C ++ Clock（）函数给出不正确的值 [英] C++ clock() function giving incorrect values

问题描述

我试图编写一个Timer类（不知道boost有一个），然后当它不起作用时，我尝试使用以下代码输出Clock（）的值：

  #include< ctime> 
 #include< iostream> 
 
 int main（）
 {
 for（int i = 0; i <50; ++ i）
 {
 std :: cout << std :: clock（）<< ; 
} 
返回0； 
} 
  

运行程序时，得到一系列的0。使用升压线程睡眠功能将时间分配得更长一些时，我也有类似的经历（尽管几秒钟后，它从0跳到10,000，并保持输出10,000）。

<我正在运行Gentoo Linux。这是平台的东西吗？ C ++的东西？发生了什么事？

编辑：奇怪的是，在经过秒（而不是毫秒）后才跳到10000。当我一次使线程休眠一秒钟时，花了五到六秒钟才达到10000。但是，如果我理解正确的话。线程花费在睡眠上的时间不会增加clock（）的计数吗？ （这有道理；如果它处于睡眠状态，为什么还要执行时钟周期？）

解决方案

clock（）返回值以微秒为单位。但是，任何低级系统调用 clock（）实现所使用的典型粒度要低得多。因此，看来您的系统上的粒度为10ms。另请注意， clock（）不能不测量实时时间-它测量程序使用的CPU时间。因此，时间流逝在您的程序控制CPU时，并且在程序暂停时冻结（例如，休眠）。

I was trying to program a Timer class (unaware that boost had one), then when that wasn't working, I tried to just output the value of clock(), using this code:

#include <ctime>
#include <iostream>

int main()
{
 for(int i = 0; i < 50; ++i)
 {
  std::cout << std::clock() << " ";
 }
 return 0;
}

When I run the program, I get a series of 0s. I have a similar experience when using boost thread sleep functions to spread out timing a little longer (although after a few seconds, it jumps from 0 to 10,000 and keeps outputting 10,000).

I'm running Gentoo Linux. Is this a platform thing? A C++ thing? What's going on?

Edit: Strangely the jump to 10000 comes after a number of seconds, not milliseconds. When I was sleeping my thread for a second at a time, it took five or six seconds to get to 10000. However, if I'm understanding correctly. The time the thread spends sleeping doesn't contribute towards the clock() count? (Which would make sense; why would it be executing clock cycles if it's sleeping?)

解决方案

The clock() return value is specified in microseconds. But typical granularity of whatever low-level system call the clock() implementation uses is much lower. So it seems that on your system the granularity is 10ms. Also note that clock() does NOT measure real time - it measures CPU time used by the program. So the time flows when your program controls the CPU, and it freezes when your program is suspended - sleeping, for example.

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