multithread and static instances inside dll/dylib

In macosx, it seems that subthreads won't terminate until after any function which is registered to the api atexit is done. see this: https://blog.youkuaiyun.com/newmandirl/article/details/80079989

When you're using any threadpool in dll/dylib, such as QThreadPool, I think you should be very careful.

Personal experience:

simple code below:

dylibModule() {
	// ..
	init_static_instances();
	// ..
	startThreadPool();
	// ..
}

~dylibModule() {
	// ..
	// may not need to call uninit_static_instances because system will automatically call ~xx() as they are static instances
	stopThreadPool();
	// ..
}

Let me tell you what will happen in this situation:

        supposing that there is a qmutex you will use in a static instance TestA:

class TestA() {
public:
	void test() {
		mutex.lock();
		//..
		mutex.unlock();
	}
	static TestA & getInstance() { static TestA instance; return instance; };
private:
	QMutex mutex;
}

there is a possibility in a time line:

// --- main-thread --->
dlclose(dylibModule);
instance::~TestA();// destruct TestA static instance
//.. destruct other static instances
~dylibModule();
// .. do something in ~dylibModule()
stopThreadPool();
// .. do something in ~dylibModule()
// <--- main-thread ---

// --- sub-thread --->// main-thread is waiting for other sub-threads because of stopThreadPool().
TestA::getInstance().test();
// crash in mutex.lock()!!!!!!!!!!!!!
// <--- sub-thread ---

Personal handler:

Make sure that threads are stopped before the static instances are destroyed. In this way we can avoid accessing invalid memory.

There are tow methods:

1.stop threads before FreeLibrary()/dlclose() outside dll/dylib. I think this method would be better than method2.

2.use a static instance inside dll/dylib, construct it after any constructor of static instances. When the instance destructor is called, stop threads. This method may still have a risk: static instances in other dll/dylib are already destroyed, but some sub-thread in this dll/dylib is still running and trying to access those instances.