dbopen - database access methods
#include <sys/types.h> #include <limits.h> #include <db.h>
DB *
dbopen(
const char *file,
int option,
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).
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. The type of the underlying access method
(and file format). 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. A pointer to a routine to
remove key/data pairs from the database.
The parameter option can be set to the following
value: 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. 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.
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. 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: Replace the key/data pair referenced
by the cursor. The cursor must have been previously
initialized. 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.) 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.) Enter the new key/data pair only if the
key did not previously exist. 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. 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: 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.) The first
key/data pair of the database is returned, and the
cursor is set or initialized to reference it. 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.) 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.
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, 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:
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:
A pointer to a byte string. 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.
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.
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: A file is
incorrectly formatted. 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 inmemory
databases.
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), recno(3)
LIBTP: Portable, Modular Transactions for UNIX, Margo
Seltzer, Michael Olson, USENIX proceedings, Winter 1992.
dbopen(3)
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