# DB->set_h_hash

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    #include <db.h>

    int
    DB->set_h_hash(DB *db,
        u_int32_t (*h_hash_fcn)(DB *, const void *bytes, u_int32_t length));

# 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 interface may be used only 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.

# Errors

The DB->set_h_hash function may fail and 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 function may fail and return a non-zero error for errors specified for other Berkeley DB and C library or system functions. If a catastrophic error has occurred, the DB->set_h_hash function may fail and return DB_RUNRECOVERY, in which case all subsequent Berkeley DB calls will fail in the same way.

# See Also

db_create, DB->associate, DB->close, DB->cursor, DB->del, DB->err, DB->errx DB->fd, DB->get, DB->pget, DB->get_byteswapped, DB->get_type, DB->join, DB->key_range, DB->open, DB->put, DB->remove, DB->rename, DB->set_alloc, DB->set_append_recno, DB->set_bt_compare, DB->set_bt_minkey, DB->set_bt_prefix, DB->set_cachesize, DB->set_dup_compare, DB->set_errcall, DB->set_errfile, DB->set_errpfx, DB->set_feedback, DB->set_flags, DB->set_h_ffactor, DB->set_h_hash, DB->set_h_nelem, DB->set_lorder, DB->set_pagesize, DB->set_paniccall, DB->set_q_extentsize, DB->set_re_delim, DB->set_re_len, DB->set_re_pad, DB->set_re_source, DB->stat, DB->sync, DB->truncate, DB->upgrade, and DB->verify.`

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