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

[Florent Monnier <monnier.florent@gmail.com>]

Le mardi 2 mars 2010 11:19:58, vous avez écrit :
> Florent Monnier <monnier.florent@gmail.com> writes:
> > Le lundi 1 mars 2010 14:24:45, Goswin von Brederlow a écrit :
> >> 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_p
> >> >tr
> >> >
> >> > 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.
> >
> > A lot of bindings wrap C pointer, it is known to be a technic that does
> > work. Dangerous that's true, be if you are very careful, it works.
> > What you can do is set the pointer to NULL when the struct is freed, and
> > then each function that uses this struct pointer can first check if the
> > pointer is NULL or not before to use it, and if it's NULL raise an
> > exception.
> 
> let x = ref None
> 
> let called_function c_ptr = x := Some c_ptr
> 
> How will you get x to be Some NULL?

I mean often in a C library pointers to C struct are just pointers to 
something abstract because a lot of lib C API do hide the struct, and provide 
a function to free the things pointed. So in your wrapper after you call the 
destroy function you can set the pointer to NULL and in the other functions 
test if the pointer is NULL before to use it to prevent a user of the wrapper 
to call a function after having called the destroy function.

>                                     Your C code does not know about the
> copy. You need to wrap the C pointer into a custom or abstract block
> first to be able to NULL it. A finalizer in a custom block can also be
> helpfull here and free the pointer when ocaml no longer needs it.
> 
> >> It is better to put the pointer into an abstract or custom block.
> >
> > You can do this too.
> 
> Imho you must. Anything else is too dangerous.

Yes it is dangerous, but as I explained with some C libraries this is the only 
possible solution. If you can do it the other way, then do, while it's indeed 
safer.

-- 
Cheers