIG" class="olink">DB_CONFIG</a>
configuration file. The syntax is a single line with the
string "set_open_flags", one or more whitespace
characters, the string "DB_PRIVATE", optionally one or
more whitespace characters and the string "on" or "off".
If the optional string is omitted, the default is "on";
for example, "set_open_flags DB_PRIVATE" or
"set_open_flags DB_PRIVATE on". Because the DB_CONFIG
file is read when the database environment is opened, it
will silently overrule configuration done before that
time.
</p>
</li>
<li>
<p><a id="envopen_DB_REGISTER"></a>
<code class="literal">DB_REGISTER</code>
</p>
<p>
Check to see if recovery needs to be performed before
opening the database environment. (For this check to be
accurate, all processes using the environment must
specify <code class="literal">DB_REGISTER</code> when opening the
environment.) If recovery needs to be performed for any
reason (including the initial use of the
<code class="literal">DB_REGISTER</code> flag), and
<code class="literal">DB_RECOVER</code> is also specified,
recovery will be performed and the open will proceed
normally. If recovery needs to be performed and
<code class="literal">DB_RECOVER</code> is not specified,
<a href="../../programmer_reference/program_errorret.html#program_errorret.DB_RUNRECOVERY" class="olink">DB_RUNRECOVERY</a>
will be returned. If recovery does not need to be
performed, the <code class="literal">DB_RECOVER</code> flag will
be ignored. See
<a href="../../programmer_reference/transapp_app.html" class="olink">Architecting Transactional Data Store applications</a>
for more information.
</p>
</li>
<li>
<p><a id="envopen_DB_SYSTEM_MEM"></a>
<code class="literal">DB_SYSTEM_MEM</code>
</p>
<p>
Allocate region memory from system shared memory instead
of from heap memory or memory backed by the filesystem.
</p>
<p>
See
<a href="../../programmer_reference/env_region.html" class="olink">Shared Memory Regions</a>
for more information.
</p>
</li>
<li>
<p><a id="envopen_DB_THREAD"></a>
<code class="literal">DB_THREAD</code>
</p>
<p>
Cause the
<a class="link" href="env.html" title="Chapter 5. The DbEnv Handle">DbEnv</a>
handle returned by <code class="methodname">DbEnv::open()</code> to be <span class="emphasis"><em>free-threaded;</em></span> that is,
concurrently usable by multiple threads in the
address space. The <code class="literal">DB_THREAD </code>
flag should be specified if the
<a class="link" href="env.html" title="Chapter 5. The DbEnv Handle">DbEnv</a> handle will be
concurrently used by more than one thread in the
process, or if any
<a class="link" href="db.html" title="Chapter 2. The Db Handle">Db</a> handles opened in
the scope of the
<a class="link" href="env.html" title="Chapter 5. The DbEnv Handle">DbEnv</a> handle will be
concurrently used by more than one thread in the
process.
</p>
<p>
This flag is required when using the Replication Manager.
</p>
<p>
You can also specify this flag in the <a href="../../programmer_reference/env_db_config.html#env_db_config.DB_CONFIG" class="olink">DB_CONFIG</a>
configuration file. The syntax is a single line with
the string "set_open_flags", one or more whitespace
characters, the string "DB_THREAD", optionally one
or more whitespace characters and the string "on" or
"off". If the optional string is omitted, the
default is "on"; for example, "set_open_flags
DB_THREAD" or "set_open_flags DB_THREAD on".
Because the DB_CONFIG file is read when the database
environment is opened, it will silently overrule
configuration done before that time.
</p>
</li>
</ul>
</div>
</div>
<div class="sect3" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h4 class="title"><a id="idp7021040"></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 Berkeley DB 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 Berkeley DB 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, Berkeley DB 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="idp7021232"></a>Errors</h3>
</div>
</div>
</div>
<p>
The <code class="methodname">DbEnv::open()</code> <span>
<span>
method may fail and throw a <a class="link" href="dbexception.html" title="Chapter 6. The DbException Class">DbException</a>
exception, encapsulating one of the following non-zero errors, or 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="idp7077936"></a>DB_RUNRECOVERY</h4>
</div>
</div>
</div>
<p>
Either the <code class="literal">DB_REGISTER</code> flag was specified, a
failure occurred, and no recovery flag was specified, or the
<code class="literal">DB_FAILCHK</code> flag was specified and recovery
was deemed necessary.
</p>
</div>
<div class="sect3" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h4 class="title"><a id="idp7020368"></a>DB_VERSION_MISMATCH</h4>
</div>
</div>
</div>
<p>
The version of the Berkeley DB library doesn't match the version
that created the database environment.
</p>
</div>
<div class="sect3" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h4 class="title"><a id="idp7062352"></a>EAGAIN</h4>
</div>
</div>
</div>
<p>
The shared memory region was locked and (repeatedly) unavailable.
</p>
</div>
<div class="sect3" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h4 class="title"><a id="idp7093440"></a>EINVAL</h4>
</div>
</div>
</div>
<p>
If the <code class="literal">DB_THREAD</code> flag was specified and fast
mutexes are not available for this architecture; The
<code class="literal">DB_HOME</code> or <code class="literal">TMPDIR</code>
environment variables were set, but empty; An incorrectly
formatted <span class="bold"><strong>NAME VALUE</strong></span> entry or
line was found; 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="idp6988664"></a>ENOENT</h4>
</div>
</div>
</div>
<p>
The file or directory does not exist.
</p>
</div>
</div>
<div class="sect2" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h3 class="title"><a id="idp7076856"></a>Class</h3>
</div>
</div>
</div>
<p>
<a class="link" href="env.html" title="Chapter 5. The DbEnv Handle">DbEnv</a>
</p>
</div>
<div class="sect2" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h3 class="title"><a id="idp7018000"></a>See Also</h3>
</div>
</div>
</div>
<p>
<a class="xref" href="env.html#envlist" title="Database Environments and Related Methods">Database Environments and Related Methods</a>
</p>
</div>
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