Re: Converting access values

From: Mark Lorenzen (mark.lorenzen_at_ofir.dk)
Date: 01/06/05 

Date: 06 Jan 2005 20:30:27 +0100

Duncan Sands <baldrick@free.fr> writes:

> Hi Mark,
>
> > We now want to copy parts ("slices") of the data to other tasks that
> > may do something interesting with these slices. The rate of data is
> > too high to simply copy the wanted slices, so instead we make "cheap
> > copies".
>
> if I understand right, you have a bunch of bytes that you would like the
> rest of your program to see as an array of an unconstrained array type.

Correct.

> The problem here is: where do you put the info about the array bounds?

Also correct. I have an access type *without* dope, but need an access
type *without* dope.

> I had a similar problem where my bunch of bytes was passed to me from
> a C library, and I didn't want to make a copy of it (too big), but wanted
> to treat it as an unconstrained array. This email I got from Steve Adams
> may interest you:
>
>
>
> Re: C array to Ada pointer to unconstrained array without copying memory
> To: comp.lang.ada@ada-france.org
>
> Duncan,
> I have had to do this for my project. What I do is not portable really but
> in general is OK if no other solution is available.
>
> Gnat does indeed use Fat pointers. *By Default*
> You can make it use thin pointers though:
>
> type var_arr is array (positive range <>) of T;
> type var_arr_ptr is access var_arr;
> for var_arr_ptr'size use DWORD; -- from memory, probably want a 'size of int
> or something
>
> The var_arr_ptr will only be a standard platform pointer size, 32 bits for
> ease in the rest of discussion.
> The pointer in *GNAT* points to the data (satisfies LRM that arr'access
> points to data) and immediately before this are
> the bounds, two ints every case i have for 1D arrays, lower then upper ->
>
> [l bound][u bound][data....]
>
> You need to do some pointer manipulations in C when first allocating them to
> set the bounds, but Gnat just treats the arrays as normal.
> Gnat is good like this since it means that C pointer is handled the same as
> Ada pointer.

Unfortunately this does not work in my case. I have to "copy" a slice
from the middle of an access, and the [l bound][u bound] values would
just overwrite some of the values in the array located before the
slice.

[cut]

I think I have to go for a buffer type like this:

type Buffer_Slice is
   record
     Data : Buffer_Ptr;
     First : Ada.Streams.Stream_element_Offset;
     Last : Ada.Streams.Stream_element_Offset;
   end record;

Where First .. Last defines the slice constraints. It is much more the
Ada-way, but it puts more responsibility on the user of the buffer
type, since he/she could easily overwrite parts of the array which is
outside the slice. It looks like my other idea is going to be very
nasty to implement.

- Mark

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