c++ - 异常调用后是否保证 std::call_once 的线程通知

Question

std::call_once 的标准示例 cppreference.com以这种方式描述异常调用的行为，据我了解，其他线程等待第一个进入 std::call_once 并且在发生异常的情况下，下一个线程将尝试执行 std::call_once。虽然在线编译器确认了这种行为，但我无法在本地重现它。对于最小的例子

#include <iostream>
#include <thread>
#include <mutex>

std::once_flag flag;

void may_throw_function(bool do_throw)
{
  if (do_throw) {
    std::cout << "throw: call_once will retry\n";
    throw std::exception();
  }
  std::cout << "Didn't throw, call_once will not attempt again\n";
}

void do_once(bool do_throw)
{
  try {
    std::call_once(flag, may_throw_function, do_throw);
  }
  catch (...) {}
}

int main()
{
    std::thread t1(do_once, true);
    std::thread t2(do_once, true);
    std::thread t3(do_once, false);
    std::thread t4(do_once, true);
    t1.join();
    t2.join();
    t3.join();
    t4.join();
}

复制自cppreference.com执行在第一次抛出后卡住并永远运行

编译是用g++-5 -std=c++14 source.cpp -pthread (version g++ (Ubuntu 5.4.0-6ubuntu1~16.04.11) 5.4.0 20160609) 或 clang++-6.0 source.cpp -pthread (version clang version 6.0.0-1ubuntu2~16.04.1 (tags/RELEASE_600/final)) .

将更多输出放入代码显示，所有线程都已启动，但只有最先抛出的线程才会结束。所有其他人似乎都在 std::call_once 声明之前等待。因此我的问题是:是否保证通知正在等待第一个线程完成的线程？

Answer 1

Is the notification of threads that are waiting for the first thread to finish guaranteed?

不涉及通知，但如果我将您的问题解释为:

Is t4.join() guaranteed to return?

是的。

[thread.once.callonce]

template<class Callable, class... Args>
 void call_once(once_flag& flag, Callable&& func, Args&&... args);

Effects: An execution of call_­once that does not call its func is a passive execution.
An execution of call_­once that calls its func is an active execution. An active execution shall call INVOKE(​std::forward<Callable>(func), std::forward<Args>(args)...).
If such a call to func throws an exception the execution is exceptional, otherwise it is returning.
An exceptional execution shall propagate the exception to the caller of call_­once. Among all executions of call_­once for any given once_­flag: at most one shall be a returning execution; if there is a returning execution, it shall be the last active execution; and there are passive executions only if there is a returning execution.

Synchronization: For any given once_­flag: all active executions occur in a total order; completion of an active execution synchronizes with the start of the next one in this total order; and the returning execution synchronizes with the return from all passive executions.

A call_once 仅当另一个call_once时才会执行返回(意思是:没有抛出)。由于观察到总顺序，因此可以保证:

• t1是主动异常执行还是被动执行；
• t2是主动异常执行还是被动执行；
• t3是主动返回执行；
• t4是主动异常执行还是被动执行；

由于被动执行返回，t4保证返回。

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由 user cpplearner 报告, a bug of pthread_once 使问题中列出的程序挂起(demo)。