如何使用升压条件变量等待线程完成处理？ [英] How do I use a boost condition variable to wait for a thread to complete processing?

问题描述

我使用的条件变量，直到另一个线程处理完它的任务队列（长的故事）来停止一个线程。因此，在一个线程我锁定和等待：

I am using a conditional variable to stop a thread until another thread has completed processing it's task queue (long story). So, on one thread I lock and wait:

boost::mutex::scoped_lock lock(m_mutex);
m_condition.wait(lock);

在另一个线程已经完成了它的任务，它标志着等待线程如下：

Once the other thread has completed it's tasks, it signals the waiting thread as follows:

boost::mutex::scoped_lock lock(m_parent.m_mutex);
m_parent.m_condition.notify_one();

我看到的问题是，除非我把它后面的指令断点（我用X code，仅供参考）等待线程不会停止等待。是的，这似乎很奇怪。没有人知道为什么这可能发生？上午我误使用条件变量？

The problem I am seeing is that the waiting thread does not stop waiting unless I set a breakpoint on the instructions following it (I am using xcode, fyi). Yes, this seems strange. Does anyone know why this might be happening? Am I mis-using the condition variable?

推荐答案

是的，你是滥用的条件变量。 条件变量都是真的只是信令机制。您还需要测试的条件。你的情况是什么可能发生的是，正在调用线程 notify_one（）调用线程等待（）前实际完成甚至开始。这就是所谓的（或者至少，在 notify_one（）呼叫在<$​​ C $ C>的wait（）电话。之前发生的事情） 错过唤醒。

Yes, you are misusing the condition variable. "Condition variables" are really just the signaling mechanism. You also need to be testing a condition. In your case what might be happening is that the thread that is calling notify_one() actually completes before the thread that calls wait() even starts. (Or at least, the notify_one() call is happening before the wait() call.) This is called a "missed wakeup."

解决方案是真正有一个包含你所关心的条件变量：

The solution is to actually have a variable which contains the condition you care about:

bool worker_is_done=false;

boost::mutex::scoped_lock lock(m_mutex);
while (!worker_is_done) m_condition.wait(lock);

和

boost::mutex::scoped_lock lock(m_mutex);
worker_is_done = true;
m_condition.notify_one();

如果 worker_is_done ==真其他线程甚至开始等待那么你会正好落在while循环而没有之前调用等待（ ）。

If worker_is_done==true before the other thread even starts waiting then you'll just fall right through the while loop without ever calling wait().

此模式是很常见的，我几乎走这么远说，如果你没有一个，而循环包装你的 condition_variable.wait（）然后你总是有一个bug。事实上，当C ++ 11采用了类似于升压的东西:: condtion_variable他们增加了一种新的等待（的），需要一个predicate拉姆达前pression（基本上它的而循环你）：

This pattern is so common that I'd almost go so far as to say that if you don't have a while loop wrapping your condition_variable.wait() then you always have a bug. In fact, when C++11 adopted something similar to the boost::condtion_variable they added a new kind of wait() that takes a predicate lambda expression (essentially it does the while loop for you):

std::condition_variable cv;
std::mutex m;
bool worker_is_done=false;


std::unique_lock<std::mutex> lk(m);
cv.wait(lk, []{return worker_is_done;});

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