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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 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 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 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, 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 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). 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. 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 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 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. 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. 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 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.
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 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 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 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 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 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 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 thang'', and was used because
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 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 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 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).
RELATED INFORMATION
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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Top of
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