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<!--$Id: db_open.so,v 10.67 2001/06/19 19:49:24 bostic Exp $-->
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<!--Copyright 1997-2001 by Sleepycat Software, Inc.-->
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<!--All rights reserved.-->
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<title>Berkeley DB: DB->open</title>
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<meta name="description" content="Berkeley DB: An embedded database programmatic toolkit.">
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<meta name="keywords" content="embedded,database,programmatic,toolkit,b+tree,btree,hash,hashing,transaction,transactions,locking,logging,access method,access methods,java,C,C++">
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<table width="100%"><tr valign=top>
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<a href="../api_c/c_index.html"><img src="../images/api.gif" alt="API"></a><a href="../reftoc.html"><img src="../images/ref.gif" alt="Ref"></a>
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DB->open(DB *db, const char *file,
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const char *database, DBTYPE type, u_int32_t flags, int mode);
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<p>The currently supported Berkeley DB file formats (or <i>access methods</i>)
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are Btree, Hash, Queue, and Recno. The Btree format is a representation
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of a sorted, balanced tree structure. The Hash format is an extensible,
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dynamic hashing scheme. The Queue format supports fast access to
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fixed-length records accessed sequentially or by logical record number.
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The Recno format supports fixed- or variable-length records, accessed
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sequentially or by logical record number, and optionally backed by a
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<p>Storage and retrieval for the Berkeley DB access methods are based on key/data
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pairs; see <a href="../api_c/dbt.html">DBT</a> for more information.
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<p>The DB->open interface opens the database represented by the
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<b>file</b> and <b>database</b> arguments for both reading and
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writing. The <b>file</b> argument is used as the name of an underlying
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file that will be used to back the database. The <b>database</b>
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argument is optional, and allows applications to have multiple databases
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in a single file. Although no <b>database</b> argument needs to be
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specified, it is an error to attempt to open a second database in a
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<b>file</b> that was not initially created using a <b>database</b>
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name. Further, the <b>database</b> argument is not supported by the
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<p>In-memory databases never intended to be preserved on disk may be
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created by setting both the <b>file</b> and <b>database</b> arguments
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to NULL. Note that in-memory databases can only ever be shared by
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sharing the single database handle that created them, in circumstances
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where doing so is safe.
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<p>The <b>type</b> argument is of type DBTYPE, and must be set to one of DB_BTREE, DB_HASH,
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DB_QUEUE, DB_RECNO, or DB_UNKNOWN. If
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<b>type</b> is DB_UNKNOWN, the database must already exist
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and DB->open will automatically determine its type. The
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<a href="../api_c/db_get_type.html">DB->get_type</a> function may be used to determine the underlying type of
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databases opened using DB_UNKNOWN.
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<p>The <b>flags</b> and <b>mode</b> arguments specify how files will be opened
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and/or created if they do not already exist.
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<p>The <b>flags</b> value must be set to 0 or by bitwise inclusively <b>OR</b>'ing together one or
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more of the following values:
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<p><dt><a name="DB_CREATE">DB_CREATE</a><dd>Create any underlying files, as necessary. If the files do not already
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exist and the DB_CREATE flag is not specified, the call will fail.
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<p><dt><a name="DB_DIRTY_READ">DB_DIRTY_READ</a><dd>Support dirty reads; that is, transactions, cursors and read operations
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in the database may request the return of modified but not yet committed
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<p><dt><a name="DB_EXCL">DB_EXCL</a><dd>Return an error if the file already exists. Underlying filesystem
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primitives are used to implement this flag. For this reason, it is only
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applicable to the file and cannot be used to test whether a database in a
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<p>The DB_EXCL flag is only meaningful when specified with the
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<p><dt><a name="DB_NOMMAP">DB_NOMMAP</a><dd>Do not map this database into process memory (see the description of the
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<a href="../api_c/env_set_mp_mmapsize.html">DB_ENV->set_mp_mmapsize</a> function for further information).
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<p><dt><a name="DB_RDONLY">DB_RDONLY</a><dd>Open the database for reading only. Any attempt to modify items in the
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database will fail, regardless of the actual permissions of any underlying
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<p><dt><a name="DB_THREAD">DB_THREAD</a><dd>Cause the DB handle returned by DB->open to be
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<i>free-threaded</i>; that is, usable by multiple threads within a
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<p><dt><a name="DB_TRUNCATE">DB_TRUNCATE</a><dd>Physically truncate the underlying file, discarding all previous
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databases it might have held. Underlying filesystem primitives are used
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to implement this flag. For this reason, it is applicable only to the
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file and cannot be used to discard databases within a file.
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<p>The DB_TRUNCATE flag cannot be transaction-protected, and it is
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an error to specify it in a transaction-protected environment.
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<p>On UNIX systems or in IEEE/ANSI Std 1003.1 (POSIX) environments, all files created by
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the access methods 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
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(see <b>umask</b>(2)). If <b>mode</b> is 0, the access methods will use a default
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mode of readable and writable by both owner and group. On Windows
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systems, the mode argument is ignored. The group ownership of created
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files is based on the system and directory defaults, and is not further
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specified by Berkeley DB.
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<p>Calling DB->open is a reasonably expensive operation, and
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maintaining a set of open databases will normally be preferable to
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repeatedly opening and closing the database for each new query.
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<p>The DB->open function returns a non-zero error value on failure and 0 on success.
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<h1>Environment Variables</h1>
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<p><dt>DB_HOME<dd>If the <b>dbenv</b> argument to <a href="../api_c/db_create.html">db_create</a> was initialized using
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<a href="../api_c/env_open.html">DB_ENV->open</a>, the environment variable <b>DB_HOME</b> may be used
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as the path of the database environment home. Specifically, DB->open
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is affected by the configuration value DB_DATA_DIR.
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<p><dt>TMPDIR<dd>If the <b>file</b> and <b>dbenv</b> arguments to DB->open are
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NULL, the environment variable <b>TMPDIR</b> may be used as a
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directory in which to create a temporary backing file.
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<p>The DB->open function may fail and return a non-zero error for the following conditions:
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<p><dt><a name="DB_OLD_VERSION">DB_OLD_VERSION</a><dd>The database cannot be opened without being first upgraded.
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<p><dt>EEXIST<dd>DB_CREATE and DB_EXCL were specified and the file exists.
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<p><dt>EINVAL<dd>An invalid flag value or parameter was specified (for example, unknown
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database type, page size, hash function, pad byte, byte order) or a flag
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value or parameter that is incompatible with the specified database.
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The <a href="../api_c/env_open.html#DB_THREAD">DB_THREAD</a> flag was specified and fast mutexes are not
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available for this architecture.
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<p>The <a href="../api_c/env_open.html#DB_THREAD">DB_THREAD</a> flag was specified to DB->open, but was not
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specified to the <a href="../api_c/env_open.html">DB_ENV->open</a> call for the environment in which the
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DB handle was created.
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<p>A backing flat text file was specified with either the <a href="../api_c/env_open.html#DB_THREAD">DB_THREAD</a>
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flag or the provided database environment supports transaction
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<p><dt>ENOENT<dd>A nonexistent <b>re_source</b> file was specified.
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<p>The DB->open function may fail and return a non-zero error for errors specified for other Berkeley DB and C library or system functions.
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If a catastrophic error has occurred, the DB->open function may fail and return
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<a href="../ref/program/errorret.html#DB_RUNRECOVERY">DB_RUNRECOVERY</a>, in which case all subsequent Berkeley DB calls will fail
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<a href="../api_c/db_create.html">db_create</a>,
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<a href="../api_c/db_associate.html">DB->associate</a>,
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<a href="../api_c/db_close.html">DB->close</a>,
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<a href="../api_c/db_cursor.html">DB->cursor</a>,
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<a href="../api_c/db_del.html">DB->del</a>,
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<a href="../api_c/db_err.html">DB->err</a>, <a href="../api_c/db_err.html">DB->errx</a>
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<a href="../api_c/db_fd.html">DB->fd</a>,
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<a href="../api_c/db_get.html">DB->get</a>,
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<a href="../api_c/db_get.html">DB->pget</a>,
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<a href="../api_c/db_get_byteswapped.html">DB->get_byteswapped</a>,
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<a href="../api_c/db_get_type.html">DB->get_type</a>,
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<a href="../api_c/db_join.html">DB->join</a>,
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<a href="../api_c/db_key_range.html">DB->key_range</a>,
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<a href="../api_c/db_open.html">DB->open</a>,
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<a href="../api_c/db_put.html">DB->put</a>,
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<a href="../api_c/db_remove.html">DB->remove</a>,
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<a href="../api_c/db_rename.html">DB->rename</a>,
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<a href="../api_c/db_set_alloc.html">DB->set_alloc</a>,
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<a href="../api_c/db_set_append_recno.html">DB->set_append_recno</a>,
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<a href="../api_c/db_set_bt_compare.html">DB->set_bt_compare</a>,
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<a href="../api_c/db_set_bt_minkey.html">DB->set_bt_minkey</a>,
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<a href="../api_c/db_set_bt_prefix.html">DB->set_bt_prefix</a>,
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<a href="../api_c/db_set_cachesize.html">DB->set_cachesize</a>,
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<a href="../api_c/db_set_dup_compare.html">DB->set_dup_compare</a>,
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<a href="../api_c/db_set_errcall.html">DB->set_errcall</a>,
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<a href="../api_c/db_set_errfile.html">DB->set_errfile</a>,
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<a href="../api_c/db_set_errpfx.html">DB->set_errpfx</a>,
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<a href="../api_c/db_set_feedback.html">DB->set_feedback</a>,
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<a href="../api_c/db_set_flags.html">DB->set_flags</a>,
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<a href="../api_c/db_set_h_ffactor.html">DB->set_h_ffactor</a>,
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<a href="../api_c/db_set_h_hash.html">DB->set_h_hash</a>,
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<a href="../api_c/db_set_h_nelem.html">DB->set_h_nelem</a>,
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<a href="../api_c/db_set_lorder.html">DB->set_lorder</a>,
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<a href="../api_c/db_set_pagesize.html">DB->set_pagesize</a>,
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<a href="../api_c/db_set_paniccall.html">DB->set_paniccall</a>,
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<a href="../api_c/db_set_q_extentsize.html">DB->set_q_extentsize</a>,
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<a href="../api_c/db_set_re_delim.html">DB->set_re_delim</a>,
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<a href="../api_c/db_set_re_len.html">DB->set_re_len</a>,
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<a href="../api_c/db_set_re_pad.html">DB->set_re_pad</a>,
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<a href="../api_c/db_set_re_source.html">DB->set_re_source</a>,
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<a href="../api_c/db_stat.html">DB->stat</a>,
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<a href="../api_c/db_sync.html">DB->sync</a>,
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<a href="../api_c/db_truncate.html">DB->truncate</a>,
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<a href="../api_c/db_upgrade.html">DB->upgrade</a>,
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<a href="../api_c/db_verify.html">DB->verify</a>.
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