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Command: dbopen | Section: 3 | Source: Digital UNIX | File: dbopen.3.gz
dbopen(3) Library Functions Manual dbopen(3)
NAME
dbopen - database access methods
SYNOPSIS
#include <sys/types.h>
#include <limits.h>
#include <db.h>
DB *
dbopen(const char *file, int flags, int 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 for-
mat 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 can 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 can be set to DB_BTREE, DB_HASH, or DB_RECNO.
The openinfo argument is a pointer to an access method specific struc-
ture 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 con-
tains 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 perform-
ing 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).
Because key/data pairs can be cached in memory, failing to sync
the file with a close or sync function may result in inconsis-
tent 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 parameter flag can 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 suc-
cess, and 1 if the specified key was not in the file.
fd A pointer to a routine that returns a file descriptor represen-
tative of the underlying database. A file descriptor referenc-
ing the same file will be returned to all processes that call
dbopen with the same file name. This file descriptor can 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 descrip-
tor on success.
get A pointer to a routine that is the interface for keyed retrieval
from the database. The address and length of the data associ-
ated with the specified key are returned in the structure refer-
enced 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 can be set to one of the following values:
R_CURSOR
Replace the key/data pair referenced by the cursor. The
cursor must have been previously 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 did not pre-
viously exist.
R_SETCURSOR
Store the key/data pair, setting or initializing the po-
sition 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 ac-
cess methods because it implies that the keys have an inherent
order that does not change.
R_IAFTER and R_IBEFORE are available only for the DB_RECNO ac-
cess method because they each imply that the access method is
able to create new keys. This is only true if the keys are or-
dered and independent (for example, record numbers).
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 ex-
ists in the file.
seq A pointer to a routine that is the interface for sequential re-
trieval 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 can 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, that is,
records inserted behind the cursor will not be returned and
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 re-
turned 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. (Ap-
plicable only to the DB_BTREE and DB_RECNO access meth-
ods.)
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 that 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. The seq routines return 2 if the
DB_RECNO access method is being used, the database file is a
character special file, and no complete key/data pairs are cur-
rently available,
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 ef-
fect and will always succeed.
The flag value can 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 that
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 can reference strings of essentially unlim-
ited length, although any two of them must be able to fit into avail-
able memory at the same time. It should be noted that the access meth-
ods provide no guarantees about byte string alignment.
RESTRICTIONS
The typedef DBT is a mnemonic for ``data base thang'', and was used be-
cause no one could think of a reasonable name that wasn't already used.
The file descriptor interface is a kluge and will be deleted in a fu-
ture version of the interface.
None of the access methods provide any form of concurrent access, lock-
ing, or transactions.
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]
A parameter had been specified (hash function, pad byte, etc.)
that is incompatible with the current file specification or not
meaningful for the function (for example, use of the cursor
without prior initialization), or a mismatch exists between the
version number of the file and the software.
The close routines may fail and set errno for any of the errors speci-
fied 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 speci-
fied for the library routine fsync(2).
RELATED INFORMATION
btree(3), hash(3), mpool(3), recno(3)
LIBTP: Portable, Modular Transactions for UNIX, Margo Seltzer, Michael
Olson, USENIX proceedings, Winter 1992. delim off
dbopen(3)