Db::set_bt_compare()
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## Db::set_bt_compare()

    #include <db_cxx.h>

    extern "C" {
        typedef int (*bt_compare_fcn_type)(DB *db, const DBT *dbt1,
                     const DBT *dbt2);
    };
    int
    Db::set_bt_compare(bt_compare_fcn_type bt_compare_fcn);

Set the Btree key comparison function. The comparison function is called whenever it is necessary to compare a key specified by the application with a key currently stored in the tree.

If no comparison function is specified, the keys are compared lexically, with shorter keys collating before longer keys.

The `Db::set_bt_compare()` method configures operations performed using the specified Db handle, not all operations performed on the underlying database.

The `Db::set_bt_compare()` method may not be called after the Db::open() method is called. If the database already exists when Db::open() is called, the information specified to `Db::set_bt_compare()` must be the same as that historically used to create the database or corruption can occur.

The `Db::set_bt_compare()` method either returns a non-zero error value or throws an exception that encapsulates a non-zero error value on failure, and returns 0 on success.

### Parameters

#### bt_compare_fcn

The bt_compare_fcn function is the application-specified Btree comparison function. The comparison function takes three parameters:

- `db`

  The db parameter is the enclosing database handle.

- `dbt1`

  The dbt1 parameter is the Dbt representing the application supplied key.

- `dbt2`

  The dbt2 parameter is the Dbt representing the current tree's key.

The bt_compare_fcn function must return an integer value less than, equal to, or greater than zero if the first key parameter is considered to be respectively less than, equal to, or greater than the second key parameter. In addition, the comparison function must cause the keys in the database to be well-ordered. The comparison function must correctly handle any key values used by the application (possibly including zero-length keys). In addition, when Btree key prefix comparison is being performed (see Db::set_bt_prefix() for more information), the comparison routine may be passed a prefix of any database key. The data and size fields of the Dbt are the only fields that may be used for the purposes of this comparison, and no particular alignment of the memory to which by the data field refers may be assumed.

### Errors

The `Db::set_bt_compare()` method may fail and throw a DbException exception, encapsulating one of the following non-zero errors, or return one of the following non-zero errors:

#### EINVAL

If the method was called after Db::open() was called; or if an invalid flag value or parameter was specified.

### Class

Db

### See Also

Database and Related Methods

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