[cfe-dev] libc++ ABI versioning

[Jorgen Tjerno]

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

- Jørgen.


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>?