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From: Natasha Kerensikova <lithiumcat@gmail.com>
Newsgroups: comp.lang.ada
Subject: Re: Importing C function with variable argument list
Date: Fri, 27 Apr 2012 16:48:13 +0000 (UTC)
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Date: 2012-04-27T16:48:13+00:00
List-Id: <comp.lang.ada>

Hello,

On 2012-04-27, Jacob Sparre Andersen <sparre@nbi.dk> wrote:
> I'm afraid I have to disappoint you.  Your example (here with test
> driver) works fine on Debian/stable (armv6l):

OK. Let's pour in some more unsafety then (what an unusual for this
forums, isn't it?)

As I said, the previous example would fail to fail when C.short is
actually the same type as C.int, so let's fix that by using
C.signed_char, though I doubt that's the case here. It would also fail
to fail the extra space required by the type promotion is already
readable, so let's write stuff too. And lastly, some alignment
constraints might also cover up a failure, so let's use several
C.signed_char, assuming they will be packed.

That leads to the following example:

procedure Test is
   function Printf_Faulty (Format : in Interfaces.C.char_array;
                           Value_1 : in Interfaces.C.signed_char;
                           Value_2 : in Interfaces.C.signed_char;
                           Value_3 : in Interfaces.C.signed_char;
                           Result : out Interfaces.C.int)
      return Interfaces.C.int;
   pragma Import (C, Printf_Faulty, "printf");

   Result, Bytes_Written : Interfaces.C.int;
begin
   Bytes_Written := Printf_Faulty ("Values: %hhd, %hhd, %hhd\n%n",
                                   42, 89, 5, Result);
   pragma Assert (Bytes_Written = Result);
end Test;


Of course there would still be no symptoms if the calling convention
somehow already includes the type promotion. For example when the first
few parameters are passed through registers rather than a stack,
or when C.signed_char values have to be aligned like C.int.

I guess that's about as far as we can go in terms of failure rate.


Natasha