[cfe-dev] Warning when comparing address of function or variable with constant?

[Matt Beaumont-Gay]

On Tue, Jan 3, 2012 at 08:49, Douglas Gregor <dgregor at apple.com> wrote:
>
> On Jan 3, 2012, at 8:20 AM, Matt Beaumont-Gay wrote:
>
>> On Tue, Jan 3, 2012 at 06:14, Ed Schouten <ed at 80386.nl> wrote:
>>> Hello all,
>>>
>>> This morning I fixed a small bug at FreeBSD that involved the following
>>> code:
>>>
>>>        void
>>>        func(struct foo *idx)
>>>        {
>>>
>>>                if (index == NULL)
>>>                        return;
>>>                ...
>>>        }
>>>
>>> The bug in this code is that we should have compared against idx -- not
>>> index. This works by accident, as index() is a function provided by our
>>> C library (BSD's strchr()).
>>>
>>> I think it is hardly ever possible that a function or variable ever
>>> resides at address 0, except in kernelspace or when using a hacked
>>> run-time linker. Does Clang have a warning for this?
>>
>> Yes, -Wbool-conversions:
>>
>> $ cat test.cc
>> int index();
>> void f(int* idx) {
>>  if (index)
>>    return;
>> }
>> $ ./build/bin/clang -fsyntax-only -Wbool-conversions test.cc
>> test.cc:3:7: warning: address of function 'index' will always evaluate
>> to 'true' [-Wbool-conversions]
>>  if (index)
>>      ^~~~~
>> test.cc:3:7: note: prefix with the address-of operator to silence this warning
>>  if (index)
>>      ^
>>      &
>> 1 warning generated.
>
>
> I think it's perfectly reasonable to extend this warning to also complain about comparisons against null pointers (with the typical caveat for weak declarations).

Oops, you're right, the current warning doesn't actually fire when the
address is used in a comparison. I agree, it should be extended to
cover comparison against NULL.

-Matt