Berkeley DB: Db::set_h_hash
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_h_hash |

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###

    #include <db_cxx.h>

    extern "C" {
        typedef u_int32_t (*h_hash_fcn_type)
    	(DB *, const void *bytes, u_int32_t length);
    };
    int
    Db::set_h_hash(h_hash_fcn_type h_hash_fcn);

### Description: Db::set_h_hash

Set a user-defined hash function; if no hash function is specified, a default hash function is used. Because no hash function performs equally well on all possible data, the user may find that the built-in hash function performs poorly with a particular data set.

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

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

The Db::set_h_hash 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

h_hash_fcn
  The h_hash_fcn parameter is the application-specified hash function.

  Application-specified hash functions take a pointer to a byte string and a length as parameters, and return a value of type u_int32_t. The hash function must handle any key values used by the application (possibly including zero-length keys).

### Errors

The Db::set_h_hash 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; the specified hash function differs from the hash function with which the database was created; or if an invalid flag value or parameter was specified.

### Class

Db

### See Also

Databases and Related Methods`

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