# Db::set_bt_compare

`

###

    #include <db_cxx.h>

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

# Description

Set the Btree key comparison function. The comparison function is called when it is necessary to compare a key specified by the application with a key currently stored in the tree. The first argument to the comparison function is the Dbt representing the application supplied key, the second is the current tree's key.

The comparison function must return an integer value less than, equal to, or greater than zero if the first key argument is considered to be respectively less than, equal to, or greater than the second key argument. 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.

If no comparison function is specified, the keys are compared lexically, with shorter keys collating before longer keys. The same comparison method must be used each time a particular Btree is opened.

The Db::set_bt_compare interface may only be used to configure Berkeley DB before the Db::open interface is called.

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.

# Errors

EINVAL
  An invalid flag value or parameter was specified.

  Called after Db::open was called.

### Class

Db

# See Also

Db::close, Db::cursor, Db::del, Db::err, Db::fd, Db::get, Db::get_byteswapped, Db::get_type, Db::join, Db::key_range, Db::open, Db::put, Db::remove, Db::set_bt_compare, Db::set_bt_minkey, Db::set_bt_prefix, Db::set_cachesize, Db::set_dup_compare, Db::set_errcall, Db::set_errfile, Db::set_errpfx, Db::set_flags, Db::set_h_ffactor, Db::set_h_hash, Db::set_h_nelem, Db::set_lorder, Db::set_malloc, Db::set_pagesize, Db::set_paniccall, Db::set_q_extentsize, Db::set_realloc, Db::set_re_delim, Db::set_re_len, Db::set_re_pad, Db::set_re_source, Db::stat, Db::sync, Db::upgrade and Db::verify.`

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