Re: Certified C compilers for safety-critical embedded systems

["Robert I. Eachus" <rieachus@comcast.net>]

Frank J. Lhota wrote:

> Was there any discussion of using access types, i.e.
> 
>     function Random (Gen : access Generator) return Result_Subtype;
> 
> This would require that each generator is aliased, and that 'Access
> attributes be used, but at least this would give some recognition to the
> fact that the generator is changed by the call.

Actually, and I think more interesting, was that we discussed making the 
generator parameter implicit, and having a per task generator value.

That idea was dropped because some people thought it would make calls to 
Random slower, and others were worried that it would force all compilers 
to support per task variables.  Per task variables did make it into Ada 
9X, but there is no standard way to use them with random number 
generators. I keep promising myself that someday I will have to write a
generic that does that, but it seems like using a sledgehammer to swat a 
fly.  Every time the issue comes up, it is easier to create a new 
generator object inside each task.

But notice that having a "default" generator would be either error prone 
in the presense of tasking, or slow because references would have to be 
via a protected object whether or not a program used random numbers in 
more than one task.  We avoided that mistake, and not by accident.  But 
it does create this anomaly that the "normal" random number generator 
interface does have this weird side effect on its parameter.

-- 
                                           Robert I. Eachus

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