c++ - 主线程退出后线程访问共享变量

Question

如果分离线程在调用线程退出并销毁共享变量后访问共享变量(例如全局变量)，在多线程 C++ 程序中会发生什么情况？

class A {
 public:
  A() { printf("Constructing A\n"); }
  ~A() { printf("Destructing A\n"); }
  void printSomething() { printf("A is printing\n"); }
}

A a;

void thread_func() {
  printf("begin thread.\n");
  sleep(3); // make sure main thread exit first
  a.printSomething();
  printf("ending thread");
}

int main(int argc, char** argv) {
  std::thread t(thread_func);
  t.detach();

  return 0;
}

程序产生:

bash$ ./a.out
Constructing A
Destructing A
bash$

似乎主线程创建了全局变量 a 并在退出时将其销毁。如果分离的子线程试图访问这个全局变量，那么 3 秒后会发生什么？

还有一个疑惑是，为什么主线程退出的时候会清空所有资源？看起来全局变量的生命周期只依赖于主线程？

Answer 1

进程在 main() 返回时退出，或者任何线程调用 exit() 或 _exit()。

但是，main() 可以调用 pthread_exit() - 这将不会终止进程。根据 Linux pthread_exit() man page :

When a thread terminates, process-shared resources (e.g., mutexes, condition variables, semaphores, and file descriptors) are not released, and functions registered using atexit(3) are not called.

After the last thread in a process terminates, the process terminates as by calling exit(3) with an exit status of zero; thus, process-shared resources are released and functions registered using atexit(3) are called.