DBOPEN(3) BSD Programmer's Manual DBOPEN(3)
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 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_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 er-
ror. 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,
unsigned int flags);
int (*fd)(const DB *db);
int (*get)(const DB *db, DBT *key, DBT *data,
unsigned int flags);
int (*put)(const DB *db, DBT *key, const DBT *data,
unsigned int flags);
int (*sync)(const DB *db, u_int flags);
int (*seq)(const DB *db, DBT *key, DBT *data,
unsigned 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 re-
turned by dbopen(), and sometimes one or more pointers to key/data struc-
tures 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, fail-
ing 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 da-
tabase.
The parameter flags may be set to the following value:
R_CURSOR Delete the record referenced by the cursor.
The cursor must have previously been initial-
ized.
del routines return -1 on error (setting errno), 0 on suc-
cess, 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 re-
trieval 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 er-
ror (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 data-
base.
The parameter flags 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 previously exist.
R_SETCURSOR Store the key/data pair, setting or initial-
izing 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 in-
herent 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 suc-
cess, 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 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 smal-
lest key greater than or equal to the speci-
fied 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 to reference it.
R_LAST The last key/data pair of the database is re-
turned, 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. (Appli-
cable only to the DB_BTREE and DB_RECNO ac-
cess 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 suc-
cess, 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 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 guaran-
tees about byte string alignment.
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] 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 the 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-
bases.
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), rcdb(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, lock-
ing, or transactions.
MirBSD #10-current January 2, 1994 4