std::future and clang with -stdlib=libstdc++ - c++11

Question

The following program fails to link with clang and -stdlib=libstdc++:
$ cat future.cpp
#include <iostream>
#include <future>
int main()
{
std::future<int> f1 = std::async([](){ return 42; });
f1.wait();
std::cout << "Magic number is: " << f1.get() << std::endl;
}
$ g++-mp-5 future.cpp -std=c++11 && ./a.out
Magic number is: 42
$ clang++-mp=3.5 future.cpp -std=c++11 && ./a.out
Magic number is: 42
When building with clang and -stdlib=libstdc++, the following linking error occurs:
$ clang++-mp-3.5 future.cpp -std=c++11 -stdlib=libstdc++ -I/opt/local/include/gcc5/c++ -I/opt/local/include/gcc5/c++/x86_64-apple-darwin14 -L/opt/local/lib/gcc5 -lstdc++ && ./a.out
Undefined symbols for architecture x86_64:
"std::__once_call", referenced from:
void std::call_once<void (std::__future_base::_State_baseV2::*)(std::function<std::unique_ptr<std::__future_base::_Result_base, std::__future_base::_Result_base::_Deleter> ()>*, bool*), std::__future_base::_State_baseV2*, std::function<std::unique_ptr<std::__future_base::_Result_base, std::__future_base::_Result_base::_Deleter> ()>*, bool*>(std::once_flag&, void (std::__future_base::_State_baseV2::*&&)(std::function<std::unique_ptr<std::__future_base::_Result_base, std::__future_base::_Result_base::_Deleter> ()>*, bool*), std::__future_base::_State_baseV2*&&, std::function<std::unique_ptr<std::__future_base::_Result_base, std::__future_base::_Result_base::_Deleter> ()>*&&, bool*&&) in future-b6480b.o
void std::call_once<void (std::thread::*)(), std::reference_wrapper<std::thread> >(std::once_flag&, void (std::thread::*&&)(), std::reference_wrapper<std::thread>&&) in future-b6480b.o
"std::__once_callable", referenced from:
void std::call_once<void (std::__future_base::_State_baseV2::*)(std::function<std::unique_ptr<std::__future_base::_Result_base, std::__future_base::_Result_base::_Deleter> ()>*, bool*), std::__future_base::_State_baseV2*, std::function<std::unique_ptr<std::__future_base::_Result_base, std::__future_base::_Result_base::_Deleter> ()>*, bool*>(std::once_flag&, void (std::__future_base::_State_baseV2::*&&)(std::function<std::unique_ptr<std::__future_base::_Result_base, std::__future_base::_Result_base::_Deleter> ()>*, bool*), std::__future_base::_State_baseV2*&&, std::function<std::unique_ptr<std::__future_base::_Result_base, std::__future_base::_Result_base::_Deleter> ()>*&&, bool*&&) in future-b6480b.o
void std::__once_call_impl<std::_Bind_simple<std::_Mem_fn<void (std::__future_base::_State_baseV2::*)(std::function<std::unique_ptr<std::__future_base::_Result_base, std::__future_base::_Result_base::_Deleter> ()>*, bool*)> (std::__future_base::_State_baseV2*, std::function<std::unique_ptr<std::__future_base::_Result_base, std::__future_base::_Result_base::_Deleter> ()>*, bool*)> >() in future-b6480b.o
void std::call_once<void (std::thread::*)(), std::reference_wrapper<std::thread> >(std::once_flag&, void (std::thread::*&&)(), std::reference_wrapper<std::thread>&&) in future-b6480b.o
void std::__once_call_impl<std::_Bind_simple<std::_Mem_fn<void (std::thread::*)()> (std::reference_wrapper<std::thread>)> >() in future-b6480b.o
ld: symbol(s) not found for architecture x86_64
clang: error: linker command failed with exit code 1 (use -v to see invocation)
However, a simple program w/o future builds just fine, e.g.:
$ cat simple.cpp
#include <iostream>
#include <future>
int main()
{
std::cout << "Magic number is: " << 42 << std::endl;
}
$ clang++-mp-3.5 simple.cpp -std=c++11 -stdlib=libstdc++ -I/opt/local/include/gcc5/c++ -I/opt/local/include/gcc5/c++/x86_64-apple-darwin14 -L/opt/local/lib/gcc5 -lstdc++ && ./a.out
Magic number is: 42
System is OSX 10.10.4 with macports.
I can't figure out what is the problem. Thanks!

Answer 1

This is an incompatibility between GCC and Clang on Mac OS X.
GCC emits a reference to ___emutls_v._ZSt15__once_callable while clang emits a reference to __ZSt15__once_callable.
Unfortunately, __ZSt15__once_callable and ___emutls_v._ZSt15__once_callable
are not compatible, so doing something like:
asm("__ZSt15__once_callable: jmp ___emutls_v._ZSt15__once_callable");
wouldn't work either.
I also came accross this LLVM bug report: http://lists.cs.uiuc.edu/pipermail/llvmbugs/2014-August/035744.html which probably means clang will never add for support GCC's emutls implementation.
Edit: It looks like support for emutls was added to clang trunk a couple of hours ago in r243438 via -femulated-tls.

Answer 2

Clang on OSX uses an older version of libstdc++. I believe its version 4.2 which does not support many of the C++11 features. If you want to use C++11 with clang on OSX your only choice is libc++.
You could also use gcc-4.x from homebrew which will include a newer version of libstdc++. Unfortunately clang will not easily link with this version of the standard library.

Answer 3

Undefined symbol std::__once_call when building GCC's standard library is explained here https://github.com/msys2/MINGW-packages/issues/5786 and should be fixed when using compiler flag -femulated-tls.