working
     with C strings, you pass the <structname>TupleDesc</structname> to
     <function>TupleDescGetAttInMetadata</function>, and then call
     <function>BuildTupleFromCStrings</function> for each row.  In the case of a
     function returning a set of tuples, the setup steps can all be done
     once during the first call of the function.
    </para>

    <para>
     Several helper functions are available for setting up the needed
     <structname>TupleDesc</structname>.  The recommended way to do this in most
     functions returning composite values is to call:
<programlisting>
TypeFuncClass get_call_result_type(FunctionCallInfo fcinfo,
                                   Oid *resultTypeId,
                                   TupleDesc *resultTupleDesc)
</programlisting>
     passing the same <literal>fcinfo</literal> struct passed to the calling function
     itself.  (This of course requires that you use the version-1
     calling conventions.)  <varname>resultTypeId</varname> can be specified
     as <literal>NULL</literal> or as the address of a local variable to receive the
     function's result type OID.  <varname>resultTupleDesc</varname> should be the
     address of a local <structname>TupleDesc</structname> variable.  Check that the
     result is <literal>TYPEFUNC_COMPOSITE</literal>; if so,
     <varname>resultTupleDesc</varname> has been filled with the needed
     <structname>TupleDesc</structname>.  (If it is not, you can report an error along
     the lines of <quote>function returning record called in context that
     cannot accept type record</quote>.)
    </para>

    <tip>
     <para>
      <function>get_call_result_type</function> can resolve the actual type of a
      polymorphic function result; so it is useful in functions that return
      scalar polymorphic results, not only functions that return composites.
      The <varname>resultTypeId</varname> output is primarily useful for functions
      returning polymorphic scalars.
     </para>
    </tip>

    <note>
     <para>
      <function>get_call_result_type</function> has a sibling
      <function>get_expr_result_type</function>, which can be used to resolve the
      expected output type for a function call represented by an expression
      tree.  This can be used when trying to determine the result type from
      outside the function itself.  There is also
      <function>get_func_result_type</function>, which can be used when only the
      function's OID is available.  However these functions are not able
      to deal with functions declared to return <structname>record</structname>, and
      <function>get_func_result_type</function> cannot resolve polymorphic types,
      so you should preferentially use <function>get_call_result_type</function>.
     </para>
    </note>

    <para>
     Older, now-deprecated functions for obtaining
     <structname>TupleDesc</structname>s are:
<programlisting>
TupleDesc RelationNameGetTupleDesc(const char *relname)
</programlisting>
     to get a <structname>TupleDesc</structname> for the row type of a named relation,
     and:
<programlisting>
TupleDesc TypeGetTupleDesc(Oid typeoid, List *colaliases)
</programlisting>
     to get a <structname>TupleDesc</structname> based on a type OID. This can
     be used to get a <structname>TupleDesc</structname> for a base or
     composite type.  It will not work for a function that returns
     <structname>record</structname>, however, and it cannot resolve polymorphic
     types.
    </para>

    <para>
     Once you have a <structname>TupleDesc</structname>, call:
<programlisting>
TupleDesc BlessTupleDesc(TupleDesc tupdesc)
</programlisting>
     if you plan to work with Datums, or:
<programlisting>
AttInMetadata *TupleDescGetAttInMetadata(TupleDesc tupdesc)
</programlisting>
     if you plan to work with C strings.  If you are writing a function
     returning set, you can save the results of these functions in the
     <structname>FuncCallContext</structname>