| DBOPEN(3) | Library Functions Manual | DBOPEN(3) |
dbopen — database
access methods
#include
<sys/types.h>
#include <db.h>
#include <fcntl.h>
#include <limits.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 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).
The
dbopen()
function 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's manual
page. If openinfo is NULL,
each access method will use defaults appropriate for the system and the
access method.
The
dbopen()
function 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)(DB *db);
int (*del)(const DB *db, const DBT *key, u_int flags);
int (*fd)(const DB *db);
int (*get)(const DB *db, const 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.
The flags argument may be set to the following value:
R_CURSORdelete routines return -1 on error (setting errno), 0 on success, and 1 if the specified key was not in the file.
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.The flags argument may be set to one of the following values:
R_CURSORR_IAFTERDB_RECNO access method.)R_IBEFOREDB_RECNO access method.)R_NOOVERWRITER_SETCURSORDB_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.
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 flags argument must be set to one of the following values:
R_CURSORDB_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_FIRSTR_LASTDB_BTREE and DB_RECNO
access methods.)R_NEXTR_FIRST flag.R_PREVR_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.
The flags argument may be set to the following value:
R_RECNOSYNCDB_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 {
void *data;
size_t size;
} DBT;
The elements of the DBT structure are defined as follows:
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]EINVAL]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 noone could think of a reasonable name that was not 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.
| January 2, 1994 | macOS 15.2 |