Section (3) dbopen
Name
dbopen — database access methods
Synopsis
#include <sys/types.h> #include <limits.h> #include <db.h> #include <fcntl.h>
DB
*dbopen( |
const char *file, |
int flags, | |
int mode, | |
DBTYPE type, | |
const void *openinfo) ; |
DESCRIPTION
Note well: This
page documents interfaces provided in glibc up until version
2.1. Since version 2.2, glibc no longer provides these
interfaces. Probably, you are looking for the APIs provided
by the libdb
library
instead.
dbopen
() is the library
interface to database files. The supported file formats are
btree, hashed and UNIX file oriented. The btree format is a
representation of a sorted, balanced tree structure. The
hashed format is an extensible, dynamic hashing scheme. The
flat-file format is a byte stream file with fixed or variable
length records. The formats and file-format-specific
information are described in detail in their respective
manual pages btree(3), hash(3), and recno(3).
dbopen
() opens file
for reading and/or
writing. Files never intended to be preserved on disk may be
created by setting the file
argument to NULL.
The flags
and
mode
arguments are as
specified to the open(2) routine, however,
only the O_CREAT
, O_EXCL
, O_EXLOCK
, O_NONBLOCK
, O_RDONLY
, O_RDWR
, O_SHLOCK
, and O_TRUNC
flags are meaningful. (Note,
opening a database file O_WRONLY
is not possible.)
The type
argument
is of type DBTYPE
(as defined
in the <
db.h
>
include file) and may be set to DB_BTREE
, DB_HASH
, or DB_RECNO
.
The openinfo
argument is a pointer to an access-method-specific structure
described in the access method_zsingle_quotesz_s manual page. If openinfo
is NULL, each access
method will use defaults appropriate for the system and the
access method.
dbopen
() returns a pointer
to a DB
structure on success
and NULL on error. The DB
structure is defined in the <
db.h
>
include file, and contains at least the following fields:
typedef struct { DBTYPE type
;int (* close
)(const DB *db);int (* del
)(const DB *db, const DBT *key, unsigned int flags);int (* fd
)(const DB *db);int (* get
)(const DB *db, DBT *key, DBT *data, unsigned int flags);int (* put
)(const DB *db, DBT *key, const DBT *data, unsigned int flags);int (* sync
)(const DB *db, unsigned int flags);int (* seq
)(const DB *db, DBT *key, DBT *data, unsigned int flags);} DB;
These elements describe a database type and a set of
functions performing various actions. These functions take a
pointer to a structure as returned by dbopen
(), and sometimes one or more
pointers to key/data structures and a flag value.
type
-
The type of the underlying access method (and file format).
close
-
A pointer to a routine to flush any cached information to disk, free any allocated resources, and close the underlying file(s). Since key/data pairs may be cached in memory, failing to sync the file with a
close
orsync
function may result in inconsistent or lost information.close
routines return −1 on error (settingerrno
) and 0 on success. del
-
A pointer to a routine to remove key/data pairs from the database.
The argument
flag
may be set to the following value:R_CURSOR
-
Delete the record referenced by the cursor. The cursor must have previously been initialized.
delete
routines return −1 on error (settingerrno
), 0 on success, and 1 if the specifiedkey
was not in the file. fd
-
A pointer to a routine which returns a file descriptor representative of the underlying database. A file descriptor referencing the same file will be returned to all processes which call
dbopen
() with the samefile
name. This file descriptor may be safely used as an argument to the fcntl(2) and flock(2) locking functions. The file descriptor is not necessarily associated with any of the underlying files used by the access method. No file descriptor is available for in memory databases.fd
routines return −1 on error (settingerrno
), and the file descriptor on success. get
-
A pointer to a routine which is the interface for keyed retrieval from the database. The address and length of the data associated with the specified
key
are returned in the structure referenced bydata
.get
routines return −1 on error (settingerrno
), 0 on success, and 1 if thekey
was not in the file. put
-
A pointer to a routine to store key/data pairs in the database.
The argument
flag
may be set to one of the following values:R_CURSOR
-
Replace the key/data pair referenced by the cursor. The cursor must have previously been initialized.
R_IAFTER
-
Append the data immediately after the data referenced by
key
, creating a new key/data pair. The record number of the appended key/data pair is returned in thekey
structure. (Applicable only to theDB_RECNO
access method.) R_IBEFORE
-
Insert the data immediately before the data referenced by
key
, creating a new key/data pair. The record number of the inserted key/data pair is returned in thekey
structure. (Applicable only to theDB_RECNO
access method.) R_NOOVERWRITE
-
Enter the new key/data pair only if the key does not previously exist.
R_SETCURSOR
-
Store the key/data pair, setting or initializing the position of the cursor to reference it. (Applicable only to the
DB_BTREE
andDB_RECNO
access methods.)
R_SETCURSOR
is available only for theDB_BTREE
andDB_RECNO
access methods because it implies that the keys have an inherent order which does not change.R_IAFTER
andR_IBEFORE
are available only for theDB_RECNO
access method because they each imply that the access method is able to create new keys. This is true only if the keys are ordered and independent, record numbers for example.The default behavior of the
put
routines is to enter the new key/data pair, replacing any previously existing key.put
routines return −1 on error (settingerrno
), 0 on success, and 1 if theR_NOOVERWRITE
flag
was set and the key already exists in the file. seq
-
A pointer to a routine which is the interface for sequential retrieval from the database. The address and length of the key are returned in the structure referenced by
key
, and the address and length of the data are returned in the structure referenced bydata
.Sequential key/data pair retrieval may begin at any time, and the position of the cursor is not affected by calls to the
del
,get
,put
, orsync
routines. Modifications to the database during a sequential scan will be reflected in the scan, that is, records inserted behind the cursor will not be returned while records inserted in front of the cursor will be returned.The flag value must be set to one of the following values:
R_CURSOR
-
The data associated with the specified key is returned. This differs from the
get
routines in that it sets or initializes the cursor to the location of the key as well. (Note, for theDB_BTREE
access method, the returned key is not necessarily an exact match for the specified key. The returned key is the smallest key greater than or equal to the specified key, permitting partial key matches and range searches.) R_FIRST
-
The first key/data pair of the database is returned, and the cursor is set or initialized to reference it.
R_LAST
-
The last key/data pair of the database is returned, and the cursor is set or initialized to reference it. (Applicable only to the
DB_BTREE
andDB_RECNO
access methods.) R_NEXT
-
Retrieve the key/data pair immediately after the cursor. If the cursor is not yet set, this is the same as the
R_FIRST
flag. R_PREV
-
Retrieve the key/data pair immediately before the cursor. If the cursor is not yet set, this is the same as the
R_LAST
flag. (Applicable only to theDB_BTREE
andDB_RECNO
access methods.)
R_LAST
andR_PREV
are available only for theDB_BTREE
andDB_RECNO
access methods because they each imply that the keys have an inherent order which does not change.seq
routines return −1 on error (settingerrno
), 0 on success and 1 if there are no key/data pairs less than or greater than the specified or current key. If theDB_RECNO
access method is being used, and if the database file is a character special file and no complete key/data pairs are currently available, theseq
routines return 2. sync
-
A pointer to a routine to flush any cached information to disk. If the database is in memory only, the
sync
routine has no effect and will always succeed.The flag value may be set to the following value:
R_RECNOSYNC
-
If the
DB_RECNO
access method is being used, this flag causes the sync routine to apply to the btree file which underlies the recno file, not the recno file itself. (See thebfname
field of the recno(3) manual page for more information.)
sync
routines return −1 on error (settingerrno
) and 0 on success.
Key/data pairs
Access to all file types is based on key/data pairs. Both keys and data are represented by the following data structure:
typedef struct { void * data
;size_t size
;} DBT;
The elements of the DBT
structure are defined as follows:
data
-
A pointer to a byte string.
size
-
The length of the byte string.
Key and data byte strings may reference strings of essentially unlimited length although any two of them must fit into available memory at the same time. It should be noted that the access methods provide no guarantees about byte string alignment.
ERRORS
The dbopen
() routine may
fail and set errno
for any of
the errors specified for the library routines open(2) and malloc(3) or the
following:
EFTYPE
-
A file is incorrectly formatted.
- EINVAL
-
A parameter has been specified (hash function, pad byte, etc.) that is incompatible with the current file specification or which is not meaningful for the function (for example, use of the cursor without prior initialization) or there is a mismatch between the version number of file and the software.
The close
routines
may fail and set errno
for any
of the errors specified for the library routines close(2), read(2), write(2), free(3), or fsync(2).
The del
,
get
, put
, and seq
routines may fail and set
errno
for any of the errors
specified for the library routines read(2), write(2), free(3) or malloc(3).
The fd
routines
will fail and set errno
to
ENOENT for in memory
databases.
The sync
routines
may fail and set errno
for any
of the errors specified for the library routine fsync(2).
BUGS
The typedef DBT
is a
mnemonic for data base thang, and was used because no one
could think of a reasonable name that wasn_zsingle_quotesz_t already
used.
The file descriptor interface is a kludge and will be deleted in a future version of the interface.
None of the access methods provide any form of concurrent access, locking, or transactions.
SEE ALSO
btree(3), hash(3), mpool(3), recno(3)
LIBTP: Portable, Modular Transactions for UNIX, Margo Seltzer, Michael Olson, USENIX proceedings, Winter 1992.
COLOPHON
This page is part of release 4.16 of the Linux man-pages
project. A
description of the project, information about reporting bugs,
and the latest version of this page, can be found at
https://www.kernel.org/doc/man−pages/.
Copyright (c) 1990, 1993 The Regents of the University of California. All rights reserved. %%%LICENSE_START(BSD_4_CLAUSE_UCB) Redistribution and use in source and binary forms, with or without modification, are permitted provided that the following conditions are met: 1. Redistributions of source code must retain the above copyright notice, this list of conditions and the following disclaimer. 2. Redistributions in binary form must reproduce the above copyright notice, this list of conditions and the following disclaimer in the documentation and/or other materials provided with the distribution. 3. All advertising materials mentioning features or use of this software must display the following acknowledgement: This product includes software developed by the University of California, Berkeley and its contributors. 4. Neither the name of the University nor the names of its contributors may be used to endorse or promote products derived from this software without specific prior written permission. THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS_zsingle_quotesz__zsingle_quotesz_ AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE. %%%LICENSE_END (#)dbopen.3 8.5 (Berkeley) 1/2/94 |