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<title>Berkeley DB: Db::set_bt_prefix</title>
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<h1>Db::set_bt_prefix</h1>
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#include <db_cxx.h>
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typedef size_t (*bt_prefix_fcn_type)(DB *, const DBT *, const DBT *);
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Db::set_bt_prefix(bt_prefix_fcn_type bt_prefix_fcn);
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<p>Set the Btree prefix function. The prefix function must return the
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number of bytes of the second key argument that would be required by
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the Btree key comparison function to determine the second key argument's
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ordering relationship with respect to the first key argument. If the
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two keys are equal, the key length should be returned. The prefix
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function must correctly handle any key values used by the application
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(possibly including zero-length keys). The <b>data</b> and
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<b>size</b> fields of the <a href="../api_cxx/dbt_class.html">Dbt</a> are the only fields that may be
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used for the purposes of this determination, and no particular alignment
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of the memory to which the <b>data</b> field refers may be assumed.
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<p>The prefix function is used to determine the amount by which keys stored
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on the Btree internal pages can be safely truncated without losing their
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uniqueness. See the <a href="../ref/am_conf/bt_prefix.html">Btree
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prefix comparison</a> section of the Berkeley DB Reference Guide for more details
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about how this works. The usefulness of this is data-dependent, but
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can produce significantly reduced tree sizes and search times in some
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<p>If no prefix function or key comparison function is specified by the
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application, a default lexical comparison function is used as the prefix
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function. If no prefix function is specified and a key comparison
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function is specified, no prefix function is used. It is an error to
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specify a prefix function without also specifying a key comparison
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<p>The Db::set_bt_prefix method configures operations performed using the specified
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<a href="../api_cxx/db_class.html">Db</a> handle, not all operations performed on the underlying
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<p>The Db::set_bt_prefix interface may not be called after the <a href="../api_cxx/db_open.html">Db::open</a>
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If the database already exists when
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<a href="../api_cxx/db_open.html">Db::open</a> is called, the information specified to Db::set_bt_prefix must
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be the same as that historically used to create the database or
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<p>The Db::set_bt_prefix method either returns a non-zero error value or throws an exception that
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encapsulates a non-zero error value on failure, and returns 0 on success.
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<p>The Db::set_bt_prefix method may fail and throw an exception or return a non-zero error for the following conditions:
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<p><dt>EINVAL<dd>An invalid flag value or parameter was specified.
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<p>Called after <a href="../api_cxx/db_open.html">Db::open</a> was called.
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<p>The Db::set_bt_prefix method may fail and throw an exception or return a non-zero error for errors specified for other Berkeley DB and C library or system methods.
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If a catastrophic error has occurred, the Db::set_bt_prefix method may fail and
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either return <a href="../ref/program/errorret.html#DB_RUNRECOVERY">DB_RUNRECOVERY</a> or throw a
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<a href="../api_cxx/runrec_class.html">DbRunRecoveryException</a>,
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in which case all subsequent Berkeley DB calls will fail in the same way.
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<a href="../api_cxx/db_class.html">Db</a>
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