DBOPEN(3) BSD Library Functions Manual DBOPEN(3)NAME
dbopen, db — database access methods
SYNOPSIS
#include <sys/types.h>
#include <limits.h>
#include <db.h>
#include <fcntl.h>
DB *
dbopen(const char *file, int flags, mode_t 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).
The dbopen() function opens file for reading and/or writing. Files never
intended to be preserved on disk may 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 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.
The DB Structure
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 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 struc‐
tures 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 underly‐
ing 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 parameter 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 rep‐
resentative 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 descrip‐
tor is not necessarily associated with any of the underly‐
ing 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 suc‐
cess.
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 data‐
base.
The parameter flag may be set to one of the following val‐
ues:
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 struc‐
ture. (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 struc‐
ture. (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 ini‐
tializing 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 suc‐
cess, 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 sequen‐
tial 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, 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:
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, per‐
mitting 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 meth‐
ods.)
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 suc‐
cess 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 guaran‐
tees 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 func‐
tion (for example, use of the cursor without prior
initialization) or there is a mismatch between the
version number of file and the software.
EFBIG The key could not be inserted due to limitations in
the DB file format (e.g., a hash database was out of
overflow pages).
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 data‐
bases.
The sync routines may fail and set errno for any of the errors specified
for the library routine fsync(2).
SEE ALSObtree(3), hash(3), mpool(3), recno(3)
Margo Seltzer and Michael Olson, "LIBTP: Portable, Modular Transactions
for UNIX", Proceedings of the 1992 Winter USENIX Technical Conference,
USENIX Association, 9-25, 1992.
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, lock‐
ing, or transactions.
BSD December 16, 2010 BSD
