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DBOPEN(3) BSD Library Functions Manual DBOPEN(3)
NAME
dbopen -- database access methods
SYNOPSIS
#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);
DESCRIPTION
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 struc-ture structure
ture 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 func-tions functions
tions 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 flags argument may be set to the following value:
R_CURSOR
Delete the record referenced by the cursor. The cursor must have previously been ini-tialized. initialized.
tialized.
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 data-base. database.
base. 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 descrip-tor descriptor
tor 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 flags argument 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 struc-ture. structure.
ture. (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 struc-ture. structure.
ture. (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 refer-ence reference
ence 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 pre-viously previously
viously 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 cur-sor cursor
sor 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_CURSOR
The data associated with the specified key is returned. This differs from the get rou-tines routines
tines 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 initial-ized initialized
ized to reference it.
R_LAST The last key/data pair of the database is returned, and the cursor is set or initial-ized initialized
ized 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 flags argument 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 follow-ing following
ing 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 pro-vide provide
vide no guarantees about byte string alignment.
ERRORS
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] An argument 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
btree(3), hash(3), mpool(3), recno(3)
Margo Seltzer and Michael Olson, LIBTP: Portable, Modular Transactions for UNIX, USENIX proceedings,
Winter 1992.
BUGS
The typedef DBT is a mnemonic for ``data base thang'', and was used because noone could think of a rea-sonable reasonable
sonable 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.
BSD January 2, 1994 BSD
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