如何确定我们是否在主线程中运行？

Question

We can get the id of the main thread by calling std::this_thread::get_id() just at the start of the main function like this answer suggests . We can then store this id in a global variable and compare against a call of std::this_thread::get_id() .

However, this forces us to change the main function. Is there a way to create a library function that does this? I was thinking about using a global variable initialized with std::this_thread::get_id() expression. Since global variables (variables with static duration) are initialized relatively early it is unlikely (but not impossible, see: deferred dynamic initialization ) that threads are spawned before these variables are initialized.

I could also initialize the global variable with a helper function which enumerates all threads and picks the one with the earliest creation time (based on this answer ).

I am very new to multithreading so any advice or guidance is extremely welcome.

Answer 1

There is no such thing as main thread. There is a thread which was launched first, but all threads are first-class citizens. By tinkering with linker flags, I can easily create a program where the thread executing main() would not be the the thread launched first.

Rethink your design.

Answer 2

EDIT

This is not a solid way of getting the main thread's ID considering what @ta.speot.is and @David Schwartz said.

You could make a static variable somewhere that initializes with the current thread's ID.

const std::thread::id MAIN_THREAD_ID = std::this_thread::get_id();

And then somewhere else:

if (std::this_thread::get_id() == MAIN_THREAD_ID)
{
    std::cout << "main thread\n";
}
else
{
    std::cout << "not main thread\n";
}

Answer 3

If you are using MFC. You can check AfxGetMainWnd() . If it return a pWnd then you have the (MFC) main thread. Disclaimer: If you have not manipulatued the m_pMainWnd Pointer.