[ocaml-ctypes] Bind a function with a type external to the library

[Jacques-Pascal Deplaix]

Thanks a lot !

I've fixed it that way:
https://github.com/LamaUrbain/ocaml-memphis/commit/7644dbc5970067be264aa5f6b610dbb68e1b5994

Sorry, I wasn't precise on which cairo binding I used. Unfortunately 
there are two bindings that look pretty much the same. Moreover the 
header is not installed with the ocaml package, so I couldn't use the C 
define.

I've another question on a different topic. I'll do another thread for that.

Thanks again,

On 01/27/2015 12:45 PM, Jeremy Yallop wrote:
> On 21 January 2015 at 22:07, Jacques-Pascal Deplaix
> <jp.deplaix at gmail.com> wrote:
>> I'm trying to bind libmemphis with ctypes and I encounter some problems with
>> one function in particular:
>> https://github.com/LamaUrbain/ocaml-memphis/blob/master/src/memphis_renderer.ml#L55
>>
>> The function takes a cairo_t in C but in OCaml I would like to use
>> Cairo.context.
>> So I tried by using Obj.magic in the overloading just below (in the code
>> linked above) but when I try to use the function it segfaults in some random
>> cairo function (memphis_renderer_draw_tile is called before and seems to
>> work).
>>
>> Is there an other way to use an OCaml type (Cairo.context) instead of the
>> abstract C type or the problem might comes from somewhere else ?
> If the cairo_t value isn't used abstractly inside libmemphis then
> passing an OCaml value instead of a cairo_t won't work.  You need to
> somehow extract the address of the underlying C object from the
> abstract Cairo representation and then pass that address to
> libmemphis.  A quick look at the Cairo source suggests that something
> along the following lines might work, but I haven't compiled or tested
> it:
>
>    /* C stub for turning a Cairo.t into a ctypes-compatible address */
>    #include "ml_cairo.h"
>    #include <caml/mlvalues.h>
>    #include <caml/alloc.h>
>    #include <stdint.h>
>
>    value cairo_address(value v)
>    {
>       return caml_copy_native_int((intptr_t)cairo_t_val(v));
>    }
>
>
>    (* ML binding for the C stub *)
>    external cairo_raw_address : Cairo.t -> nativeint =
>      "cairo_address"
>
>    let ctypes_ptr_of_cairo_t : Cairo.t -> unit Ctypes.ptr =
>      fun c -> Ctypes.ptr_of_raw_address (cairo_raw_address c)