db_dbt
typedef struct {
void *data;
u_int32_t size;
u_int32_t ulen;
u_int32_t dlen;
u_int32_t doff;
u_int32_t flags;
} DBT;
KEY/DATA PAIRS
Storage and retrieval for the DB access methods are based
on key/data pairs. Both key and data items are
represented by the DBT data structure.
Key and data byte strings may reference strings of
essentially unlimited length, although any two keys must
fit into available memory at the same time so that they
may be compared, and any one data item must fit into
available memory so that it may be returned.
In order to ensure compatibility with future releases of
DB, all fields of the DBT structure that are not
explicitly set should be initialized to 0 before the first
time the structure is used. Do this by declaring the
structure external or static, or by calling the C library
routine bzero(3) or memset(3).
By default, the flags structure element is expected to be
0. In this default case, when being provided a key or
data item by the application, the DB package expects the
data structure element to point to a byte string of size
bytes. When returning a key/data item to the application,
the DB package will store into the data structure element
a pointer to a byte string of size bytes. By default, the
memory referenced by this stored pointer is only valid
until the next call to the DB package using the DB handle
returned by db_open.
The access methods provide no guarantees about byte string
alignment, and applications are responsible for
maintaining any necessary alignment. Use the
DB_DBT_USERMEM flag to cause returned items to be placed
in memory of arbitrary alignment.
The elements of the DBT structure are defined as follows:
void *data;
A pointer to a byte string.
u_int32_t size;
The length of data, in bytes.
u_int32_t ulen;
The size of the user's buffer (referenced by data),
in bytes. This location is not written by the DB
functions. See the DB_DBT_USERMEM flag for more
information.
u_int32_t dlen;
The length of the partial record being read or
written by the application, in bytes. See the
DB_DBT_PARTIAL flag for more information.
u_int32_t doff;
The offset of the partial record being read or
written by the application, in bytes. See the
DB_DBT_PARTIAL flag for more information.
u_int32_t flags;
The flags value is specified by or'ing together one
or more of the following values:
DB_DBT_MALLOC
Ignored except when retrieving information from
a database, e.g., a DB->get or DBcursor->c_get
call. This flag causes DB to allocate memory
for the returned key or data item (using
malloc(3), or the user-specified malloc
function) and return a pointer to it in the data
field of the key or data DBT structure. The
allocated memory becomes the responsibility of
the calling application. It is an error to
specify both DB_DBT_MALLOC and DB_DBT_USERMEM.
DB_DBT_USERMEM
Ignored except when retrieving information from
a database, e.g., a DB->get or DBcursor->c_get
call. The data field of the key or data
structure must reference memory that is at least
ulen bytes in length. If the length of the
requested item is less than or equal to that
number of bytes, the item is copied into the
memory referenced by the data field. Otherwise,
an error is returned, the size field is set to
the length needed for the requested item, and
the errno variable is set to ENOMEM. It is an
error to specify both DB_DBT_MALLOC and
DB_DBT_USERMEM.
DB_DBT_PARTIAL
Ignored except when specified for a data
parameter, where this flag causes the partial
retrieval or storage of an item. If the calling
application is doing a get, the dlen bytes
starting doff bytes from the beginning of the
retrieved data record are returned as if they
comprised the entire record. If any or all of
the specified bytes do not exist in the record,
the get is successful and the existing bytes or
0 bytes are returned.
For example, if the data portion of a retrieved
record was 100 bytes, and a partial retrieval
was done using a DBT having a dlen field of 20
and a doff field of 85, the get call would
succeed, the data field would reference the last
15 bytes of the record, and the size field would
be set to 15.
If the calling application is doing a put, the
dlen bytes starting doff bytes from the
beginning of the specified key's data record are
replaced by the data specified by the data and
size structure elements. If dlen is smaller
than size, the record will grow, and if dlen is
larger than size, the record will shrink. If
the specified bytes do not exist, the record
will be extended using nul bytes as necessary,
and the put call will succeed.
It is an error to attempt a partial put using
the DB->put function in a database that supports
duplicate records. Partial puts in databases
supporting duplicate records must be done using
a db_cursor(3) function. It is an error to
attempt a partial put with differing dlen and
size values in a recno database with fixed-
length records.
For example, if the data portion of a retrieved
record was 100 bytes, and a partial put was done
using a DBT having a dlen field of 20, a doff
field of 85, and a size field of 30, the
resulting record would be 115 bytes in length,
where the last 30 bytes would be those specified
by the put call.
The default algorithm of associating returned key or data
items with the DB handle returned by db_open(3) will
obviously not work when DB handles are being used
concurrently by multiple threads within a process, i.e,
when DB_THREAD was specified to db_open(3). When multiple
threads are using the returned DB handle concurrently,
either the DB_DBT_MALLOC or DB_DBT_USERMEM flags must be
specified for any DBT used for key or data retrieval.
LOGICAL RECORD NUMBERS
In all cases for the recno access method, and when calling
the db->get and cursor->c_get functions with the
DB_SET_RECNO flag specified, the data field of the key
must be a pointer to a memory location of type db_recno_t,
as typedef'd in the <db.h> include file. This type is a
32-bit unsigned type, (which limits the number of logical
records in a recno database, and the maximum logical
record which may be directly retrieved from a btree
database, to 4,294,967,296). The size field of the key
should be the size of that type, e.g., in the C
programming language, ``sizeof(db_recno_t)''.
Logical record numbers are 1-based, not 0-based, i.e., the
first record in the database is record number 1.
BUGS
The DB access methods provide no guarantees about byte
string alignment, and applications are responsible for
maintaining any necessary alignment.
The name DBT is a mnemonic for ``data base thang'', and
was used because noone could think of a reasonable name
that wasn't already in use somewhere else.
SEE ALSO
The DB library is a family of groups of functions that
provides a modular programming interface to transactions
and record-oriented file access. The library includes
support for transactions, locking, logging and file page
caching, as well as various indexed access methods. Many
of the functional groups (e.g., the file page caching
functions) are useful independent of the other DB
functions, although some functional groups are explicitly
based on other functional groups (e.g., transactions and
logging). For a general description of the DB package,
see db_intro(3).
db_archive(1), db_checkpoint(1), db_deadlock(1), db_dump(1),
db_load(1), db_recover(1), db_stat(1), db_intro(3),
db_appinit(3), db_cursor(3), db_dbm(3), db_internal(3),
db_lock(3), db_log(3), db_mpool(3), db_open(3), db_thread(3),
db_txn(3)
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