[cfe-dev] libc++ ABI versioning

[Jorgen Tjerno]

So why is this not an ABI-breaking change?

What causes a program built against the earlier version of
std::ifstream to correctly execute on a machine that has the latest
version of std::ifstream that expects a larger layout?


- Jørgen.


On Wed, Jun 11, 2014 at 6:48 AM, Marshall Clow <mclow.lists at gmail.com> wrote:
>
> On Jun 9, 2014, at 7:26 PM, Jorgen Tjerno <jorgen at uberent.com> wrote:
>> On Mon, Jun 9, 2014 at 6:56 PM, Richard Smith <richard at metafoo.co.uk> wrote:
>>> On Mon, Jun 9, 2014 at 6:32 PM, Jorgen Tjerno <jorgen at uberent.com> wrote:
>>>>
>>>> I've noticed that Xcode 4.6 and Xcode 5.1 ship with headers that have
>>>> different layouts for std::ifstream. sizeof(std::ifstream) is 448 for
>>>> 4.6, and 576 on 5.1. You can see my test case here:
>>>> https://gist.github.com/anonymous/bd0db5e4bcb307b70506
>>>>
>>>> How does clang/libc++ ensure that the executable runs with the right
>>>> libc++ version for headers it was built with? I can run the binaries
>>>> produced by Xcode 4.6 on a machine that has a newer libc++, and I can
>>>> run the binaries produced by 5.1 on a machine that has an older
>>>> libc++. I would assume that if I built with headers that give me a
>>>> smaller size than what is expected by libc++, that would be cause the
>>>> newer libc++ to trash memory beyond the end of my object. Am I missing
>>>> something?
>>>>
>>>> I was unable to find any resources on how libc++ ABI versioning works
>>>> other than the "major ABI version" -- which is 1.
>>>>
>>>> The specific problem I'm seeing is that it seems that when I link with
>>>> a static library that was built with the older libc++ version, a
>>>> translation unit built with the new libc++ ends up using the wrong
>>>> (old) value for the construction symbol at runtime
>>>>
>>>> (__ZTCNSt3__114basic_ifstreamIcNS_11char_traitsIcEEEE0_NS_13basic_istreamIcS2_EE,
>>>> or '
>>>>   construction vtable for std::__1::basic_istream<char,
>>>> std::__1::char_traits<char> >-in-std::__1::basic_ifstream<char,
>>>> std::__1::char_traits<char> >' according to c++filt).
>>>
>>>
>>> Looks like the difference is probably due to the std::filebuf subobject. Did
>>> that type also change size? Did std::basic_streambuf<char>?
>>
>> Yep, filebuf did, and that does seem to account for the difference in size.
>>
>> themac:~ jorgen$ ./foo-modern
>> sizeof(std::ifstream)=576, offset=0
>> sizeof(std::istream)=168, offset=0
>> sizeof(std::ios)=152, offset=424
>> sizeof(std::ios_base)=136, offset=424
>> sizeof(std::filebuf)=408, offset=0
>> sizeof(std::basic_streambuf<char>)=64, offset=0
>>
>> themac:~ jorgen$ ./foo
>> sizeof(std::ifstream)=448, offset=0
>> sizeof(std::istream)=168, offset=0
>> sizeof(std::ios)=152, offset=296
>> sizeof(std::ios_base)=136, offset=296
>> sizeof(std::filebuf)=280, offset=0
>> sizeof(std::basic_streambuf<char>)=64, offset=0
>
> Looks like this commit was the culprit:
>
> r162601 | hhinnant | 2012-08-24 14:20:56 -0700 (Fri, 24 Aug 2012) | 31 lines
>
> It added a new member variable
>         state_type __st_last_;
>
> to the basic_filebuf object, and that variable is 128 bytes long.
>
> — Marshall
>
>