RE: [Caml-list] FW: CamlIDL: Returning a whole array of cows

[Jakob Lichtenberg <jakobl@windows.microsoft.com>]

Christopher,

I probably added to the confusion by providing get_cows_inout in the first place :-)

Let me clarify: I am interested in camlidl exposing the function:

  val get_cows : unit -> cow array

My included get_cows_inout simply demonstrate my current workaround where I hack the cows API, but the ML code that use get_cows_inout is incredibly ugly.

Thanks,

- Jakob


-----Original Message-----
From: christopherleeconway@gmail.com [mailto:christopherleeconway@gmail.com] On Behalf Of Christopher L Conway
Sent: Monday, November 05, 2007 1:28 PM
To: Jakob Lichtenberg
Cc: caml-list@inria.fr
Subject: Re: [Caml-list] FW: CamlIDL: Returning a whole array of cows

Jakob,

Sorry for the confusion: my suggestion was for your get_cows_inout
function (where the integer input constrains the length of the
output). For get_cows, just use:

[size_is(*len)] cow *get_cows([ignore] int* len);

Regards,
Chris

On 11/5/07, Jakob Lichtenberg <jakobl@windows.microsoft.com> wrote:
> Christopher,
>
> Thanks for the immediate reply,
>
> Even if I can change the interface, you suggestion provides an ML API with the wrong ML type:
>   val get_cows : int -> cow array
>
> I want a type:
>   val get_cows : unit -> cow array
>
> Why: The ML user has no idea how many cows will be returned and can hence not be expected to provide this as an argument!
>
> Thanks,
>
> - Jakob
>
> -----Original Message-----
> From: christopherleeconway@gmail.com [mailto:christopherleeconway@gmail.com] On Behalf Of Christopher L Conway
> Sent: Monday, November 05, 2007 12:51 PM
> To: Jakob Lichtenberg
> Cc: caml-list@inria.fr
> Subject: Re: [Caml-list] FW: CamlIDL: Returning a whole array of cows
>
> Jakob,
>
> I sent out pretty much the same question to the list just last week
> and also didn't get a satisfactory answer (see here:
> http://caml.inria.fr/pub/ml-archives/caml-list/2007/10/efb73229fa177fb413e663a2c1ad28b4.en.html).
>  It seems that when you add a size_is annotation to a parameter, the
> size is treated as a pre-condition, not as a post-condition. If you
> have the freedom to change the C API, you might try:
>
> [size_is(*outputlen)] cow *get_cows_inout([in] int inputlen, [ignore]
> int *outputlen);
>
> IIRC, this will give you the correct behavior. The alternative is
> hand-code the wrapper.
>
> Regards,
> Chris
>
> On 11/5/07, Jakob Lichtenberg <jakobl@windows.microsoft.com> wrote:
> >
> >
> >
> >
> > Resending due to no answer.
> >
> >
> >
> > - Jakob
> >
> >
> >
> >
> >
> > From: Jakob Lichtenberg
> >  Sent: Monday, September 17, 2007 1:11 PM
> >  To: 'caml-list@inria.fr'
> >  Subject: CamlIDL: Returning a whole array of cows
> >
> >
> >
> > 1. Summary: Stub code generated by Camlidl seems to call camlidl_malloc with
> > an uninitialized size.
> >
> >
> >
> >
> >
> > 2. Details:
> >
> >
> >
> > I am declaring a function 'void get_cows([out] int* len, [length_is(*len),
> > size_is(*len), out] cow** cows);' that creates and returns an array of cows.
> >  (Each cow is just a pointer to a structure.)  The generated stub code seems
> > incorrect:
> >
> >
> >
> > cow_stubs.c:
> >
> > value camlidl_cow_get_cows(value _unit)
> >
> > {
> >
> >   int *len; /*out*/
> >
> >   cow **cows; /*out*/
> >
> >   int _c1;
> >
> >   mlsize_t _c2;
> >
> >   value _v3;
> >
> >   value _vres;
> >
> >
> >
> >   struct camlidl_ctx_struct _ctxs = { CAMLIDL_TRANSIENT, NULL };
> >
> >   camlidl_ctx _ctx = &_ctxs;
> >
> >   len = &_c1;
> >
> >   cows = camlidl_malloc(*len * sizeof(cow *), _ctx);
> >
> >   get_cows(len, cows);
> >
> >   _vres = camlidl_alloc(*len, 0);
> >
> >   Begin_root(_vres)
> >
> >     for (_c2 = 0; _c2 < *len; _c2++) {
> >
> >       _v3 = camlidl_c2ml_cow_cow(&*cows[_c2], _ctx);
> >
> >       modify(&Field(_vres, _c2), _v3);
> >
> >     }
> >
> >   End_roots()
> >
> >   camlidl_free(_ctx);
> >
> >   return _vres;
> >
> > }
> >
> >
> >
> > As you can see camlidl_malloc is called with an uninitialized value.
> >
> >
> >
> > Is this a bug in camlidl, or am I writing my IDL file incorrectly.  Another
> > way to ask:  How do I declared that a function reserves an array
> >
> >
> >
> >
> >
> > 3. Implementation details:
> >
> >
> >
> > This is my C api I'd like to access from ocaml:
> >
> >
> >
> > cow.h:
> >
> > typedef struct _cow {
> >
> >   char* name;
> >
> >   int age;
> >
> > } *cow;
> >
> >
> >
> > cow get_dummy_cow();
> >
> >
> >
> > void get_cows_inout(int inputlen, int *outputlen, cow ca[]); // Write cows
> > to 'ca', however not more than 'inputlen' elements.  Write number of cows
> > written to '*outputlen'.
> >
> >
> >
> > void get_cows(int* len, cow **ca); // Malloc array for cows.  Save number of
> > elements to *len, save address for array in '*ca'.
> >
> >
> >
> > void print_cow(cow o);
> >
> >
> >
> > I'd like to access this API from OCaml using CamlIDL.  I use the following
> > idl file:
> >
> >
> >
> > cow.idl:
> >
> > typedef [abstract] void* cow;
> >
> >
> >
> > [pointer_default(ref)] interface Cow {
> >
> >
> >
> > cow get_dummy_cow(void);
> >
> >
> >
> > void get_cows_inout([in] int inputlen, [out] int * outputlen,
> >
> >
> > [in,out,size_is(inputlen),length_is(*outputlen)] cow d[]);
> >
> >
> >
> > void get_cows([out] int* len, [length_is(*len), size_is(*len), out] cow**
> > cows);
> >
> >
> >
> > void print_cow(cow o);
> >
> >
> >
> > }
> >
> >
> >
> > I compile this using:
> >
> >
> >
> >                 camlidl cow.idl
> >
> >
> >
> > And use it from the following ML program:
> >
> >
> >
> > mlmain.ml:
> >
> > let main use_inout =
> >
> >   let cows =
> >
> >     if use_inout then Cow.get_cows_inout(Array.create 3
> > (Cow.get_dummy_cow()))
> >
> >     else Cow.get_cows()
> >
> >   in
> >
> >   Array.iter Cow.print_cow cows
> >
> >
> >
> > let _ = main true;
> >
> >
> >
> > This works fine.  However, if I change the call to main to 'main false'  I
> > get a crash.
> >
> >
> >
> > Thanks,
> >
> >
> >
> > -        Jakob
> >
> >
> >
> > PS. I know that I am leaking memory - that I can fix with a simple
> > quote(dealloc, "free(*cows);");
> > _______________________________________________
> > Caml-list mailing list. Subscription management:
> > http://yquem.inria.fr/cgi-bin/mailman/listinfo/caml-list
> > Archives: http://caml.inria.fr
> > Beginner's list:
> > http://groups.yahoo.com/group/ocaml_beginners
> > Bug reports: http://caml.inria.fr/bin/caml-bugs
> >
> >
>
>
>