glibc中的localtime方法源码分析

localtime方法会加锁，当TZ环境变量为空或者变更时，还会读取文件，还有个问题就是这个方法返回的指针是一个全局变量，可以使用redis无锁的localtime方法来优化这个性能。
localtime方法调用链：localtime -> __localtime64 -> __tz_convert（加锁、调用tzset_internal方法解释TZ环境变量，如果为空或者改变，则会尝试读取TZ文件）

/* The C Standard says that localtime and gmtime return the same pointer.  */
struct tm _tmbuf;


/* Return the `struct tm' representation of *T in local time,
   using *TP to store the result.  */
struct tm *
__localtime64_r (const __time64_t *t, struct tm *tp)
{
  return __tz_convert (*t, 1, tp);
}

/* Provide a 32-bit variant if needed.  */

#if __TIMESIZE != 64

struct tm *
__localtime_r (const time_t *t, struct tm *tp)
{
  __time64_t t64 = *t;
  return __localtime64_r (&t64, tp);
}
libc_hidden_def (__localtime64_r)

#endif

weak_alias (__localtime_r, localtime_r)


/* Return the `struct tm' representation of *T in local time.  */
struct tm *
__localtime64 (const __time64_t *t)
{
  /* 使用全局变量tmbuf，但是__tz_convert方法里面会加锁 */
  return __tz_convert (*t, 1, &_tmbuf);
}
libc_hidden_def (__localtime64)

/* Provide a 32-bit variant if needed.  */

#if __TIMESIZE != 64

struct tm *
localtime (const time_t *t)
{
  __time64_t t64 = *t;
  return __localtime64 (&t64);
}
libc_hidden_def (localtime)

#endif

struct tm *
__tz_convert (__time64_t timer, int use_localtime, struct tm *tp)
{
  long int leap_correction;
  int leap_extra_secs;

  __libc_lock_lock (tzset_lock);

  /* Update internal database according to current TZ setting.
     POSIX.1 8.3.7.2 says that localtime_r is not required to set tzname.
     This is a good idea since this allows at least a bit more parallelism.  */
  tzset_internal (tp == &_tmbuf && use_localtime); /* 方法内部会读取TZ环境变量，并更根据需要读取TZ文件 */

  if (__use_tzfile)
    __tzfile_compute (timer, use_localtime, &leap_correction,
		      &leap_extra_secs, tp);
  else
    {
      if (! __offtime (timer, 0, tp))
	tp = NULL;
      else
	__tz_compute (timer, tp, use_localtime);
      leap_correction = 0L;
      leap_extra_secs = 0;
    }

  __libc_lock_unlock (tzset_lock);

  if (tp)
    {
      if (! use_localtime)
	{
	  tp->tm_isdst = 0;
	  tp->tm_zone = "GMT";
	  tp->tm_gmtoff = 0L;
	}

      if (__offtime (timer, tp->tm_gmtoff - leap_correction, tp))
        tp->tm_sec += leap_extra_secs;
      else
	tp = NULL;
    }

  return tp;
}

redis版无锁localtime

static int is_leap_year(time_t year) {
    if (year % 4) return 0;         /* A year not divisible by 4 is not leap. */
    else if (year % 100) return 1;  /* If div by 4 and not 100 is surely leap. */
    else if (year % 400) return 0;  /* If div by 100 *and* 400 is not leap. */
    else return 1;                  /* If div by 100 and not by 400 is leap. */
}

void nolocks_localtime(struct tm *tmp, time_t t, time_t tz, int dst) {
    const time_t secs_min = 60;
    const time_t secs_hour = 3600;
    const time_t secs_day = 3600*24;

    t -= tz;                            /* Adjust for timezone. */
    t += 3600*dst;                      /* Adjust for daylight time. */
    time_t days = t / secs_day;         /* Days passed since epoch. */
    time_t seconds = t % secs_day;      /* Remaining seconds. */

    tmp->tm_isdst = dst;
    tmp->tm_hour = seconds / secs_hour;
    tmp->tm_min = (seconds % secs_hour) / secs_min;
    tmp->tm_sec = (seconds % secs_hour) % secs_min;

    /* 1/1/1970 was a Thursday, that is, day 4 from the POV of the tm structure
     * where sunday = 0, so to calculate the day of the week we have to add 4
     * and take the modulo by 7. */
    tmp->tm_wday = (days+4)%7;

    /* Calculate the current year. */
    tmp->tm_year = 1970;
    while(1) {
        /* Leap years have one day more. */
        time_t days_this_year = 365 + is_leap_year(tmp->tm_year);
        if (days_this_year > days) break;
        days -= days_this_year;
        tmp->tm_year++;
    }
    tmp->tm_yday = days;  /* Number of day of the current year. */

    /* We need to calculate in which month and day of the month we are. To do
     * so we need to skip days according to how many days there are in each
     * month, and adjust for the leap year that has one more day in February. */
    int mdays[12] = {31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31};
    mdays[1] += is_leap_year(tmp->tm_year);

    tmp->tm_mon = 0;
    while(days >= mdays[tmp->tm_mon]) {
        days -= mdays[tmp->tm_mon];
        tmp->tm_mon++;
    }

    tmp->tm_mday = days+1;  /* Add 1 since our 'days' is zero-based. */
    tmp->tm_year -= 1900;   /* Surprisingly tm_year is year-1900. */
}