[SystemSafety] What do we know about software reliability?

Derek M Jones derek at knosof.co.uk
Wed Sep 16 09:20:48 CEST 2020


Hugues,

> You do not mention cause of software unreliability, but of the hardware which executes it.

I bring this up because it sets an upper bound on what might be
considered an acceptable reliability of the software (or lower bound,
depending on how things are measured).

> My question is on software reliability.

There is missing source, or A is not assigned to, and
its value is random.

If the language definition says a boolean true is any non-zero
value, the probability of being false is: 1 divided by
(number of possible values representable in A).

> 
> Regards
> 
> Hugues
> 
> 
> 
>> Le 16 sept. 2020 à 03:05, Derek M Jones <derek at knosof.co.uk> a écrit :
>>
>> Hugues
>>> if A then
>>>    do_nothing
>>> else
>>>    fail --potentially hurt and kill people
>>> end if
>>> end
>>> The specification of the software is to do nothing;
>>> NB: I'm not asking if it is the best implementation, whatever the criteria are, but just : "is it reliable?"
>>
>> The "if A then"
>> could be unreliable, in that a cosmic ray could flip its value.
>>
>> There are research compilers that reduce the likelihood of a
>> cosmic ray altering the behavior of a program:
>> http://shape-of-code.coding-guidelines.com/2011/11/07/compiling-to-reduce-the-impact-of-soft-errors-on-program-output/
>>
>> Circa 2011, 1-bit of a 4GB RAM had its value flipped roughly every 33
>> hours as a result of a cosmic ray hitting the upper atmosphere.
>>
>>> regards,
>>> Hugues
>>> ----- Mail original -----
>>>> De: "Peter Bernard Ladkin" <ladkin at causalis.com>
>>>> À: systemsafety at lists.techfak.uni-bielefeld.de
>>>> Envoyé: Mardi 15 Septembre 2020 19:58:45
>>>> Objet: Re: [SystemSafety] What do we know about software reliability?
>>>>
>>>> Bev and I and Dewi have a colleague who poses the following question.
>>>>
>>>> "We have clients who have installed hundreds of [examples of our kit]
>>>> over the last ten years, and
>>>> have never seen any failure. They want to use it in further systems
>>>> that they build. What arguments
>>>> do we/they need to provide in order validly to justify such further
>>>> use?"
>>>>
>>>> So, what is the answer to that question?
>>>>
>>>> PBL
>>>>
>>>> Prof. Peter Bernard Ladkin, Bielefeld, Germany
>>>> Styelfy Bleibgsnd
>>>> Tel+msg +49 (0)521 880 7319  www.rvs-bi.de
>>>>
>>>>
>>>>
>>>>
>>>>
>>>>
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>>
>> -- 
>> Derek M. Jones           Evidence-based software engineering
>> tel: +44 (0)1252 520667  blog:shape-of-code.coding-guidelines.com
>> _______________________________________________
>> The System Safety Mailing List
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> 

-- 
Derek M. Jones           Evidence-based software engineering
tel: +44 (0)1252 520667  blog:shape-of-code.coding-guidelines.com


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