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camlidl and 2D arrays with the [out] attribute
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Date: 2008-02-15 (20:46)
From: Hezekiah M. Carty <hcarty@a...>
Subject: Re: [Caml-list] camlidl and 2D arrays with the [out] attribute
On Fri, Feb 15, 2008 at 3:15 PM, Brian Hurt <> wrote:
> Hezekiah M. Carty wrote:
>  >I am attempting to use camlidl to wrap a function with the following C
>  >prototype portion (see [1] for the full prototype):
>  >
>  >... void c_plriddata( ... int nptsx, ... int nptsy, [out,
>  >size_is(nptsx,nptsy)] double ** zg ...);
>  >
>  >There is a lot more in the full prototype [1], but these are the
>  >important bits for this question.  In this form, the 2D zg array is
>  >not allocated as I would have expected.  Rather than allocating a
>  >rectangular nptsx by nptsy array, the generated C is:
>  >
>  >zg = camlidl_malloc(nptsx * sizeof(double *), _ctx);
>  >
>  >
>  >
>  I think the problem is with your C code.  zg is defined as a pointer to
>  a pointer- not a pointer to an array!  And yes, Virginia, there is a
>  difference.  If c_plriddata is accessing this pointer as a pointer to an
>  array, you probably have a bug in your C code.
>  Brian

The C code in question is from the PLplot library[1].  From what I
understand, zg is treated as a two dimensional array in the
c_plgriddata code - zg[i][j] to access an element.  There are a set of
functions plAlloc2dGrid(zg, xdim, ydim) and plFree2dGrid(zg, xdim,
ydim) use in the C API which respectively allocate and free a 2D array
using a (double** zg).  I have written the interface for this
c_plgriddata function by hand using these plAlloc2dGrid and
plFree2dGrid functions, but I would like to be able to use camlidl to
do the work instead.  As I mentioned, camlidl works as I expected for
other functions which take a (double**) argument with an [in]
attribute and treat them as 2D double arrays.  While it may not be
possible, I would ideally like camlidl to do the same memory
allocation for an [out] double** as it does for an [in] double **.

As I mentioned in my post, camlidl currently generates this line to
allocate zg when it has just an [out] attribute:

zg = camlidl_malloc(nptsx * sizeof(double *), _ctx);

Something along the lines of a set of camlidl_malloc(nptsy *
sizeof(double), _ctx) calls for each of the nptsx double* pointers
allocated in the above line should do the trick of allocating the
appropriate memory.  This is what camlidl does for parameters with an
[in] attribute, since it needs to copy the OCaml float array array to
a C 2D array.  I do not know how to tell camlidl to do this for
parameters with an [out] attribute, or if there is a way to tell it to
do this.

The c_plgriddata function does work properly in C applications and
with my hand-written wrapper [2], so I don't think that the problem is
in the PLplot library.


[1] -
[2] - The working hand-written wrapper code:

Hezekiah M. Carty
Graduate Research Assistant
University of Maryland
Department of Atmospheric and Oceanic Science