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From: Pascal Brisset <pascal.brisset@cnet.francetelecom.fr>
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Date: Tue, 3 Nov 1998 10:32:51 +0100 (MET)
To: Thierry Bravier <thierry.bravier@dassault-aviation.fr>
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Subject: Re: problem with ocamlmktop (contd)
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Pascal Brisset writes:
 > (3) catching a C++ exception generated by a C++ primitive called
 >     through a Caml callback.

Oops, I shouldn't have included the last part... Here is a tentative
list of issues regarding exceptions and Caml/C++ interfacing.
I hope others will contribute; this is definitely tricky.

* g++-2.8.1 seems to link C/C++ reliably, therefore nesting Caml/C++
  and C++/Caml calls without exceptions on either side should be safe.

* C++ primitive raising a Caml exception:

  The following code is incorrect because mlraise uses siglongjmp,
  which does not unwind the C++ stack (obj will not be destroyed):
   |   extern "C" value caml_stub(value) {
   |     Object obj(...);
   |     mlraise(...);
   |   }

  This might be safe for practical purposes:
   |   extern "C" value caml_stub(value) {
   |     try { /* pure C++ stuff */; }
   |     catch (...) { failwith("Uncaught C++ exception"); }
   |   }
  but only a C++ expert could tell whether the exception's destructor
  (if any) will be called...

* Callback raising a Caml exception:

  This is unsafe for the same reason:
   |   extern "C" value caml_stub(value) {
   |     Object obj(...);
   |     callback(...);
   |   }

* C++ primitive throwing a C++ exception:

  We might want to have the interpreter translate all C++ exceptions
  into Caml exceptions:
   |   Instruct(C_CALL1):
   |     Setup_for_c_call;
   |     try { accu = cprim[*pc](accu); }
   |     catch (...) {
   |       local_roots = initial_local_roots;
   |       failwith("Uncaught C++ exception");
   |     }
   |     Restore_after_c_call;
   |     ...
  The local_roots assignment is meant to handle exceptions thrown from
  within a Begin_roots/End_roots block (i.e. don't let failwith()
  allocate memory while local_roots points to an obsolete stack
  frame). Is this enough ?

* Caml callback throwing a C++ exception (i.e. trying to embed Caml
  code in C++ code transparently like in my example):

  I believe similar changes are required in order to return from
  interprete() gracefully (catch the C++ exception, handle things like
  external_raise, callback_depth and local_roots, then re-throw the
  exception). Has anyone done this ?

- Pascal Brisset <pascal.brisset@cnet.francetelecom.fr> +33296051928 -
- France Telecom CNET DTL/MSV | 2 av Pierre Marzin | F-22307 Lannion -