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<th colspan="3" align="center">DbEnv::open()</th>
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<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="idp51819184"></a>Parameters</h3>
</div>
</div>
</div>
<div class="sect3" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h4 class="title"><a id="idp51757792"></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="idp51760888"></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>
<p>
This flag implies the environment will only be accessed by a single process
(although that process may be multithreaded). 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>
This flag should not be specified if more than a single process is accessing the
environment because it is likely to cause database corruption and unpredictable
behavior. For example, if both a server application and 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>) are expected to access the
environment, the <code class="literal">DB_PRIVATE</code> flag should not be specified.
</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="idp51816424"></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="idp51830904"></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="idp51871560"></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="idp51831704"></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="idp51891432"></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="idp51905504"></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="idp51825048"></a>ENOSPC</h4>
</div>
</div>
</div>
<p>
HP-UX only: Due to the constraints of the PA-RISC memory architecture,
HP-UX does not allow a process to map a file into its address space
multiple times. For this reason, each Berkeley DB environment may be
opened only once by a process on HP-UX; that is, calls to
<code class="methodname">DbEnv::open()</code> will fail if the specified Berkeley DB environment has
been opened and not subsequently closed.
</p>
</div>
<div class="sect3" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h4 class="title"><a id="idp51857464"></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="idp51869400"></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="idp51877080"></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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