[C++-sig] Re: Boost.Python and GCC 3.4.2

[David Abrahams]

Patrick Hartling <patrick.hartling at gmail.com> writes:

> On Wed, 29 Sep 2004 13:05:57 -0400, David Abrahams
> <dave at boost-consulting.com> wrote:
>> Patrick Hartling <patrick.hartling at gmail.com> writes:
>> 
>> > The code that GCC is complaining about is the type cast in the last
>> > term of the last statement in the following:
>> >
>> > template <class Data>
>> > struct additional_instance_size
>> > {
>> >     typedef instance<Data> instance_data;
>> >     typedef instance<char> instance_char;
>> >     static const std::size_t value = sizeof(instance_data) -
>> > ((size_t)(&((instance_char *)0)->storage));
>> >
>> >
>> > };
>> >
>> > I haven't run into this problem before, and I am not sure how to go
>> > about fixing it (and submitting a patch if appropriate).  I did find a
>> > semi-related discussion on another mailing list with a reference to
>> > the relevant part of the C++ Standard:
>> >
>> >    http://www.cygwin.com/ml/ecos-discuss/2003-02/msg00214.html
>> >
>> > I have seen reports from other people on this list saying that they
>> > are using GCC 3.4 to compile Boost.Python without problems, so I may
>> > be doing something wrong.  I have attached some simple test code that
>> > reproduces the compiler error.  Is there a way to work around this, or
>> > is it a GCC bug?
>> 
>> It is a GCC bug, but not where you think.  This expression:
>> 
>>    (&((instance_char *)0)->storage)
>> 
>> is generated by GCC's C library sizeof() macro.
>
> Do you mean offsetof() here?  

Uh, yeah, sorry.

> I should have looked at the expansion of
> BOOST_PYTHON_OFFSETOF before going straight to the full output from
> the preprocessor.
>
>> sizeof is required to
>> return an integral constant, by whatever means.  So the GCC people
>> need to stick in some special compiler magic to make it happy.
>
> After digging into this some more, I think that this is a FreeBSD bug.
>  GCC 3.4.2 works just fine on Fedora Core 2 (though I haven't tried
> other Linux distributions).  On FreeBSD, stddef.h expands offsetof()
> to __offsetof() (from sys/cdefs.h).  __offsetof() is what generates
> the above code that GCC 3.4.2 does not like.  I'll let the FreeBSD
> folks know about this in hopes of seeing it fixed before FreeBSD 5.3
> gets released.
>
> Thanks for pointing me in the right direction on this.

sure thing.  Sorry for the sizeof misdirection.

-- 
Dave Abrahams
Boost Consulting
http://www.boost-consulting.com