<!--$Id: env_open.html,v 1.2 2004/03/30 01:21:42 jtownsen Exp $--> <!--Copyright 1997-2003 by Sleepycat Software, Inc.--> <!--All rights reserved.--> <!--See the file LICENSE for redistribution information.--> <html> <head> <title>Berkeley DB: DB_ENV->open</title> <meta name="description" content="Berkeley DB: An embedded database programmatic toolkit."> <meta name="keywords" content="embedded,database,programmatic,toolkit,b+tree,btree,hash,hashing,transaction,transactions,locking,logging,access method,access methods,Java,C,C++"> </head> <body bgcolor=white> <a name="2"><!--meow--></a> <table width="100%"><tr valign=top> <td> <h3>DB_ENV->open</h3> </td> <td align=right> <a href="../api_c/api_index.html"><img src="../images/api.gif" alt="API"></a> <a href="../ref/toc.html"><img src="../images/ref.gif" alt="Ref"></a> </td></tr></table> <hr size=1 noshade> <tt> <h3><pre> #include <db.h> <p> int DB_ENV->open(DB_ENV *dbenv, char *db_home, u_int32_t flags, int mode); <p> int DB_ENV->get_home(DB_ENV *dbenv, const char **homep); <p> int DB_ENV->get_open_flags(DB_ENV *dbenv, u_int32_t *flagsp); </pre></h3> <hr size=1 noshade> <h3>Description: DB_ENV->open</h3> <a name="3"><!--meow--></a> <a name="4"><!--meow--></a> <p>The DB_ENV->open 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 DB_ENV->open method returns a non-zero error value on failure and 0 on success. If DB_ENV->open fails, the <a href="../api_c/env_close.html">DB_ENV->close</a> method should be called to discard the <a href="../api_c/env_class.html">DB_ENV</a> handle. </p> <h3>Parameters</h3> <p><dl compact> <p><dt><b>db_home</b><dd> The <b>db_home</b> parameter is the database environment's home directory. For more information on <b>db_home</b>, and filename resolution in general, see <a href="../ref/env/naming.html">Berkeley DB File Naming</a>. The environment variable <b>DB_HOME</b> may be used as the path of the database home, as described in <a href="../ref/env/naming.html">Berkeley DB File Naming</a>. <p><dt><b>flags</b><dd> The <b>flags</b> parameter specifies the subsystems that are initialized and how the application's environment affects Berkeley DB file naming, among other things. The <b>flags</b> parameter must be set to 0 or by bitwise inclusively <b>OR</b>'ing together one or more of the following values: <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><dl compact> <p><dt><a name="DB_JOINENV">DB_JOINENV</a><dd>Join an existing environment. This option allows applications to join an existing environment without knowing which Berkeley DB subsystems the environment supports. <p><dt><a name="DB_INIT_CDB">DB_INIT_CDB</a><dd>Initialize locking for the <a href="../ref/cam/intro.html">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 DB_INIT_CDB flag is DB_INIT_MPOOL. <p><dt><a name="DB_INIT_LOCK">DB_INIT_LOCK</a><dd>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 href="../utility/db_deadlock.html">db_deadlock</a> and <a href="../api_c/lock_detect.html">DB_ENV->lock_detect</a> for more information. <p><dt><a name="DB_INIT_LOG">DB_INIT_LOG</a><dd>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><dt><a name="DB_INIT_MPOOL">DB_INIT_MPOOL</a><dd>Initialize the shared memory buffer pool subsystem. This subsystem should be used whenever an application is using any Berkeley DB access method. <p><dt><a name="DB_INIT_REP">DB_INIT_REP</a><dd>Initialize the replication subsystem. This subsystem should be used whenever an application plans on using replication. The DB_INIT_REP flag requires the DB_INIT_TXN and DB_INIT_LOCK flags also be configured. <p><dt><a name="DB_INIT_TXN">DB_INIT_TXN</a><dd>Initialize the transaction subsystem. This subsystem should be used when recovery and atomicity of multiple operations are important. The DB_INIT_TXN flag implies the DB_INIT_LOG flag. </dl> <p>The second group of flags govern what recovery, if any, is performed when the environment is initialized:</p> <p><dl compact> <p><dt><a name="DB_RECOVER">DB_RECOVER</a><dd>Run normal recovery on this environment before opening it for normal use. If this flag is set, the DB_CREATE flag must also be set because the regions will be removed and re-created. <p><dt><a name="DB_RECOVER_FATAL">DB_RECOVER_FATAL</a><dd>Run catastrophic recovery on this environment before opening it for normal use. If this flag is set, the DB_CREATE flag must also be set because the regions will be removed and re-created. </dl> <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 DB_ENV->open method; or it should remove the environment after recovery is completed, leaving creation of the correctly sized environment to a subsequent call to DB_ENV->open.</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 DB_RECOVER 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 DB_CREATE and DB_RECOVER flags during startup.</p> <p>The DB_ENV->open function returns successfully if DB_RECOVER or DB_RECOVER_FATAL is specified and no log files exist, so it is necessary to ensure that all necessary log files are present before running recovery. For further information, consult <a href="../utility/db_archive.html">db_archive</a> and <a href="../utility/db_recover.html">db_recover</a>.</p> <p>The third group of flags govern file-naming extensions in the environment:</p> <p><dl compact> <a name="5"><!--meow--></a> <p><dt><a name="DB_USE_ENVIRON">DB_USE_ENVIRON</a><dd>The Berkeley DB process' environment may be permitted to specify information to be used when naming files; see <a href="../ref/env/naming.html">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 DB_USE_ENVIRON flag is set. <p><dt><a name="DB_USE_ENVIRON_ROOT">DB_USE_ENVIRON_ROOT</a><dd>The Berkeley DB process' environment may be permitted to specify information to be used when naming files; see <a href="../ref/env/naming.html">Berkeley DB File Naming</a>. Because permitting users to specify which files are used can create security problems, if the DB_USE_ENVIRON_ROOT flag is set, environment information will be used for file naming only for users with appropriate permissions (for example, users with a user-ID of 0 on UNIX systems). </dl> <p>Finally, there are a few additional unrelated flags:</p> <p><dl compact> <p><dt><a name="DB_CREATE">DB_CREATE</a><dd>Cause Berkeley DB subsystems to create any underlying files, as necessary. <p><dt><a name="DB_LOCKDOWN">DB_LOCKDOWN</a><dd>Lock shared Berkeley DB environment files and memory-mapped databases into memory. <p><dt><a name="DB_PRIVATE">DB_PRIVATE</a><dd>Specify that 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 potentially accessible to more than a single process. Second, mutexes are only configured to work between threads. <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 the Berkeley DB utility <a href="../utility/db_stat.html">db_stat</a> are expected to access the environment, the DB_PRIVATE flag should not be specified.</p> <p><dt><a name="DB_SYSTEM_MEM">DB_SYSTEM_MEM</a><dd>Allocate memory from system shared memory instead of from memory backed by the filesystem. See <a href="../ref/env/region.html">Shared Memory Regions</a> for more information. <p><dt><a name="DB_THREAD">DB_THREAD</a><dd>Cause the <a href="../api_c/env_class.html">DB_ENV</a> handle returned by DB_ENV->open to be <i>free-threaded</i>; that is, usable by multiple threads within a single address space. </dl> <p><dt><b>mode</b><dd> On UNIX systems or in IEEE/ANSI Std 1003.1 (POSIX) environments, all files created by Berkeley DB are created with mode <b>mode</b> (as described in <b>chmod</b>(2)) and modified by the process' umask value at the time of creation (see <b>umask</b>(2)). If <b>mode</b> is 0, Berkeley DB will use a default mode of readable and writable by both owner and group. On Windows systems, the mode parameter is ignored. The group ownership of created files is based on the system and directory defaults, and is not further specified by Berkeley DB. </dl> <h3>Errors</h3> <p>The DB_ENV->open method may fail and return one of the following non-zero errors:</p> <p><dl compact> <p><dt>EAGAIN<dd>The shared memory region was locked and (repeatedly) unavailable. </dl> <p><dl compact> <p><dt>EINVAL<dd>If the DB_THREAD flag was specified and fast mutexes are not available for this architecture; The DB_HOME or TMPDIR environment variables were set, but empty; An incorrectly formatted <b>NAME VALUE</b> entry or line was found; or if an invalid flag value or parameter was specified. </dl> <p><dl compact> <p><dt>ENOSPC<dd>HP-UX only: a previously created Berkeley DB environment for this process still exists. </dl> <p><dl compact> <p><dt>ENOENT<dd>The file or directory does not exist. </dl> <hr size=1 noshade> <h3>Description: DB_ENV->get_home</h3> <p>The DB_ENV->get_home method returns the database environment home directory.</p> <p>The DB_ENV->get_home method may be called at any time during the life of the application.</p> <hr size=1 noshade> <h3>Description: DB_ENV->get_open_flags</h3> <p>The DB_ENV->get_open_flags method returns the open method flags.</p> <p>The DB_ENV->get_open_flags method may not be called before the DB_ENV->open method has been called.</p> <p>The DB_ENV->get_open_flags method returns a non-zero error value on failure and 0 on success. </p> <h3>Parameters</h3> <p><dl compact> <p><dt><b>flagsp</b><dd> The DB_ENV->get_open_flags method returns the open method flags in <b>flagsp</b>. </dl> <hr size=1 noshade> <h3>Class</h3> <a href="../api_c/env_class.html">DB_ENV</a> <h3>See Also</h3> <a href="../api_c/env_list.html">Database Environments and Related Methods</a> </tt> <table width="100%"><tr><td><br></td><td align=right> <a href="../api_c/api_index.html"><img src="../images/api.gif" alt="API"></a><a href="../ref/toc.html"><img src="../images/ref.gif" alt="Ref"></a> </td></tr></table> <p><font size=1><a href="../sleepycat/legal.html">Copyright (c) 1996-2003</a> <a href="http://www.sleepycat.com">Sleepycat Software, Inc.</a> - All rights reserved.</font> </body> </html>