--------- time/localtime.c 和 time/tzset.c

localtime_r中调用了tzset_internal来设置时区，入口参数为always=0，所以理论上只要第一次初次化过了，就不需初始化了。参考下面的代码。
但是由于引入了静态变量is_initialized，在多线程环境下，这种实现代码是有问题的。无法保证并发执行环境下的正确性。

---- (time/tzset.c) -------

/* Interpret the TZ envariable. */
static void
internal_function
tzset_internal (always)
     int always;
{
static int is_initialized;
register const char *tz;
register size_t l;
char *tzbuf;
unsigned short int hh, mm, ss;
unsigned short int whichrule;

if (is_initialized && !always)
    return;
is_initialized = 1;
...........
}

但是mktime不是这样的，

---- (time/mktime.c) -------

/* Convert *TP to a time_t value. */
time_t
mktime (tp)
     struct tm *tp;
{
#ifdef _LIBC
/* POSIX.1 8.1.1 requires that whenever mktime() is called, the
     time zone names contained in the external variable `tzname' shall
     be set as if the tzset() function had been called. */
__tzset ();
#endif

return __mktime_internal (tp, my_mktime_localtime_r, &localtime_offset);
}

由于_LIBC被定义，所以tzset将每次都被调用，而tzset的代码是这样的

---- (time/tzset.c) -------

void
__tzset (void)
{
__libc_lock_lock (tzset_lock);

tzset_internal (1);

if (!__use_tzfile)
    {
      /* Set `tzname'. */
      __tzname[0] = (char *) tz_rules[0].name;
      __tzname[1] = (char *) tz_rules[1].name;
    }

__libc_lock_unlock (tzset_lock);
}