# Dbt

`

###

    #include <db_cxx.h>

    class Dbt {
    public:
    	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(void *data, size_t size);
    	Dbt();
    	Dbt(const Dbt &);
    	Dbt &operator = (const Dbt &);
    	~Dbt();

    	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);
    };

# Description

This manual page describes the specific details of the Dbt class, used to encode keys and data items in a database.

### 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 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.

The Dbt class provides simple access to an underlying data structure, whose elements can be examined or changed using the set_ or get_ methods. The remainder of the manual page sometimes refers to these accesses using the underlying name; for example, ulen rather than Dbt::get_ulen and Dbt::set_ulen. Dbt can be subclassed, providing a way to associate with it additional data or references to other structures.

The constructors set all elements of the underlying structure to zero. The constructor with two arguments has the effect of setting all elements to zero except for the data and size elements.

In the case in which 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.

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.

The elements of the structure underlying the Dbt class are defined as follows:

void *data;
  A pointer to a byte string. This element is accessed using Dbt::get_data and Dbt::set_data, and may be initialized using one of the constructors.

u_int32_t size;
  The length of data, in bytes. This element is accessed using Dbt::get_size and Dbt::set_size, and may be initialized using the constructor with two arguments.

u_int32_t ulen;
  The size of the user's buffer (referred to by data), in bytes. This location is not written by the Db methods.

  Note that applications can determine the length of a record by setting the ulen to 0 and checking the return value found in size. See the DB_DBT_USERMEM flag for more information.

  This element is accessed using Dbt::get_ulen and Dbt::set_ulen.

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. This element is accessed using Dbt::get_dlen, and Dbt::set_dlen.

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. This element is accessed using Dbt::get_doff and Dbt::set_doff.

u_int32_t flags;
  This element is accessed using Dbt::get_flags and Dbt::set_flags.

  The flags value must be set to 0 or by bitwise inclusively OR'ing together one or more of the following values:

  DB_DBT_MALLOC
    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 method), and return a pointer to it in the data field of the key or data Dbt object. 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.

  DB_DBT_REALLOC
    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 method), and return a pointer to it in the data field of the key or data Dbt object. 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.

  DB_DBT_USERMEM
    The data field of the key or data object 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 referred to by the data field. Otherwise, the size field is set to the length needed for the requested item, and the error ENOMEM 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 ENOMEM error or throws an exception encapsulating an ENOMEM.

  DB_DBT_PARTIAL
    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 objects. 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 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.

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.

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