详解C++11 线程休眠函数

C++ 11之前并未提供专门的休眠函数。c语言的sleep、usleep其实都是系统提供的函数，不同的系统函数的功能还有些差异。

在Windows系统中，sleep的参数是毫秒。

sleep(2*1000); //sleep for 2 seconds

在类Unix系统中，sleep()函数的单位是秒。

sleep(2);  //sleep for 2 seconds

从C++11开始，中C++标准库提供了专门的线程休眠函数，使得你的代码可以独立于不同的平台。

std::this_thread::sleep_for

std::this_thread::sleep_untill

1. 让线程休眠一段时间

std::this_thread::sleep_for用于Block当前线程一段时间。

template< class Rep, class Period >
void sleep_for( const std::chrono::duration<Rep, Period>& sleep_duration );

sleep的时间间隔从纳秒到小时都有具体的定义。

std::chrono::nanoseconds
std::chrono::microseconds
std::chrono::milliseconds
std::chrono::seconds
std::chrono::minutes
std::chrono::hours

比如我们想要一个线程休眠100ms。

std::this_thread::sleep_for(std::chrono::milliseconds(100));

我们想要一个线程休眠1分钟：

std::this_thread::sleep_for(std::chrono::minutes(1));

完整的代码示例：

#include <iostream>
#include <chrono>
#include <thread>
 
int main() {
  std::cout << "Hello waiter\n" << std::flush;

  auto start = std::chrono::high_resolution_clock::now();

  std::this_thread::sleep_for(std::chrono::milliseconds(2000));
  
  auto end = std::chrono::high_resolution_clock::now();
  
  std::chrono::duration<double, std::milli> elapsed = end-start;
  std::cout << "Waited " << elapsed.count() << " ms\n";
}

输出：

Hello waiter
Waited 2000.12 ms

2. 休眠直至到一个时间点

template< class Clock, class Duration >
void sleep_until( const std::chrono::time_point<Clock,Duration>& sleep_time )

sleep_until会阻塞当前线程直至未来某个时间点到达。

#include <iostream>
#include <thread>
#include <chrono>

// Print Current Time
void print_time_point(std::chrono::system_clock::time_point timePoint) {
  std::time_t timeStamp = std::chrono::system_clock::to_time_t(timePoint);
  std::cout << std::ctime(&timeStamp) << std::endl;
}

void threadFunc() {
  std::cout<<"Current Time :: ";
  // Print Current Time
  print_time_point(std::chrono::system_clock::now());
  // create a time point pointing to 10 second in future
  std::chrono::system_clock::time_point timePoint =
      std::chrono::system_clock::now() + std::chrono::seconds(10);
  std::cout << "Going to Sleep Until :: "; print_time_point(timePoint);
  // Sleep Till specified time point
  // Accepts std::chrono::system_clock::time_point as argument
  std::this_thread::sleep_until(timePoint);
  std::cout<<"Current Time :: ";
  // Print Current Time
  print_time_point(std::chrono::system_clock::now());
}

int main() {
  std::thread th(&threadFunc);
  th.join();

  return 0;
}

程序输出：

Current Time :: Sun Oct 11 02:57:38 2020

Going to Sleep Until :: Sun Oct 11 02:57:48 2020

Current Time :: Sun Oct 11 02:57:48 2020

参考材料

1.https://baike.baidu.com/item/sleep%E5%87%BD%E6%95%B0/6735027

2.https://thispointer.com/how-to-put-a-thread-to-sleep-in-c11-sleep_for-sleep_until/

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