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dbopen(3)
NAME
dbopen - database access methods
SYNOPSIS
#include <sys/types.h>
#include <limits.h>
#include <db.h>
DB *
dbopen(
const char *file,
int option,
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 can be created by setting the file
parameter to NULL.
The option 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 options 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 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 option);
int (*fd)(const DB *db);
int (*get)(const DB *db, DBT *key, DBT *data, u_int option);
int (*put)(const DB *db, DBT *key, const DBT *data, u_int option);
int (*sync)(const DB *db, u_int option);
int (*seq)(const DB *db, DBT *key, DBT *data, u_int option);
} DB;
These elements describe a database type and a set of functions performing
various actions. The functions take a pointer to a structure as returned by
dbopen(), and sometimes one or more pointers to key/data structures and a
option 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 inconsistent or lost information.
The 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 option can be set to the following value:
R_CURSOR
Delete the record referenced by the cursor. The cursor must
have previously been initialized.
The del 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 that returns a file descriptor representative of
the underlying database. A file descriptor referencing 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.
The fd routines return -1 on error (setting errno), and the file
descriptor 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 associated with the
specified key are returned in the structure referenced by data. The 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 option 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 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 did 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 that 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 (for example, record numbers).
The default behavior of the put routines is to enter the new
key/data pair, replacing any previously existing key.
The put routines return -1 on error (setting errno), 0 on
success, and 1 if the R_NOOVERWRITE option was set and the key
already exists in the file.
seq A pointer to a routine that 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 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 option 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 option.
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
option. (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.
The 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 currently 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 effect and will
always succeed.
The option value can be set to the following value:
R_RECNOSYNC
If the DB_RECNO access method is being used, this option 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.)
The 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 can reference strings of essentially unlimited
length, although any two of them must be able to fit into available memory
at the same time. It should be noted that the access methods provide no
guarantees about byte string alignment.
RESTRICTIONS
The typedef DBT is a mnemonic for ``data base thing'', which was used
because no one could think of a reasonable 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.
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 for any of the following
errors:
[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 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),
ormalloc(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)
LIBTP: Portable, Modular Transactions for UNIX, Margo Seltzer, Michael
Olson, USENIX proceedings, Winter 1992.
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Index for
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Alphabetical
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