dbopen - database access methods
#include <sys/types.h>
#include <fcntl.h>
#include <limits.h>
#include <db.h>
DB *
dbopen(const char *file, int flags, int mode, DBTYPE type,
const void *openinfo);
The dbopen() function 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 flatfile
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_NOFOLLOW,
O_NONBLOCK,
O_RDONLY, O_RDWR, O_SHLOCK, O_SYNC, 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);
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(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 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.
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
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 byte string alignment.
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).
btree(3), hash(3), mpool(3), recno(3)
Margo Seltzer and Michael Olson, "LIBTP: Portable, Modular
Transactions
for UNIX", USENIX proceedings, Winter 1992.
The typedef DBT is a mnemonic for ``data base thang'', and
was used because
no one could think of a reasonable name that wasn'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.
OpenBSD 3.6 January 2, 1994
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