[cfe-dev] : Analyzer ignoring the effects of a function call?

[Anna Zaks]

Hi Don,

It would be helpful if you could file bugzilla bug reports for the error messages that you find especially obscure.

Thanks!
Anna.
On Oct 2, 2011, at 5:54 PM, Don Quixote de la Mancha wrote:

> Inside your if statement, a is always 0.  So the expression can be
> simplified to "c = b;".
> 
> I've found clang's static analyzer of immense help with my iPhone App,
> but the messages it presents are sometimes rather obscure in their
> meaning.
> 
> Don Quixote de la Mancha
> quixote at dulcineatech.com
> 
> On Sun, Oct 2, 2011 at 5:48 PM, Arjun Singri <arjunsingri at gmail.com> wrote:
>> But how did it arrive at the conclusion that 'a' was '0' when it is clearly
>> unknown? If I don't provide an intial value for 'a', it still reports the
>> same warning. Does that mean that it is interpreting that 'a' is set to zero
>> inside the function?
>> 
>> On Sun, Oct 2, 2011 at 5:35 PM, John McCall <rjmccall at apple.com> wrote:
>>> 
>>> On Oct 2, 2011, at 4:59 PM, Arjun Singri wrote:
>>> 
>>> I think I am getting a false positive for IdempotentOperationChecker which
>>> is affecting another checker I am writing. Here is the program I am running
>>> the clang analyzer on:
>>> #include<stdio.h>
>>>   int main(void)
>>>   {
>>>       int a = 5;
>>>       int b = 4;
>>>       int c = 10;
>>>       scanf("%d", &a);
>>>       if (a + b == 4) {
>>>           c = a + b;
>>>       }
>>>       return c;
>>>   }
>>> I get:
>>> $clang --analyze d.cpp
>>> 
>>> 
>>> d.cpp:12:15: warning: The left operand to '+' is always 0
>>>         c = a + b;
>>>                         ~  ^
>>> Why is it ignoring the scanf function call? Is this expected because the
>>> analyzer doesn't do inter-procedural analysis? But even then I think it
>>> shouldn't ignore the effects of the function call. Now this is happening
>>> because the LHSVal.isConstant(0) call inside the IdempotentOperationChecker
>>> is evaluating to true.
>>> 
>>> It's not ignoring the scanf call:  if it were, it would think that 'a' was
>>> 5, not 0.  No, here it's presumably analyzing the dominating if condition
>>> and doing basic algebra.
>>> John.
>> 
>> 
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>> 
> 
> 
> 
> -- 
> Don Quixote de la Mancha
> quixote at dulcineatech.com
> 
>    Custom Software Development for the iPhone and Mac OS X
>    http://www.dulcineatech.com/custom-software-development/
> 
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