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CamlIDL: Returning a whole array of cows
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Date: -- (:)
From: Jakob Lichtenberg <jakobl@w...>
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);");