on result
of type <span class=font-it>ty</span> is converted from C to Caml. The function can then check
the <span class=font-it>ty</span> value for values indicating an error condition, and raise the
appropriate exception. If in addition the <span class=syntax-token>errorcode</span> attribute is
provided, the conversion from C to Caml is suppressed: values of type
<span class=font-it>ty</span> are only passed to <span class=nonterminal>fn</span> for error checking, then discarded.</p>
<h3 id="sec27" class=