我使用 GetSystemTimeAdjustment 进行了一些测试在 Windows 7 上运行，并得到了一些我无法解释的有趣结果。据我所知，如果系统时间是定期同步的，则此方法应该返回，如果是，则以哪个时间间隔和哪个增量更新( see GetSystemTimeAdjustment function on MSDN )。

从这里我遵循，如果我查询系统时间，例如使用 GetSystemTimeAsFileTime重复我应该要么没有变化(系统时钟尚未更新)，要么变化是 GetSystemTimeAdjustment 检索到的增量的倍数. 问题一:这个假设正确吗？

现在考虑以下测试代码:

#include <windows.h>
#include <iostream>
#include <iomanip>

int main()
{
    FILETIME fileStart;
    GetSystemTimeAsFileTime(&fileStart);
    ULARGE_INTEGER start;
    start.HighPart = fileStart.dwHighDateTime;
    start.LowPart = fileStart.dwLowDateTime;

    for (int i=20; i>0; --i)
    {
        FILETIME timeStamp1;
        ULARGE_INTEGER ts1;

        GetSystemTimeAsFileTime(&timeStamp1);

        ts1.HighPart = timeStamp1.dwHighDateTime;
        ts1.LowPart  = timeStamp1.dwLowDateTime;

        std::cout << "Timestamp: " << std::setprecision(20) << (double)(ts1.QuadPart - start.QuadPart) / 10000000 << std::endl;

    }

    DWORD dwTimeAdjustment = 0, dwTimeIncrement = 0, dwClockTick;
    BOOL fAdjustmentDisabled = TRUE;
    GetSystemTimeAdjustment(&dwTimeAdjustment, &dwTimeIncrement, &fAdjustmentDisabled);

    std::cout << "\nTime Adjustment disabled: " << fAdjustmentDisabled
        << "\nTime Adjustment: " << (double)dwTimeAdjustment/10000000
        << "\nTime Increment: " << (double)dwTimeIncrement/10000000 << std::endl;

}

Timestamp: 0
Timestamp: 0.0025000000000000001
Timestamp: 0.0074999999999999997
Timestamp: 0.01
Timestamp: 0.012500000000000001
Timestamp: 0.014999999999999999
Timestamp: 0.017500000000000002
Timestamp: 0.022499999999999999
Timestamp: 0.025000000000000001
Timestamp: 0.0275
Timestamp: 0.029999999999999999
Timestamp: 0.032500000000000001
Timestamp: 0.035000000000000003
Timestamp: 0.040000000000000001
Timestamp: 0.042500000000000003
Timestamp: 0.044999999999999998
Timestamp: 0.050000000000000003
Timestamp: 0.052499999999999998
Timestamp: 0.055
Timestamp: 0.057500000000000002

Time Adjustment disabled: 0
Time Adjustment: 0.0156001
Time Increment: 0.0156001

最佳答案

GetSystemTimeAsFileTime API 以文件时间格式提供对系统挂钟的访问。

64 位 FILETIME 结构以 100ns 为单位接收系统时间作为 FILETIME，该时间自 1601 年 1 月 1 日起已过期。对 GetSystemTimeAsFileTime 的调用通常需要 10 ns 到 15 ns。

为了调查此 API 提供的系统时间的真实准确性，需要讨论与时间值一起出现的粒度。换句话说:系统时间多久更新一次？第一个估计值由隐藏的 API 调用提供:

NTSTATUS NtQueryTimerResolution(OUT PULONG MinimumResolution, 
                                OUT PULONG MaximumResolution, 
                                OUT PULONG ActualResolution);

NTSTATUS NtSetTimerResolution(IN ULONG RequestedResolution,
                              IN BOOLEAN Set,
                              OUT PULONG ActualResolution);

MMRESULT timeBeginPeriod(UINT uPeriod);

MMRESULT timeGetDevCaps(LPTIMECAPS ptc, UINT cbtc );

typedef struct {
  UINT wPeriodMin;
  UINT wPeriodMax;
} TIMECAPS;

FILETIME FileTime,LastFileTime;
long long DueTime,LastTime;
long FileTimeTransitionPeriod; 

GetSystemTimeAsFileTime(&FileTime);
for (int i = 0; i < 20; i++) {
  LastFileTime.dwLowDateTime = FileTime.dwLowDateTime;
  while (FileTime.dwLowDateTime == LastFileTime.dwLowDateTime) GetSystemTimeAsFileTime(&FileTime); 
  // enough to just look at the low part to catch the transition
  CopyMemory(&DueTime,&FileTime,sizeof(FILETIME));
  CopyMemory(&LastTime,&LastFileTime,sizeof(FILETIME));
  FileTimeTransitionPeriod = (long)(DueTime-LastTime);
  fprintf(stdout,"transition period: % 7.4lf ms)\n",(double)(FileTimeTransitionPeriod)/10000);
}   

// WARNING: This code consumes 100% of the cpu for 20 file time increments.
// At the standard file time increment of 15.625 ms this corresponds to 312.5ms!

关于c++ - Windows 7 计时功能 - 如何正确使用 GetSystemTime Adjustment？，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/7685762/