《linux 内核完全剖析》 mktime.c

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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;
}