[cfe-dev] libc++ ABI versioning

[Marshall Clow]

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