Set a user defined hash method; if no hash method is specified, a default
hash method is used. Since 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
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
if 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 interface may only be used to configure Berkeley DB before
the DB->open interface is called.
The DB->set_h_hash function returns a non-zero error value on failure and 0 on success.