[cfe-dev] Safe use of offsetof?

[Jeffrey Walton]

I'm working with some source code that does the following. Its
declared as a static method.

167  long
168  cbdata::MakeOffset()
169  {
170      cbdata *zero = (cbdata *)0L;
171      void **dataOffset = &zero->data;
172      return (long)dataOffset;
173  }

Under UBSan, the code creates an entry for each use of MakeOffset():

    runtime error: member access within null pointer of type 'cbdata'

The code is not amicable to use of offsetof(...). I think its because
there's a Stack member, but I don't know the C/C++ language rules like
most folks.

   cbdata.cc:176:19: error: offset of on non-POD type 'struct cbdata'

But I'm fairly certain (correct me here) it falls under case 2 listed
at http://gcc.gnu.org/ml/gcc/2003-11/msg00281.html:

    2) Those cases where any reasonable implementation (and all
       implementations we know of) give the right answer, but ISO
       C++ says "undefined".

Is there a way to code MakeOffset() so that UBSan will be happy so as
to avoid the runtime error? Or will this have to go into an ignore
list?

Thanks in advance.