DBOPEN(3) UNIX System V (January 2, 1994) DBOPEN(3)
NAME [Toc] [Back]
dbopen - database access methods
SYNOPSIS [Toc] [Back]
#include <sys/types.h>
#include <limits.h>
#include <db.h>
DB *
dbopen(const char *file, int flags, int mode, DBTYPE type,
const void *openinfo);
DESCRIPTION [Toc] [Back]
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 parameter 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'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, u_int flags);
int (*fd)(const DB *db);
int (*get)(const DB *db, DBT *key, DBT *data, u_int flags);
Page 1 (printed 4/30/98)
DBOPEN(3) UNIX System V (January 2, 1994) DBOPEN(3)
int (*put)(const DB *db, DBT *key, const DBT *data,
u_int flags);
int (*sync)(const DB *db, u_int flags);
int (*seq)(const DB *db, DBT *key, DBT *data, u_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 or
sync function may result in inconsistent or lost
information. Close routines return -1 on error
(setting errno) and 0 on success.
del A pointer to a routine to remove key/data pairs from
the database.
The parameter 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 (setting errno), 0
on success, and 1 if the specified key 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 same file
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
(setting errno), 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
Page 2 (printed 4/30/98)
DBOPEN(3) UNIX System V (January 2, 1994) DBOPEN(3)
in the structure referenced by data. Get routines
return -1 on error (setting errno), 0 on success, and 1
if the key was not in the file.
put A pointer to a routine to store key/data pairs in the
database.
The parameter 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 the key structure. (Applicable only
to the DB_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 the key structure. (Applicable only
to the DB_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 and DB_RECNO
access methods.)
R_SETCURSOR is available only for the DB_BTREE and
DB_RECNO access methods because it implies that the
keys have an inherent order which does not change.
R_IAFTER and R_IBEFORE are available only for the
DB_RECNO access method because they each imply that the
access method is able to create new keys. This is only
true 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.
Page 3 (printed 4/30/98)
DBOPEN(3) UNIX System V (January 2, 1994) DBOPEN(3)
Put routines return -1 on error (setting errno), 0 on
success, and 1 if the R_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 by data.
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, or sync
routines. Modifications to the database during a
sequential scan will be reflected in the scan, i.e.
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 the
DB_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
and DB_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
Page 4 (printed 4/30/98)
DBOPEN(3) UNIX System V (January 2, 1994) DBOPEN(3)
same as the R_LAST flag. (Applicable only to the
DB_BTREE and DB_RECNO access methods.)
R_LAST and R_PREV are available only for the DB_BTREE
and DB_RECNO access methods because they each imply
that the keys have an inherent order which does not
change.
Seq routines return -1 on error (setting errno), 0 on
success and 1 if there are no key/data pairs less than
or greater than the specified or current key. If the
DB_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, the
seq 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 the bfname field of the
recno(3) manual page for more information.)
Sync routines return -1 on error (setting errno) 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
Page 5 (printed 4/30/98)
DBOPEN(3) UNIX System V (January 2, 1994) DBOPEN(3)
byte string alignment.
ERRORS [Toc] [Back]
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).
SEE ALSO [Toc] [Back]
btree(3), hash(3), mpool(3), recno(3)
LIBTP: Portable, Modular Transactions for UNIX, Margo
Seltzer, Michael Olson, USENIX proceedings, Winter 1992.
BUGS [Toc] [Back]
The typedef DBT is a mnemonic for ``data base thang'', and
was used because noone could think of a reasonable name that
wasn't already used.
The file descriptor interface is a kluge 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.
This version of berkeley db (1.85) is free software which is
not developed nor maintained by SGI. It is known to have
some bugs that are unlikely to get fixed (See NOTES below)
Page 6 (printed 4/30/98)
DBOPEN(3) UNIX System V (January 2, 1994) DBOPEN(3)
in particular, certain cursor and certain deletion/overwrite
operations are known to have problems, up to corrupting
databases, and should be avoided according to
http://www.sleepycat.com/db.185.html. See hash(3), and
btree(3) for details.
NOTES [Toc] [Back]
The default hash function in this version of db is the
Fowler/Vo/Noll hash which gives better distributions (less
collisions) on average than the publicly released version.
This version of berkeley db is 1.85. A newer, enhanced
version db-2.x requires licensing. Check out
http://www.sleepycat.com/ for details.
Page 7 (printed 4/30/98)
[ Back ]
|
