如果没有本机 Clock_gettime 支持，如何在 Windows 上实现纳秒级精确计时？

将clock_gettime移植到Windows

clock_gettime函数是一个POSIX标准函数，它提供了一种以纳秒精度获取当前时间的方法。但是，Windows 本身不支持它。

将代码移植到 Windows

要将使用clock_gettime 的代码移植到 Windows，您可以使用 Win32 实现替换函数API。以下是如何执行此操作的示例：

<code class="C++">#include <Windows.h>

// Returns the file time offset for Windows.
LARGE_INTEGER getFILETIMEoffset() {
    SYSTEMTIME s;
    FILETIME f;
    LARGE_INTEGER t;

    s.wYear = 1970;
    s.wMonth = 1;
    s.wDay = 1;
    s.wHour = 0;
    s.wMinute = 0;
    s.wSecond = 0;
    s.wMilliseconds = 0;
    SystemTimeToFileTime(&amp;s, &amp;f);
    t.QuadPart = f.dwHighDateTime;
    t.QuadPart <<= 32;
    t.QuadPart |= f.dwLowDateTime;
    return (t);
}

// Replacement for clock_gettime function.
int clock_gettime(int X, struct timeval *tv) {
    LARGE_INTEGER t;
    FILETIME f;
    double microseconds;
    static LARGE_INTEGER offset;
    static double frequencyToMicroseconds;
    static int initialized = 0;
    static BOOL usePerformanceCounter = 0;

    if (!initialized) {
        LARGE_INTEGER performanceFrequency;
        initialized = 1;
        usePerformanceCounter = QueryPerformanceFrequency(&amp;performanceFrequency);
        if (usePerformanceCounter) {
            QueryPerformanceCounter(&amp;offset);
            frequencyToMicroseconds = (double)performanceFrequency.QuadPart / 1000000.;
        } else {
            offset = getFILETIMEoffset();
            frequencyToMicroseconds = 10.;
        }
    }
    if (usePerformanceCounter) QueryPerformanceCounter(&amp;t);
    else {
        GetSystemTimeAsFileTime(&amp;f);
        t.QuadPart = f.dwHighDateTime;
        t.QuadPart <<= 32;
        t.QuadPart |= f.dwLowDateTime;
    }

    t.QuadPart -= offset.QuadPart;
    microseconds = (double)t.QuadPart / frequencyToMicroseconds;
    t.QuadPart = microseconds;
    tv->tv_sec = t.QuadPart / 1000000;
    tv->tv_usec = t.QuadPart % 1000000;
    return (0);
}</code>

实现替换函数后，您可以在代码中使用它来获取纳秒精度的当前时间。请注意，您可能需要对代码进行额外的修改，以解决不同平台上 Clock_gettime 行为的任何差异。

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