Linking problems due to symbols with abi::cxx11?

Question

We recently caught a report because of GCC 5.1, libstdc++ and Dual ABI. It seems Clang is not aware of the GCC inline namespace changes, so it generates code based on one set of namespaces or symbols, while GCC used another set of namespaces or symbols. At link time, there are problems due to missing symbols.

If I am parsing the Dual ABI page correctly, it looks like a matter of pivoting on _GLIBCXX_USE_CXX11_ABI and abi::cxx11 with some additional hardships. More reading is available on Red Hat's blog at GCC5 and the C++11 ABI and The Case of GCC-5.1 and the Two C++ ABIs.

Below is from a Ubuntu 15 machine. The machine provides GCC 5.2.1.

$ 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]

How can I generate a binary with symbols using both decorations ("coexistence" as the Red Hat blog calls it)?

Or, what are the options available to us?

---

I'm trying to achieve an "it just works" for users. I don't care if there are two weak symbols with two different behaviors (std::string lacks copy-on-write, while std::string[abi:cxx11] provides copy-on-write). Or, one can be an alias for the other.

Debian has a boatload of similar bugs at Debian Bug report logs: Bugs tagged libstdc++-cxx11. Their solution was to rebuild everything under the new ABI, but it did not handle the corner case of mixing/matching compilers modulo the ABI changes.

In the Apple world, I think this is close to a fat binary. But I'm not sure what to do in the Linux/GCC world. Finally, we don't control how the distro's build the library, and we don't control what compilers are used to link an applications with the library.

Answer 1

Disclaimer, the following is not tested in production, use at your own risk.

You can yourself release your library under dual ABI. This is more or less analogous to OSX "fat binary", but built entirely with C++.

The easiest way to do so would be to compile the library twice: with -D_GLIBCXX_USE_CXX11_ABI=0 and with -D_GLIBCXX_USE_CXX11_ABI=1. Place the entire library under two different namespaces depending on the value of the macro:

#if _GLIBCXX_USE_CXX11_ABI
#  define DUAL_ABI cxx11 __attribute__((abi_tag("cxx11")))
#else
#  define DUAL_ABI cxx03
#endif

namespace CryptoPP {
  inline namespace DUAL_ABI {
    // library goes here
  }
}

Now your users can use CryptoPP::whatever as usual, this maps to either CryptoPP::cxx11::whatever or CryptoPP::cxx03::whatever depending on the ABI selected.

Note, the GCC manual says that this method will change mangled names of everything defined in the tagged inline namespace. In my experience this doesn't happen.

The other method would be tagging every class, function, and variable with __attribute__((abi_tag("cxx11"))) if _GLIBCXX_USE_CXX11_ABI is nonzero. This attribute nicely adds [cxx11] to the output of the demangler. I think that using a namespace works just as well though, and requires less modification to the existing code.

In theory you don't need to duplicate the entire library, only functions and classes that use std::string and std::list, and functions and classes that use these functions and classes, and so on recursively. But in practice it's probably not worth the effort, especially if the library is not very big.