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<title>Berkeley DB: Db::set_h_hash</title>
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<h1>Db::set_h_hash</h1>
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#include <db_cxx.h>
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typedef u_int32_t (*h_hash_fcn_type)
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(DB *, const void *bytes, u_int32_t length);
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Db::set_h_hash(h_hash_fcn_type h_hash_fcn);
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<p>Set a user-defined hash method; if no hash method is specified, a default
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hash method is used. Because no hash method performs equally well on all
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possible data, the user may find that the built-in hash method performs
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poorly with a particular data set. User-specified hash functions must
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take a pointer to a byte string and a length as arguments, and return a
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The hash function must handle any key values used by the application
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(possibly including zero-length keys).
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<p>If a hash method is specified, <a href="../api_cxx/db_open.html">Db::open</a> will attempt to determine
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whether the hash method specified is the same as the one with which the
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database was created, and will fail if it detects that it is not.
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<p>The Db::set_h_hash method configures operations performed using the specified
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<a href="../api_cxx/db_class.html">Db</a> handle, not all operations performed on the underlying
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<p>The Db::set_h_hash interface may not be called after the <a href="../api_cxx/db_open.html">Db::open</a>
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If the database already exists when
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<a href="../api_cxx/db_open.html">Db::open</a> is called, the information specified to Db::set_h_hash must
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be the same as that historically used to create the database or
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<p>The Db::set_h_hash method either returns a non-zero error value or throws an exception that
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encapsulates a non-zero error value on failure, and returns 0 on success.
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<p>The Db::set_h_hash method may fail and throw an exception or return a non-zero error for the following conditions:
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<p><dt>EINVAL<dd>An invalid flag value or parameter was specified.
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<p>Called after <a href="../api_cxx/db_open.html">Db::open</a> was called.
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<p>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.
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If a catastrophic error has occurred, the Db::set_h_hash method may fail and
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either return <a href="../ref/program/errorret.html#DB_RUNRECOVERY">DB_RUNRECOVERY</a> or throw a
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<a href="../api_cxx/runrec_class.html">DbRunRecoveryException</a>,
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in which case all subsequent Berkeley DB calls will fail in the same way.
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<a href="../api_cxx/db_class.html">Db</a>
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<a href="../api_cxx/db_list.html">Databases and Related Methods</a>
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