#include <db.h>
typedef struct {
void *app_data;
void *data;
u_int32_t size;
u_int32_t ulen;
u_int32_t dlen;
u_int32_t doff;
u_int32_t flags;
} DBT;
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. (The name DBT is a mnemonic for data base thang, and was used because no one could think of a reasonable name that wasn't already in use somewhere else.) 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.
All fields of the DBT structure that are not explicitly set should be initialized to nul bytes before the first time the structure is used. Do this by declaring the structure external or static, or by calling the C library routine memset(3).
By default, the flags structure element is expected to
be set to 0. In this default case,
when the application is providing Berkeley DB a key or data item to store into the database,
Berkeley DB expects the data
structure element
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
structure element
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.
The elements of the DBT structure are defined as follows:
void *app_data;
Optional field that can be used to pass information through Berkeley DB API calls into user-defined callback functions. For example, this field may be accessed to pass user-defined content when implementing the callback used by DB->set_dup_compare() .
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 (to which data refers), in bytes. This location is not written by the Berkeley DB functions.
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.
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 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.
The difference between
DB_DBT_MALLOC and
DB_DBT_REALLOC
is that the latter will call
realloc(3) instead of
malloc(3),
so the allocated memory will be grown as
necessary instead of the application doing repeated
free/malloc calls.
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.
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 DBcursor->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.