localtime居然不可重入，踩坑了

最近踩坑了，全都拜localtime所赐，因为localtime居然不可重入：

[email protected]:~$ man localtime
CTIME(3)                                             Linux Programmer's Manual                                            CTIME(3)

NAME
       asctime,  ctime,  gmtime, localtime, mktime, asctime_r, ctime_r, gmtime_r, localtime_r - transform date and time to broken-
       down time or ASCII

SYNOPSIS
       #include <time.h>

       char *asctime(const struct tm *tm);
       char *asctime_r(const struct tm *tm, char *buf);

       char *ctime(const time_t *timep);
       char *ctime_r(const time_t *timep, char *buf);

       struct tm *gmtime(const time_t *timep);
       struct tm *gmtime_r(const time_t *timep, struct tm *result);

       struct tm *localtime(const time_t *timep);
       struct tm *localtime_r(const time_t *timep, struct tm *result);

       time_t mktime(struct tm *tm);

   Feature Test Macro Requirements for glibc (see feature_test_macros(7)):

       asctime_r(), ctime_r(), gmtime_r(), localtime_r():
              _POSIX_C_SOURCE >= 1 || _XOPEN_SOURCE || _BSD_SOURCE || _SVID_SOURCE || _POSIX_SOURCE

DESCRIPTION
       The ctime(), gmtime() and localtime() functions all take an argument of data type time_t, which represents  calendar  time.
       When  interpreted  as  an  absolute  time  value,  it  represents the number of seconds elapsed since the Epoch, 1970-01-01
       00:00:00 +0000 (UTC).

       The asctime() and mktime() functions both take an argument representing broken-down time, which is a  representation  sepa[m
       rated into year, month, day, and so on.

       Broken-down time is stored in the structure tm, which is defined in <time.h> as follows:

           struct tm {
               int tm_sec;    /* Seconds (0-60) */
               int tm_min;    /* Minutes (0-59) */
               int tm_hour;   /* Hours (0-23) */
               int tm_mday;   /* Day of the month (1-31) */
               int tm_mon;    /* Month (0-11) */
               int tm_year;   /* Year - 1900 */
               int tm_wday;   /* Day of the week (0-6, Sunday = 0) */
               int tm_yday;   /* Day in the year (0-365, 1 Jan = 0) */
               int tm_isdst;  /* Daylight saving time */
           };

真的需要小心一点。