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<title>Berkeley DB: Db::set_bt_compare</title>
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<h1>Db::set_bt_compare</h1>
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#include <db_cxx.h>
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typedef int (*bt_compare_fcn_type)(DB *, const DBT *, const DBT *);
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Db::set_bt_compare(bt_compare_fcn_type bt_compare_fcn);
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<p>Set the Btree key comparison function. The comparison function is
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called when it is necessary to compare a key specified by the
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application with a key currently stored in the tree. The first argument
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to the comparison function is the <a href="../api_cxx/dbt_class.html">Dbt</a> representing the
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application supplied key; the second is the current tree's key.
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<p>The comparison function must return an integer value less than, equal
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to, or greater than zero if the first key argument is considered to be
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respectively less than, equal to, or greater than the second key
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argument. In addition, the comparison function must cause the keys in
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the database to be <i>well-ordered</i>. The comparison function
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must correctly handle any key values used by the application (possibly
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including zero-length keys). In addition, when Btree key prefix
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comparison is being performed (see <a href="../api_cxx/db_set_bt_prefix.html">Db::set_bt_prefix</a> for more
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information), the comparison routine may be passed a prefix of any
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database key. The <b>data</b> and <b>size</b> fields of the
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<a href="../api_cxx/dbt_class.html">Dbt</a> are the only fields that may be used for the purposes of
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this comparison, and no particular alignment of the memory to which
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by the <b>data</b> field refers may be assumed.
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<p>If no comparison function is specified, the keys are compared lexically,
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with shorter keys collating before longer keys.
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<p>The Db::set_bt_compare 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_compare 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_compare 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_compare 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_compare 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_compare 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_compare 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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