在Linux上的时区转换C API，任何人吗？

问题描述

我在找东西，我presumed会很简单 - 给定的本地Unix时间在一个特定的时间段（指定为字符串，如美国/纽约 - 注意，是的不是的我的本地时间），拿在格林尼治标准​​时间对应的时间价值。即沿东西线

I'm looking for something that I presumed would be very simple - given local Unix time in a specific time zone (specified as a string, e.g., "America/New_York" - note that's not my local time), get the corresponding time value in GMT. I.e., something along the lines of

time_t get_gmt_time(time_t local_time,
                    const char* time_zone);

作为看似简单，因为它听起来，最接近我能找到来自timegm的手册页以下code片断：

As deceptively simple as it sounds, the closest I could find was the following code snippet from timegm's man page:

       #include <time.h>
       #include <stdlib.h>

       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;
       }

有一定有比这挑衅不是线程安全的憎恶一个更好的办法，对不对？右??

There gotta be a better way than this belligerently not thread-safe abomination, right? Right??

推荐答案

想在这里添加更多的细节。

Wanted to add a bit more detail here.

如果您尝试以下操作：

#include <stdio.h>
#include <time.h>    /* defines 'extern long timezone' */

int main(int argc, char **argv)
{
    time_t t, lt, gt;
    struct tm tm;

    t = time(NULL);
    lt = mktime(localtime(&t));
    gt = mktime(gmtime(&t));

    printf( "(t = time(NULL)) == %x,\n"
        "mktime(localtime(&t)) == %x,\n"
        "mktime(gmtime(&t)) == %x\n"
        "difftime(...) == %f\n"
        "timezone == %d\n", t, lt, gt,
        difftime(gt, lt), timezone);
    return 0;
}

你会发现，时区转换确保：

you'll notice that timezone conversions make sure that:

• mktime（本地时间（T））==牛逼和


• mktime（gmtime的（T））== T +时区，结果
  因此：


• difftime（mktime（gmtime的（T）），mktime（本地时间（T）））==时区结果
  （后者是任何初始化的全局变量 tzset（）或任何时区转换函数的调用）。

• mktime(localtime(t)) == t, and
• mktime(gmtime(t)) == t + timezone,
  therefore:
• difftime(mktime(gmtime(t)), mktime(localtime(t))) == timezone
  (the latter is a global variable initialized by either tzset() or the invocation of any timezone conversion function).

上面的示例输出：

$ TZ=GMT ./xx
(t = time(NULL)) == 4dd13bac,
mktime(localtime(&t)) == 4dd13bac,
mktime(gmtime(&t)) == 4dd13bac
difftime(...) == 0.000000
timezone == 0

$ TZ=EST ./xx
(t = time(NULL)) == 4dd13baf,
mktime(localtime(&t)) == 4dd13baf,
mktime(gmtime(&t)) == 4dd181ff
difftime(...) == 18000.000000
timezone == 18000

$ TZ=CET ./xx
(t = time(NULL)) == 4dd13bb2,
mktime(localtime(&t)) == 4dd13bb2,
mktime(gmtime(&t)) == 4dd12da2
difftime(...) == -3600.000000
timezone == -3600

在这个意义上说，你试图别倒着 - time_t的被视为的绝对的参与联合国* X，即总相对于划时代（0:00 UTC在01/01/1970）。

In that sense, you're attempting to "do it backwards" - time_t is treated as absolute in UN*X, i.e. always relative to the "EPOCH" (0:00 UTC on 01/01/1970).

UTC和当前时区（最后一个 tzset（）调用）之间的区别是总是在外长时区全球

The difference between UTC and the current timezone (last tzset() call) is always in the external long timezone global.

这不摆脱环境操纵uglyness的，但你可以节省自己的通过 mktime去努力（）。

That doesn't get rid of the environment manipulation uglyness, but you can save yourself the effort of going through mktime().

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