Re: [Caml-list] How to pass C pointers to Caml

[Goswin von Brederlow <goswin-v-b@web.de>]

Florent Monnier <monnier.florent@gmail.com> writes:

> Le lundi 1 mars 2010 04:55:00, Jianzhou Zhao a écrit :
>> I have been calling OCaml code from C in my project.
>> The C code has some pointers to C structures.
>> I got 'seg fault' when calling the OCaml function receiving
>> C structure pointers.
>> 
>> 18.7 at http://caml.inria.fr/pub/docs/manual-ocaml/manual032.html
>> gives the examples that pass int into OCaml. These examples work for me.
>> But, Does OCaml support to pass C structure pointers to OCaml?
>
> Yes it does. Just cast your pointer to the type value.
>
> In this tutorial there is an example "Pointers to C structures":
> http://www.linux-nantes.org/~fmonnier/OCaml/ocaml-wrapping-c.php#ref_ptr
>
> the pointer to a C struct is wrapped on the ocaml side by an abstract type 
> called "t" here, and it is provided back to C with print_t / dump_ptr.

The problem with this trivial approach is that ocaml can store the
pointer somewhere. When the C pointer is freeed then ocaml has a
dangling pointer. Worse, if the GC allocates a new heap then the pointer
might suddenly point into the heap and then BOOM.

It is better to put the pointer into an abstract or custom block.

MfG
        Goswin