Originální popis anglicky:
dbopen - database access methods
Návod, kniha: Library Functions Manual
#include <sys/types.h>
#include <limits.h>
#include <db.h>
DB *
dbopen(const char *file, int flags, int mode, DBTYPE type,
const void *openinfo);
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);
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 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.
Access to all file types is based on key/data pairs. Both keys and data are
represented by the following data structure:
typedef struct {
} 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)
LIBTP: Portable, Modular Transactions for UNIX, Margo Seltzer, Michael
Olson, USENIX proceedings, Winter 1992.
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 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.
