NAME
dbopen - database access methods
SYNOPSIS
#include <sys/types.h>
#include <limits.h>
#include <db.h>
#include <fcntl.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 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).
dbopen() opens file for reading and/or writing. Files never intended
to be preserved on disk may be created by setting the file argument 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.
dbopen() 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, unsigned int flags);
int (*fd)(const DB *db);
int (*get)(const DB *db, DBT *key, DBT *data,
unsigned int flags);
int (*put)(const DB *db, DBT *key, const DBT *data,
unsigned int flags);
int (*sync)(const DB *db, unsigned int flags);
int (*seq)(const DB *db, DBT *key, DBT *data,
unsigned 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 argument 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 argument 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, that is,
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.
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).
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.
SEE ALSO
btree(3), hash(3), mpool(3), recno(3)
LIBTP: Portable, Modular Transactions for UNIX, Margo Seltzer, Michael
Olson, USENIX proceedings, Winter 1992.
COLOPHON
This page is part of release 3.24 of the Linux man-pages project. A
description of the project, and information about reporting bugs, can
be found at http://www.kernel.org/doc/man-pages/.