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NNTP-Posting-Date: Wed, 13 Jul 2011 13:00:37 -0500
Date: Wed, 13 Jul 2011 11:00:28 -0700
From: Patricia Shanahan <pats@acm.org>
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Subject: Re: Arithmetic overflow checking
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On 7/13/2011 10:38 AM, Gene Wirchenko wrote:
> On Tue, 12 Jul 2011 16:09:04 -0700 (PDT), lewbloch
> <lewbloch@gmail.com>  wrote:
>
>> Gene Wirchenko wrote:
>>> lewbloch wrote:
>>>> Martin Gregorie wrote:
>>>
>>> [snip]
>>>
>>>>> [1] The instrument causing the problem was an unmodified Ariane 4 SRI
>>>>> which raised an out-of-limits exception when the normal Ariane 5
>>>>> trajectory exceeded a permitted Ariane 4 horizontal velocity limit.
>>>
>>>       ...the Ariane 5 having more powerful engines.
>>>
>>>> In other words, this was a case where there *was* an out-of-range
>>>> exception, thus it makes the exact opposite point to the one Gene
>>>> presumably wanted to support.
>>>
>>>       The data I read was that the exception was not handled.  IIRC,
>>> debugging got interpreted as navigational data.
>
>> Precisely.  There was an exception, and it was not handled.  Having
>> the exception was not enough.
>
>       No surprise there.  Most of us understand that exceptions have to
> be handled as well as thrown.

I think the lesson we need to learn from the Ariane failure is that
adding a check removes one type of risk, but the cost of two forms of
additional risk:

1. The check gives a false positive result.

2. There is a bug in the handling of the error report.

Patricia