将FILETIME转换为可移植的时间单位

Question

How would I go about converting a Windows FILETIME object into a time_t or raw seconds/milliseconds? I'm porting some code from Windows to Unix so I cannot rely on the Windows API functions.

Answer 1

A FILETIME is defined as

Contains a 64-bit value representing the number of 100-nanosecond intervals since January 1, 1601 (UTC).

So to convert it to a Unix time, it's just a matter of subtracting the two epoch times and converting from 100-nanosecond intervals to seconds/millisconds. Any number of tools/sites will tell you that the two epochs are 134774 days (or 11644473600 seconds) apart. Therefore:

void convert_filetime(struct timeval *out_tv, const FILETIME *filetime)
{
    // Microseconds between 1601-01-01 00:00:00 UTC and 1970-01-01 00:00:00 UTC
    static const uint64_t EPOCH_DIFFERENCE_MICROS = 11644473600000000ull;

    // First convert 100-ns intervals to microseconds, then adjust for the
    // epoch difference
    uint64_t total_us = (((uint64_t)filetime->dwHighDateTime << 32) | (uint64_t)filetime->dwLowDateTime) / 10;
    total_us -= EPOCH_DIFFERENCE_MICROS;

    // Convert to (seconds, microseconds)
    out_tv->tv_sec = (time_t)(total_us / 1000000);
    out_tv->tv_usec = (useconds_t)(total_us % 1000000);
}