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Last updated：2026-10-12

C++ std::thread Implementation Internals

A deep dive into std::thread

When we write a lambda and pass it to a thread, when exactly are the thread creation and the task invocation triggered?

auto t = std::thread([this] {
  // do something
});

This thread is passed a lambda, and the question this section sets out to answer is: when is this lambda invoked? At construction time? Or at some point when it gets called?

Come, let us step into the era of source code.

1. _M_make_routine

The code analyzed in this section is, respectively:

1) Header file

https://github.com/gcc-mirror/gcc/blob/releases/gcc-4.9/libstdc%2B%2B-v3/include/std/thread

  1. Implementation file

https://github.com/gcc-mirror/gcc/blob/releases/gcc-4.9/libstdc%2B%2B-v3/src/c%2B%2B11/thread.cc

  1. posix header

https://github.com/gcc-mirror/gcc/blob/releases/gcc-4.9/libgcc/gthr-posix.h

The code above will call the following constructor:

template<typename _Callable, typename... _Args>
explicit 
thread(_Callable&& __f, _Args&&... __args)
{
  _M_start_thread(_M_make_routine(std::__bind_simple(
          std::forward<_Callable>(__f),
          std::forward<_Args>(__args)...)));
}

__f is the lambda we passed in; here the lambda is bound together with the arguments via bind and then handed to _M_make_routine, which constructs the _Impl.

template<typename _Callable>
      shared_ptr<_Impl<_Callable>>
      _M_make_routine(_Callable&& __f)
      {
    // Create and allocate full data structure, not base.
    return std::make_shared<_Impl<_Callable>>(std::forward<_Callable>(__f));
      }

And what is stored inside _Impl is exactly the result of the bind from before, _M_func.

template<typename _Callable>
      struct _Impl : public _Impl_base
      {
    _Callable       _M_func;

    _Impl(_Callable&& __f) : _M_func(std::forward<_Callable>(__f))
    { }

    void
    _M_run() { _M_func(); }
      };

Then the shared_ptr<_Impl<_Callable>> created by _M_make_routine is handed over to _M_start_thread.

2. _M_start_thread

_M_start_thread calls __gthread_create, and execute_native_thread_routine is passed here.

void
  thread::_M_start_thread(__shared_base_type __b)
  {
    if (!__gthread_active_p())
#if __EXCEPTIONS
      throw system_error(make_error_code(errc::operation_not_permitted),
             "Enable multithreading to use std::thread");
#else
      __throw_system_error(int(errc::operation_not_permitted));
#endif

    __b->_M_this_ptr = __b;
    int __e = __gthread_create(&_M_id._M_thread,
                   &execute_native_thread_routine, __b.get());
    if (__e)
    {
      __b->_M_this_ptr.reset();
      __throw_system_error(__e);
    }
  }

Key point 1: execute_native_thread_routine

The code below gives several pieces of information:

First: why extern "C"

Second: _M_run() is called

  extern "C"
  {
    static void*
    execute_native_thread_routine(void* __p)
    {
      thread::_Impl_base* __t = static_cast<thread::_Impl_base*>(__p);
      thread::__shared_base_type __local;
      __local.swap(__t->_M_this_ptr);

      __try
    {
      __t->_M_run();
    }
      __catch(const __cxxabiv1::__forced_unwind&)
    {
      __throw_exception_again;
    }
      __catch(...)
    {
      std::terminate();
    }

      return 0;
    }
  } // extern "C"

Key point 2: __gthread_create

It calls pthread_create

static inline int
__gthread_create (__gthread_t *__threadid, void *(*__func) (void*),
          void *__args)
{
  return __gthrw_(pthread_create) (__threadid, NULL, __func, __args);
}

Alright, let us sort out the flow above. When we construct a std::thread, we pass in a custom callback (a lambda) that executes our business logic; the std::thread constructor initiates the pthread_create call and passes execute_native_thread_routine to pthread_create in C style. When pthread_create executes, it runs execute_native_thread_routine, and the _M_run() inside is then invoked — this corresponds to the invocation of our lambda function.

This section has given us a deeper understanding of thread invocation. Did you learn something? Feel free to share.

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