Re: Importing C function with variable argument list

[Adam Beneschan <adam@irvine.com>]

On Thursday, April 12, 2012 3:08:28 AM UTC-7, Markus Schöpflin wrote:
> 
>      It may work on some platforms to directly interface to a varargs function 
> by providing a specific Ada profile for a particular call. However, this does 
> not work on all platforms, since there is no guarantee that the calling 
> sequence for a two argument normal C function is the same as for calling a 
> varargs C function with the same two arguments.

Right.  My recollection is that on some RISC-type machines, the normal calling conventions require that subprograms know the exact size of the parameter frame getting passed to it, and they adjust the stack pointer by that amount before returning.  I'm not entirely sure I'm correct; perhaps the caller does the adjusting on all of those machines.  But if I'm right, then C functions with varargs would definitely have to follow a different convention, and the RM note is wrong.

I hadn't realized this problem in the RM before.  But it looks to me like B.3(75) should be removed, or at least changed to say that it's correct only on some platforms whose calling conventions meet certain conditions.

> So the note from the Ada RM does not seem to be followed by GNAT

I don't think GNAT has a choice whether to follow it or not, since GNAT can't be expected to know what the code of the imported C subprogram is going to look like.  All it can do is obey what the user tells it to do.

                        -- Adam