[lldb-dev] Possible bug in the LLDB Python Scripting Example (And the API itself)

[Spundun Bhatt]

Thank you Enrico and Zachary,

I've updated my nullpointer checking code as follows.

def IsNullPtr(ptr):
if ptr.GetType().IsPointerType() and ptr.GetValueAsUnsigned() == 0:
return True;
else:
return False;

That looks much more stable now.

I hope someone reads this thread and corrects the example to fix the null
pointer checks.

-S


On Mon, Feb 16, 2015 at 10:44 AM, Enrico Granata <egranata at apple.com> wrote:

>
> On Feb 16, 2015, at 10:11 AM, Zachary Turner <zturner at google.com> wrote:
>
> The issue is that the example on the website is broken because it checks
> GetValue() == None. So either the example is wrong
>
>
> I would posit that the example is wrong - I don’t think anyone updated it
> in the longest time, definitely not me
>
> or the implementation is wrong. Checking GetValue() against None is the
> intuitive thing one would do though, so it seems desirable to make that work
>
>
> I disagree.
>
> In our API, generally, an answer of None means “could not compute this” -
> for instance, your SBValue refers to a variable that is out of scope, or is
> outright invalid
> Conflating the case of “I could compute this, the answer is 0” with “I
> could not compute this” feels wrong - also, imagining this from the
> perspective of, say, an IDE, it makes for more convoluted code:
>
> value = obj.GetValue()
> if value == None:
> if value.IsValid(): value = “NULL” #argh - None now means two things!!!!!
>
> I would also recommend against GetValue() for numeric comparisons - I
> think even right now, that API returns “NULL” the string for null pointers
> and “nil” for the ObjC version thereof - and if we could reliably tell C++
> >= 11, I would have no objection to making it return “nullptr” when
> applicable
> If you need a numeric value to compare - GetValueAsSigned/Unsigned() are
> the APIs to go to - and even those should NOT return None to mean NULL, but
> the correct bit pattern to mean NULL on the underlying system (which we are
> assuming to be 0 :-)
>
> On Mon, Feb 16, 2015 at 9:19 AM Enrico Granata <egranata at apple.com> wrote:
>
> I may be clearly misunderstanding what you are trying to say here, but my
>> expectation is that given
>>
>> T *ptr = nullptr;
>>
>> the way to check if "ptr" is a nullptr would be
>>
>> sbvalueForPtr.GetValueAsUnsigned() == 0
>>
>> given that sbvalueForPtr.GetType().IsPointerType() is true
>>
>> As for the special case of a shared_ptr<>, that is a class that has an
>> instance variable of pointer type. To check for NULL-ness, you're gonna
>> have to retrieve the child. That requires you to have some knowledge of the
>> internals of your standard C++ library.
>>
>> An alternative would be (and I am not sure if that is plugged in at the
>> moment - if not feel free to ask for it, or provide a patch to that effect)
>> to use the recently added ability for synthetic children to provide a
>> numeric value. One could imagine wiring things up so that the
>> shared_ptr<>'s value is the underlying pointer value. Then no child
>> fetching would be required.
>>
>> Sent from my iPhone
>>
>> On Feb 15, 2015, at 11:23 PM, Spundun Bhatt <spundun at gmail.com> wrote:
>>
>> (Using OSX 10.10.1, XCode 6.1.1 6A2008a, lldb-320.4.156, Apple LLVM
>> version 6.0 (clang-600.0.56) (based on LLVM 3.5svn) )
>>
>> Hi all,
>> I have just started using LLDB and its Python scripting interface.
>>
>> I may have stumbled upon a deep bug related to null pointer treatment.
>>
>> http://lldb.llvm.org/scripting.html I tried to follow this scripting
>> tutorial.
>>
>> http://pastebin.com/S0RhVG3s This is the output of my interaction with
>> the lldb and the python script. (I haven't modified any part of the example
>> code there)
>>
>> It seems like `if left_child_ptr.GetValue() == None:` expression (and
>> other similar expressions) doesn't evaluate to true for null pointers
>>
>> I was able to cook up a python check for null pointer:
>>
>> def IsNullPtr(ptr):
>>
>>   ptr_string = str(ptr.GetValue())
>>
>>   if re.search('[1-9a-fA-F]', ptr_string):
>>
>>     return False;
>>
>>   else:
>>
>>     return True;
>>
>> I discussed this on the irc channel and zturner thought it looked like a
>> bug and that I should post it here.
>>
>> Hope this helps.
>> Please let me know if there is a standard way to do null pointer check
>> through the python API. Especially a check for nullptr shared_ptrs.
>>
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>
> Thanks,
> *- Enrico*
> [image: 📩] egranata@.com ☎️ 27683
>
>
>
>
>
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