Re: Ocaml C interface - Usage of custom blocks

Re: Ocaml C interface - Usage of custom blocks

[Dominik Brugger]

Is there a "best practice" for returning C data to OCaml 
which was allocated by malloc?

One way to do this is given by the curses library example
in the OCaml reference manual in section 18.6:

value curses_initscr(value unit)
{
  CAMLparam1 (unit);
  CAMLreturn ((value) initscr());  /* OK to coerce directly from WINDOW * to
                              value since that's a block created by malloc() */
}

But section 18.2.3 of the manual points out, that it is potentially
dangerous to free C data, as it might be reclaimed by the OCaml GC.
So what happens if the data is never explicitly freed? Does the OCaml heap
space grow until there is no more memory available to the C part of the
OCaml program?

In my opinion the only way to avoid these problems is the usage of
OCaml custom blocks.

Dominik

Re: [Caml-list] Re: Ocaml C interface - Usage of custom blocks

[Lukasz Stafiniak]

2005/9/22, Dominik Brugger <dominikbrugger@gmx.de>:
> Is there a "best practice" for returning C data to OCaml
> which was allocated by malloc?
>

My bet is on custom blocks.

> But section 18.2.3 of the manual points out, that it is potentially
> dangerous to free C data, as it might be reclaimed by the OCaml GC.

No. It is dangerous to free C data, because you might still use them
on the OCaml side. GC will not reclaim "malloc pointers".

> So what happens if the data is never explicitly freed? Does the OCaml heap
> space grow until there is no more memory available to the C part of the
> OCaml program?

The data (allocated by malloc) is not on OCaml heap. You have memory
leak on the C side.
>
> In my opinion the only way to avoid these problems is the usage of
> OCaml custom blocks.

My too. The difficulty comes, when the data is also refered by other C
data structures. I've solved it by reference counters, decremented
explicitly on C side and by finalisation on OCaml side.

Lukasz

Re: [Caml-list] Re: Ocaml C interface - Usage of custom blocks

[Damien Doligez]

On Sep 22, 2005, at 20:33, Lukasz Stafiniak wrote:

> 2005/9/22, Dominik Brugger <dominikbrugger@gmx.de>:
>
>> Is there a "best practice" for returning C data to OCaml
>> which was allocated by malloc?
>
> My bet is on custom blocks.

Here is the official word from the OCaml team: custom blocks are
the way to go.

Have a nice day,

-- Damien