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From: Damien Doligez <damien.doligez@inria.fr>
To: caml-list <caml-list@yquem.inria.fr>
Cc: Joel Stanley <jstanley@galois.com>
Subject: Re: [Caml-list] Using the C FFI to wrap an OCaml library
Date: Thu, 7 Feb 2008 23:26:54 +0100	[thread overview]
Message-ID: <24097E85-C08E-4547-986B-4EF91B7692C3@inria.fr> (raw)
In-Reply-To: <85FB0157-5721-45F1-9440-A3021913BA1F@galois.com>


On 2008-02-07, at 00:54, Joel Stanley wrote:

> 1. Can the CAMLparam*/CAMLlocal*/CAMLreturn macros be used to safely
> carry values of type 'value' between C functions (even C functions  
> that
> have the appropriate CAMLparam/CAMLreturn invocations present)?

Yes, they are designed for that.

> I
> thought this worked, but more extensive testing has yielded some hangs
> with the stack trace looking like:
>
>    #0  0x00002b98 in caml_oldify_local_roots ()
>    #1  0x00004dd7 in caml_empty_minor_heap ()
>    #2  0x00004f28 in caml_minor_collection ()
>    #3  0x00003501 in caml_garbage_collection ()
>    #4  0x00011888 in caml_call_gc ()
>    #5  0x00013577 in run_solver ()
>    #6  0x00013cb4 in main ()
>
> where run_solver here is the C function that is passed an opaque  
> object
> reference (elided by a value of type 'value') from another C function,
> and is calling methods repeatedly on the provided object via  
> caml_callback.

This could be anything, the most likely is that you used "return"
somewhere instead of "CAMLreturn".

> Looking at the macro expansions, I'm suspicious about the safety  
> between
> C functions,

Could you elaborate on what makes you say that?  The macros carefully
implement a stack discipline designed for nested calls.

> and wonder if the only way to carry data is use
> caml_register_global_root (and manage my own memory if I need dynamic
> allocation).

You don't want to do that, it would be too inefficient.

> 2. As a follow-up question to #1, the OCaml values may need to be
> carried across yet another FFI, in this case, C <-> Poly/ML .  Even if
> using the macros was a safe way to carry values between C functions,  
> I"m
> not sure I can easily replicate the macros on the other side of the  
> FFI,
> so am wondering if an explicit memory management approach using
> caml_register_global_root will work.  E.g.,
>
>  value* alloc_value() {
>    value* p = malloc(sizeof(value));
>    caml_register_global_root(p);
>    return p;
>  }


In order to make this work, you have to explain to Poly/ML that every
access to the value must be done through the value*.  Or you need to  
make
sure that Poly/ML code never allocates in the OCaml heap, and never  
calls
back to a C function that does.

I think it's more reasonable to just copy the data between worlds  
instead
of trying to share pointers between OCaml and Poly/ML.

-- Damien


  reply	other threads:[~2008-02-07 22:26 UTC|newest]

Thread overview: 5+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2008-02-06 23:54 Joel Stanley
2008-02-07 22:26 ` Damien Doligez [this message]
2008-02-08 21:53   ` [Caml-list] " Joel Stanley
2008-02-09  4:07     ` Jonathan Bryant
2008-02-12 21:58       ` Damien Doligez

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