clock_gettime_win.c

#if defined(_WIN32) || defined(_WIN64)
#include <windows.h>
#include <time.h>

#define MS_PER_SEC      1000ULL     // MS = milliseconds
#define US_PER_MS       1000ULL     // US = microseconds
#define HNS_PER_US      10ULL       // HNS = hundred-nanoseconds (e.g., 1 hns = 100 ns)
#define NS_PER_US       1000ULL

#define HNS_PER_SEC     (MS_PER_SEC * US_PER_MS * HNS_PER_US)
#define NS_PER_HNS      (100ULL)    // NS = nanoseconds
#define NS_PER_SEC      (MS_PER_SEC * US_PER_MS * NS_PER_US)

int clock_gettime_monotonic(struct timespec *tv)
{
    static LARGE_INTEGER ticksPerSec;
    LARGE_INTEGER ticks;
    double seconds;

    if (!ticksPerSec.QuadPart) {
        QueryPerformanceFrequency(&ticksPerSec);
        if (!ticksPerSec.QuadPart) {
            errno = ENOTSUP;
            return -1;
        }
    }

    QueryPerformanceCounter(&ticks);

    seconds = (double) ticks.QuadPart / (double) ticksPerSec.QuadPart;
    tv->tv_sec = (time_t)seconds;
    tv->tv_nsec = (long)((ULONGLONG)(seconds * NS_PER_SEC) % NS_PER_SEC);

    return 0;
}

int clock_gettime_realtime(struct timespec *tv)
{
    FILETIME ft;
    ULARGE_INTEGER hnsTime;

    GetSystemTimeAsFileTime(&ft);

    hnsTime.LowPart = ft.dwLowDateTime;
    hnsTime.HighPart = ft.dwHighDateTime;

    // To get POSIX Epoch as baseline, subtract the number of hns intervals from Jan 1, 1601 to Jan 1, 1970.
    hnsTime.QuadPart -= (11644473600ULL * HNS_PER_SEC);

    // modulus by hns intervals per second first, then convert to ns, as not to lose resolution
    tv->tv_nsec = (long) ((hnsTime.QuadPart % HNS_PER_SEC) * NS_PER_HNS);
    tv->tv_sec = (long) (hnsTime.QuadPart / HNS_PER_SEC);

    return 0;
}

int clock_gettime(clockid_t type, struct timespec *tp)
{
    if (type == CLOCK_MONOTONIC)
    {
        return clock_gettime_monotonic(tp);
    }
    else if (type == CLOCK_REALTIME)
    {
        return clock_gettime_realtime(tp);
    }

    errno = ENOTSUP;
    return -1;
}

#endif