为什么 `std::exit` 没有按预期触发析构函数？

Question

#include <cstdlib>
#include <thread>
#include <chrono>
#include <iostream>

using namespace std;
using namespace std::literals;

struct A
{
    int n_ = 0;
    A(int n) : n_(n) { cout << "A:" << n_ << endl; }
    ~A() { cout << "~A:" << n_ << endl; }
};

A a1(1);

int main()
{
    std::thread([]()
    {
        static A a2(2);
        thread_local A a3(3);
        std::this_thread::sleep_for(24h);
    }).detach();

    static A a4(4);
    thread_local A a5(5);

    std::this_thread::sleep_for(1s);
    std::exit(0);
}

我的编译器是 clang 5.0 和 -std=c++1z。

输出结果如下：

A:1
A:2
A:4
A:5
A:3
~A:5
~A:2
~A:4
~A:1

注意没有~A:3，表示对象A a3没有被销毁

然而，根据cppref：

std::exit causes normal program termination to occur. Several cleanup steps are performed:

The destructors of objects with thread local storage duration ... are guaranteed to be called.

Answer 1

这里的问题是，当您退出进程时，线程将（在大多数现代多任务操作系统上）被强行杀死。这种线程的终止发生在 OS 级别，并且 OS 对对象或析构函数一无所知。

Answer 2

具有线程存储持续时间的对象保证只为调用exit的线程销毁。引用 C++14 (N4140)，[support.start.term] 18.5/8（强调我的）：

[[noreturn]] void exit(int status)
The function exit() has additional behavior in this International Standard:

First, objects with thread storage duration and associated with the current thread are destroyed. Next, objects with static storage duration are destroyed and functions registered by calling atexit are called. See 3.6.3 for the order of destructions and calls. (Automatic objects are not destroyed as a result of calling exit().) If control leaves a registered function called by exit because the function does not provide a handler for a thrown exception, std::terminate() shall be called (15.5.1).

Next, all open C streams (as mediated by the function signatures declared in <cstdio>) with unwritten buffered data are flushed, all open C streams are closed, and all files created by calling tmpfile() are removed.

Finally, control is returned to the host environment. If status is zero or EXIT_SUCCESS, an implementation-defined form of the status successful termination is returned. If status is EXIT_FAILURE, an implementation-defined form of the status unsuccessful termination is returned. Otherwise the status returned is implementation-defined.

因此，该标准不保证销毁具有与调用 exit.

以外的其他线程关联的线程存储持续时间的对象

为什么 `std::exit` 没有按预期触发析构函数？

Why does `std::exit` not trigger destructors as expected?

c++

destructor

global-variables

thread-local-storage

c++11