# DB->set_bt_compare

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    #include <db.h>

    int
    DB->set_bt_compare(DB *db,
        int (*bt_compare_fcn)(DB *, const DBT *, const DBT *));

# 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, and no particular alignment of the memory to which by the data field refers may be assumed.

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 interface may not be called after the DB->open interface 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.

# Errors

The DB->set_bt_compare method may fail and return a non-zero error for the following conditions:

EINVAL
  An invalid flag value or parameter was specified.

  Called after DB->open was called.

The DB->set_bt_compare method may fail and return a non-zero error for errors specified for other Berkeley DB and C library or system functions. If a catastrophic error has occurred, the DB->set_bt_compare method may fail and return DB_RUNRECOVERY, in which case all subsequent Berkeley DB calls will fail in the same way.

# Class

DB

# See Also

Databases and Related Methods`

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