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FW: CamlIDL: Returning a whole array of cows
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Date: 2007-11-05 (18:32)
From: Jakob Lichtenberg <jakobl@w...>
Subject: FW: CamlIDL: Returning a whole array of cows
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:

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);
    for (_c2 = 0; _c2 < *len; _c2++) {
      _v3 = camlidl_c2ml_cow_cow(&*cows[_c2], _ctx);
      modify(&Field(_vres, _c2), _v3);
  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:

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:

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:

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()
  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.


-        Jakob

PS. I know that I am leaking memory - that I can fix with a simple quote(dealloc, "free(*cows);");