Berkeley DB: Db::set_dup_compare
Description: Berkeley DB: An embedded database programmatic toolkit.
Keywords: embedded,database,programmatic,toolkit,b+tree,btree,hash,hashing,transaction,transactions,locking,logging,access method,access methods,Java,C,C++

### Db::set_dup_compare |

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###

    #include <db_cxx.h>

    extern "C" {
        typedef int (*dup_compare_fcn_type)(DB *, const DBT *, const DBT *);
    };
    int
    Db::set_dup_compare(dup_compare_fcn_type dup_compare_fcn);

### Description: Db::set_dup_compare

Set the duplicate data item comparison function. The comparison function is called whenever it is necessary to compare a data item specified by the application with a data item currently stored in the database. Calling Db::set_dup_compare implies calling Db::set_flags with the DB_DUPSORT flag.

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

The Db::set_dup_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_dup_compare must be the same as that historically used to create the database or corruption can occur.

The Db::set_dup_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

dup_compare_fcn
  The dup_compare_fcn function is the application-specified duplicate data item comparison function. The function takes three arguments:

  db
    The db parameter is the enclosing database handle.

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

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

  The dup_compare_fcn function must return an integer value less than, equal to, or greater than zero if the first data item parameter is considered to be respectively less than, equal to, or greater than the second data item parameter. In addition, the comparison function must cause the data items in the set to be well-ordered. The comparison function must correctly handle any data item values used by the application (possibly including zero-length data items). 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 the data field refers may be assumed.

### Errors

The Db::set_dup_compare method may fail and throw DbException, encapsulating one of the following non-zero errors, or return one of the following non-zero errors:

EINVAL
  An invalid flag value or parameter was specified.

### Class

Db

### See Also

Databases and Related Methods`

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