std :: mktime和时区信息

mktime()使用tzname来检测时区.tzset()从TZ环境变量初始化tzname变量.如果TZ变量出现在环境中但其值为空或其值无法正确解释,则使用UTC.

根据timegm联机帮助页的便携式(非线程安全)版本

   #include 
   #include 

   time_t
   my_timegm(struct tm *tm)
   {
       time_t ret;
       char *tz;

       tz = getenv("TZ");
       setenv("TZ", "", 1);
       tzset();
       ret = mktime(tm);
       if (tz)
           setenv("TZ", tz, 1);
       else
           unsetenv("TZ");
       tzset();
       return ret;
   }

Eric S Raymond在他的文章" Time,Clock,and Calendar Programming In C"中发表了一个线程安全版本

time_t my_timegm(register struct tm * t)
/* struct tm to seconds since Unix epoch */
{
    register long year;
    register time_t result;
#define MONTHSPERYEAR   12      /* months per calendar year */
    static const int cumdays[MONTHSPERYEAR] =
        { 0, 31, 59, 90, 120, 151, 181, 212, 243, 273, 304, 334 };

    /*@ +matchanyintegral @*/
    year = 1900 + t->tm_year + t->tm_mon / MONTHSPERYEAR;
    result = (year - 1970) * 365 + cumdays[t->tm_mon % MONTHSPERYEAR];
    result += (year - 1968) / 4;
    result -= (year - 1900) / 100;
    result += (year - 1600) / 400;
    if ((year % 4) == 0 && ((year % 100) != 0 || (year % 400) == 0) &&
        (t->tm_mon % MONTHSPERYEAR) < 2)
        result--;
    result += t->tm_mday - 1;
    result *= 24;
    result += t->tm_hour;
    result *= 60;
    result += t->tm_min;
    result *= 60;
    result += t->tm_sec;
    if (t->tm_isdst == 1)
        result -= 3600;
    /*@ -matchanyintegral @*/
    return (result);
}