Re: bash-4.3 bug report

[Chet Ramey]

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On 4/14/14, 11:19 AM, Eric Blake wrote:
> On 04/14/2014 08:50 AM, Chet Ramey wrote:
>> On 4/14/14, 5:34 AM, David Binderman wrote:
>>> Hello there,
>>>
>>>  [bind.c:2238]: (style) Array index 'j' is used before limits check.
>>>
>>> Source code is
>>>
>>>           for (j = 0; invokers[j] && j < 5; j++)
>>>
>>> Suggest new code
>>>
>>>           for (j = 0; (j < 5) && (invokers[j] != NULL); j++)
>>
>> Can you give me a use case for which this makes a difference?
> 
> It silences static code checkers and avoids undefined C behavior.

Sorry, I just don't see it.  Maybe I'm missing the undefined C behavior
part.  `invokers' is a pointer to an array of strings that is guaranteed
to be null-terminated; we only want to print at most five members.  Say
there are fewer than five strings: the loop will break when it encounters
the null terminator.  Say there are more: the loop will break after
printing five strings.

Say there are four or fewer strings; the loop will break when invokers[j]
== 0.  Say there are five strings; the loop will break when j == 5 because
invokers[5] == 0.  Say there are six or more strings; the loop will break
when j == 5 because j >= 5, but invokers[j] will be valid.

I'm a little under the weather this morning, and a little sluggish, but I
just don't see the problem.

- -- 
``The lyf so short, the craft so long to lerne.'' - Chaucer
                 ``Ars longa, vita brevis'' - Hippocrates
Chet Ramey, ITS, CWRU    chet@case.edu    http://cnswww.cns.cwru.edu/~chet/
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