[Caml-list] Matching exceptions in C code

["Richard W.M. Jones" <rich@annexia.org>]

I'm trying to catch a Unix_error exception in C code, and extract the
errno from it.  Getting the errno is fine, but the problem is matching
on the Unix.Unix_error exception.

My original code used the documented method:

  retv = caml_callbackN_exn (*cb, 4, args);

  if (Is_exception_result (retv)) {
    retv = Extract_exception (retv);
    if (Field (retv, 0) == *caml_named_value ("Unix.Unix_error"))
      ...

However the if statement never matched the exception.  (I verified on
the OCaml side that the correct exception is being thrown).

After reading:

  http://caml.inria.fr/pub/ml-archives/caml-list/2006/05/097f63cfb39a80418f95c70c3c520aa8.en.html
  http://caml.inria.fr/pub/ml-archives/caml-list/2009/06/797e2f797f57b8ea2a2c0e431a2df312.en.html

it seems I'm not the first person to have this problem.  It may be
that Unix is linked twice, but I'm not really sure about that.  I'm
using -output-obj with unix.cmxa linked into the object file, and also
-lunix in the link step, which is the documented way to do things, but
I don't know if that means the Unix module initializes itself twice.

Anyway, I modified the code to this, which *works* and is actually
more convenient than the documented method (the real code matches on
several other system exceptions as well):

    ...
    retv = Extract_exception (retv);
    exn_name = String_val (Field (Field (retv, 0), 0));
    if (strcmp (exn_name, "Unix.Unix_error") == 0) {
      int errcode = code_of_unix_error (Field (retv, 1));

The questions are:

(a) Will this continue to work in future?  It seems to be
undocumented.

(b) Could we have some sort of official function to hide the details?

Rich.