Re: Ada/C [GNAT]: Accessing C arrays from Ada

["David C. Hoos, Sr." <david.c.hoos.sr@ada95.com>]

Hi Michael,
Michael K Rohan <mrohan@ix.netcom.com> wrote in article
<32225A96.4512F115@ix.netcom.com>...
> Hello,
> 
> I'm trying to figure out the best way to use a C pointer as an array
> from Ada.  I have a C service function which returns the number of items
> and a pointer to a vector of the items:
> 
>         Query(int *n, item** items);
> 
> Interfacing from Ada is OK: I can get the number of items and the
> pointer as an "access all Item", but how do I access the individual
> items?
> 
> Thank you, in advance, for any help/info,
> Michael.
> 
Assuming that the C function Query allocates memory for the items, you need
to do something like the following:

Here is a library-level Ada procedure which demonstrates what you want to
do.   It has been tested with gnat 3.05 on IRIX 5.3]
VADS 623.53.3 on IRIX 5.3 and IRIX 6.2, and with gnat 3.04a on Win95.

Note that there are many other ways this could have been done.  For
example, if we were not looking for Ada 83 compatibility, we could use the
Interfaces.C.Pointers package from Ada 95's Annex B.

This approach shows how to declare an array which takes on the attributes
'FIRST, 'LAST 'LENGTH, etc., corresponding to the number of elements
returned by the C function.  This allows passing the Ada array to another
Ada subprogram, for example.

with System;
with Text_IO;
procedure Do_Something is
  subtype ITEM_TYPE is INTEGER;  -- Ada Equivalent of the C item type
  -- We use INTEGER here only for example.
  
  -- The type of an array of items
  type ITEM_ARRAY_TYPE is array (POSITIVE range <>) of ITEM_TYPE;
  
  -- The Ada specification of the C function   
  procedure Query (
      Number_Of_Items_Address    : in System.ADDRESS;
      Item_Array_Address_Address : in System.ADDRESS
      );
  
  -- The ada "implementation" of the C function is to interface to the C
  -- function's implementation.
  pragma INTERFACE (C, Query);
  
  The_Item_Array_Address : System.ADDRESS;
  -- This is where we will tell the Query function to put the address of
the
  -- array of items it has allocated.
  
  The_Number_Of_Items    : NATURAL;
  -- This is where we will tell the Query function to put the number of
items
  -- in the array.
begin
  -- Call the query function.
  Query (
    Number_Of_Items_Address    => The_Number_Of_Items'ADDRESS,
    Item_Array_Address_Address => The_Item_Array_Address'ADDRESS
    );
    
  -- Now that we know how many items are in the array we can declare the
array
  -- object.
  declare
    -- This is required for Ada 95.  See further note below.
    The_Constant_Item_Array_Address : constant System.ADDRESS := 
        The_Item_Array_Address;
	
    -- This is the array declaration.
    The_Item_Array : ITEM_ARRAY_TYPE (1 .. The_Number_Of_Items);
    
    -- This is the address clause which defines the location of the array
of
    -- items.  NOTE:  In Ada95, Address clauses must have constant
addresses,
    -- hence the need to declare the constant above.  In Ada83 we could
have
    -- just used the variable set by Query, directly.
    for The_Item_Array use at The_Constant_Item_Array_Address;
    
  begin
    -- Here we demonstrate that we have the correct data
    for i in The_Item_Array'RANGE loop
      Text_IO.Put_Line (
          "Item (" & INTEGER'IMAGE (i) & " ) =>" &
          INTEGER'IMAGE (The_Item_Array (i))
          );
    end loop;
  end;
end Do_Something;
-- Example C source file for test 
-- #define length(a) (sizeof (a) / sizeof (* a)) 
-- static int data [] = {1,2,4,8,16,32,64,128};
-- /* NOTE: In a real implementation, the data is probably in dynamic
memory,
--    so, one would probably want an interface to "free", so that the
memory
--    could be deallocated from Ada code. */
-- void query (int * n, int ** items)
-- {
--   * n = length (data);
--   * items = data;
-- }
-- 

David C. Hoos, Sr.,
http://www.dbhwww.com
http://www.ada95.com