[Caml-list] Hybrid ML/C programs and exceptions

[Caml-list] Hybrid ML/C programs and exceptions

[Tom Buscher]

Hi,

We have a hybrid O'Caml/C application in which ML calls C,
which calls ML, like this:

  ML -> C -> ML
   1    2    3

The "lower" ML(3) raises an exception, which is dealt with
by C(2) using one of the callback_exn functions. When the C
function returns, the "upper" ML function generates a bus
error on Solaris 8, on writing to a mutable global variable.
We are using O'Caml 3.04 with the bytecode compiler.

Attached is a sample program of the same structure that
reproduces this problem. The problem appears to be caused by
the handling of the RAISE opcode in interprete(), executing
the "lower" ML bytecode. It sets extern_sp to a value which
reflects the program state before the C layer was called.

Is this program structure allowed?

Regards,

Tom, Trevor and Jeff

------------------------------------------------------------
FILE: top.ml
------------------------------------------------------------
exception Bot;;
let bot() = raise Bot 
let _ = Callback.register "bot" bot

external foo : unit -> unit = "foo"

let r = ref 1;;
r := 2;
foo();
r := 3

------------------------------------------------------------
FILE: foo.c
------------------------------------------------------------
#include <caml/mlvalues.h>
#include <caml/callback.h>
#include <caml/memory.h>

CAMLprim void
foo(value unit)
{
        CAMLparam1(unit);
        callback_exn(*caml_named_value("bot"), Val_int(0));
        CAMLreturn0;
}

------------------------------------------------------------
Build command (with gcc-2.95.3):

  ocamlc -custom -o top top.ml foo.c
------------------------------------------------------------
Tom Buscher                       "For every complex problem,
GNP Computers, Inc.                there is a simple solution.
tbuscher@gnp.com                   And it is wrong."
fon: +1 626.305.8484
fax: +1 626.599.3115                         -- H. L. Mencken
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Re: [Caml-list] Hybrid ML/C programs and exceptions

[Remi VANICAT]

Tom Buscher <tbuscher@gnp.com> writes:

> Hi,
> 
> We have a hybrid O'Caml/C application in which ML calls C,
> which calls ML, like this:
> 
>   ML -> C -> ML
>    1    2    3
> 
> The "lower" ML(3) raises an exception, which is dealt with
> by C(2) using one of the callback_exn functions. When the C
> function returns, the "upper" ML function generates a bus
> error on Solaris 8, on writing to a mutable global variable.
> We are using O'Caml 3.04 with the bytecode compiler.
> 
> Attached is a sample program of the same structure that
> reproduces this problem. The problem appears to be caused by
> the handling of the RAISE opcode in interprete(), executing
> the "lower" ML bytecode. It sets extern_sp to a value which
> reflects the program state before the C layer was called.
> 
> Is this program structure allowed?
> 
> Regards,
> 
> Tom, Trevor and Jeff
> 
> ------------------------------------------------------------
> FILE: top.ml
> ------------------------------------------------------------
> exception Bot;;
> let bot() = raise Bot 
> let _ = Callback.register "bot" bot
> 
> external foo : unit -> unit = "foo"
> 
> let r = ref 1;;
> r := 2;
> foo();
> r := 3
> 
> ------------------------------------------------------------
> FILE: foo.c
> ------------------------------------------------------------
> #include <caml/mlvalues.h>
> #include <caml/callback.h>
> #include <caml/memory.h>
> 
> CAMLprim void
> foo(value unit)
> {
>         CAMLparam1(unit);
>         callback_exn(*caml_named_value("bot"), Val_int(0));
>         CAMLreturn0;
> }

this seem suspicious, CAMLprim should have type

CAMLprim value foo(value)

CAMLreturn0 is only usefull in the case of something like

ML call C witch call C witch call ML

(for example it useful for Gtk binding, where there are C callback
which then call ML).

by the way, changing foo.c with :

#include <caml/mlvalues.h>
#include <caml/callback.h>
#include <caml/memory.h>

CAMLprim value
foo(value unit)
{
        CAMLparam1(unit);
        callback_exn(*caml_named_value("bot"), Val_int(0));
        CAMLreturn(Val_unit);
}

doesn't change the problem...

-- 
Rémi Vanicat
vanicat@labri.u-bordeaux.fr
http://dept-info.labri.u-bordeaux.fr/~vanicat
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Re: [Caml-list] Hybrid ML/C programs and exceptions

[Xavier Leroy]

> We have a hybrid O'Caml/C application in which ML calls C,
> which calls ML, like this:
> The "lower" ML(3) raises an exception, which is dealt with
> by C(2) using one of the callback_exn functions. When the C
> function returns, the "upper" ML function generates a bus
> error on Solaris 8, on writing to a mutable global variable.
> We are using O'Caml 3.04 with the bytecode compiler.
> 
> Is this program structure allowed?

Yes, it is, and if you compile with the native-code compiler ocamlopt,
it works fine.  However, you've hit a bug in the bytecode interpreter
that causes callback_exn to misbehave.  This will be fixed shortly.
Thanks for reporting the problem.

- Xavier Leroy
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