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<title>Berkeley DB: Db::set_h_compare</title>
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Db::set_h_compare
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#include &lt;db_cxx.h&gt;

extern "C" {
 typedef int (*compare_fcn_type)(DB *db, const DBT *dbt1, const DBT *dbt2);
};
int
Db::set_h_compare(compare_fcn_type compare_fcn);
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Description: Db::set_h_compare
Set the Hash key comparison function. The comparison function is called
whenever it is necessary to compare a key specified by the application
with a key currently stored in the database.
If no comparison function is specified, a byte-by-byte comparison is
performed.
The Db::set_h_compare method configures operations performed using the specified
<a href="../../db4-devel-4.7.25/api_cxx/db_class.html">Db</a> handle, not all operations performed on the underlying
database.
The Db::set_h_compare method may not be called after the <a href="../../db4-devel-4.7.25/api_cxx/db_open.html">Db::open</a> method is called.
If the database already exists when
<a href="../../db4-devel-4.7.25/api_cxx/db_open.html">Db::open</a> is called, the information specified to Db::set_h_compare must
be the same as that historically used to create the database or
corruption can occur.
The Db::set_h_compare 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.

Parameters 
 compare_fcn<ul compact><li>The compare_fcn function is the application-specified Hash
comparison function. The comparison function takes three parameters:
 
db<ul compact><li>The db parameter is the enclosing database handle.</ul>
dbt1<ul compact><li>The dbt1 parameter is the <a href="../../db4-devel-4.7.25/api_cxx/dbt_class.html">Dbt</a> representing the
application supplied key.</ul>
dbt2<ul compact><li>The dbt2 parameter is the <a href="../../db4-devel-4.7.25/api_cxx/dbt_class.html">Dbt</a> representing the
current database's key.</ul>
 </ul>
The compare_fcn function must return an integer value less
than, equal to, or greater than zero if the first key parameter is
considered to be respectively less than, equal to, or greater than the
second key parameter. The comparison function must correctly handle any
key values used by the application (possibly including zero-length
keys). The data and size fields of the <a href="../../db4-devel-4.7.25/api_cxx/dbt_class.html">Dbt</a> are
the only fields that may be used for the purposes of this comparison,
and no particular alignment of the memory to which by the data
field refers may be assumed.
 
 Errors
The Db::set_h_compare method
may fail and throw
<a href="../../db4-devel-4.7.25/api_cxx/except_class.html">DbException</a>,
encapsulating one of the following non-zero errors, or return one of
the following non-zero errors:
 
EINVAL<ul compact><li>If the method was called after <a href="../../db4-devel-4.7.25/api_cxx/db_open.html">Db::open</a> was called; or if an
invalid flag value or parameter was specified.</ul>
 
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 Class
<a href="../../db4-devel-4.7.25/api_cxx/db_class.html">Db</a>
 See Also
<a href="../../db4-devel-4.7.25/api_cxx/db_list.html">Databases and Related Methods</a>
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