NAME
dbopen,
db —
database
access methods
SYNOPSIS
#include <sys/types.h>
#include <limits.h>
#include <db.h>
#include <fcntl.h>
DB *
dbopen(
const char
*file,
int flags,
mode_t mode,
DBTYPE type,
const void *openinfo);
DESCRIPTION
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).
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 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.
The DB Structure
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 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.
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 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
- 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.
-
-
EFBIG
- The key could not be inserted due to limitations in the DB
file format (e.g., a hash database was out of overflow pages).
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, Proceedings of the 1992
Winter USENIX Technical Conference, USENIX Association,
9-25, 1992.
BUGS
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, locking, or
transactions.