477 lines
13 KiB
Groff
477 lines
13 KiB
Groff
.\" Copyright (c) 1990, 1993
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.\" The Regents of the University of California. All rights reserved.
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.\"
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.\" Redistribution and use in source and binary forms, with or without
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.\" modification, are permitted provided that the following conditions
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.\" are met:
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.\" 1. Redistributions of source code must retain the above copyright
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.\" notice, this list of conditions and the following disclaimer.
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.\" 2. Redistributions in binary form must reproduce the above copyright
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.\" notice, this list of conditions and the following disclaimer in the
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.\" documentation and/or other materials provided with the distribution.
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.\" 3. All advertising materials mentioning features or use of this software
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.\" must display the following acknowledgement:
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.\" This product includes software developed by the University of
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.\" California, Berkeley and its contributors.
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.\" 4. Neither the name of the University nor the names of its contributors
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.\" may be used to endorse or promote products derived from this software
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.\" without specific prior written permission.
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.\"
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.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
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.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
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.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
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.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
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.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
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.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
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.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
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.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
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.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
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.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
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.\" SUCH DAMAGE.
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.\"
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.\" @(#)dbopen.3 8.5 (Berkeley) 1/2/94
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.\"
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.TH DBOPEN 3 "January 2, 1994"
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.UC 7
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.SH NAME
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dbopen \- database access methods
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.SH SYNOPSIS
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.nf
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.ft B
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#include <sys/types.h>
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#include <limits.h>
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#include <db.h>
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DB *
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dbopen(const char *file, int flags, int mode, DBTYPE type,
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.ti +5
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const void *openinfo);
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.ft R
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.fi
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.SH DESCRIPTION
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.IR Dbopen
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is the library interface to database files.
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The supported file formats are btree, hashed and UNIX file oriented.
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The btree format is a representation of a sorted, balanced tree structure.
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The hashed format is an extensible, dynamic hashing scheme.
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The flat-file format is a byte stream file with fixed or variable length
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records.
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The formats and file format specific information are described in detail
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in their respective manual pages
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.IR btree (3),
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.IR hash (3)
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and
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.IR recno (3).
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.PP
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Dbopen opens
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.I file
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for reading and/or writing.
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Files never intended to be preserved on disk may be created by setting
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the file parameter to NULL.
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.PP
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The
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.I flags
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and
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.I mode arguments
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are as specified to the
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.IR open (2)
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routine, however, only the O_CREAT, O_EXCL, O_EXLOCK, O_NONBLOCK,
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O_RDONLY, O_RDWR, O_SHLOCK and O_TRUNC flags are meaningful.
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(Note, opening a database file O_WRONLY is not possible.)
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.\"Three additional options may be specified by
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.\".IR or 'ing
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.\"them into the
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.\".I flags
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.\"argument.
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.\".TP
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.\"DB_LOCK
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.\"Do the necessary locking in the database to support concurrent access.
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.\"If concurrent access isn't needed or the database is read-only this
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.\"flag should not be set, as it tends to have an associated performance
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.\"penalty.
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.\".TP
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.\"DB_SHMEM
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.\"Place the underlying memory pool used by the database in shared
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.\"memory.
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.\"Necessary for concurrent access.
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.\".TP
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.\"DB_TXN
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.\"Support transactions in the database.
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.\"The DB_LOCK and DB_SHMEM flags must be set as well.
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.PP
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The
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.I type
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argument is of type DBTYPE (as defined in the <db.h> include file) and
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may be set to DB_BTREE, DB_HASH or DB_RECNO.
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.PP
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The
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.I openinfo
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argument is a pointer to an access method specific structure described
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in the access method's manual page.
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If
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.I openinfo
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is NULL, each access method will use defaults appropriate for the system
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and the access method.
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.PP
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.I Dbopen
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returns a pointer to a DB structure on success and NULL on error.
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The DB structure is defined in the <db.h> include file, and contains at
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least the following fields:
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.sp
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.nf
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typedef struct {
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.RS
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DBTYPE type;
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int (*close)(const DB *db);
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int (*del)(const DB *db, const DBT *key, u_int flags);
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int (*fd)(const DB *db);
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int (*get)(const DB *db, DBT *key, DBT *data, u_int flags);
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int (*put)(const DB *db, DBT *key, const DBT *data,
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.ti +5
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u_int flags);
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int (*sync)(const DB *db, u_int flags);
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int (*seq)(const DB *db, DBT *key, DBT *data, u_int flags);
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.RE
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} DB;
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.fi
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.PP
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These elements describe a database type and a set of functions performing
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various actions.
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These functions take a pointer to a structure as returned by
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.IR dbopen ,
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and sometimes one or more pointers to key/data structures and a flag value.
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.TP
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type
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The type of the underlying access method (and file format).
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.TP
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close
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A pointer to a routine to flush any cached information to disk, free any
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allocated resources, and close the underlying file(s).
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Since key/data pairs may be cached in memory, failing to sync the file
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with a
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.I close
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or
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.I sync
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function may result in inconsistent or lost information.
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.I Close
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routines return -1 on error (setting
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.IR errno )
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and 0 on success.
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.TP
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del
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A pointer to a routine to remove key/data pairs from the database.
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.IP
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The parameter
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.I flag
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may be set to the following value:
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.RS
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.TP
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R_CURSOR
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Delete the record referenced by the cursor.
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The cursor must have previously been initialized.
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.RE
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.IP
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.I Delete
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routines return -1 on error (setting
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.IR errno ),
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0 on success, and 1 if the specified
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.I key
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was not in the file.
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.TP
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fd
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A pointer to a routine which returns a file descriptor representative
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of the underlying database.
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A file descriptor referencing the same file will be returned to all
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processes which call
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.I dbopen
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with the same
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.I file
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name.
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This file descriptor may be safely used as an argument to the
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.IR fcntl (2)
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and
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.IR flock (2)
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locking functions.
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The file descriptor is not necessarily associated with any of the
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underlying files used by the access method.
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No file descriptor is available for in memory databases.
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.I Fd
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routines return -1 on error (setting
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.IR errno ),
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and the file descriptor on success.
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.TP
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get
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A pointer to a routine which is the interface for keyed retrieval from
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the database.
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The address and length of the data associated with the specified
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.I key
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are returned in the structure referenced by
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.IR data .
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.I Get
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routines return -1 on error (setting
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.IR errno ),
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0 on success, and 1 if the
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.I key
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was not in the file.
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.TP
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put
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A pointer to a routine to store key/data pairs in the database.
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.IP
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The parameter
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.I flag
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may be set to one of the following values:
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.RS
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.TP
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R_CURSOR
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Replace the key/data pair referenced by the cursor.
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The cursor must have previously been initialized.
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.TP
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R_IAFTER
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Append the data immediately after the data referenced by
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.IR key ,
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creating a new key/data pair.
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The record number of the appended key/data pair is returned in the
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.I key
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structure.
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(Applicable only to the DB_RECNO access method.)
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.TP
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R_IBEFORE
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Insert the data immediately before the data referenced by
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.IR key ,
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creating a new key/data pair.
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The record number of the inserted key/data pair is returned in the
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.I key
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structure.
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(Applicable only to the DB_RECNO access method.)
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.TP
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R_NOOVERWRITE
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Enter the new key/data pair only if the key does not previously exist.
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.TP
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R_SETCURSOR
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Store the key/data pair, setting or initializing the position of the
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cursor to reference it.
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(Applicable only to the DB_BTREE and DB_RECNO access methods.)
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.RE
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.IP
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R_SETCURSOR is available only for the DB_BTREE and DB_RECNO access
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methods because it implies that the keys have an inherent order
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which does not change.
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.IP
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R_IAFTER and R_IBEFORE are available only for the DB_RECNO
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access method because they each imply that the access method is able to
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create new keys.
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This is only true if the keys are ordered and independent, record numbers
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for example.
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.IP
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The default behavior of the
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.I put
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routines is to enter the new key/data pair, replacing any previously
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existing key.
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.IP
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.I Put
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routines return -1 on error (setting
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.IR errno ),
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0 on success, and 1 if the R_NOOVERWRITE
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.I flag
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was set and the key already exists in the file.
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.TP
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seq
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A pointer to a routine which is the interface for sequential
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retrieval from the database.
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The address and length of the key are returned in the structure
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referenced by
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.IR key ,
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and the address and length of the data are returned in the
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structure referenced
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by
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.IR data .
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.IP
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Sequential key/data pair retrieval may begin at any time, and the
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position of the ``cursor'' is not affected by calls to the
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.IR del ,
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.IR get ,
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.IR put ,
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or
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.I sync
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routines.
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Modifications to the database during a sequential scan will be reflected
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in the scan, i.e. records inserted behind the cursor will not be returned
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while records inserted in front of the cursor will be returned.
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.IP
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The flag value
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.B must
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be set to one of the following values:
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.RS
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.TP
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R_CURSOR
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The data associated with the specified key is returned.
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This differs from the
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.I get
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routines in that it sets or initializes the cursor to the location of
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the key as well.
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(Note, for the DB_BTREE access method, the returned key is not necessarily an
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exact match for the specified key.
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The returned key is the smallest key greater than or equal to the specified
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key, permitting partial key matches and range searches.)
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.TP
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R_FIRST
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The first key/data pair of the database is returned, and the cursor
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is set or initialized to reference it.
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.TP
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R_LAST
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The last key/data pair of the database is returned, and the cursor
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is set or initialized to reference it.
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(Applicable only to the DB_BTREE and DB_RECNO access methods.)
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.TP
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R_NEXT
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Retrieve the key/data pair immediately after the cursor.
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If the cursor is not yet set, this is the same as the R_FIRST flag.
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.TP
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R_PREV
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Retrieve the key/data pair immediately before the cursor.
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If the cursor is not yet set, this is the same as the R_LAST flag.
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(Applicable only to the DB_BTREE and DB_RECNO access methods.)
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.RE
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.IP
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R_LAST and R_PREV are available only for the DB_BTREE and DB_RECNO
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access methods because they each imply that the keys have an inherent
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order which does not change.
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.IP
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.I Seq
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routines return -1 on error (setting
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.IR errno ),
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0 on success and 1 if there are no key/data pairs less than or greater
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than the specified or current key.
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If the DB_RECNO access method is being used, and if the database file
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is a character special file and no complete key/data pairs are currently
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available, the
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.I seq
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routines return 2.
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.TP
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sync
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A pointer to a routine to flush any cached information to disk.
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If the database is in memory only, the
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.I sync
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routine has no effect and will always succeed.
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.IP
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The flag value may be set to the following value:
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.RS
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.TP
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R_RECNOSYNC
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If the DB_RECNO access method is being used, this flag causes
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the sync routine to apply to the btree file which underlies the
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recno file, not the recno file itself.
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(See the
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.I bfname
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field of the
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.IR recno (3)
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manual page for more information.)
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.RE
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.IP
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.I Sync
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routines return -1 on error (setting
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.IR errno )
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and 0 on success.
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.SH "KEY/DATA PAIRS"
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Access to all file types is based on key/data pairs.
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Both keys and data are represented by the following data structure:
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.PP
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typedef struct {
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.RS
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void *data;
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.br
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size_t size;
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.RE
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} DBT;
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.PP
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The elements of the DBT structure are defined as follows:
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.TP
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data
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A pointer to a byte string.
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.TP
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size
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The length of the byte string.
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.PP
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Key and data byte strings may reference strings of essentially unlimited
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length although any two of them must fit into available memory at the same
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time.
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It should be noted that the access methods provide no guarantees about
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byte string alignment.
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.SH ERRORS
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The
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.I dbopen
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routine may fail and set
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.I errno
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for any of the errors specified for the library routines
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.IR open (2)
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and
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.IR malloc (3)
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or the following:
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.TP
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[EFTYPE]
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A file is incorrectly formatted.
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.TP
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[EINVAL]
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A parameter has been specified (hash function, pad byte etc.) that is
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incompatible with the current file specification or which is not
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meaningful for the function (for example, use of the cursor without
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prior initialization) or there is a mismatch between the version
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number of file and the software.
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.PP
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The
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.I close
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routines may fail and set
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.I errno
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for any of the errors specified for the library routines
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.IR close (2),
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.IR read (2),
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.IR write (2),
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.IR free (3),
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or
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.IR fsync (2).
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.PP
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The
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.IR del ,
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.IR get ,
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.I put
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and
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.I seq
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routines may fail and set
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.I errno
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for any of the errors specified for the library routines
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.IR read (2),
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.IR write (2),
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.IR free (3)
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or
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.IR malloc (3).
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.PP
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The
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.I fd
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routines will fail and set
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.I errno
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to ENOENT for in memory databases.
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.PP
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The
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.I sync
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routines may fail and set
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.I errno
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for any of the errors specified for the library routine
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.IR fsync (2).
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.SH "SEE ALSO"
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.IR btree (3),
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.IR hash (3),
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.IR mpool (3),
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.IR recno (3)
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.sp
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.IR "LIBTP: Portable, Modular Transactions for UNIX" ,
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Margo Seltzer, Michael Olson, USENIX proceedings, Winter 1992.
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.SH BUGS
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The typedef DBT is a mnemonic for ``data base thang'', and was used
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because noone could think of a reasonable name that wasn't already used.
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.PP
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The file descriptor interface is a kluge and will be deleted in a
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future version of the interface.
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.PP
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None of the access methods provide any form of concurrent access,
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locking, or transactions.
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