DB
*
dbopen(
const char *file
, int flags
, mode_t mode
, DBTYPE type
, const void *openinfo
)
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
(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);
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 , and sometimes one or more pointers to key/data structures and a flag value.
type
close
close
or
sync
function may result in inconsistent or lost information.
close
routines return -1 on error (setting
errno)
and 0 on success.
del
The parameter
flag
may be set to the following value:
R_CURSOR
delete
routines return -1 on error (setting
errno),
0 on success, and 1 if the specified
key
was not in the file.
fd
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
key
are returned 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
The parameter
flag
may be set to one of the following values:
R_CURSOR
R_IAFTER
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
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
R_SETCURSOR
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.
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
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
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
R_LAST
DB_BTREE
and
DB_RECNO
access methods.)
R_NEXT
R_FIRST
flag.
R_PREV
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
sync
routine has no effect and will always succeed.
The flag value may be set to the following value:
R_RECNOSYNC
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.
typedef struct {
void *data;
size_t size;
} DBT;
The elements of the DBT structure are defined as follows:
data
size
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.
EFTYPE
EINVAL
EFBIG
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).
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.