.\" Copyright (c) 1990 The Regents of the University of California.
.\" %sccs.include.redist.man%
.\" @(#)dbopen.3 5.17 (Berkeley) %G%
dbopen \- database access methods
dbopen(const char *file, int flags, int mode, DBTYPE type,
is the library interface to database files.
The supported file formats are btree, hashed and record 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
The formats and file format specific information are described in detail
in their respective manual pages
for reading and/or writing.
Files never intended to be preserved on disk may be created by setting
the file parameter to NULL.
routine, however, only the O_CREAT, O_EXCL, O_RDONLY, O_RDWR, O_TRUNC
and O_WRONLY flags are meaningful.
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.
argument is a pointer to an access method specific structure described
in the access method's manual page.
is NULL, each access method will use defaults appropriate for the system
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:
int (*close)(const DB *db);
int (*del)(const DB *db, const DBT *key, u_int flags);
int (*get)(const DB *db, DBT *key, DBT *data, u_int flags);
int (*put)(const DB *db, const DBT *key, const DBT *data,
int (*sync)(const DB *db);
int (*seq)(const DB *db, DBT *key, DBT *data, u_int flags);
These elements a database type and a set of functions performing various
These functions take a pointer to a structure as returned by
and sometimes one or more pointers to key/data structures and a flag 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).
Since key/data pairs may be cached in memory, failing to sync the file
function may result in inconsistent or lost information.
routines return -1 on error (setting
A pointer to a routine to remove key/data pairs from the database.
may be set to the following value:
Delete the record referenced by the cursor.
routines return -1 on error (setting
0 on success, and 1 if the specified
A pointer to a routine which is the interface for keyed retrieval from
The address and length of the data associated with the specified
are returned in the structure referenced by
routines return -1 on error (setting
0 on success, and 1 if the
A pointer to a routine to store key/data pairs in the database.
must be set to one of the following values:
Append the data immediately after the data referenced by
creating a new key/data pair.
(This implies that the access method is able to create new keys,
i.e. the keys are ordered and independent, for example, record numbers.
Applicable only to the DB_RECNO access method.)
Insert the data immediately before the data referenced by
creating a new key/data pair.
(This implies that the access method is able to create new keys,
i.e. the keys are ordered and independent, for example, record numbers.
Applicable only to the DB_RECNO access method.)
Enter the new key/data pair only if the key does not previously exist.
The default behavior of the
routines is to enter the new key/data pair, replacing any previously
routines return -1 on error (setting
0 on success, and 1 if the R_NOOVERWRITE
was set and the key already exists in the file.
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
and the address and length of the data are returned in the
Sequential key/data pair retrieval may begin at any time, and the
position of the ``cursor'' is not affected by calls to the
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:
The data associated with the specified key is returned.
routines in that it sets the ``cursor'' to the location of the
(This implies that the access method has a implicit order which does
Applicable only to the DB_BTREE and DB_RECNO access methods.)
The first key/data pair of the database is returned.
The last key/data pair of the database is returned.
(This implies that the access method has a implicit order which does
Applicable only to the DB_BTREE and DB_RECNO access methods.)
Retrieve the key/data pair immediately after the key/data pair most recently
The cursor is moved to the returned key/data pair.
is set to R_NEXT the first time the
routine is called, the first key/data pair of the database is returned.
Retrieve the key/data pair immediately before the key/data pair most recently
The cursor is moved to the returned key/data pair.
is set to R_PREV the first time the
routine is called, the last key/data pair of the database is returned.
(This implies that the access method has a implicit order which does
Applicable only to the DB_BTREE and DB_RECNO access methods.)
routines return -1 on error (setting
0 on success, 1 if there are no more key/data pairs available.
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
A pointer to a routine to flush any cached information to disk.
If the database is in memory only, the
routine has no effect and will always succeed.
routines return -1 on error (setting
Access to all file types is based on key/data pairs.
Both keys and data are represented by the following data structure:
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 may reference strings of essentially unlimited
length although any two of them must fit into available memory at the same
It should be noted that the access methods provide no guarantees about
for any of the errors specified for the library routines
A file is incorrectly formatted.
A parameter has been specified (hash function, pad byte etc.) that is
incompatible with the current file specification or there is a mismatch
between the version number of file and the software.
routines may fail and set
for any of the errors specified for the library routines
routines may fail and set
for any of the errors specified for the library routines
routines may fail and set
for any of the errors specified for the library routine
The typedef DBT is a mnemonic for ``data base thang'', and was used
because noone could think of a reasonable name that wasn't already used.
None of the access methods provide any form of concurrent access,
locking, or transactions.