Re: [Caml-list] Matching exceptions in C code

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

On Wed, Jun 21, 2017 at 04:50:06PM +0100, John Whitington wrote:
> >>    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));
> > 
> > So I found out today that this doesn't work for every exception.
> > 
> > For 'End_of_file' in particular, 'exn_name' ends up pointing to a bit
> > of random memory (unfortunately it's not NULL or otherwise easily
> > detectable), and so the program crashes in the strcmp.
> 
> End_of_file, Invalid_argument, and Failure are not defined in pervasives.ml, they are “internal” in some sense. Perhaps this is the cause?

Actually it's not a random pointer, it's a pointer made of ASCII
characters.  In other words it seems there is one less indirection.
This code works for me:

  if (Tag_val (Field (exn, 0)) == String_tag)
    exn_name = String_val (Field (exn, 0));    // for End_of_file
  else
    exn_name = String_val (Field (Field (exn, 0), 0));  // for others

but I feel like I'm well outside the bounds of valid, supportable use
of the OCaml VM here.

Rich.