Re: [PATCH] test-stddef: test offsetof compliance

[Paolo Bonzini]

On 08/16/2010 11:52 PM, Paul Eggert wrote:
> On 08/16/10 23:16, Eric Blake wrote:
> > The problem is how do we provide a working offsetof replacement, given
> > that there is no way using standard C89 or C++ constructs to do it?  All
> > implementations either use something like this which abuses undefined
> > behavior within the standard but works without warning for that compiler:
> > #define offsetof(__a,__b) ((size_t)(&(((__a*)0)->__b)))
>
> That's a perfectly reasonable replacement, for compilers
> where offsetof is broken.  It works for all such compilers that
> we know of.  If some new compiler comes up, we can deal with it
> then.

Agreed.  GCC only provides the builtin for three reasons, all related to 
C++: 1) because the definition above is not an integral constant 
expression, while offsetof's result is; 2) to support templates properly 
(which it doesn't anyway, see http://gcc.gnu.org/PR17310), and 3) to 
warn and/or fail when given strange C++ constructs involving overloaded 
operator[].

In C, the macro will work in practice with all compilers.  That said, if 
the only problem we find is a non-parenthesized body, that is not huge.

Paolo