[cfe-dev] -fcatch-undefined-behavior false positive with readdir()?

[Eli Friedman]

On Wed, Sep 26, 2012 at 1:44 PM, Sean McBride <sean at rogue-research.com> wrote:
> On Wed, 26 Sep 2012 12:59:23 -0700, Richard Smith said:
>
>>> It prints 8, for every iteration of the loop.  I guess that's correct
>>> since the first field of the struct is a uint64_t.
>>
>>
>>What pointer values does it print? Are they 8 byte aligned?
>
> Aha!  So I changed your printf() to:
>
>         printf ("%p %zd r=%lu\n", d, _Alignof(*d), (uintptr_t)d % 8);
>
> and now:
>
> 0x7fb758800000 8 r=0
> 0x7fb758800018 8 r=0
> 0x7fb758800030 8 r=0
> 0x7fb75880004c 8 r=4
> 0x7fb758800070 8 r=0
> 0x7fb758800094 8 r=4
>
> Indeed the first non-aligned one crashes.  If I add an 'if' and skip all the non 8 byte aligned ones, -fcatch-undefined-behavior no longer complains.
>
> So this is an OS X bug then?  (I'm pretty sure my code snippet itself is correct.)

Yes, it appears so.  (Not sure whether it's a bug in the header or a
bug in the implementation.)

-Eli