c++ wait_until奇怪的超时行为

Question

我正在尝试用 C++ 编写一种线程池。该代码在 OSX 中运行良好，但在 Linux 下我遇到了一个奇怪的行为。 经过一些调试后，我发现问题是由于对 std::condition_variable::wait_until 的调用造成的，我一定是以错误的方式进行的。

对于下面的代码，我希望循环每三秒循环一次:

#include <mutex>
#include <chrono>
#include <iostream>
#include <memory>
#include <condition_variable>
#include <thread>

using namespace std;

typedef std::chrono::steady_clock                           my_clock;
typedef std::chrono::duration<float, std::ratio<1> >        seconds_duration;
typedef std::chrono::time_point<my_clock, seconds_duration> timepoint;

timepoint my_begin = my_clock::now();

float timepointToFloat(timepoint time) {
  return time.time_since_epoch().count() - my_begin.time_since_epoch().count();
}

void printNow(std::string mess) {
  timepoint now = my_clock::now();
  cout << timepointToFloat(now) << " " << mess  << endl;;
};

void printNow(std::string mess, timepoint time ) {
  timepoint now = my_clock::now();
  cout << timepointToFloat(now) << " " << mess  << " " << timepointToFloat(time) << endl;;
};

int main() {
  mutex _global_mutex;
  condition_variable _awake_global_execution;
  auto check_predicate = [](){
    cout << "predicate called" << endl;
    return false;
  };

  while (true) {
    { // Expected to loop every three seconds
      unique_lock<mutex> lock(_global_mutex);
      timepoint planned_awake = my_clock::now() + seconds_duration(3);
      printNow("wait until", planned_awake);
      _awake_global_execution.wait_until(lock, planned_awake, check_predicate);
    }
    printNow("finish wait, looping");
  }

  return 0;
}

但是，有时我得到的输出是:

<X> wait until <X+3>
predicate called
(...hangs here for a long time)

(其中 X 是一个数字)，所以似乎超时没有安排在三秒后。有时我会得到:

<X> wait until <X+3>
predicate called
predicate called
<X> finish wait, looping
<X> wait until <X+3> (another loop)
predicate called
predicate called
<X> finish wait, looping
(...continue looping without waiting)

所以看起来超时是在几秒钟后安排的。我想我在超时时间点上搞砸了，但我不知道我做错了什么。

如果可能相关，这段代码在 OSX 中运行良好，而在 Linux 中(Ubuntu 16.04，gcc 5.4，用“g++ main.cc -std=c++11 -pthread”编译)我遇到了奇怪的问题行为。

我怎样才能让它工作？

Answer 1

尝试将超时设置为时钟的持续时间:

auto planned_awake = my_clock::now() + 
                     std::chrono::duration_cast<my_clock::duration>(secon‌​ds_duration(3));