prepared state.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="ecpg-transactions-exec-sql-autocommit-on">
      <term><literal>EXEC SQL SET AUTOCOMMIT TO ON</literal></term>
      <listitem>
       <para>
        Enable autocommit mode.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="ecpg-transactions-exec-sql-autocommit-off">
      <term><literal>EXEC SQL SET AUTOCOMMIT TO OFF</literal></term>
      <listitem>
       <para>
        Disable autocommit mode.  This is the default.
       </para>
      </listitem>
     </varlistentry>
    </variablelist>
   </para>
  </sect2>

  <sect2 id="ecpg-prepared">
   <title>Prepared Statements</title>

   <para>
    When the values to be passed to an SQL statement are not known at
    compile time, or the same statement is going to be used many
    times, then prepared statements can be useful.
   </para>

   <para>
    The statement is prepared using the
    command <literal>PREPARE</literal>.  For the values that are not
    known yet, use the
    placeholder <quote><literal>?</literal></quote>:
<programlisting>
EXEC SQL PREPARE stmt1 FROM "SELECT oid, datname FROM pg_database WHERE oid = ?";
</programlisting>
   </para>

   <para>
    If a statement returns a single row, the application can
    call <literal>EXECUTE</literal> after
    <literal>PREPARE</literal> to execute the statement, supplying the
    actual values for the placeholders with a <literal>USING</literal>
    clause:
<programlisting>
EXEC SQL EXECUTE stmt1 INTO :dboid, :dbname USING 1;
</programlisting>
   </para>

   <para>
    If a statement returns multiple rows, the application can use a
    cursor declared based on the prepared statement.  To bind input
    parameters, the cursor must be opened with
    a <literal>USING</literal> clause:
<programlisting>
EXEC SQL PREPARE stmt1 FROM "SELECT oid,datname FROM pg_database WHERE oid &gt; ?";
EXEC SQL DECLARE foo_bar CURSOR FOR stmt1;

/* when end of result set reached, break out of while loop */
EXEC SQL WHENEVER NOT FOUND DO BREAK;

EXEC SQL OPEN foo_bar USING 100;
...
while (1)
{
    EXEC SQL FETCH NEXT FROM foo_bar INTO :dboid, :dbname;
    ...
}
EXEC SQL CLOSE foo_bar;
</programlisting>
   </para>

   <para>
    When you don't need the prepared statement anymore, you should
    deallocate it:
<programlisting>
EXEC SQL DEALLOCATE PREPARE <replaceable>name</replaceable>;
</programlisting>
   </para>

   <para>
    For more details about <literal>PREPARE</literal>,
    see <xref linkend="ecpg-sql-prepare"/>. Also
    see <xref linkend="ecpg-dynamic"/> for more details about using
    placeholders and input parameters.
   </para>
  </sect2>
 </sect1>

 <sect1 id="ecpg-variables">
  <title>Using Host Variables</title>

  <para>
   In <xref linkend="ecpg-commands"/> you saw how you can execute SQL
   statements from an embedded SQL program.  Some of those statements
   only used fixed values and did not provide a way to insert
   user-supplied values into statements or have the program process
   the values returned by the query.  Those kinds of statements are
   not really useful in real applications.  This section explains in
   detail how you can pass data between your C program and the
   embedded SQL statements using a simple mechanism called
   <firstterm>host variables</firstterm>. In an embedded SQL program we
   consider the SQL statements to be <firstterm>guests</firstterm> in the C
   program code which is the <firstterm>host language</firstterm>. Therefore
   the variables of the C program are called <firstterm>host
   variables</firstterm>.
  </para>

  <para>
   Another way to exchange values between PostgreSQL backends and ECPG
   applications is the use of SQL descriptors, described
   in <xref linkend="ecpg-descriptors"/>.
  </para>

  <sect2 id="ecpg-variables-overview">
   <title>Overview</title>

   <para>
    Passing data between the C program and the SQL statements is
    particularly simple