Re: Ada to 'C' parameter passing problem

[rod.chapman@praxis-cs.co.uk (Rod Chapman)]

Pat.Donahue@msfc.nasa.gov (Patrick) wrote in message news:<ea3b54bb.0302180739.3cb6c159@posting.google.com>...
> I am using GNAT Ada on Linux.  I am trying to port from a Sun
> (Solaris) implementation.  I didn't write the Ada or 'C' myself, but
> am trying to make it work on Linux.
<snip>

This code looks mildly horrible.  All that messing about with
'Address and Unchecked_Conversion is in very poor style.

My advice:

1) Work out what the true Ada mode of each parameter should be.
Just because C forces pass-pointer-by-copy mechanism doesn't mean
that you should try to emulate that directly in Ada.  Is each
pointed-to parameter intended to be read, updated or both?  That
tells you that each parameter should be "in" , "out", or "in out"
in Ada terms.

2) What type is each by-reference parameter actually pointing at?
(Integer or "zero-terminated bounded array of character" are
probable answers) - that tells you the Ada types that
you should be passing.

3) Check out Annex B.3 - The package Interfaces.C gives you
lots of stuff (e.g. for turning an Ada String into a null-terminated
C string) that's useful.  Also read up the small print on
what Pragma Import does when you use Convention "C" - that
defines how Ada parameters are passed to C - very useful.

4) A C function that has multiple side-effects via
by-reference parameters shouldn't
be a function in Ada at all - it's a procedure!

That should allow you to write a clean Ada binding.

 - Rod Chapman, SPARK Team, Praxis Critical Systems