#include <db.h>
int
DB->set_bt_compare(DB *db,
int (*bt_compare_fcn)(DB *db, const DBT *dbt1, const DBT *dbt2));
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 returns a non-zero error value on failure and 0 on success.
The bt_compare_fcn function is the application-specified Btree comparison function. The comparison function takes three parameters:
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.
The DB->set_bt_compare()
method may fail and return one of the following non-zero errors:
If the method was called after DB->open() was called; or if an invalid flag value or parameter was specified.