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社区首页 >问答首页 >从gettimeofday创建ntp时间戳

从gettimeofday创建ntp时间戳
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Stack Overflow用户
提问于 2010-04-15 08:27:57
回答 2查看 9K关注 0票数 1

我需要使用gettimeofday计算ntp时间戳。下面是我如何使用方法注释来实现的。你们觉得还不错吧?(减去错误检查)。另外,这里有一个codepad链接。

#include <unistd.h>
#include <sys/time.h>

const unsigned long EPOCH = 2208988800UL; // delta between epoch time and ntp time
const double NTP_SCALE_FRAC = 4294967295.0; // maximum value of the ntp fractional part

int main()
{
  struct timeval tv;
  uint64_t ntp_time;
  uint64_t tv_ntp;
  double tv_usecs;

  gettimeofday(&tv, NULL);
  tv_ntp = tv.tv_sec + EPOCH;

  // convert tv_usec to a fraction of a second
  // next, we multiply this fraction times the NTP_SCALE_FRAC, which represents
  // the maximum value of the fraction until it rolls over to one. Thus,
  // .05 seconds is represented in NTP as (.05 * NTP_SCALE_FRAC)
  tv_usecs = (tv.tv_usec * 1e-6) * NTP_SCALE_FRAC;

  // next we take the tv_ntp seconds value and shift it 32 bits to the left. This puts the 
  // seconds in the proper location for NTP time stamps. I recognize this method has an 
  // overflow hazard if used after around the year 2106
  // Next we do a bitwise OR with the tv_usecs cast as a uin32_t, dropping the fractional
  // part
  ntp_time = ((tv_ntp << 32) | (uint32_t)tv_usecs);
}
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页面原文内容由Stack Overflow提供。腾讯云小微IT领域专用引擎提供翻译支持
原文链接:

https://stackoverflow.com/questions/2641954

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