Windows 上的 UTC 时间戳 [英] UTC time stamp on Windows

问题描述

我在 C 中有一个带有 UTC 时间戳的缓冲区，我每十秒广播一次该缓冲区.问题是两个数据包的时间差不一致.经过 5 到 10 次迭代后，时间差变为 9、11 和 10.请帮助我解决这个问题.

I have a buffer with the UTC time stamp in C, I broadcast that buffer after every ten seconds. The problem is that the time difference between two packets is not consistent. After 5 to 10 iterations the time difference becomes 9, 11 and then again 10. Kindly help me to sort out this problem.

我使用 作为 UTC 时间.

I am using <time.h> for UTC time.

推荐答案

休眠 X 毫秒的线程并不能保证休眠精确到那么多毫秒.我假设您有一个类似于以下内容的声明:

A thread that sleeps for X milliseconds is not guaranteed to sleep for precisely that many milliseconds. I am assuming that you have a statement that goes something like:

while(1) {
  ...
  sleep(10); // Sleep for 10 seconds.
  // fetch timestamp and send
}

如果您在循环检查中睡眠时间较短(例如 20 毫秒)，直到时间到期，您将获得更准确的时间计量.当您休眠 10 秒时，您的线程会进一步脱离底层操作系统的即时调度优先级.

You will get a more accurate gauge of time if you sleep for shorter periods (say 20 milliseconds) in a loop checking until the time has expired. When you sleep for 10 seconds, your thread gets moved further out of the immediate scheduling priority of the underlying OS.

您可能还考虑到发送时间戳所花费的时间可能会有所不同，具体取决于网络条件等，如果您执行 sleep(10) -> send ->sleep(10) 类型的循环，则时间采取发送将实际添加到下一个睡眠(10).

You might also take into account that the time taken to send the timestamps may vary, depending on network conditions, etc, if you do a sleep(10) -> send ->sleep(10) type of loop, the time taken to send will be added onto the next sleep(10) in real terms.

试试这样的东西(原谅我，我的 C 有点生疏):

Try something like this (forgive me, my C is a little rusty):

bool expired = false;
double last, current;
double t1, t2;
double difference = 0;

while(1) {
   ...
   last = (double)clock();
   while(!expired) {
      usleep(200); // sleep for 20 milliseconds
      current = (double)clock();
      if(((current - last) / (double)CLOCKS_PER_SEC) >= (10.0 - difference))
        expired = true;
   }
   t1 = (double)clock();
   // Set and send the timestamp.
   t2 = (double)clock();
   //
   // Calculate how long it took to send the stamps.
   // and take that away from the next sleep cycle.
   //
   difference = (t2 - t1) / (double)CLOCKS_PER_SEC;
   expired = false;
 }

如果您不介意使用标准 C 库，您可以考虑使用 Windows 的高分辨率计时器功能，例如 QueryPerformanceFrequency/QueryPerformanceCounter 函数.

If you are not bothered about using the standard C library, you could look at using the high resolution timer functionality of windows such as QueryPerformanceFrequency/QueryPerformanceCounter functions.

LONG_INTEGER freq;
LONG_INTEGER t2, t1;
//
// Get the resolution of the timer.
//
QueryPerformanceFrequency(&freq);

// Start Task.
QueryPerformanceCounter(&t1);

... Do something ....

QueryPerformanceCounter(&t2);

// Very accurate duration in seconds.
double duration = (double)(t2.QuadPart - t1.QuadPart) / (double)freq.QuadPart;

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