boost::scoped_lock 似乎没有锁定 std::cout

Question

我正在使用 boost 1.54.0 和 Visual Studio 2010。代码：

#include <iostream>
#include "boost/thread/thread.hpp"
#include "boost/thread/mutex.hpp"

boost::mutex mx1;

void func1()
{
    {
        boost::mutex::scoped_lock(mx1);
        std::cout << "Thread " << boost::this_thread::get_id() << " starting work." << std::endl;
    }
    int x = 0;
    for (int i=0; i<100; i++)
        x++;
    {
        boost::mutex::scoped_lock(mx1);
        std::cout << "Thread " << boost::this_thread::get_id() << " finished." << std::endl;
    }
}

int main(void)
{
    boost::thread thread1(&func1);
    boost::thread thread2(&func1);
    thread1.join();
    thread2.join();
    return 0;
}

大约一半的时间我得到以下信息（显然，线程 ID 和执行顺序不同）：

Thread Thread 15b0 starting work.
1a18 starting work.
Thread 15b0 finished.
Thread 1a18 finished.

...而不是这个（这是我所期望的）：

Thread 15b0 starting work.
Thread 1a18 starting work.
Thread 15b0 finished.
Thread 1a18 finished.

但是，使用

mx1.lock();
std::cout << "Thread " << boost::this_thread::get_id() << " starting work." << std::endl;
mx1.unlock();

...似乎没有问题。

输出似乎总是遵循相同的模式。我是否错误地使用了互斥锁，还是与 std::cout 有关？

Answer 1

替换

    boost::mutex::scoped_lock(mx1);

与

    boost::mutex::scoped_lock lock(mx1);

你成为了使用范围锁最常见的错字的受害者:-)