如何分解Unix时间用C [英] How to decompose unix time in C

问题描述

这看起来似乎任何人都不应该要做的，但我正在一个内核模块上的嵌入式系统（的OpenWRT），其中似乎 time.h中的确实的包括的timespec 和 time_t的类型，而 clock_gettime 和 gmtime的的功能，但确实的不的包括本地时间，的ctime ，时间，或者关键的是， TM 键入

This seems like something no one should ever have to do, but I'm working on a kernel module for an embedded system (OpenWRT) in which it seems that time.h does include the timespec and time_t types, and the clock_gettime and gmtime functions, but does not include localtime, ctime, time, or, critically, the tm type.

当我试图从gmtime的返回指针转换为我自己的结构，我得到段错误。

When I attempt to cast the return pointer from gmtime to my own struct, I get a segfault.

所以我想我会满足于解决问题的两种方法之一，it'd是巨大的，弄清楚如何获得访问该缺少类型，或者，如何推出自己的方法分解UNIX时间戳。

So I guess I'd be content to solve the problem either of two ways—it'd be great to figure out how to get access to that missing type, or alternatively, how to roll my own method for decomposing a unix timestamp.

推荐答案

这应该是准确的（填写的删节仿结构TM ，我的年使用共同的时代，而不是一个1900 CE纪元）：

This should be accurate (fills out a cut-down imitation of a struct tm, my year uses Common Era instead of a 1900 CE epoch):

struct xtm
{
    unsigned int year, mon, day, hour, min, sec;
};

#define YEAR_TO_DAYS(y) ((y)*365 + (y)/4 - (y)/100 + (y)/400)

void untime(unsigned long unixtime, struct xtm *tm)
{
    /* First take out the hour/minutes/seconds - this part is easy. */

    tm->sec = unixtime % 60;
    unixtime /= 60;

    tm->min = unixtime % 60;
    unixtime /= 60;

    tm->hour = unixtime % 24;
    unixtime /= 24;

    /* unixtime is now days since 01/01/1970 UTC
     * Rebaseline to the Common Era */

    unixtime += 719499;

    /* Roll forward looking for the year.  This could be done more efficiently
     * but this will do.  We have to start at 1969 because the year we calculate here
     * runs from March - so January and February 1970 will come out as 1969 here.
     */
    for (tm->year = 1969; unixtime > YEAR_TO_DAYS(tm->year + 1) + 30; tm->year++)
        ;

    /* OK we have our "year", so subtract off the days accounted for by full years. */
    unixtime -= YEAR_TO_DAYS(tm->year);

    /* unixtime is now number of days we are into the year (remembering that March 1
     * is the first day of the "year" still). */

    /* Roll forward looking for the month.  1 = March through to 12 = February. */
    for (tm->mon = 1; tm->mon < 12 && unixtime > 367*(tm->mon+1)/12; tm->mon++)
        ;

    /* Subtract off the days accounted for by full months */
    unixtime -= 367*tm->mon/12;

    /* unixtime is now number of days we are into the month */

    /* Adjust the month/year so that 1 = January, and years start where we
     * usually expect them to. */
    tm->mon += 2;
    if (tm->mon > 12)
    {
        tm->mon -= 12;
        tm->year++;
    }

    tm->day = unixtime;
}

我的道歉对于所有的幻数。 367 *月/ 12是一个巧妙的方法来产生日历天30/31序列。计算适用于年中开始在三月直到修正到了最后，这让事情变得简单，因为那么闰日落在一个年的结束。

My apologies for all the magic numbers. 367*month/12 is a neat trick to generate the 30/31 day sequence of the calendar. The calculation works with years that start in March until the fixup at the end, which makes things easy because then the leap day falls at the end of a "year".

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