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Using the C FFI to wrap an OCaml library
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Date: 2008-02-07 (22:26)
From: Damien Doligez <damien.doligez@i...>
Subject: Re: [Caml-list] Using the C FFI to wrap an OCaml library

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  
sure that Poly/ML code never allocates in the OCaml heap, and never  
back to a C function that does.

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

-- Damien