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

### Db::set_h_compare |

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###

    #include <db_cxx.h>

    extern "C" {
        typedef int (*compare_fcn_type)(DB *db, const DBT *dbt1, const DBT *dbt2);
    };
    int
    Db::set_h_compare(compare_fcn_type compare_fcn);

### Description: Db::set_h_compare

Set the Hash 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 database.

If no comparison function is specified, a byte-by-byte comparison is performed.

The Db::set_h_compare method configures operations performed using the specified Db handle, not all operations performed on the underlying database.

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

The Db::set_h_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

compare_fcn
  The compare_fcn function is the application-specified Hash 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 database's key.

  The 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. The comparison function must correctly handle any key values used by the application (possibly including zero-length keys). 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_h_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
  If the method was called after Db::open was called; or if an invalid flag value or parameter was specified.

### Class

Db

### See Also

Databases and Related Methods`

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