FreeBSD Man Pagesdbopen(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 argu-
ment 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)(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
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 data-
base.
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 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 represen-
tative 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 rou-
tines 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 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. (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. (Applicable only to the DB_RECNO access method.)
R_NOOVERWRITE
Enter the new key/data pair only if the key does not pre-
viously exist.
R_SETCURSOR
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 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 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.
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 suc-
cess.
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 guaran-
tees about byte string alignment.
ERRORS
The dbopen() routine may fail and set errno for any of the errors speci-
fied 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 func-
tion (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 data-
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 reasonable name that wasn't 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, lock-
ing, or transactions.
FreeBSD 5.4 January 2, 1994 FreeBSD 5.4
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