Windows / Posix compatible clock_gettime(REALTIME) ffi bindings.

clock_gettime.lua

local ffi = require 'ffi'
local bit = require 'bit'

local clock_gettime

if ffi.os == 'Windows' then
    local UNIX_TIME_START = 0x019DB1DED53E8000LL
    local TICKS_PER_SECOND = 10000000LL

    ffi.cdef [[
        typedef struct _FILETIME {
            int dwLowDateTime;
            int dwHighDateTime;
        } FILETIME, *PFILETIME, *LPFILETIME;

        void GetSystemTimeAsFileTime(
            LPFILETIME lpSystemTimeAsFileTime
        );
    ]]

    function clock_gettime()
        local struct = assert(ffi.new('FILETIME[1]'))

        ffi.C.GetSystemTimeAsFileTime(struct[0])

        local low = ffi.cast('int64_t', struct[0].dwLowDateTime)
        local high = ffi.cast('int64_t', struct[0].dwHighDateTime)

        local windows_time = bit.lshift(high, 32) + low
        local unix_time = windows_time - UNIX_TIME_START

        return tonumber(unix_time / TICKS_PER_SECOND) + tonumber(unix_time % TICKS_PER_SECOND) /
                   tonumber(TICKS_PER_SECOND)
    end
else
    local CLOCK_REALTIME = 0
    local NS_PER_SEC = 1e9

    ffi.cdef [[
        typedef struct timespec {
            long    tv_sec;
            long    tv_nsec;
        } accurate_realtime;

        int clock_gettime( int clk_id, struct timespec *tp );
    ]]

    function clock_gettime()
        local struct = assert(ffi.new('accurate_realtime[1]'))

        ffi.C.clock_gettime(CLOCK_REALTIME, struct)

        local sec = tonumber(struct[0].tv_sec)
        local nsec = tonumber(struct[0].tv_nsec)

        return sec + nsec / NS_PER_SEC
    end
end

return clock_gettime