Library Version 18.1.40

DB->set_h_hash()
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## DB->set_h_hash()

    #include <db.h>

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

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 returns a non-zero error value on failure and 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 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

Database and Related Methods

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