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I am trying to get the number of ms since jan 01 1970 using this. However when I try to compile I get

"Error: cannot open source file 'sys/time.h'"

I am using Visual Studio.

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HXSP1947
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2 Answers2

4

The sys/time.h header is a POSIX header and as such is not typically found on a Windows machine, and certainly not in a Windows SDK. For Visual Studio you need to include time.h rather than sys/time.h, as described in the documentation.

But the functions in the Visual Studio time.h don't give you millisecond precision. There is not gettimeofday(). The best you can do is Unix time in seconds. Now, millisecond precision Unix time on Windows was the subject of this question: How can i get UTCTime in millisecond since January 1, 1970 in c language. However, the accepted answer is dire and really should be deleted.

After a bit more diffing, I think you will find what you need here: Convert Windows Filetime to second in Unix/Linux. The workflow goes like this:

  1. Call GetSystemTime() to get the system time.
  2. Call SystemTimeToFileTime to convert to file time.
  3. Use the conversion detailed in the above link to convert from file time to Unix time.

I think this is what you need, but bear in mind I am not remotely fluent in C++:

#include <windows.h>

long long GetUnixTime()
{
    const long long WINDOWS_TICK = 10000000LL;
    const long long SEC_TO_UNIX_EPOCH = 11644473600LL;

    SYSTEMTIME st;
    GetSystemTime(&st);

    FILETIME ft;
    if (!SystemTimeToFileTime(&st, &ft))
        return -1;

    ULARGE_INTEGER uli;
    uli.LowPart = ft.dwLowDateTime;
    uli.HighPart = ft.dwHighDateTime;
    return uli.QuadPart / WINDOWS_TICK - SEC_TO_UNIX_EPOCH;
}
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David Heffernan
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  • This still didn't solve the problem since gettimeofday() is now not recognized (I am copying verbatim from the solution in the link i provided). – HXSP1947 Jan 28 '14 at 20:06
  • There's a difference between solving your problem, and answering your question. Anyway, I think the final link that I offer should do the trick. – David Heffernan Jan 28 '14 at 20:09
  • don't append wMilliseconds! They are already counted in the code above. – David Heffernan Jan 29 '14 at 07:10
  • +1 I'd suggest to use `GetSystemTimeAsFileTime()` as replacement for `GetSystemTime()` and `SystemTimeToFileTime()` to make the code even smarter. – Arno Feb 27 '14 at 15:51
4

If you are using C++ 11, you can use std::chrono. 

#include <chrono>

long long GetUnixTimeChrono()
{
    auto timeSinceEpoch = std::chrono::system_clock::now().time_since_epoch();
    return std::chrono::duration_cast<std::chrono::milliseconds>(timeSinceEpoch).count();
}

Otherwise, you can do this:

#include <Windows.h>

long long GetUnixTime()
{
    SYSTEMTIME sysUnixEpoch;
    sysUnixEpoch.wYear = 1970;
    sysUnixEpoch.wMonth = 1;
    sysUnixEpoch.wDayOfWeek = 4;
    sysUnixEpoch.wDay = 1;
    sysUnixEpoch.wHour = 0;
    sysUnixEpoch.wMinute = 0;
    sysUnixEpoch.wSecond = 0;
    sysUnixEpoch.wMilliseconds = 0;

    FILETIME unixEpoch;
    SystemTimeToFileTime(&sysUnixEpoch, &unixEpoch);

    ULARGE_INTEGER unixEpochValue;
    unixEpochValue.HighPart = unixEpoch.dwHighDateTime;
    unixEpochValue.LowPart = unixEpoch.dwLowDateTime;

    FILETIME systemTime;
    GetSystemTimeAsFileTime(&systemTime);

    ULARGE_INTEGER systemTimeValue;
    systemTimeValue.HighPart = systemTime.dwHighDateTime;
    systemTimeValue.LowPart = systemTime.dwLowDateTime;

    long long diffHundredNanos = systemTimeValue.QuadPart - unixEpochValue.QuadPart;
    return diffHundredNanos / 10000;
}
Derek J S
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