# Db::set_h_hash

`

###

    #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

Set a user-defined hash method; if no hash method is specified, a default hash method is used. Because no hash method performs equally well on all possible data, the user may find that the built-in hash method performs poorly with a particular data set. User-specified hash functions must take a pointer to a byte string and a length as arguments, 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).

If a hash method is specified, Db::open will attempt to determine whether the hash method specified is the same as the one with which the database was created, and will fail if it detects that it is not.

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 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_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.

# Errors

The Db::set_h_hash method may fail and throw an exception or 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_h_hash method may fail and throw an exception or return a non-zero error for errors specified for other Berkeley DB and C library or system methods. If a catastrophic error has occurred, the Db::set_h_hash method may fail and either return DB_RUNRECOVERY or throw a DbRunRecoveryException, 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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