docs/api_cxx/dbt_class.html

<!--$Id: dbt_class.so,v 10.1 2002/08/24 18:22:30 bostic Exp $-->
<!--$Id: dbt_cxx.so,v 10.72 2007/02/27 00:41:24 mjc Exp $-->
<!--Copyright (c) 1997,2008 Oracle.  All rights reserved.-->
<!--See the file LICENSE for redistribution information.-->
<html>
<head>
<title>Berkeley DB: Dbt</title>
<meta name="description" content="Berkeley DB: An embedded database programmatic toolkit.">
<meta name="keywords" content="embedded,database,programmatic,toolkit,btree,hash,hashing,transaction,transactions,locking,logging,access method,access methods,Java,C,C++">
</head>
<body bgcolor=white>
<table width="100%"><tr valign=top>
<td>
<b>Dbt</b>
</td>
<td align=right>
<a href="../api_cxx/api_core.html"><img src="../images/api.gif" alt="API"></a>
<a href="../ref/toc.html"><img src="../images/ref.gif" alt="Ref"></a></td>
</tr></table>
<hr size=1 noshade>
<tt>
<b><pre>
#include &lt;db_cxx.h&gt;
<p>
class Dbt {
public:
	Dbt(void *data, size_t size);
	Dbt();
	Dbt(const Dbt &);
	Dbt &operator = (const Dbt &);
	~Dbt();
<p>
	void *get_data() const;
	void set_data(void *);
<p>
	u_int32_t get_size() const;
	void set_size(u_int32_t);
<p>
	u_int32_t get_ulen() const;
	void set_ulen(u_int32_t);
<p>
	u_int32_t get_dlen() const;
	void set_dlen(u_int32_t);
<p>
	u_int32_t get_doff() const;
	void set_doff(u_int32_t);
<p>
	u_int32_t get_flags() const;
	void set_flags(u_int32_t);
<p>
	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);
};
</pre></b>
<hr size=1 noshade>
<b>Description: Dbt</b>
<p>This information describes the specific details of the Dbt class,
used to encode keys and data items in a database.</p>
<a name="2"><!--meow--></a>
<b>Key/Data Pairs</b>
<p>Storage and retrieval for the <a href="../api_cxx/db_class.html">Db</a> 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
<a href="../ref/am_misc/dbsizes.html">Database limits</a> for more
information.</p>
<p>The Dbt class provides simple access to an underlying data
structure, whose elements can be examined or changed using the usual
<b>set</b> or <b>get</b> methods.  Dbt can be subclassed,
providing a way to associate with it additional data or references to
other structures.</p>
<p>The constructors set all elements of the underlying structure to zero.
The constructor with two parameters has the effect of setting all elements
to zero except for the <b>data</b> and <b>size</b> elements.</p>
<p>In the case in which the <b>flags</b> 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 <b>data</b> object to point to a byte
string of <b>size</b> bytes.  When returning a key/data item to the
application, Berkeley DB will store into the <b>data</b> object a pointer to
a byte string of <b>size</b> bytes, and the memory to which the pointer
refers will be allocated and managed by Berkeley DB.</p>
<p>Access to Dbt objects is not re-entrant.  In particular, if
multiple threads simultaneously access the same Dbt object using
<a href="../api_cxx/db_class.html">Db</a> 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.</p>
<p>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.</p>
<p>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.</p>
<p>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.</p>
<hr size=1 noshade>
<b>Description: Dbt::set_data</b>
<p>Set the data array.</p>
<b>Parameters</b> <br>
 <b>data</b><ul compact><li>The <b>data</b> parameter is an array of bytes to be used to set the
content for the Dbt.</ul>
<br>
<hr size=1 noshade>
<b>Description: Dbt::get_data</b>
<p>Return the data array.</p>
<hr size=1 noshade>
<b>Description: Dbt::set_recno_key_data</b>
<p>Initialize the data array from a logical record number.  Recno database
records are ordered by integer keys starting at 1.  When the
Dbt::set_recno_key_data method is called, the data, size and offset
fields in the Dbt are implicitly set to hold a byte array
representation of the integer key.</p>
<b>Parameters</b> <br>
 <b>recno</b><ul compact><li>The <b>recno</b> parameter logical record number used to initialize the
data array.</ul>
<br>
<hr size=1 noshade>
<b>Description: Dbt::get_recno_key_data</b>
<p>Return an object from the data array, expecting that data to be a
logical record number.</p>
<hr size=1 noshade>
<b>Description: Dbt::set_offset</b>
<p>Set the byte offset into the data array.</p>
<p>The number of bytes offset into the <b>data</b> array determine the
portion of the array actually used.  This element is accessed using
Dbt::get_offset and Dbt::set_offset.</p>
<b>Parameters</b> <br>
 <b>offset</b><ul compact><li>The <b>offset</b> parameter is the byte offset into the data array.</ul>
<br>
<hr size=1 noshade>
<b>Description: Dbt::get_offset</b>
<p>Return the byte offset into the data array.</p>
<hr size=1 noshade>
<b>Description: Dbt::set_size</b>
<p>Set the byte size of the data array.</p>
<b>Parameters</b> <br>
 <b>size</b><ul compact><li>The <b>size</b> parameter is the size of the data array in bytes.</ul>
<br>
<hr size=1 noshade>
<b>Description: Dbt::get_size</b>
<p>Return the data array size.</p>
<hr size=1 noshade>
<b>Description: Dbt::set_ulen</b>
<p>Set the byte size of the user-specified buffer.</p>
<p>Note that applications can determine the length of a record by setting
the <b>ulen</b> to 0 and checking the return value found in <b>size</b>.
See the DB_DBT_USERMEM flag for more information.</p>
<b>Parameters</b> <br>
 <b>ulen</b><ul compact><li>The <b>ulen</b> parameter the size of the data array in bytes.</ul>
<br>
<hr size=1 noshade>
<b>Description: Dbt::get_ulen</b>
<p>Return the length in bytes of the user-specified buffer.</p>
<hr size=1 noshade>
<b>Description: Dbt::set_dlen</b>
<p>Set the byte length of the partial record being read or written by the
application, in bytes.  See the DB_DBT_PARTIAL flag for more
information.</p>
<b>Parameters</b> <br>
 <b>dlen</b><ul compact><li>The <b>dlen</b> parameter is the length of the partial record in bytes.</ul>
<br>
<hr size=1 noshade>
<b>Description: Dbt::get_dlen</b>
<p>Return the length of the partial record, in bytes.</p>
<hr size=1 noshade>
<b>Description: Dbt::set_doff</b>
<p>Set the offset of the partial record being read or written by the
application, in bytes.  See the DB_DBT_PARTIAL flag for more
information.</p>
<b>Parameters</b> <br>
 <b>doff</b><ul compact><li>The <b>doff</b> parameter is the offset of the partial record.</ul>
<br>
<hr size=1 noshade>
<b>Description: Dbt::get_doff</b>
<p>Return the offset of the partial record, in bytes.</p>
<hr size=1 noshade>
<b>Description: Dbt::set_flags</b>
<p>Set the object flag value.</p>
<b>Parameters</b> <br>
 <b>flags</b><ul compact><li>The <b>flags</b> parameter is Dbt flag value.</ul>
The <b>flags</b> parameter must be set to 0 or by bitwise inclusively <b>OR</b>'ing together one
or more of the following values:
<br>
<b><a name="DB_DBT_MALLOC">DB_DBT_MALLOC</a></b><ul compact><li>When this flag is set, Berkeley DB will allocate memory for the returned key
or data item (using <b>malloc</b>(3) or the user-specified malloc
method), and return a pointer to it in the <b>data</b> 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
<b>data</b> field.
<p>It is an error to specify more than one of DB_DBT_MALLOC,
DB_DBT_REALLOC, and DB_DBT_USERMEM.</p></ul>
<b><a name="DB_DBT_REALLOC">DB_DBT_REALLOC</a></b><ul compact><li>When this flag is set Berkeley DB will allocate memory for the returned key
or data item (using <b>realloc</b>(3) or the user-specified realloc
method), and return a pointer to it in the <b>data</b> 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
<b>data</b> field.
<p>It is an error to specify more than one of DB_DBT_MALLOC,
DB_DBT_REALLOC, and DB_DBT_USERMEM.</p></ul>
<a name="3"><!--meow--></a>
<b><a name="DB_DBT_USERMEM">DB_DBT_USERMEM</a></b><ul compact><li>The <b>data</b> field of the key or data object must refer to memory
that is at least <b>ulen</b> 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 <b>data</b> field.
Otherwise, the <b>size</b> fields of both the key and data Dbt
objects are set to the length needed for the requested item, and the
error DB_BUFFER_SMALL is returned.
<p>It is an error to specify more than one of DB_DBT_MALLOC,
DB_DBT_REALLOC, and DB_DBT_USERMEM.</p></ul>
<br>
<p>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.</p>
<br>
<b><a name="DB_DBT_PARTIAL">DB_DBT_PARTIAL</a></b><ul compact><li>Do partial retrieval or storage of an item.  If the calling application
is doing a get, the <b>dlen</b> bytes starting <b>doff</b> 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.
<p>For example, if the data portion of a retrieved record was 100 bytes,
and a partial retrieval was done using a Dbt having a <b>dlen</b>
field of 20 and a <b>doff</b> field of 85, the get call would succeed,
the <b>data</b> field would refer to the last 15 bytes of the record,
and the <b>size</b> field would be set to 15.</p>
<p>If the calling application is doing a put, the <b>dlen</b> bytes starting
<b>doff</b> bytes from the beginning of the specified key's data record
are replaced by the data specified by the <b>data</b> and <b>size</b>
objects.
If <b>dlen</b> is smaller than <b>size</b>, the record will grow; if
<b>dlen</b> is larger than <b>size</b>, 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.</p>
<p>It is an error to attempt a partial put using the <a href="../api_cxx/db_put.html">Db::put</a>
method in a database that supports duplicate records.
Partial puts in databases supporting duplicate records must be done
using a <a href="../api_cxx/dbc_class.html">Dbc</a> method.</p>
<p>It is an error to attempt a partial put with differing <b>dlen</b> and
<b>size</b> values in Queue or Recno databases with fixed-length records.</p>
<p>For example, if the data portion of a retrieved record was 100 bytes,
and a partial put was done using a Dbt having a <b>dlen</b>
field of 20, a <b>doff</b> field of 85, and a <b>size</b> 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.</p></ul>
<b><a name="DB_DBT_APPMALLOC">DB_DBT_APPMALLOC</a></b><ul compact><li>After an application-supplied callback routine passed to either
<a href="../api_cxx/db_associate.html">Db::associate</a> or <a href="../api_cxx/db_set_append_recno.html">Db::set_append_recno</a> is executed, the
<b>data</b> field of a Dbt may refer to memory allocated with
<b>malloc</b>(3) or <b>realloc</b>(3).  In that case, the
callback sets the DB_DBT_APPMALLOC flag in the Dbt so
that Berkeley DB will call <b>free</b>(3) to deallocate the memory when it
is no longer required.</ul>
<b><a name="DB_DBT_MULTIPLE">DB_DBT_MULTIPLE</a></b><ul compact><li>Set in a secondary key creation callback routine passed to
<a href="../api_cxx/db_associate.html">Db::associate</a> to indicate that multiple secondary keys should be
associated with the given primary key/data pair.  If set, the
<b>size</b> field indicates the number of secondary keys and the
<b>data</b> field refers to an array of that number of Dbt
structures.
<p>The DB_DBT_APPMALLOC flag may be set on any of the Dbt
structures to indicate that their <b>data</b> field needs to be
freed.</p></ul>
<br>
<br>
<hr size=1 noshade>
<b>Description: Dbt::get_flags</b>
<p>Return the object flag value.</p>
</tt>
<table width="100%"><tr><td><br></td><td align=right>
<a href="../api_cxx/api_core.html"><img src="../images/api.gif" alt="API"></a><a href="../ref/toc.html"><img src="../images/ref.gif" alt="Ref"></a>
</td></tr></table>
<p><font size=1>Copyright (c) 1996,2008 Oracle.  All rights reserved.</font>
</body>
</html>