Re: Using Ada (or SPARK) in Ada-unaware environment

[Niklas Holsti <nobody@nowhere.fi>]

Maciej Sobczak wrote:
> Hi,
> 
> Imagine a control system (to be developed) which needs to interact with 
> some devices. Those devices are supplied with C drivers and libraries.

Been there a couple of times.

> What is the recommended practice for developing this control system in 
> Ada (or SPARK)? I basically see two options for this:
> 
> 1. Enjoy Ada's ability to interface with C libraries (pragma Import) and 
> write everything in Ada.

I have always used this option. It works well *unless* the C 
libraries try to interact with the OS/kernel using signals or 
timers (sleeps), or create their own threads.

Sometimes the C library interface (C header files) relies heavily 
on #define macros, which can make writing the Ada specs rather 
boring. It helps to put one layer of application-specific C 
routines between the C libraries and the Ada code, presenting a 
simpler, application-specific C-Ada interface.

> 2. Write separate programs in C (or C++) that will be responsible only 
> for talking to the devices via their C access libraries. Write the main 
> controller in Ada, as another separate program, and use some form of 
> interprocess communication to have all those components talk.

I would do that only if the C libraries use the OS features I 
mentioned above, in which case it would be easier to isolate the C 
code in its own process (= program). Still, you would need an Ada 
run-time (tasking) that connects well to the interprocess 
communication primitives, eg. lets an Ada task wait on an OS mutex 
or signal.

> The advantage of the second option is that the controller part can be 
> easily tested in isolation or in a fake environment.

Hmmm... I have usually done that in option 1 by implementing a 
test/fake version of the C libraries themselves. The only reason 
why this could be simpler in option 2 is if the test/fake code 
must be written in C and needs to use the OS features mentioned 
above. In option 1, the test/fake code can be written in Ada and 
use Ada tasking and I/O if necessary, which seems a lot simpler to me.

My 2 cents...

-- 
Niklas Holsti
Tidorum Ltd
niklas holsti tidorum fi
       .      @       .