1<!--$Id: db_get.so,v 10.95 2007/11/07 16:51:38 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->get</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->get</b> 14</td> 15<td align=right> 16<a href="../api_c/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.h> 23<p> 24int 25DB->get(DB *db, 26 DB_TXN *txnid, DBT *key, DBT *data, u_int32_t flags); 27<p> 28int 29DB->pget(DB *db, 30 DB_TXN *txnid, DBT *key, DBT *pkey, DBT *data, u_int32_t flags); 31</pre></b> 32<hr size=1 noshade> 33<b>Description: DB->get</b> 34<p>The DB->get method retrieves key/data pairs from the database. The 35address and length of the data associated with the specified <b>key</b> 36are returned in the structure to which <b>data</b> refers.</p> 37<p>In the presence of duplicate key values, DB->get will return the 38first data item for the designated key. Duplicates are sorted by insert 39order, except where this order has been overridden by cursor operations. 40<b>Retrieval of duplicates requires the use of cursor operations.</b> 41See <a href="../api_c/dbc_get.html">DBcursor->get</a> for details.</p> 42<p>When called on a database that has been made into a secondary index 43using the <a href="../api_c/db_associate.html">DB->associate</a> method, the DB->get and 44DB->pget methods return the key from the secondary index and the data 45item from the primary database. In addition, the DB->pget method 46returns the key from the primary database. In databases that are not 47secondary indices, the DB->pget method will always fail.</p> 48<p>The DB->get method will return <a href="../ref/program/errorret.html#DB_NOTFOUND">DB_NOTFOUND</a> if the specified key is not in the database. 49The DB->get method will return <a href="../ref/program/errorret.html#DB_KEYEMPTY">DB_KEYEMPTY</a> if the database is a Queue or Recno database and 50the specified key exists, but was never explicitly created by the 51application or was later deleted. 52Unless otherwise specified, the DB->get method 53returns a non-zero error value on failure 54and 0 on success. 55</p> 56<b>Parameters</b> <br> 57 <b>data</b><ul compact><li>The data <a href="../api_c/dbt_class.html">DBT</a> operated on.</ul> 58 <b>flags</b><ul compact><li>The <b>flags</b> parameter must be set to 0 or 59one of the following values: 60<br> 61<b><a name="DB_CONSUME">DB_CONSUME</a></b><ul compact><li>Return the record number and data from the available record closest to 62the head of the queue, and delete the record. The record number will 63be returned in <b>key</b>, as described in <a href="../api_c/dbt_class.html">DBT</a>. The data will 64be returned in the <b>data</b> parameter. A record is available if it 65is not deleted and is not currently locked. The underlying database 66must be of type Queue for DB_CONSUME to be specified.</ul> 67<b><a name="DB_CONSUME_WAIT">DB_CONSUME_WAIT</a></b><ul compact><li>The DB_CONSUME_WAIT flag is the same as the DB_CONSUME 68flag, except that if the Queue database is empty, the thread of control 69will wait until there is data in the queue before returning. The 70underlying database must be of type Queue for DB_CONSUME_WAIT 71to be specified. 72<p>If lock or transaction timeouts have been specified, the DB->get method 73with the DB_CONSUME_WAIT flag 74may return <a href="../ref/program/errorret.html#DB_LOCK_NOTGRANTED">DB_LOCK_NOTGRANTED</a>. 75This failure, by itself, does not require the enclosing transaction be 76aborted.</p></ul> 77<b><a name="DB_GET_BOTH">DB_GET_BOTH</a></b><ul compact><li>Retrieve the key/data pair only if both the key and data match the 78arguments. 79<p>When used with the DB->pget method version of this method on a 80secondary index handle, return the secondary key/primary key/data tuple 81only if both the primary and secondary keys match the arguments. It is 82an error to use the DB_GET_BOTH flag with the DB->get 83version of this method and a secondary index handle.</p></ul> 84<b><a name="DB_SET_RECNO">DB_SET_RECNO</a></b><ul compact><li>Retrieve the specified numbered key/data pair from a database. Upon 85return, both the <b>key</b> and <b>data</b> items will have been 86filled in. 87<p>The <b>data</b> field of the specified <b>key</b> must be a pointer 88to a logical record number (that is, a <b>db_recno_t</b>). This record 89number determines the record to be retrieved.</p> 90<p>For DB_SET_RECNO to be specified, the underlying database must be 91of type Btree, and it must have been created with the DB_RECNUM flag.</p></ul> 92<br> 93In addition, the following flags may be set by 94bitwise inclusively <b>OR</b>'ing them into the <b>flags</b> parameter: 95<br> 96<b><a name="DB_IGNORE_LEASE">DB_IGNORE_LEASE</a></b><ul compact><li>Return the data item irrespective of the state of master leases. The 97item will be returned under all conditions: if master leases are not 98configured, if the request is made to a client, if the request is made 99to a master with a valid lease, or if the request is made to a master 100without a valid lease.</ul> 101<b><a name="DB_MULTIPLE">DB_MULTIPLE</a></b><ul compact><li>Return multiple data items in the buffer to which the <b>data</b> 102parameter refers. 103<p>In the case of Btree or Hash databases, all of the data items associated 104with the specified key are entered into the buffer. In the case of 105Queue or Recno databases, all of the data items in the database, 106starting at, and subsequent to, the specified key, are entered into the 107buffer.</p> 108<p>The buffer to which the <b>data</b> parameter refers must be provided 109from user memory (see <a href="../api_c/dbt_class.html#DB_DBT_USERMEM">DB_DBT_USERMEM</a>). The buffer must be at 110least as large as the page size of the underlying database, aligned for 111unsigned integer access, and be a multiple of 1024 bytes in size. If 112the buffer size is insufficient, then upon return from the call the size 113field of the <b>data</b> parameter will have been set to an estimated 114buffer size, and the error DB_BUFFER_SMALL is returned. (The size is an estimate as the 115exact size needed may not be known until all entries are read. It is 116best to initially provide a relatively large buffer, but applications 117should be prepared to resize the buffer as necessary and repeatedly call 118the method.)</p> 119<p>The DB_MULTIPLE flag may only be used alone, or with the 120DB_GET_BOTH and DB_SET_RECNO options. The 121DB_MULTIPLE flag may not be used when accessing databases made 122into secondary indices using the <a href="../api_c/db_associate.html">DB->associate</a> method.</p> 123<p>See 124<a href="../api_c/dbt_bulk.html#DB_MULTIPLE_INIT">DB_MULTIPLE_INIT</a> 125for more information.</p></ul> 126<b><a name="DB_READ_COMMITTED">DB_READ_COMMITTED</a></b><ul compact><li>Configure a transactional get operation to have degree 2 isolation (the 127read is not repeatable).</ul> 128<b><a name="DB_READ_UNCOMMITTED">DB_READ_UNCOMMITTED</a></b><ul compact><li>Configure a transactional get operation to have degree 1 isolation, 129reading modified but not yet committed data. Silently ignored if the 130<a href="../api_c/db_open.html#DB_READ_UNCOMMITTED">DB_READ_UNCOMMITTED</a> flag was not specified when the underlying 131database was opened.</ul> 132<b><a name="DB_RMW">DB_RMW</a></b><ul compact><li>Acquire write locks instead of read locks when doing the read, if 133locking is configured. Setting this flag can eliminate deadlock during 134a read-modify-write cycle by acquiring the write lock during the read 135part of the cycle so that another thread of control acquiring a read 136lock for the same item, in its own read-modify-write cycle, will not 137result in deadlock. 138<p>Because the DB->get method will not hold locks across 139Berkeley DB calls in non-transactional operations, the <a href="../api_c/dbc_get.html#DB_RMW">DB_RMW</a> flag 140to the DB->get call is meaningful only in the presence of 141transactions.</p></ul> 142<br></ul> 143 <b>key</b><ul compact><li>The key <a href="../api_c/dbt_class.html">DBT</a> operated on.</ul> 144 <b>pkey</b><ul compact><li>The <b>pkey</b> parameter is the return key from the primary database.</ul> 145 <b>txnid</b><ul compact><li>If the operation is part of an application-specified transaction, the 146<b>txnid</b> parameter is a transaction handle returned from 147<a href="../api_c/txn_begin.html">DB_ENV->txn_begin</a>; if the operation is part of a Berkeley DB Concurrent Data Store group, the 148<b>txnid</b> parameter is a handle returned from 149<a href="../api_c/env_cdsgroup_begin.html">DB_ENV->cdsgroup_begin</a>; otherwise NULL. 150If no transaction handle is 151specified, but the 152operation occurs in a transactional 153database, 154the operation will be implicitly transaction protected. 155</ul> 156<br> 157<br><b>Errors</b> 158<p>The DB->get method 159may fail and return one of the following non-zero errors:</p> 160<br> 161<b>DB_BUFFER_SMALL</b><ul compact><li>The requested item could not be returned due to undersized buffer.</ul> 162<br> 163<br> 164<b>DB_LOCK_DEADLOCK</b><ul compact><li>A transactional database environment operation was selected to resolve 165a deadlock.</ul> 166<b>DB_LOCK_NOTGRANTED</b><ul compact><li>A Berkeley DB Concurrent Data Store database environment configured for lock timeouts was unable 167to grant a lock in the allowed time.</ul> 168<br> 169<br> 170<b>DB_LOCK_NOTGRANTED</b><ul compact><li>The DB_CONSUME_WAIT flag was specified, lock or transaction 171timers were configured and the lock could not be granted before the wait-time expired.</ul> 172<br> 173<br> 174<b>DB_REP_HANDLE_DEAD</b><ul compact><li>The database handle has been invalidated because a replication election 175unrolled a committed transaction.</ul> 176<br> 177<br> 178<b>DB_REP_LEASE_EXPIRED</b><ul compact><li>The operation failed because the site's replication master lease has expired.</ul> 179<br> 180<br> 181<b>DB_REP_LOCKOUT</b><ul compact><li>The operation was blocked by client/master synchronization.</ul> 182<br> 183<br> 184<b>DB_SECONDARY_BAD</b><ul compact><li>A secondary index references a nonexistent primary key.</ul> 185<br> 186<br> 187<b>EINVAL</b><ul compact><li>If a record number of 0 was specified; 188the <a href="../api_c/env_open.html#DB_THREAD">DB_THREAD</a> flag was specified to the <a href="../api_c/db_open.html">DB->open</a> method and 189none of the <a href="../api_c/dbt_class.html#DB_DBT_MALLOC">DB_DBT_MALLOC</a>, <a href="../api_c/dbt_class.html#DB_DBT_REALLOC">DB_DBT_REALLOC</a> or 190<a href="../api_c/dbt_class.html#DB_DBT_USERMEM">DB_DBT_USERMEM</a> flags were set in the <a href="../api_c/dbt_class.html">DBT</a>; 191the DB->pget method was called with a <a href="../api_c/db_class.html">DB</a> handle that does not 192refer to a secondary index; or if an 193invalid flag value or parameter was specified.</ul> 194<br> 195<hr size=1 noshade> 196<br><b>Class</b> 197<a href="../api_c/db_class.html">DB</a> 198<br><b>See Also</b> 199<a href="../api_c/db_list.html">Databases and Related Methods</a> 200</tt> 201<table width="100%"><tr><td><br></td><td align=right> 202<a href="../api_c/api_core.html"><img src="../images/api.gif" alt="API"></a><a href="../ref/toc.html"><img src="../images/ref.gif" alt="Ref"></a> 203</td></tr></table> 204<p><font size=1>Copyright (c) 1996,2008 Oracle. 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