既然 Windows 本身不提供'clock_gettime”函数，那么如何在 Windows 上实现该函数呢？

将clock_gettime移植到Windows

clock_gettime系统调用，通常用于测量经过的时间和检索当前时间，在本机中不可用Windows 操作系统。但是，有一些方法可以实现此函数的替换。

Windows 实现

下面是一个示例代码实现，模拟 Windows 的clock_gettime 功能：

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

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(&s, &f);
    t.QuadPart = f.dwHighDateTime;
    t.QuadPart <<= 32;
    t.QuadPart |= f.dwLowDateTime;
    return (t);
}

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(&performanceFrequency);
        if (usePerformanceCounter) {
            QueryPerformanceCounter(&offset);
            frequencyToMicroseconds = (double)performanceFrequency.QuadPart / 1000000.;
        } else {
            offset = getFILETIMEoffset();
            frequencyToMicroseconds = 10.;
        }
    }
    if (usePerformanceCounter) QueryPerformanceCounter(&t);
    else {
        GetSystemTimeAsFileTime(&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>

使用示例

要在Windows代码中使用clock_gettime替换函数，您可以调整原始QNX实现中提供的示例用法：

<code class="c++">#define BILLION 1000000000L;

struct timespec start_time;
struct timespec stop_time;

void startMyTestFunc() {
    clock_gettime(CLOCK_REALTIME, &start_time);
    // ... additional code
    cout << "The execution time of func" << calculateExecutionTime();
}

double calculateExecutionTime() {
    clock_gettime(CLOCK_REALTIME, &stop_time);

    double dSeconds = (stop_time.tv_sec - start_time.tv_sec);

    double dNanoSeconds = (double)(stop_time.tv_nsec - start_time.tv_nsec) / BILLION;

    return dSeconds + dNanoSeconds;
}</code>

此修改后的代码现在将使用 Windows 特定的 Clock_gettime 实现来测量 Windows 系统上的执行时间。

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