Re: GNAT parameter passing, C-style?

[Marin David Condic <nobody@noplace.com>]

You're talking about the ordering of parameters as they are pushed on 
the stack? I know of no mechanism to control that in the Ada standard 
and I doubt that the standard could possibly specify anything that is 
such a low level implementation detail. What about machines with large 
register sets? Compilers would want to optimize by putting parameters in 
registers and that might mess up the order you think you're getting. 
Different platforms will have different parameter passing mechanisms and 
conventions.

What you refer to as "C-Style" and "Pascal-Style" is probably misleading 
as well. What C compiler on what platform? Likewise for Pascal? AFAIK, 
there is no requirement in C or Pascal to use any particular mechanisms 
for parameter passing (beyond C allowing you to specify that something 
should live in a register.) If you had two different compiler writers 
working on two different platforms, you'd likely get two different 
mechanisms for parameter passing. Should the stack pointer increment or 
decrement as it puts items on the stack? It depends - what's the 
convention on the processor, OS, 
other-compilers-you-wish-to-be-compatible-with combinations? Does the 
machine even have a stack pointer?

If there is any such thing as an Ada mechanism, it would be through the 
pragmas for interfacing to other languages. Indicating that you are 
calling (or being called from) a C program should try to enforce C 
conventions. But there is no one, single "C-Style" anyway, so you'd have 
to know if your Ada compiler lined up with a particular C compiler. With 
Gnat, it is both an Ada and a C compiler, so it is likely to be 
consistent with itself. But Gnat couldn't promise you compatibility with 
someone else's C compiler.

MDC


Luke A. Guest wrote:
> Hi,
> 
> I've just written a program to prove it, but am I correct in saying that
> the GNAT procedure/function calling mechanism is to pass parameters from
> right-to-left (C-style), rather than left-to-right (PASCAL-style)?
> 
> I was also wondering if there is an Ada standard way to changing the
> mechanism? Or this compiler specific?
> 
> Thanks,
> Luke.
> 


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