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From: emery@goldfinger.mitre.org (David Emery)
Newsgroups: comp.lang.ada
Subject: Re: C-Ada Import of struct's -- Help
Date: 18 Oct 94 09:48:47
Organization: The Mitre Corp., Bedford, MA.
Message-ID: <EMERY.94Oct18094847@goldfinger.mitre.org>
References: <mmcnett.782451256@sparc53.cs.uiuc.edu>
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In-reply-to: mmcnett@sparc53.cs.uiuc.edu's message of 18 Oct 94 03:34:16 GMT
Date: 1994-10-18T09:48:47+00:00
List-Id: <comp.lang.ada>

Given
  struct queue_noncyc_s {
    char *debugname;	      /* name for debugging purposes */
    int length;		      /* length (max items+1) of queue */
    int first, last;	      /* begin and end of queue */
    Qitem *entries;	      /* array[length] of entries */
  };

I come up with the following Ada data structure
  with System;
  package C_Types is
    type char_star is new System.address;
    type c_int is new integer;
  end C_Types;

  type queue_noncyc_s is record
    debugname : C_Types.char_star;
    length : C_Types.c_int;
    first, last  : C_Types.c_int;
    entries : System.address;  -- handle specially
  end record;
(There are 'standard tricks' for handling char * values.  See my paper
 in Tri-Ada '90.)

Now one problem is determining -how- the record is passed to C.  Some
C compilers support passing structs (required by ANSI C, I believe):
	int pass_struct (struct rec bar);
others do not, and you pass the address of the struct (Original K&R C):
	int pass_addr (struct rec *bar);
The latter case is more portable, and easier to represent.  

So, given our Ada type that we want to pass to C...
	q_rec : queue_noncyc_s;

To pass it to pass_addr() we have:
	function pass_addr (addr : system.address) return c_types.int;
	...
	pass_addr (q_rec'address);	-- pretty straightforward

A problem occurs with pass_struct().  I've seen cases where the C
compiler passes the structure, and other cases where it passes the
address of the structure.  And I've seen Ada compilers that did not
support the C compiler's choice.  

You'll either have to determine how your compiler does this, or,
better, write some wrapper code in C to reduce the problem to the
'known case'.  Write this C code, then call it from Ada.

	int pass_addr (bar)
		struct rec *bar;
	{	
		return pass_struct (*bar);	
		/*  calls original routine */
	};	

The Ada call for this is the as shown above.  

				dave
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