Getting length of array from C

Getting length of array from C

[Chuck]

Is there some way of getting an ada array length from C interface code?
I have to try and accomplish this while minimizing the number of
changes to the original Ada code.  Any ideas?

Thanks,
Chuck

Re: Getting length of array from C

[Simon Wright]

"Chuck" <cwinters15@comcast.net> writes:

> Is there some way of getting an ada array length from C interface
> code?  I have to try and accomplish this while minimizing the number
> of changes to the original Ada code.  Any ideas?

Afraid I can't tell which way round you want this to work! Do you have
an Ada array that you need C code to be able to get at, or a C array
that you need Ada code to be able to get at? What's the concept of
operation?

Typically C code needs/can provide the address of the first element of
the array and a count of the number of elements.

Depending on your compiler, you may find something like

  procedure C_Proc (A : My_Array; Count : Integer);
  pragma Import (C, C_Proc, "c_proc");

helpful. You might need to declare a derived array type to get the
representation to be compatible:

  type My_C_Array is new My_Array;
  pragma Convention (C, My_C_Array);

-- 
Simon Wright                               100% Ada, no bugs.

Re: Getting length of array from C

[Martin Krischik]

Chuck wrote:

> Is there some way of getting an ada array length from C interface code?
> I have to try and accomplish this while minimizing the number of
> changes to the original Ada code.  Any ideas?

I did a lot of C interfacing and there are quite a few option open. Usualy
you need two parameters one for the array one for the size.

A very interesting hint - if you declare the following type inside a
procedure:

  type C_Array is array 0 .. Number - 1 of Interfaces.C.int;

then Number does not need to a constant. And of course the other way around
as well:

  type Ada_Array is array 1 .. Number of Integer;


Martin

-- 
mailto://krischik@users.sourceforge.net
http://www.ada.krischik.com

Re: Getting length of array from C

[Chuck]

Here's the deal.  I currently have some Ada code.  Quite a bit of it.
The originators of the code had done some C interfacing with the Ada
code.  So, I have some procedures like the following:

procedure Write_Array( my_array : in My_Array_Type );
pragma Import( C, Write_Array, "c_write_array" );

The C function prototype from the header file I was given is:
void c_write_array( void *array );

In the C code is there anyway I can get the length of the incoming
array?  So, is Ada actually passing in a structure or an array?
Hope this clears things up.

Chuck

Re: Getting length of array from C

[Martin Krischik]

Chuck wrote:

> Here's the deal.  I currently have some Ada code.  Quite a bit of it.
> The originators of the code had done some C interfacing with the Ada
> code.  So, I have some procedures like the following:
> 
> procedure Write_Array( my_array : in My_Array_Type );
> pragma Import( C, Write_Array, "c_write_array" );

Well, at least GNAT is very upset if My_Array_Type is not a fixed array
type. You are allowed to do it - but there is warning about the use of the
dope vector.

http://en.wikipedia.org/wiki/Dope_vector
 
> The C function prototype from the header file I was given is:
> void c_write_array( void *array );

That does not look right. If My_Array_Type is fixed then there should be
typedef documenting that. If My_Array_Type in not fixed then indeed a
length parameter is missing.

> In the C code is there anyway I can get the length of the incoming
> array?  So, is Ada actually passing in a structure or an array?
> Hope this clears things up.

If My_Array_Type is fixed the the size is fixed. If My_Array_Type is not
fixed then the code is broken because someone forgot to pass on the dope
vector.

Martin
-- 
mailto://krischik@users.sourceforge.net
http://www.ada.krischik.com

Re: Getting length of array from C

[Ludovic Brenta]

"Chuck" writes:
> Here's the deal.  I currently have some Ada code.  Quite a bit of it.
> The originators of the code had done some C interfacing with the Ada
> code.  So, I have some procedures like the following:
>
> procedure Write_Array( my_array : in My_Array_Type );
> pragma Import( C, Write_Array, "c_write_array" );
>
> The C function prototype from the header file I was given is:
> void c_write_array( void *array );
>
> In the C code is there anyway I can get the length of the incoming
> array?  So, is Ada actually passing in a structure or an array?
> Hope this clears things up.
>
> Chuck

No length information is passed to, or expected by the C function.
The "designers" (if they call themselves that) of this old code don't
want you to be bothered with array lengths :)

Because of the pragma Import, there is no dope vector either; the C
function just gets a void* pointing at the first element in the array.

This reminds me of the C library's null-terminated strings.  Of
course, all the functions in the C library that fail to use a length
parameter are deprecated (e.g. strdup is deprecated in favour of
strndup).

You should look for a design document that explains where the array
ends; there may be a terminating element at the end of the array, or
perhaps the array has a "well-known size" somewhere.  If My_Array_Type
is constrained, you're in luck because you'll know the size at compile
time.

-- 
Ludovic Brenta.

Re: Getting length of array from C

[Dave Thompson]

On Tue, 01 Feb 2005 21:07:08 +0100, Ludovic Brenta
<ludovic.brenta@insalien.org> wrote:
<snip>
> This reminds me of the C library's null-terminated strings.  Of
> course, all the functions in the C library that fail to use a length
> parameter are deprecated (e.g. strdup is deprecated in favour of
> strndup).
> 
Presumably you mean strncpy vs strcpy and strncat vs strcat, and the
wide-string wcs* versions similarly. These latter are not deprecated
by any C standard (yet), and the former are NOT just exact
replacements-plus-size for the latter -- this is a fairly frequent
source of problems posted to comp.lang.c. Neither is sprintf
deprecated in favor of snprintf, which is only standard as of C99.

(If you want the gritty, *ncpy pads if short but doesn't terminate if
long; *ncat always terminates but requires the _available/remaining_
size minus 1 not the total size. Oh, and snprintf was fairly widely
available pre-C99 but often with a different return value for the
overflow case. Oh joy.)

There is work in WG14 (and J11) currently on a proposed "TR"
(effectively a standardized option) for "secure" versions of library
routines that are exact replacements. If this is widely adopted it
might become required and the older forms deprecated around '09 or so.
Berkeley strlcpy and strlcat already are exact replacements, which
some people promote as preferable, but they are not standard, not even
POSIX/SUS.

To be clear, even in the 'n' 'l' and 's' versions only the size aka
maximum length (or a variant of it) is an added/explicit parameter;
the _current_ length is still done by NUL-termination. If you want
explicitly counted strings you have to build them yourself with mem*,
although of course in C++ you can package the result as classes with
methods and if you want operators -- or just use std::string which is
already pointer+counts like Ada Unbounded_String.

strdup is not in any C standard, although it is POSIX/SUS; strndup
doesn't exist at all and probably doesn't need to since strdup's
target is newly allocated large enough space not an existing buffer.

> You should look for a design document that explains where the array
> ends; there may be a terminating element at the end of the array, or
> perhaps the array has a "well-known size" somewhere.  If My_Array_Type
> is constrained, you're in luck because you'll know the size at compile
> time.

- David.Thompson1 at worldnet.att.net

Re: Getting length of array from C

[Jeffrey Carter]

Chuck wrote:

> procedure Write_Array( my_array : in My_Array_Type );
> pragma Import( C, Write_Array, "c_write_array" );
> 
> The C function prototype from the header file I was given is:
> void c_write_array( void *array );

There isn't enough information here to know what c_write_array is doing 
with the array. You may have to look at the body to know how to use it.

> In the C code is there anyway I can get the length of the incoming
> array?  So, is Ada actually passing in a structure or an array?
> Hope this clears things up.

The only way for C to know the length is to pass it as a 2nd parameter. 
Ada will pass a convention-C pointer to the array's 1st element.

-- 
Jeff Carter
"You cheesy lot of second-hand electric donkey-bottom biters."
Monty Python & the Holy Grail
14

Re: Getting length of array from C

[Steve]

How is "My_Array_Type" defined?

Steve
(The Duck)

"Chuck" <cwinters15@comcast.net> wrote in message 
news:1107263868.447580.257310@z14g2000cwz.googlegroups.com...
> Here's the deal.  I currently have some Ada code.  Quite a bit of it.
> The originators of the code had done some C interfacing with the Ada
> code.  So, I have some procedures like the following:
>
> procedure Write_Array( my_array : in My_Array_Type );
> pragma Import( C, Write_Array, "c_write_array" );
>
> The C function prototype from the header file I was given is:
> void c_write_array( void *array );
>
> In the C code is there anyway I can get the length of the incoming
> array?  So, is Ada actually passing in a structure or an array?
> Hope this clears things up.
>
> Chuck
>