1<!--$Id: db_open.so,v 10.124 2007/10/24 16:06:06 bostic Exp $--> 2<!--Copyright (c) 1997,2008 Oracle. All rights reserved.--> 3<!--See the file LICENSE for redistribution information.--> 4<html> 5<head> 6<title>Berkeley DB: Db::open</title> 7<meta name="description" content="Berkeley DB: An embedded database programmatic toolkit."> 8<meta name="keywords" content="embedded,database,programmatic,toolkit,btree,hash,hashing,transaction,transactions,locking,logging,access method,access methods,Java,C,C++"> 9</head> 10<body bgcolor=white> 11<table width="100%"><tr valign=top> 12<td> 13<b>Db::open</b> 14</td> 15<td align=right> 16<a href="/api_cxx/api_core.html"><img src="/images/api.gif" alt="API"></a> 17<a href="/ref/toc.html"><img src="/images/ref.gif" alt="Ref"></a></td> 18</tr></table> 19<hr size=1 noshade> 20<tt> 21<b><pre> 22#include <db_cxx.h> 23<p> 24int 25Db::open(DbTxn *txnid, const char *file, 26 const char *database, DBTYPE type, u_int32_t flags, int mode); 27<p> 28int 29Db::get_dbname(const char **filenamep, const char **dbnamep); 30<p> 31int 32Db::get_multiple() 33<p> 34int 35Db::get_open_flags(u_int32_t *flagsp); 36<p> 37int 38Db::get_transactional() 39</pre></b> 40<hr size=1 noshade> 41<b>Description: Db::open</b> 42<p>The Db::open method opens the database represented by the <b>file</b> 43and <b>database</b> parameters for both reading and writing.</p> 44<p>The currently supported Berkeley DB file formats (or <i>access 45methods</i>) are Btree, Hash, Queue, and Recno. The Btree format is a 46representation of a sorted, balanced tree structure. The Hash format 47is an extensible, dynamic hashing scheme. The Queue format supports 48fast access to fixed-length records accessed sequentially or by logical 49record number. The Recno format supports fixed- or variable-length 50records, accessed sequentially or by logical record number, and 51optionally backed by a flat text file.</p> 52<p>Storage and retrieval for the Berkeley DB access methods are based on key/data 53pairs; see <a href="/api_cxx/dbt_class.html">Dbt</a> for more information.</p> 54<p>Calling Db::open is a relatively expensive operation, and 55maintaining a set of open databases will normally be preferable to 56repeatedly opening and closing the database for each new query.</p> 57<p>The Db::open method 58either returns a non-zero error value 59or throws an exception that encapsulates a non-zero error value on 60failure, and returns 0 on success. 61If Db::open fails, the <a href="/api_cxx/db_close.html">Db::close</a> method must be called to 62discard the <a href="/api_cxx/db_class.html">Db</a> handle. 63</p> 64<b>Parameters</b> <br> 65 <b>database</b><ul compact><li>The <b>database</b> parameter is optional, and allows applications to 66have multiple databases in a single file. Although no <b>database</b> 67parameter needs to be specified, it is an error to attempt to open a 68second database in a <b>file</b> that was not initially created using 69a <b>database</b> name. Further, the <b>database</b> parameter is not 70supported by the Queue format. Finally, when opening multiple databases 71in the same physical file, it is important to consider locking and 72memory cache issues; see <a href="/ref/am/opensub.html">Opening 73multiple databases in a single file</a> for more information. 74<p>In-memory databases never intended to be preserved on disk may be 75created by setting the <b>file</b> parameter to NULL. If the 76<b>database</b> parameter is also NULL, the database is strictly 77temporary and cannot be opened by any other thread of control, thus the 78database can only be accessed by sharing the single database handle that 79created it, in circumstances where doing so is safe. If the 80<b>database</b> parameter is not set to NULL, the database can be opened 81by other threads of control and will be replicated to client sites in 82any replication group.</p></ul> 83 <b>file</b><ul compact><li>The <b>file</b> parameter is used as the name of an underlying file that 84will be used to back the database; see <a href="/ref/env/naming.html">File naming</a> for more information. 85<p>In-memory databases never intended to be preserved on disk may be 86created by setting the <b>file</b> parameter to NULL.</p></ul> 87<p>When using a Unicode build on Windows (the default), the <b>file</b> 88argument will be interpreted as a UTF-8 string, which is equivalent to 89ASCII for Latin characters.</p> 90 <b>flags</b><ul compact><li>The <b>flags</b> parameter must be set to 0 or by bitwise inclusively <b>OR</b>'ing together one 91or more of the following values: 92<br> 93<b><a name="DB_AUTO_COMMIT">DB_AUTO_COMMIT</a></b><ul compact><li>Enclose the Db::open call within a transaction. If the call 94succeeds, the open operation will be recoverable and all subsequent 95database modification operations based on this handle will be 96transactionally protected. If the call fails, no database will have 97been created.</ul> 98<b><a name="DB_CREATE">DB_CREATE</a></b><ul compact><li>Create the database. If the database does not already exist and the 99DB_CREATE flag is not specified, the Db::open will fail.</ul> 100<b><a name="DB_EXCL">DB_EXCL</a></b><ul compact><li>Return an error if the database already exists. The DB_EXCL 101flag is only meaningful when specified with the DB_CREATE 102flag.</ul> 103<b><a name="DB_MULTIVERSION">DB_MULTIVERSION</a></b><ul compact><li>Open the database with support for <a href="/ref/transapp/read.html">multiversion concurrency control</a>. This will cause updates to the 104database to follow a copy-on-write protocol, which is required to 105support snapshot isolation. The DB_MULTIVERSION flag requires 106that the database be transactionally protected during its open and is 107not supported by the queue format.</ul> 108<b><a name="DB_NOMMAP">DB_NOMMAP</a></b><ul compact><li>Do not map this database into process memory (see the 109<a href="/api_cxx/env_set_mp_mmapsize.html">DbEnv::set_mp_mmapsize</a> method for further information).</ul> 110<b><a name="DB_RDONLY">DB_RDONLY</a></b><ul compact><li>Open the database for reading only. Any attempt to modify items in the 111database will fail, regardless of the actual permissions of any 112underlying files.</ul> 113<b><a name="DB_READ_UNCOMMITTED">DB_READ_UNCOMMITTED</a></b><ul compact><li>Support transactional read operations with degree 1 isolation. Read 114operations on the database may request the return of modified but not 115yet committed data. This flag must be specified on all <a href="/api_cxx/db_class.html">Db</a> 116handles used to perform dirty reads or database updates, otherwise 117requests for dirty reads may not be honored and the read may block.</ul> 118<b><a name="DB_THREAD">DB_THREAD</a></b><ul compact><li>Cause the <a href="/api_cxx/db_class.html">Db</a> handle returned by Db::open to be 119<i>free-threaded</i>; that is, concurrently usable by multiple 120threads in the address space.</ul> 121<b><a name="DB_TRUNCATE">DB_TRUNCATE</a></b><ul compact><li>Physically truncate the underlying file, discarding all previous 122databases it might have held. Underlying filesystem primitives are used 123to implement this flag. For this reason, it is applicable only to the 124file and cannot be used to discard databases within a file. 125<p>The DB_TRUNCATE flag cannot be lock or transaction-protected, 126and it is an error to specify it in a locking or transaction-protected 127environment.</p></ul> 128<br></ul> 129 <b>mode</b><ul compact><li>On Windows systems, the mode parameter is ignored. 130<p>On UNIX systems or in IEEE/ANSI Std 1003.1 (POSIX) environments, files created by the database open 131are created with mode <b>mode</b> (as described in <b>chmod</b>(2)) 132and modified by the process' umask value at the time of creation (see 133<b>umask</b>(2)). Created files are owned by the process owner; the 134group ownership of created files is based on the system and directory 135defaults, and is not further specified by Berkeley DB. System shared memory 136segments created by the database open are created with mode <b>mode</b>, unmodified 137by the process' umask value. If <b>mode</b> is 0, the database open will use a 138default mode of readable and writable by both owner and group.</p></ul> 139 <b>txnid</b><ul compact><li>If the operation is part of an application-specified transaction, the 140<b>txnid</b> parameter is a transaction handle returned from 141<a href="/api_cxx/txn_begin.html">DbEnv::txn_begin</a>; if the operation is part of a Berkeley DB Concurrent Data Store group, the 142<b>txnid</b> parameter is a handle returned from 143<a href="/api_cxx/env_cdsgroup_begin.html">DbEnv::cdsgroup_begin</a>; otherwise NULL. 144If no transaction handle is 145specified, but the 146DB_AUTO_COMMIT flag is specified, 147the operation will be implicitly transaction protected. 148Note that transactionally protected operations on a <a href="/api_cxx/db_class.html">Db</a> handle 149requires the <a href="/api_cxx/db_class.html">Db</a> handle itself be transactionally protected 150during its open. Also note that the transaction must be committed before 151the handle is closed; see <a href="/ref/program/scope.html">Berkeley DB 152handles</a> for more information.</ul> 153 <b>type</b><ul compact><li>The <b>type</b> parameter is of type DBTYPE, and must be set to one of 154<a name="DB_BTREE">DB_BTREE</a>, <a name="DB_HASH">DB_HASH</a>, <a name="DB_QUEUE">DB_QUEUE</a>, 155<a name="DB_RECNO">DB_RECNO</a>, or <a name="DB_UNKNOWN">DB_UNKNOWN</a>. If <b>type</b> is 156DB_UNKNOWN, the database must already exist and Db::open will 157automatically determine its type. The <a href="/api_cxx/db_get_type.html">Db::get_type</a> method may be used 158to determine the underlying type of databases opened using DB_UNKNOWN.</ul> 159<br> 160<br><b>Environment Variables</b> 161<p>If the database was opened within a database environment, the 162environment variable <b>DB_HOME</b> may be used as the path of the 163database environment home.</p> 164<p>Db::open is affected by any database directory specified using 165the <a href="/api_cxx/env_set_data_dir.html">DbEnv::set_data_dir</a> method, or by setting the "set_data_dir" string 166in the environment's <a href="/ref/env/db_config.html#DB_CONFIG">DB_CONFIG</a> file.</p> 167<br> 168<b>TMPDIR</b><ul compact><li>If the <b>file</b> and <b>dbenv</b> parameters to Db::open are 169NULL, the environment variable <b>TMPDIR</b> may be used as a 170directory in which to create temporary backing files</ul> 171<br> 172<br><b>Errors</b> 173<br> 174<b>ENOENT</b><ul compact><li>The file or directory does not exist.</ul> 175<br> 176<p>The Db::open method 177may fail and throw 178<a href="/api_cxx/except_class.html">DbException</a>, 179encapsulating one of the following non-zero errors, or return one of 180the following non-zero errors:</p> 181<br> 182<b>DB_OLD_VERSION</b><ul compact><li>The database cannot be opened without being first upgraded.</ul> 183<br> 184<br> 185<b>EEXIST</b><ul compact><li>DB_CREATE and DB_EXCL were specified and the database exists.</ul> 186<br> 187<br> 188<b>EINVAL</b><ul compact><li>If an unknown database type, page size, hash function, pad byte, byte 189order, or a flag value or parameter that is incompatible with the 190specified database was specified; 191the <a href="/api_cxx/env_open.html#DB_THREAD">DB_THREAD</a> flag was specified and fast mutexes are not 192available for this architecture; 193the <a href="/api_cxx/env_open.html#DB_THREAD">DB_THREAD</a> flag was specified to Db::open, but was 194not specified to the <a href="/api_cxx/env_open.html">DbEnv::open</a> call for the environment in 195which the <a href="/api_cxx/db_class.html">Db</a> handle was created; 196a backing flat text file was specified with either the <a href="/api_cxx/env_open.html#DB_THREAD">DB_THREAD</a> 197flag or the provided database environment supports transaction 198processing; or if an 199invalid flag value or parameter was specified.</ul> 200<br> 201<br> 202<b>ENOENT</b><ul compact><li>A nonexistent <b>re_source</b> file was specified.</ul> 203<br> 204<br> 205<b>DB_REP_HANDLE_DEAD</b><ul compact><li>The database handle has been invalidated because a replication election 206unrolled a committed transaction.</ul> 207<br> 208<br> 209<b>DB_REP_LOCKOUT</b><ul compact><li>The operation was blocked by client/master synchronization.</ul> 210<br> 211<p>If a transactional database environment operation was selected to 212resolve a deadlock, the Db::open method will fail and 213either return <a href="/ref/program/errorret.html#DB_LOCK_DEADLOCK">DB_LOCK_DEADLOCK</a> or 214throw a <a href="/api_cxx/deadlock_class.html">DbDeadlockException</a> exception.</p> 215<p>If a Berkeley DB Concurrent Data Store database environment configured for lock timeouts was unable 216to grant a lock in the allowed time, the Db::open method will fail and 217either return <a href="/ref/program/errorret.html#DB_LOCK_NOTGRANTED">DB_LOCK_NOTGRANTED</a> or 218throw a <a href="/api_cxx/lockng_class.html">DbLockNotGrantedException</a> exception.</p> 219<hr size=1 noshade> 220<b>Description: Db::get_dbname</b> 221<p>The Db::get_dbname method returns the current filename and database 222name.</p> 223<b>Parameters</b> <br> 224 <b>filenamep</b><ul compact><li>The <b>filenamep</b> parameter references memory into which 225a pointer to the current filename is copied.</ul> 226 <b>dbnamep</b><ul compact><li>The <b>dbnamep</b> parameter references memory into which 227a pointer to the current database name is copied.</ul> 228<br> 229<p>The Db::get_dbname method may be called at any time during the life of the 230application.</p> 231<p>The Db::get_dbname method 232either returns a non-zero error value 233or throws an exception that encapsulates a non-zero error value on 234failure, and returns 0 on success. 235</p> 236<hr size=1 noshade> 237<b>Description: <a href="/api_cxx/db_open.html">Db::get_multiple</a></b> 238<p>The <a href="/api_cxx/db_open.html">Db::get_multiple</a> method returns non-zero if the <a href="/api_cxx/db_class.html">Db</a> handle 239references a physical file supporting multiple databases.</p> 240<p>In this case, the <a href="/api_cxx/db_class.html">Db</a> handle is a handle on a database whose key 241values are the names of the databases stored in the physical file and 242whose data values are opaque objects. No keys or data values may be 243modified or stored using the database handle.</p> 244<p>The <a href="/api_cxx/db_open.html">Db::get_multiple</a> method may not be called before the Db::open method is called.</p> 245<hr size=1 noshade> 246<b>Description: Db::get_open_flags</b> 247<p>The Db::get_open_flags method returns the current open method flags.</p> 248<p>The Db::get_open_flags method may not be called before the Db::open method is called.</p> 249<p>The Db::get_open_flags method 250either returns a non-zero error value 251or throws an exception that encapsulates a non-zero error value on 252failure, and returns 0 on success. 253</p> 254<b>Parameters</b> <br> 255 <b>flagsp</b><ul compact><li>The Db::get_open_flags method returns the 256current open method flags in <b>flagsp</b>.</ul> 257<br> 258<hr size=1 noshade> 259<b>Description: Db::get_transactional</b> 260<p>The Db::get_transactional method returns non-zero if the <a href="/api_cxx/db_class.html">Db</a> 261handle has been opened in a transactional mode.</p> 262<p>The Db::get_transactional method may be called at any time during the life of the 263application.</p> 264<hr size=1 noshade> 265<br><b>Class</b> 266<a href="/api_cxx/db_class.html">Db</a> 267<br><b>See Also</b> 268<a href="/api_cxx/db_list.html">Databases and Related Methods</a> 269</tt> 270<table width="100%"><tr><td><br></td><td align=right> 271<a href="/api_cxx/api_core.html"><img src="/images/api.gif" alt="API"></a><a href="/ref/toc.html"><img src="/images/ref.gif" alt="Ref"></a> 272</td></tr></table> 273<p><font size=1>Copyright (c) 1996,2008 Oracle. All rights reserved.</font> 274</body> 275</html> 276