<?xml version="1.0" encoding="UTF-8" standalone="no"?> <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"> <html xmlns="http://www.w3.org/1999/xhtml"> <head> <meta http-equiv="Content-Type" content="text/html; charset=UTF-8" /> <title>Chapter 4. The DBT Handle</title> <link rel="stylesheet" href="apiReference.css" type="text/css" /> <meta name="generator" content="DocBook XSL Stylesheets V1.73.2" /> <link rel="start" href="index.html" title="Berkeley DB C API Reference" /> <link rel="up" href="index.html" title="Berkeley DB C API Reference" /> <link rel="prev" href="dbcset_priority.html" title="DBcursor->set_priority()" /> <link rel="next" href="DB_MULTIPLE_INIT.html" title="DB_MULTIPLE_INIT" /> </head> <body> <div xmlns="" class="navheader"> <div class="libver"> <p>Library Version 11.2.5.3</p> </div> <table width="100%" summary="Navigation header"> <tr> <th colspan="3" align="center">Chapter 4. The DBT Handle </th> </tr> <tr> <td width="20%" align="left"><a accesskey="p" href="dbcset_priority.html">Prev</a> </td> <th width="60%" align="center"> </th> <td width="20%" align="right"> <a accesskey="n" href="DB_MULTIPLE_INIT.html">Next</a></td> </tr> </table> <hr /> </div> <div class="chapter" lang="en" xml:lang="en"> <div class="titlepage"> <div> <div> <h2 class="title"><a id="dbt"></a>Chapter 4. The DBT Handle </h2> </div> </div> </div> <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><p> This flag is ignored when used with the <code class="literal">pkey</code> parameter on <a class="link" href="dbget.html" title="DB->get()">DB->pget()</a> or <a class="link" href="dbcget.html" title="DBcursor->get()">DBcursor->pget()</a>. </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><li><p><a id="dbt_DB_DBT_READONLY"></a> <code class="literal">DB_DBT_READONLY</code> </p><p> 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. </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="dbsort_multiple.html" title="DB->sort_multiple()">DB->sort_multiple()</a> </td> <td>Sort a set of DBTs</td> </tr> <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> </table> </div> </div> </div> <div class="navfooter"> <hr /> <table width="100%" summary="Navigation footer"> <tr> <td width="40%" align="left"><a accesskey="p" href="dbcset_priority.html">Prev</a> </td> <td width="20%" align="center"> </td> <td width="40%" align="right"> <a accesskey="n" href="DB_MULTIPLE_INIT.html">Next</a></td> </tr> <tr> <td width="40%" align="left" valign="top">DBcursor->set_priority() </td> <td width="20%" align="center"> <a accesskey="h" href="index.html">Home</a> </td> <td width="40%" align="right" valign="top"> DB_MULTIPLE_INIT</td> </tr> </table> </div> </body> </html>