#include <db_cxx.h>
class Dbt {
public:
Dbt(void *data, size_t size);
Dbt();
Dbt(const Dbt &);
Dbt &operator = (const Dbt &);
~Dbt();
void *get_data() const;
void set_data(void *);
u_int32_t get_size() const;
void set_size(u_int32_t);
u_int32_t get_ulen() const;
void set_ulen(u_int32_t);
u_int32_t get_dlen() const;
void set_dlen(u_int32_t);
u_int32_t get_doff() const;
void set_doff(u_int32_t);
u_int32_t get_flags() const;
void set_flags(u_int32_t);
DBT *Dbt::get_DBT();
const DBT *Dbt::get_const_DBT() const;
static Dbt *Dbt::get_Dbt(DBT *dbt);
static const Dbt *Dbt::get_const_Dbt(const DBT *dbt);
};
The Dbt class is used to encode key and data
items in a Berkeley DB database.
Storage and retrieval for the Db access methods are
based on key/data pairs. Both key and data items are represented by
Dbt objects.
Key and data byte strings may refer to strings of zero length up to strings of essentially
unlimited length. See Database limits for
more information.
In the case when the flags structure element is set to
0,
when the application is providing Berkeley DB a key or data item to store into the database,
Berkeley DB expects the data
object
to point to a byte string of size bytes. When returning a
key/data item to the application, Berkeley DB will store into the data
object
a pointer to a byte string of size bytes,
and the memory to which the pointer refers will be
allocated and managed by Berkeley DB. Note that using the default flags
for returned Dbts is only compatible with
single threaded usage of Berkeley DB.
Access to Dbt objects is not re-entrant. In particular, if multiple
threads simultaneously access the same Dbt object using
Db API calls, the results are undefined, and may
result in a crash. One easy way to avoid problems is to use Dbt
objects that are constructed as stack variables.
Each Dbt object has an associated DBT struct,
which is used by the underlying implementation of Berkeley DB and its C-language API. The
Dbt::get_DBT() method returns a pointer to this struct. Given a
const Dbt object, Dbt::get_const_DBT()
returns a const pointer to the same struct.
Given a DBT struct, the Dbt::get_Dbt() method
returns the corresponding Dbt object, if there is one. If the
DBT object was not associated with a Dbt (that is,
it was not returned from a call to Dbt::get_DBT()), then the
result of Dbt::get_Dbt() is undefined. Given a const
DBT struct, Dbt::get_const_Dbt() returns the
associated const Dbt object, if there is one.
These methods may be useful for Berkeley DB applications including both C and C++ language software. It should not be necessary to use these calls in a purely C++ application.
Dbt::set_data(void *data)
Set the data array.
The data parameter is an array
of bytes to be used to set the content for the
Dbt.
Dbt::get_data()
Return the data array.
Dbt::set_size(u_int32_t size)
Sets the byte size of the data array, in bytes.
Dbt::get_size()
Return the data array size.
Dbt::set_ulen(u_int32_t value)
Set the byte size of the user-specified buffer.
Note that applications can determine the length of a record by setting
the ulen field to 0 and checking the return value in the
size field. See
the DB_DBT_USERMEM flag for more information.
Dbt::get_ulen()
Return the length in bytes of the user-specified buffer.
Note that applications can determine the length of a record by setting
the ulen field to 0 and checking the return value in the
size field. See
the DB_DBT_USERMEM flag for more information.
Dbt::set_dlen(u_int32_t dlen)
Set the
length of the partial record being read or written by the
application, in bytes. See the DB_DBT_PARTIAL
flag for more information.
Dbt::get_dlen()
Return the length of the partial record, in bytes.
Dbt::set_doff(u_int32_t value)
Sets the
offset of the partial record being read or written by the application,
in bytes. See the DB_DBT_PARTIAL flag for more information.
Dbt::get_doff()
Return the offset of the partial record, in bytes.
Dbt::set_flags(u_int32_t flags)
Set the object flag value.
The flags parameter must be set to 0 or by bitwise inclusively OR'ing together one or more of the following values:
Set this flag on a Dbt used for the data portion of a record to indicate that the Dbt stores external file data. If this flag is set, and if the database otherwise supports external files, then the data contained by this Dbt will be stored as an external file, regardless of whether it exceeds the external file threshold in size.
When this flag is set, Berkeley DB will 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. Because any allocated memory becomes the
responsibility of the calling application, the caller
must determine whether memory was allocated using the
returned value of the
data field.
It is an error to specify more than one of
DB_DBT_MALLOC,
DB_DBT_REALLOC, and
DB_DBT_USERMEM.
When this flag is set Berkeley DB will allocate memory for the returned key or data item (using realloc(3), or the user-specified realloc function), and return a pointer to it in the data field of the key or data DBT structure. Because any allocated memory becomes the responsibility of the calling application, the caller must determine whether memory was allocated using the returned value of the data field.
It is an error to specify more than one of
DB_DBT_MALLOC,
DB_DBT_REALLOC, and
DB_DBT_USERMEM.
The data
field of the key or data structure must refer
to 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
to which the
data
field refers. Otherwise, the
size
field is set to the length needed for the requested
item, and the error
DB_BUFFER_SMALL is returned.
It is an error to specify more than one of
DB_DBT_MALLOC,
DB_DBT_REALLOC, and
DB_DBT_USERMEM.
If DB_DBT_MALLOC or
DB_DBT_REALLOC is specified, Berkeley DB
allocates a properly sized byte array to contain the data. This
can be convenient if you know little about the nature of the
data, specifically the size of data in the database. However, if
your application makes repeated calls to retrieve keys or data,
you may notice increased garbage collection due to this
allocation. If you know the maximum size of data you are
retrieving, you might decrease the memory burden and speed your
application by allocating your own byte array and using
DB_DBT_USERMEM. Even if you don't know the
maximum size, you can use this option and reallocate your array
whenever your retrieval API call returns an
DB_BUFFER_SMALL error or throws an exception
encapsulating an DB_BUFFER_SMALL.
Do 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 any existing 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 refer to 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; 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() method in a database that supports duplicate records. Partial puts in databases supporting duplicate records must be done using a Dbc::put() method.
It is an error to attempt a partial put with differing dlen and size values in Queue or Recno databases 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.
This flag is ignored when used with the
pkey parameter on
DB->pget() or
DBcursor->pget().
After an application-supplied callback routine passed to
either
Db::associate()
or
Db::set_append_recno()
is executed, the
data
field of a DBT may refer to memory allocated with
malloc(3)
or
realloc(3).
In that case,
the callback sets the
DB_DBT_APPMALLOC
flag in the DBT
so that Berkeley DB will call
free(3)
to deallocate the
memory when it is no longer required.
Set in a secondary key creation callback routine passed to Db::associate() to indicate that multiple secondary keys should be associated with the given primary key/data pair. If set, the size field indicates the number of secondary keys and the data field refers to an array of that number of DBT structures.
The DB_DBT_APPMALLOC flag may be set on any of the DBT
structures to indicate that their
data
field needs to be
freed.
When this flag is set Berkeley DB will not write into the DBT. This may be set on key values in cases where the key is a static string that cannot be written and Berkeley DB might try to update it because the application has set a user defined comparison function.