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From: Simon Wright <simon@pogner.demon.co.uk>
Subject: Re: ada to c interfaces and CHARS_PTR
Date: 2000/03/11
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 <slrn8cg0rv.mlp.gisle@spurv.ii.uib.no>
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gisle@spurv.ii.uib.no (Gisle S�lensminde) writes:

> C has not 'in out' parameters, only 'in' parameters. To pass the 
> pointer to the string you can use system'address. str'address is
> equivalent to &str in C. The following Ada code can replace 
> the Ada code of your last posting:

Gisle,

If you do this, you're missing a lot of typechecking.

Certainly on GNAT the compiler has a deep understanding of "convention
C" (this is covered in Annex B.3 (63) ff). I think the one you want is
(68) -- a GNAT chars_ptr is (eventually) a
System.Storage_Elements.Integer_Address -- but if your compiler
doesn't handle this level of interfacing properly I think you should
send it back for a refund!

--------- strs.adb -----------------------------------------------------
with Interfaces.C.Strings;
procedure Strs is
  procedure O_Str (O_Str : out Interfaces.C.Strings.Chars_Ptr);
  pragma Import (C, O_Str, "o_str");
  procedure I_Str (I_Str : Interfaces.C.Strings.Chars_Ptr);
  pragma Import (C, I_Str, "i_str");
  Str : Interfaces.C.Strings.Chars_Ptr;
begin
  O_Str (Str);
  I_Str (Str);
end Strs;
------------------------------------------------------------------------

--------- strs_c.c -----------------------------------------------------
#include <stdio.h>
#include <stdlib.h>

char sig[] = "a message from o_str()";

void o_str(char **s)
{
  *s = sig;
}

void i_str(char *s)
{
  printf("i_str(%s).\n", s);
}
------------------------------------------------------------------------

gcc -c strs_c.c
gnatmake strs -largs strs_c.o

-Simon