<function id="libpq-PQfn">PQfn</function><indexterm><primary>PQfn</primary></indexterm>
   requests execution of a server function via the fast-path interface:
<synopsis>
PGresult *PQfn(PGconn *conn,
               int fnid,
               int *result_buf,
               int *result_len,
               int result_is_int,
               const PQArgBlock *args,
               int nargs);

typedef struct
{
    int len;
    int isint;
    union
    {
        int *ptr;
        int integer;
    } u;
} PQArgBlock;
</synopsis>
  </para>

  <para>
   The <parameter>fnid</parameter> argument is the OID of the function to be
   executed.  <parameter>args</parameter> and <parameter>nargs</parameter> define the
   parameters to be passed to the function; they must match the declared
   function argument list.  When the <parameter>isint</parameter> field of a
   parameter structure is true, the <parameter>u.integer</parameter> value is sent
   to the server as an integer of the indicated length (this must be
   2 or 4 bytes); proper byte-swapping occurs.  When <parameter>isint</parameter>
   is false, the indicated number of bytes at <parameter>*u.ptr</parameter> are
   sent with no processing; the data must be in the format expected by
   the server for binary transmission of the function's argument data
   type.  (The declaration of <parameter>u.ptr</parameter> as being of
   type <type>int *</type> is historical; it would be better to consider
   it <type>void *</type>.)
   <parameter>result_buf</parameter> points to the buffer in which to place
   the function's return value.  The caller must have allocated sufficient
   space to store the return value.  (There is no check!) The actual result
   length in bytes will be returned in the integer pointed to by
   <parameter>result_len</parameter>.  If a 2- or 4-byte integer result
   is expected, set <parameter>result_is_int</parameter> to 1, otherwise
   set it to 0.  Setting <parameter>result_is_int</parameter> to 1 causes
   <application>libpq</application> to byte-swap the value if necessary, so that it
   is delivered as a proper <type>int</type> value for the client machine;
   note that a 4-byte integer is delivered into <parameter>*result_buf</parameter>
   for either allowed result size.
   When <parameter>result_is_int</parameter> is 0, the binary-format byte string
   sent by the server is returned unmodified. (In this case it's better
   to consider <parameter>result_buf</parameter> as being of
   type <type>void *</type>.)
  </para>

  <para>
   <function>PQfn</function> always returns a valid
   <structname>PGresult</structname> pointer, with
   status <literal>PGRES_COMMAND_OK</literal> for success
   or <literal>PGRES_FATAL_ERROR</literal> if some problem was encountered.
   The result status should be
   checked before the result is used.   The caller is responsible for
   freeing  the  <structname>PGresult</structname>  with
   <xref linkend="libpq-PQclear"/> when it is no longer needed.
  </para>

  <para>
   To pass a NULL argument to the function, set
   the <parameter>len</parameter> field of that parameter structure
   to <literal>-1</literal>; the <parameter>isint</parameter>
   and <parameter>u</parameter> fields are then irrelevant.
  </para>

  <para>
   If the function returns NULL, <parameter>*result_len</parameter> is set
   to <literal>-1</literal>, and <parameter>*result_buf</parameter> is not
   modified.
  </para>

  <para>
   Note that it is not possible to handle set-valued results when using
   this interface.  Also, the function must be a plain function, not an
   aggregate, window function, or procedure.
  </para>

 </sect1>

 <sect1 id="libpq-notify">
  <title>Asynchronous Notification</title>

  <indexterm zone="libpq-notify">
   <primary>NOTIFY</primary>
   <secondary>in libpq</secondary>
  </indexterm>

  <para>
   <productname>PostgreSQL</productname> offers asynchronous notification
   via the <command>LISTEN</command> and <command>NOTIFY</command>