tm结构体的定义在time.h里面
struct tm {
int tm_sec;
int tm_min;
int tm_hour;
int tm_mday;
int tm_mon;
int tm_year;
int tm_wday;
int tm_yday;
int tm_isdst;
};
/*
* linux/kernel/mktime.c
*
* (C) 1991 Linus Torvalds
*/
#include <time.h>
/*
* This isn‘t the library routine, it is only used in the kernel.
* as such, we don‘t care about years<1970 etc, but assume everything
* is ok. Similarly, TZ etc is happily ignored. We just do everything
* as easily as possible. Let‘s find something public for the library
* routines (although I think minix times is public).
*/
/*
* PS. I hate whoever though up the year 1970 - couldn‘t they have gotten
* a leap-year instead? I also hate Gregorius, pope or no. I‘m grumpy.
*/
#define MINUTE 60 //一分钟60秒，这里60是以秒为单位计数
#define HOUR (60*MINUTE) //一小时60分钟
#define DAY (24*HOUR) //一天24小时
#define YEAR (365*DAY) //一年365天
/* interestingly, we assume leap-years */
static int month[12] = {//初始化每个月开始的秒数，即以秒为单位的起始时间
0,
DAY*(31),
DAY*(31+29),
DAY*(31+29+31),
DAY*(31+29+31+30),
DAY*(31+29+31+30+31),
DAY*(31+29+31+30+31+30),
DAY*(31+29+31+30+31+30+31),
DAY*(31+29+31+30+31+30+31+31),
DAY*(31+29+31+30+31+30+31+31+30),
DAY*(31+29+31+30+31+30+31+31+30+31),
DAY*(31+29+31+30+31+30+31+31+30+31+30)
};
long kernel_mktime(struct tm * tm)
{
long res;
int year;
year = tm->tm_year - 70;//从1970年开始计时，year得到的是和70年的年差
/* magic offsets (y+1) needed to get leapyears right.*/
res = YEAR*year + DAY*((year+1)/4);//闰年多一天
res += month[tm->tm_mon];//时间精确到月
/* and (y+2) here. If it wasn‘t a leap-year, we have to adjust */
if (tm->tm_mon>1 && ((year+2)%4))
//因为是从1970年算起的，这里year+2就能耦合上闰年。。。换而言之，1972年是闰年，但是年差是2，这里补上2就对齐了。。。。
res -= DAY;//year不是闰年，则减一天
res += DAY*(tm->tm_mday-1);
res += HOUR*tm->tm_hour;
res += MINUTE*tm->tm_min;
res += tm->tm_sec;//时间精确到秒
return res;
}