mark：linux获取系统时间

static const int64_t kNumMillisecsPerSec = INT64_C(1000);
static const int64_t kNumMicrosecsPerSec = INT64_C(1000000);
static const int64_t kNumNanosecsPerSec = INT64_C(1000000000);

static const int64_t kNumMicrosecsPerMillisec =
    kNumMicrosecsPerSec / kNumMillisecsPerSec;
static const int64_t kNumNanosecsPerMillisec =
    kNumNanosecsPerSec / kNumMillisecsPerSec;
static const int64_t kNumNanosecsPerMicrosec =
    kNumNanosecsPerSec / kNumMicrosecsPerSec;


int64_t SystemTimeNanos() {
  int64_t ticks = 0;

  struct timespec ts;
  clock_gettime(CLOCK_PROCESS_CPUTIME_ID, &ts);
  ticks = kNumNanosecsPerSec * static_cast(ts.tv_sec) +
            static_cast(ts.tv_nsec);
  return ticks;
}

CLOCK_REALTIME:系统实时时间,随系统实时时间改变而改变,即从UTC1970-1-1 0:0:0开始计时,中间时刻如果系统时间被用户该成其他,则对应的时间相应改变

CLOCK_MONOTONIC:从系统启动这一刻起开始计时,不受系统时间被用户改变的影响

CLOCK_PROCESS_CPUTIME_ID:本进程到当前代码系统CPU花费的时间

CLOCK_THREAD_CPUTIME_ID:本线程到当前代码系统CPU花费的时间

参考：

clock_gettime(3): clock/time functions - Linux man pageThe function clock_getres() finds the resolution (precision) of the specified clock clk_id, and, if res is non-NULL, stores it in the struct timespec pointed to by res.https://linux.die.net/man/3/clock_gettime