scoped thread wrapper for std::thread

Question

I am trying to make a scoped thread.

#include <iostream>
#include <thread>

class ScopedThread {
 public:
    template< class Function, class... Args>
    explicit ScopedThread( int id, Function&& f, Args&&... args)
        : m_thread( std::ref(f), std::forward<Args>(args)...)
        , id(std::move(id)) {
    }



    int getId() const { return id; }

    ~ScopedThread() { m_thread.join(); }
 private:
    std::thread m_thread;
    int id;

};

class Worker {
 public:
    Worker(int id): thd(id, &Worker::work, this) { }

    void work() {
       for( int i = 0; i < 10; i++)
        std::cout << "I am working" << std::endl;
    }


 private:
    ScopedThread thd;
};

int main() {
    Worker(1);
    Worker(2);
    Worker(3);
    Worker(4);
}

When I run the code, it dumps core.

#0  0x00007ffcfbbc6380 in ?? ()
#1  0x00000000004026c9 in std::_Mem_fn<void (Worker::*)()>::operator()<, void>(Worker*) const (this=0x7f0b43551de0, __object=0x7ffcfbbc63c0)
    at /usr/bin/../lib/gcc/x86_64-linux-gnu/4.8/../../../../include/c++/4.8/functional:601
#2  0x00000000004025cd in std::__invoke<void (Worker::*)(), Worker*> (__f=@0x7ffcfbbc6360: (void (Worker::*)(Worker * const)) 0x7ffcfbbc6380, this adjustment 4198629,
    __args=<unknown type in /home/asit/cpp/scope_thd, CU 0x0, DIE 0x2abf>) at /usr/bin/../lib/gcc/x86_64-linux-gnu/4.8/../../../../include/c++/4.8/functional:247
#3  0x0000000000402532 in std::reference_wrapper<void (Worker::*)()>::operator()<Worker*>(Worker*&&) const (this=0x1b27048,
    __args=<unknown type in /home/asit/cpp/scope_thd, CU 0x0, DIE 0x57fb>) at /usr/bin/../lib/gcc/x86_64-linux-gnu/4.8/../../../../include/c++/4.8/functional:467
#4  0x00000000004024d2 in std::_Bind_simple<std::reference_wrapper<void (Worker::*)()> (Worker*)>::_M_invoke<0ul>(std::_Index_tuple<0ul>) (this=0x1b27040)
    at /usr/bin/../lib/gcc/x86_64-linux-gnu/4.8/../../../../include/c++/4.8/functional:1731
#5  0x0000000000402485 in std::_Bind_simple<std::reference_wrapper<void (Worker::*)()> (Worker*)>::operator()() (this=0x1b27040)
    at /usr/bin/../lib/gcc/x86_64-linux-gnu/4.8/../../../../include/c++/4.8/functional:1720
#6  0x0000000000402119 in std::thread::_Impl<std::_Bind_simple<std::reference_wrapper<void (Worker::*)()> (Worker*)> >::_M_run() (this=0x1b27028)
    at /usr/bin/../lib/gcc/x86_64-linux-gnu/4.8/../../../../include/c++/4.8/thread:115
#7  0x00007f0b44103a60 in ?? () from /usr/lib/x86_64-linux-gnu/libstdc++.so.6
#8  0x00007f0b43920184 in start_thread (arg=0x7f0b43552700) at pthread_create.c:312
#9  0x00007f0b4364d37d in clone () at ../sysdeps/unix/sysv/linux/x86_64/clone.S:111

Can someone tell me how do I forward member functions and arguments to the underlying std::thread class ? I have observed that, the segmentation fault comes only in clang++, not in gcc.

My objective is to make the wrapper class completely replaceable with std::thread class. Wrapper class takes a new argument for thread id.

Answer 1

There are several issues with your implementation of ScopedThread.

1. There's no need to deal with Function&& f separately. Just handle it as part of the args... pack.

2. There's no need to move id.

   template< class... Args>
   explicit ScopedThread( int id, Args&&... args)
       : m_thread( std::forward<Args>(args)...)
       , id(id) {
   }
   

3. You should make sure that your thread is joinable before calling .join().

   ~ScopedThread() { if(m_thread.joinable()) m_thread.join(); }
   

Applying these changes prevents the segmentation fault on clang++.

---

The culprit is std::ref(f) - you're creating a temporary reference_wrapper and passing it to std::thread's constructor which uses std::invoke to call it.

According to g++ and UndefinedBehaviorSanitizer:

/usr/local/gcc-head/include/c++/7.0.1/bits/invoke.h:73:46:

runtime error: member call on misaligned address 0x7fff939ec8d3 for type 'struct Worker', which requires 8 byte alignment

The problem is that you're creating a reference to a temporary using std::ref(f), where the temporary is &Worker::work.

Copying f instead of using std::ref doesn't cause any segfault or UndefinedBehaviorSanitizer diagnostic.