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How to pass C pointers to Caml
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Date: 2010-03-03 (11:02)
From: Goswin von Brederlow <goswin-v-b@w...>
Subject: Re: [Caml-list] How to pass C pointers to Caml
Florent Monnier <monnier.florent@gmail.com> writes:

> 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.

It is always possible to wrap the C pointer into an abstract or custom
block. The pointer can still become dangling when the C side frees the
target before ocaml forgets the block but, as the GC does not look into
abstract/custom blocks, it will never result in the GC exploding because
a dangling pointer points into the GC heap.