Re: Interfacing with C: access/out parameters

[Jeffrey Carter <spam@spam.com>]

Jano wrote:

> Helper : aliased Datum;
> For Helper'Address use Datum'Address;
> if Thin_Package.Blah (Helper'Access) = Error_Value then
>    raise My_Error;
> end if;

Assuming that you meant "Data" in the declaration of Helper, this is 
probably not a good way to do this. I would advise:

procedure Blah (Datum : out Data) is
    Helper : aliased Data;
begin -- Blah
    if Thin_Package.Blah (Helper'access) = Error_Value then
       raise My_Error;
    end if;

    Datum := Helper;
end Blah;

Before anyone starts complaining about the "inefficiency" of using a 
second variable and doing an assignment, I ask

1. What are the explicit timing requirements for this software?

2. Does it fail to meet these requirements?

3. If so, is eliminating this assignment the only way to meet the timing 
requirements?

I have never encountered SW where these could all be answered "Yes".

Probably you don't want your software to be using C-convention types for 
anything except the interface to C, so you'd want your thick binding to 
have an Ada-convention type for Datum, and assign the fields of Helper 
to it. Horrors! Multiple assignments!

-- 
Jeff Carter
"Now look, Col. Batguano, if that really is your name."
Dr. Strangelove
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