c++ - try_lock_for 未按预期工作

Question

我正在摆弄一些 C++ 中的代码，由于某种原因不想工作，我将它缩小到这种情况:

#include <thread>
#include <atomic>
#include <chrono>
#include <mutex>
#include <iostream>

using namespace std;

void test()
{
  timed_mutex m;
  m.lock();
  std::cout << "Can i have the lock? " << m.try_lock() << std::endl;
  std::cout << "in test(), should block for 10 seconds" << std::endl;
  bool got_lock = m.try_lock_for(std::chrono::seconds(10));
  std::cout << "Now i've blocked, got the lock: " << got_lock << std::endl;
  m.unlock();
}

int main()
{
  thread t = thread(&test);
  t.join();

  return EXIT_SUCCESS;
}

问题是 test() 根本不会阻塞，即使 try_lock 返回 false。有没有我忽略的东西，或者这是 gcc 中的错误，或者我接下来应该去哪里找出问题所在？感谢任何建议和帮助!

我像这样编译了这个小程序:g++ -pthread -std=c++11 threads.cpp -o threads如果有任何帮助，这是 gcc 和我的操作系统的版本:

g++ --version
g++ (GCC) 4.7.2
Copyright (C) 2012 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

uname -a
Linux *computername* 3.6.11-1-ARCH #1 SMP PREEMPT Tue Dec 18 08:57:15 CET 2012 x86_64 GNU/Linux

Answer 1

您的代码行为未定义。 std::timed_mutex 具有非递归所有权 语义。禁止在同一个线程上第二次获取锁(包括try_lock家族)。

C++11 标准 30.4.1.3.1 [thread.timedmutex.class]/p3/b2:(感谢 Howard Hinnant)

3 The behavior of a program is undefined if:

• a thread that owns a timed_mutex object calls lock(), try_lock(), try_lock_for(), or try_lock_until() on that object, or

C++11 标准 30.4.1.2 [thread.mutex.requirements.mutex]/p6-7:

---

已编辑:

how i am going to "work around this" or get it to behave the way i want? Should i use a recursive mutex instead?

一般来说，出于异常安全的考虑，不鼓励获取/释放互斥对象的锁。如果您改用 unique_lock 对象，owns_lock() 成员函数可能会帮助您。同时，递归互斥锁对您的目的毫无用处，因为“递归”仅意味着“我(一个线程)在我已经拥有锁时可以获得两次或更多次锁。”

void test()
{
  std::timed_mutex m;
  std::unique_lock<decltype(m)> lk(m, std::defer_lock);

  // acquire lock
  lk.lock();
  // You can query locked status via unique_lock object
  std::cout << "Do I have own lock? " << lk.owns_lock() << std::endl;
  // release lock
  lk.unlock();
}