Re: Passing C-style flags to C subprograms

[Jeffrey Carter <spam.jrcarter.not@spam.not.acm.org>]

On 03/30/2012 11:18 AM, Natasha Kerensikova wrote:
>
>     type These_Flags is record
>        Allow_Stuff : Boolean := False;
>        Do_Otherwise : Boolean := False;
>        Special_Feature : Boolean := False;
>     end record;
>     for These_Flags use record
>        Allow_Stuff at 0 range 0 .. 0;
>        Do_Otherwise at 0 range 1 .. 1;
>        Special_Feature at 0 range 2 .. Interfaces.C.int'Size;
>     end record;
>     for These_Flags'Size use Interfaces.C.int'Size;

I don't see how this could work. You've specified Interfaces.C.Int'Size + 1 bits 
in These_Flags, and then specified that These_Flags is Interfaces.C.Int'Size 
bits in size.

It's probably better to have 3 Boolean fields and a padding field that takes up 
the additional bits. That will work as long as you don't move to a big-endian 
machine.

If you're worried about portability, the modular type approach is probably 
better. Brukardt has pointed out some pros and cons of both approaches.

If you're really worried about portability, then you should make sure you pass 
the same kind to C that C expects. If the C functions take an int, then you 
should pass an Interfaces.C.Int to them. This doesn't prevent using either of 
the approaches, but does involve an unchecked conversion to or from 
Interfaces.C.Int hidden in the lowest level of your code.

There are at least 2 parts to your problem: How you make this appear to the rest 
of your Ada code, and how you implement the low-level interface to C. There 
could also be a 3rd part that sits in between those, something more Ada-like 
than the low-level interface but less application-specific than the interface to 
this specific application.

-- 
Jeff Carter
"Friends don't let friends program in C++."
Ludovic Brenta
114