Re: How to provide coexisting std::string's (with and without abi:cxx11) in GCC 5.1 and above?

[Markus Trippelsdorf <markus at trippelsdorf dot de>]

On 2016.03.24 at 07:25 -0400, Jeffrey Walton wrote:
> On Thu, Mar 24, 2016 at 6:54 AM, Markus Trippelsdorf
> <markus@trippelsdorf.de> wrote:
> > On 2016.03.24 at 06:50 -0400, Jeffrey Walton wrote:
> >> We caught a report due to _GLIBCXX_USE_CXX11_ABI and abi:cxx11. We are
> >> upstream, and it appears Debian built the library using GCC and
> >> _GLIBCXX_USE_CXX11_ABI was in effect. A user then came along with
> >> Clanf and compiled the userland program. The link failed. This is
> >> expected given our understanding of the landscape.
> >>
> >> We found "GCC5 and the C++11 ABI",
> >> https://developerblog.redhat.com/2015/02/05/gcc5-and-the-c11-abi/. The
> >> pages says:
> >>
> >>     Providers of such libraries or interfaces need to consider
> >>     whether they want to provide ABI coexistence, like libstdc++
> >>     does, or require their users to rebuild.
> >>
> >> I read the Red Hat blog post, but its not clear to me how to
> >> accomplish the coexistence. That is, I want to ensure the library that
> >> Debian builds has a symbol in both namespace so linking can occur with
> >> either GCC or Clang and "things just work" for the user.
> >>
> >> How is libstdc++ providing symbols in both namespaces? How do I ensure
> >> the symbol is present in both namesspaces so "things just work" for a
> >> user?
> >
> > It will not work until this clang bug gets fixed:
> > https://llvm.org/bugs/show_bug.cgi?id=23529
> 
> Thanks Markus.
> 
> Please forgive my ignorance... What is the "coexistence" that is
> discussed? Naively, I thought it would have been something like this
> contrived example:
> 
>   $ cat test.cxx
>   #include <string>
> 
>   std::string foo __attribute__ ((visibility ("default")));
>   std::string bar __attribute__ ((visibility ("default")));
> 
>   $ g++ -g3 -O2 -shared test.cxx -o test.so
> 
>   $ nm test.so | grep _Z3
>   ...
>   0000201c B _Z3barB5cxx11
>   00002034 B _Z3fooB5cxx11
> 
>   $ echo _Z3fooB5cxx11 _Z3barB5cxx11 | c++filt
>   foo[abi:cxx11]  bar[abi:cxx11]
>   foo  bar
> 
> In the above contrived example, foo and bar are in both namespaces.
> That is, the symbol coexists and "things just work".

Consider the following example:

markus@x4 /tmp % cat test.cpp
#include <string>

struct Test {
  __attribute__((used)) std::string foo() { return "foo"; };
};

markus@x4 /tmp % g++ -c test.cpp && nm test.o | grep _ZN4Test
0000000000000000 W _ZN4Test3fooB5cxx11Ev
markus@x4 /tmp % clang++ -c test.cpp && nm test.o | grep _ZN4Test
0000000000000000 W _ZN4Test3fooEv

-- 
Markus