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<h1>DB->set_bt_compare</h1>
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<h3><pre>
#include <db.h>
<p>
int
DB->set_bt_compare(DB *db,
int (*bt_compare_fcn)(DB *, const DBT *, const DBT *));
</pre></h3>
<h1>Description</h1>
<p>Set the Btree key comparison function. The comparison function is
called when it is necessary to compare a key specified by the
application with a key currently stored in the tree. The first argument
to the comparison function is the <a href="../api_c/dbt_class.html">DBT</a> representing the
application supplied key; the second is the current tree's key.
<p>The comparison function must return an integer value less than, equal
to, or greater than zero if the first key argument is considered to be
respectively less than, equal to, or greater than the second key
argument. In addition, the comparison function must cause the keys in
the database to be <i>well-ordered</i>. The comparison function
must correctly handle any key values used by the application (possibly
including zero-length keys). In addition, when Btree key prefix
comparison is being performed (see <a href="../api_c/db_set_bt_prefix.html">DB->set_bt_prefix</a> for more
information), the comparison routine may be passed a prefix of any
database key. The <b>data</b> and <b>size</b> fields of the
<a href="../api_c/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 <b>data</b> field refers may be assumed.
<p>If no comparison function is specified, the keys are compared lexically,
with shorter keys collating before longer keys.
<p>The DB->set_bt_compare method configures operations performed using the specified
<a href="../api_c/db_class.html">DB</a> handle, not all operations performed on the underlying
database.
<p>The DB->set_bt_compare interface may not be called after the <a href="../api_c/db_open.html">DB->open</a>
interface is called.
If the database already exists when
<a href="../api_c/db_open.html">DB->open</a> is called, the information specified to DB->set_bt_compare must
be the same as that historically used to create the database or
corruption can occur.
<p>The DB->set_bt_compare method returns a non-zero error value on failure and 0 on success.
<h1>Errors</h1>
<p>The DB->set_bt_compare method may fail and return a non-zero error for the following conditions:
<p><dl compact>
<p><dt>EINVAL<dd>An invalid flag value or parameter was specified.
<p>Called after <a href="../api_c/db_open.html">DB->open</a> was called.
</dl>
<p>The DB->set_bt_compare method 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_bt_compare method may fail and
return <a href="../ref/program/errorret.html#DB_RUNRECOVERY">DB_RUNRECOVERY</a>,
in which case all subsequent Berkeley DB calls will fail in the same way.
<h1>Class</h1>
<a href="../api_c/db_class.html">DB</a>
<h1>See Also</h1>
<a href="../api_c/db_list.html">Databases and Related Methods</a>
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