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<h2 class="title"><a id="dbt"></a>Chapter 4.
The DBT Handle
</h2>
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<pre class="programlisting">#include <db.h>
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;
</pre>
<p>
Storage and retrieval for the <a class="link" href="db.html" title="Chapter 2. The DB Handle">DB</a> access methods are
based on key/data pairs. Both key and data items are represented by
<span>
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.)
</span>
Key and data byte strings may refer to strings of zero length up to strings of essentially
unlimited length. See <a href="../../programmer_reference/am_misc_dbsizes.html" class="olink">Database limits</a> for
more information.
</p>
<p>
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
<span class="bold"><strong>memset</strong></span>(3).
</p>
<p>
<span>
By default, the <span class="bold"><strong>flags</strong></span> structure element is expected to
be set to 0. In this default case,
</span>
when the application is providing Berkeley DB a key or data item to store into the database,
Berkeley DB expects the <span class="bold"><strong>data</strong></span>
<span>
structure element
</span>
to point to a byte string of <span class="bold"><strong>size</strong></span> bytes. When returning a
key/data item to the application, Berkeley DB will store into the <span class="bold"><strong>data</strong></span>
<span>
structure element
</span>
a pointer to a byte string of <span class="bold"><strong>size</strong></span> 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 <code class="classname">DBT</code>s is only compatible with
single threaded usage of Berkeley DB.
</p>
<p>
The elements of the DBT structure are defined as follows:
</p>
<div class="itemizedlist">
<ul type="disc">
<li>
<p>
<span class="bold"><strong>void *data;</strong></span>
</p>
<p>
A pointer to a byte string.
</p>
</li>
<li>
<p>
<span class="bold"><strong>u_int32_t size;</strong></span>
</p>
<p>
The length of <span class="bold"><strong>data</strong></span>, in bytes.
</p>
</li>
<li>
<p>
<span class="bold"><strong>u_int32_t ulen;</strong></span>
</p>
<p>
The size of the user's buffer (to which <span class="bold"><strong>data</strong></span> refers), in bytes. This
location is not written by the Berkeley DB functions.
</p>
<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 <code class="literal">ulen</code> field to 0 and checking the return value in the
<span class="bold"><strong>size</strong></span> field. See
the <code class="literal">DB_DBT_USERMEM</code> flag for more information.
</p>
</li>
<li>
<p>
<span class="bold"><strong>u_int32_t dlen;</strong></span>
</p>
<p>
<span>
The
</span>
length of the partial record being read or written by the
application, in bytes. See the <code class="literal">DB_DBT_PARTIAL</code>
flag for more information.
</p>
</li>
<li>
<p>
<span class="bold"><strong>u_int32_t doff;</strong></span>
</p>
<p>
<span>
The
</span>
offset of the partial record being read or written by the application,
in bytes. See the <code class="literal">DB_DBT_PARTIAL</code> flag for more information.
</p>
</li>
<li>
<p>
<span class="bold"><strong>u_int32_t flags;</strong></span>
</p>
<p>
The <span class="bold"><strong>flags</strong></span> parameter must be set to 0 or
by bitwise inclusively <span class="bold"><strong>OR</strong></span>'ing together
one or more of the following values:
</p>
<span>
<div class="itemizedlist"><ul type="circle"><li><p><a id="dbt_DB_DBT_MALLOC"></a>
<code class="literal">DB_DBT_MALLOC</code>
</p><p>
When this flag is set, Berkeley DB will allocate memory
for the returned key or data item (using
<span class="bold"><strong>malloc</strong></span>(3), or
the user-specified malloc function), and return a
pointer to it in the
<span class="bold"><strong>data</strong></span> field of the key or data
<code class="literal">DBT</code>
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
<span class="bold"><strong>data</strong></span> field.
</p><p>
It is an error to specify more than one of
<code class="literal">DB_DBT_MALLOC</code>,
<code class="literal">DB_DBT_REALLOC</code>, and
<code class="literal">DB_DBT_USERMEM</code>.
</p></li><li><p><a id="dbt_DB_DBT_REALLOC"></a>
<code class="literal">DB_DBT_REALLOC</code>
</p><p>
When this flag is set Berkeley DB will allocate memory
for the returned key or data item (using
<span class="bold"><strong>realloc</strong></span>(3), or
the user-specified realloc function), and return a
pointer to it in the
<span class="bold"><strong>data</strong></span> 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
<span class="bold"><strong>data</strong></span> field.
</p><p>
The difference between
<code class="literal">DB_DBT_MALLOC</code> and
<code class="literal">DB_DBT_REALLOC</code>
is that the latter will call
<span class="bold"><strong>realloc</strong></span>(3) instead of
<span class="bold"><strong>malloc</strong></span>(3),
so the allocated memory will be grown as
necessary instead of the application doing repeated
free/malloc calls.
</p><p>
It is an error to specify more than one of
<code class="literal">DB_DBT_MALLOC</code>,
<code class="literal">DB_DBT_REALLOC</code>, and
<code class="literal">DB_DBT_USERMEM</code>.
</p></li><li><p><a id="dbt_DB_DBT_USERMEM"></a>
<code class="literal">DB_DBT_USERMEM</code>
</p><p>
The <span class="emphasis"><em>data</em></span>
field of the key or data structure must refer
to memory that is at least
<span class="emphasis"><em>ulen</em></span>
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
<span class="emphasis"><em>data</em></span>
field refers. Otherwise, the
<span class="emphasis"><em>size</em></span>
field is set to the length needed for the requested
item, and the error
<code class="literal">DB_BUFFER_SMALL</code> is returned.
</p><p>
It is an error to specify more than one of
<code class="literal">DB_DBT_MALLOC</code>,
<code class="literal">DB_DBT_REALLOC</code>, and
<code class="literal">DB_DBT_USERMEM</code>.
</p></li></ul></div>
<div class="itemizedlist"><ul type="circle"><li><p><a id="dbt_DB_DBT_PARTIAL"></a>
<code class="literal">DB_DBT_PARTIAL</code>
</p><p>
Do partial retrieval or storage of an item. If the
calling application is doing a get, the
<span class="bold"><strong>dlen</strong></span> bytes
starting
<span class="bold"><strong>doff</strong></span>
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><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
<span class="bold"><strong>dlen</strong></span>
field of 20 and a
<span class="bold"><strong>doff</strong></span>
field of 85,
the get call would succeed, the
<span class="bold"><strong>data</strong></span>
field would refer
to the last 15 bytes of the record, and the
<span class="bold"><strong>size</strong></span>
field would be set to 15.
</p><p>
If the calling application is doing a put, the
<span class="bold"><strong>dlen</strong></span>
bytes starting
<span class="bold"><strong>doff</strong></span>
bytes from the beginning of the
specified key's data record are replaced by the data
specified by the
<span class="bold"><strong>data</strong></span>
and
<span class="bold"><strong>size</strong></span>
structure elements. If
<span class="bold"><strong>dlen</strong></span>
is smaller than
<span class="bold"><strong>size</strong></span>
the record will grow; if
<span class="bold"><strong>dlen</strong></span>
is larger than
<span class="bold"><strong>size</strong></span>
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 class="xref" href="dbput.html" title="DB->put()">DB->put()</a>
method in a database that supports duplicate
records. Partial puts in databases supporting duplicate
records must be done using a
<a class="xref" href="dbcput.html" title="DBcursor->put()">DBcursor->put()</a>
method.
</p><p>
It is an error to attempt a partial put with differing
<span class="bold"><strong>dlen</strong></span>
and
<span class="bold"><strong>size</strong></span>
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
<span class="bold"><strong>dlen</strong></span>
field of 20, a
<span class="bold"><strong>doff</strong></span>
field of 85, and a
<span class="bold"><strong>size</strong></span>
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></li><li><p><a id="dbt_DB_DBT_APPMALLOC"></a>
<code class="literal">DB_DBT_APPMALLOC</code>
</p><p>
After an application-supplied callback routine passed to
either
<a class="xref" href="dbassociate.html" title="DB->associate()">DB->associate()</a>
or
<a class="xref" href="dbset_append_recno.html" title="DB->set_append_recno()">DB->set_append_recno()</a>
is executed, the
<span class="bold"><strong>data</strong></span>
field of a DBT may refer to memory allocated with
<span class="bold"><strong>malloc</strong></span>(3)
or
<span class="bold"><strong>realloc</strong></span>(3).
In that case,
the callback sets the
<code class="literal">DB_DBT_APPMALLOC</code>
flag in the DBT
so that Berkeley DB will call
<span class="bold"><strong>free</strong></span>(3)
to deallocate the
memory when it is no longer required.
</p></li><li><p><a id="dbt_DB_DBT_MULTIPLE"></a>
<code class="literal">DB_DBT_MULTIPLE</code>
</p><p>
Set in a secondary key creation callback routine passed
to
<a class="xref" href="dbassociate.html" title="DB->associate()">DB->associate()</a>
to indicate that multiple secondary
keys should be associated with the given primary
key/data pair. If set, the
<span class="bold"><strong>size</strong></span>
field indicates the
number of secondary keys and the
<span class="bold"><strong>data</strong></span>
field refers to an
array of that number of DBT structures.
</p><p>
The <code class="literal">DB_DBT_APPMALLOC</code> flag may be set on any of the DBT
structures to indicate that their
<span class="bold"><strong>data</strong></span>
field needs to be
freed.
</p></li></ul></div>
</span>
</li>
</ul>
</div>
<div class="sect1" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h2 class="title" style="clear: both"><a id="dbtlist"></a>DBT and Bulk Operations</h2>
</div>
</div>
</div>
<div class="navtable">
<table border="1" width="80%">
<thead>
<tr>
<th>DBT and Bulk Operations</th>
<th>Description</th>
</tr>
</thead>
<tbody>
<tr>
<td>
<a class="xref" href="DB_MULTIPLE_INIT.html" title="DB_MULTIPLE_INIT">DB_MULTIPLE_INIT</a>
</td>
<td>Initialize bulk get retrieval</td>
</tr>
<tr>
<td>
<a class="xref" href="DB_MULTIPLE_NEXT.html" title="DB_MULTIPLE_NEXT">DB_MULTIPLE_NEXT</a>
</td>
<td>Next bulk get retrieval</td>
</tr>
<tr>
<td>
<a class="xref" href="DB_MULTIPLE_KEY_NEXT.html" title="DB_MULTIPLE_KEY_NEXT">DB_MULTIPLE_KEY_NEXT</a>
</td>
<td>Next bulk get retrieval</td>
</tr>
<tr>
<td>
<a class="xref" href="DB_MULTIPLE_RECNO_NEXT.html" title="DB_MULTIPLE_RECNO_NEXT">DB_MULTIPLE_RECNO_NEXT</a>
</td>
<td>Next bulk get retrieval</td>
</tr>
<tr>
<td>
<a class="xref" href="DB_MULTIPLE_WRITE_INIT.html" title="DB_MULTIPLE_WRITE_INIT">DB_MULTIPLE_WRITE_INIT</a>
</td>
<td>Initialize a bulk buffer to hold key/data pairs</td>
</tr>
<tr>
<td>
<a class="xref" href="DB_MULTIPLE_WRITE_NEXT.html" title="DB_MULTIPLE_WRITE_NEXT">DB_MULTIPLE_WRITE_NEXT</a>
</td>
<td>Append a data item to a bulk buffer</td>
</tr>
<tr>
<td>
<a class="xref" href="DB_MULTIPLE_RESERVE_NEXT.html" title="DB_MULTIPLE_RESERVE_NEXT">DB_MULTIPLE_RESERVE_NEXT</a>
</td>
<td>Reserve space for a data item in a bulk buffer</td>
</tr>
<tr>
<td>
<a class="xref" href="DB_MULTIPLE_KEY_WRITE_NEXT.html" title="DB_MULTIPLE_KEY_WRITE_NEXT">DB_MULTIPLE_KEY_WRITE_NEXT</a>
</td>
<td>Append a key / data pair to a bulk buffer</td>
</tr>
<tr>
<td>
<a class="xref" href="DB_MULTIPLE_KEY_RESERVE_NEXT.html" title="DB_MULTIPLE_KEY_RESERVE_NEXT">DB_MULTIPLE_KEY_RESERVE_NEXT</a>
</td>
<td>Reserve space for a key / data pair in a bulk buffer</td>
</tr>
<tr>
<td>
<a class="xref" href="DB_MULTIPLE_RECNO_WRITE_INIT.html" title="DB_MULTIPLE_RECNO_WRITE_INIT">DB_MULTIPLE_RECNO_WRITE_INIT</a>
</td>
<td>Initialize a bulk buffer to hold recno/data pairs</td>
</tr>
<tr>
<td>
<a class="xref" href="DB_MULTIPLE_RECNO_WRITE_NEXT.html" title="DB_MULTIPLE_RECNO_WRITE_NEXT">DB_MULTIPLE_RECNO_WRITE_NEXT</a>
</td>
<td>Append a record number / data pair to a bulk buffer</td>
</tr>
<tr>
<td>
<a class="xref" href="DB_MULTIPLE_RECNO_RESERVE_NEXT.html" title="DB_MULTIPLE_RECNO_RESERVE_NEXT">DB_MULTIPLE_RECNO_RESERVE_NEXT</a>
</td>
<td>Reserve space for a record number / data pair in a bulk buffer</td>
</tr>
</tbody>
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