Question

I just compiled and installed clang+llvm 3.0 on my ubuntu 10.04, and also libc++ from svn. As the status in libc++ shows thread support is complete, I wanted to try std::async. So I follow the example given by Anthony Williams in

http://www.justsoftwaresolutions.co.uk/threading/multithreading-in-c++0x-part-8-futures-and-promises.html

And just make minor change to make it compile:

#include <future>
#include <iostream>

int calculate_the_answer_to_LtUaE()
{
  return 42;
}

void do_stuff()
{
  std::cout << "doing stuff" << std::endl;
}

int main()
{
  std::future<int> the_answer=std::async(calculate_the_answer_to_LtUaE);
  do_stuff();
  std::cout<<"The answer to life, the universe and everything is "
    <<the_answer.get()<<std::endl;
}

And I compile with

clang++ –std=c++0x -stdlib=libc++ -lpthread async.cpp

However, it runs and always finish with a core dump:

doing stuff
The answer to life, the universe and everything is Aborted (core dumped)

I check the core dump and it shows stack like this (which I don’t quite get a hint)

#0  0x00007fd0a1a7ba75 in raise () from /lib/libc.so.6
#1  0x00007fd0a1a7f5c0 in abort () from /lib/libc.so.6
#2  0x00007fd0a22a735b in std::exception_ptr::~exception_ptr (this=) at ../src/exception.cpp:130
#3  0x0000000000404178 in void std::__1::__assoc_state::set_value(int&&) ()
#4  0x00000000004051ae in _ZNSt3__119__async_assoc_stateIiNS_12__async_funcIPFivEJEEEE9__executeEv ()
#5  0x0000000000404e00 in _ZNSt3__114__thread_proxyINS_5tupleIJMNS_19__async_assoc_stateIiNS_12__async_funcIPFivEJEEEEEFvvEPS7_EEEEEPvSC_ ()
#6  0x00007fd0a250f9ca in start_thread () from /lib/libpthread.so.0
#7  0x00007fd0a1b2e70d in clone () from /lib/libc.so.6
#8  0x0000000000000000 in ?? ()

Anybody has an idea why?

Answer 1

I ran your example on OS X Lion, using:

clang++ -std=c++0x -stdlib=libc++ async.cpp

And the program output:

doing stuff
The answer to life, the universe and everything is 42

Inspecting the source of libc++ as suggested by moshbear’s comment I see:

exception_ptr::~exception_ptr() _NOEXCEPT
{
#if HAVE_DEPENDENT_EH_ABI
    __cxa_decrement_exception_refcount(__ptr_);
#else
    #warning exception_ptr not yet implemented
    ::abort();
#endif  // __APPLE__
}

It appears to me that ~exception_ptr() has not been ported to ubuntu 10.04. This is a low-level facility not implementable in portable C++. Work on creating a GPL-free implementation of this level is ongoing at libc++abi. I can assure you that libc++abi is not ready for prime time at this time.

There has also been an independent effort at this low-level library at: https://github.com/pathscale/libcxxrt . I do not know the status of this library nor whether it has been ported to ubuntu.